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Final Report

Biofertiliser management: best

practice for agronomic benefit &

odour control

This report examines the potential links between digestate stability and digestate odour potential. It also examines the impacts of separation on digestate nutrient characteristics, and recommends spreading techniques that minimize odour potential while maximising beneficial use of the ammonium content of digestates („biofertilisers‟).

Project code: OAV036-210 Date: September 2011

WRAP‟s vision is a world without waste, where resources are used sustainably. We work with businesses and individuals to help them reap the benefits of reducing waste, develop sustainable products and use resources in an efficient way. Find out more at www.wrap.org.uk

Document reference: [e.g. WRAP, 2006, Report Name (WRAP Project TYR009-19. Report prepared by…..Banbury, WRAP]

Written by: Phil Wallace, Gwyn Harris (SKM Enviros), Jim Frederickson, Graham Howell (The Open University)

Edited by: David Tompkins, WRAP

Front cover photography: Shallow injection of digestate into grass (Picture courtesy of Bryan Lewens, AnDigestion)

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Biofertiliser management: best practice for agronomic benefit & odour

control 1

Executive summary

The PAS110 specification includes a compulsory digestate stability test – developed to demonstrate that input

materials have genuinely been subjected to a digestion process and have not simply been passed through the

system. This Residual Biogas Potential (RBP) test is time consuming to perform, and feedback from industry has

questioned whether the current RBP (stability) limit is set at an appropriate level.

In addition, there have been documented complaints about odours associated with land-spreading of digestate.

This could be due to use of inappropriate spreading practise, or spreading of digestates that might be deemed

„unstable‟, but this was difficult to determine without a clear understanding of the links between stability and

odour. The impacts of separating digestate (into separate fibre and liquor fractions) on stability and nutrient

content were also unknown.

Possible links between odour potential and stability, as well as odour potential and nutrient partitioning associated

with separation of whole digestate into liquor and fibre fractions were investigated. The potential for a

Sequencing Batch Reactor (SBR) approach to increase stability, reduce odour potential and convert ammonium to

nitrate was also investigated. The findings are summarized below:

Anaerobic treatment of biodegradable wastes was shown to be capable of producing digestates with low

odour potentials. However, the relationship between digestate stability and odour potential was not clear since some anomalies were found, in which some stabilised digestates passed the RBP test, but still had

elevated odour potentials. Best practice (for example: Defra, 20101) suggests that digestates be band-spread

at the soil surface or injected into the soil. These measures will reduce ammonia losses and minimise potential odour emissions.

It was recommended that the current RBP threshold be maintained at the level of 0.25 L/g VS, which was

shown to be comparable with the range of RBP values found for cattle slurries, and which seemed to be best associated with digestates with low to medium odour potentials.

Separating the digestate into liquor and fibre fractions generally had little impact on the nutrient profiles of

the different fractions. However, since the liquor fraction comprised by far the majority of the total digestate mass, this represents the most significant reserve of nutrients that could be recovered for agronomic benefit.

Subjecting whole digestates to sequencing batch reactor (SBR) tests successfully converted the ammoniacal-

nitrogen to nitrate-nitrogen. Digestate stability was also greatly increased, and the BOD (Biological Oxygen Demand), COD (Chemical Oxygen Demand), total VFAs (Volatile Fatty Acids)2 and dissolved carbon lowered.

The odour potential and hydrogen sulphide were eliminated – demonstrating the potential of this technique to

reduce digestate handling issues.

1 http://www.defra.gov.uk/publications/files/rb209-fertiliser-manual-110412.pdf 2 BOD and COD are proxy measures for the stability of materials, while VFAs are naturally produced as part of biogas production – and their excessive presence may indicate inefficient digestion

http://www.defra.gov.uk/publications/files/rb209-fertiliser-manual-110412.pdf

Biofertiliser management: best practice for agronomic benefit & odour

control 2

Contents

1.0 Introduction ................................................................................................................................ 4 1.1 Biofertilisers – quality and certification .................................................................................. 4 1.2 Digestate separation ............................................................................................................ 4 1.3 Whole digestate .................................................................................................................. 5 1.4 Liquor ................................................................................................................................ 5 1.5 Solid fraction/fibre ............................................................................................................... 5 1.6 Digestate testing ................................................................................................................. 5 1.7 Project aims........................................................................................................................ 5

2.0 Methodology ............................................................................................................................... 7 2.1 Types of materials ............................................................................................................... 7 2.2 Separation .......................................................................................................................... 8 2.3 Chemical analyses ............................................................................................................... 9 2.4 Odour tests ........................................................................................................................ 9 2.5 Odour potential – apparatus and procedure ........................................................................... 9 2.6 Odour concentration measurements ...................................................................................... 9 2.7 Hedonic Tone Assessment.................................................................................................. 10 2.8 SBR tests .......................................................................................................................... 10

2.8.1 Background .......................................................................................................... 10 2.8.2 Objectives for SBR trials ........................................................................................ 11 2.8.3 The Treatment Units ............................................................................................. 11 2.8.4 Operation ............................................................................................................. 11 2.8.5 Digestates treated ................................................................................................. 12

3.0 Results and discussion ............................................................................................................. 12 3.1 Chemical characteristics of digestate ................................................................................... 12

3.1.1 Basics .................................................................................................................. 12 3.1.2 Nutrients and typical loading rates .......................................................................... 12 3.1.3 Impacts of separation on nutrients, dry matter and Volatile Solids in digestate ........... 13 3.1.4 Potentially Toxic Elements (PTEs) ........................................................................... 15

3.2 Stability ............................................................................................................................ 15 3.2.1 RBP stability values ............................................................................................... 15

3.3 Impacts of separation on stability and odour potential .......................................................... 17 3.3.1 Odour Potentials ................................................................................................... 17 3.3.2 Relationship between stability (RBP value) and Odour Potential ................................ 18 3.3.3 Characterisation of odour ...................................................................................... 20 3.3.4 Alternative metrics for determining digestate stability or degree of digestion .............. 21 3.3.5 Total volatile fatty acid (VFA) concentration ............................................................ 22 3.3.6 BOD and COD/BOD ratio ....................................................................................... 23 3.3.7 Dissolved carbon (DC) ........................................................................................... 24 3.3.8 Hedonic tone ........................................................................................................ 25

3.4 Treatment options/additives ............................................................................................... 27 3.4.1 SBR ..................................................................................................................... 27 3.4.2 Sewer discharge consents ...................................................................................... 29 3.4.3 Additives .............................................................................................................. 30 3.4.3.1 Addition of trace elements ..................................................................................... 30 3.4.3.2 Addition of various chemicals and micro-organisms to control odour .......................... 30 3.4.4 Previous research and support systems ................................................................... 31 3.4.5 Application ........................................................................................................... 31 3.4.5.1 Liquor and Whole Digestate application ................................................................... 32 3.4.5.2 Application – solid fraction/fibre ............................................................................. 33 3.4.6 Other factors for consideration regarding application ................................................ 33 3.4.6.1 Application at correct rate ...................................................................................... 33 3.4.6.2 Odour and other emissions .................................................................................... 33 3.4.6.3 Timing of Applications and Cropping ....................................................................... 34 3.4.7 Spreading equipment sources ................................................................................ 34

Biofertiliser management: best practice for agronomic benefit & odour control 3

4.0 Conclusions and recommendations .......................................................................................... 36 5.0 References ................................................................................................................................ 37 Appendix 1 Data tables ......................................................................................................................... 38 Appendix 2 Hedonic tone graphs .......................................................................................................... 48 Appendix 3 ............................................................................................................................................ 60


1.0 Introduction

Anaerobic digestion (AD) is not a new technology in the UK, but has only recently been used here on any

commercial scale to process inputs such as food wastes, farm manures and slurries, and biomass crops.

Anaerobic digestion results in two main outputs: biogas (a source of renewable energy) and digestate (a source

of crop-available nutrients).

PAS110:2010 is the UK‟s specification for whole digestate, separated liquor and separated fibre derived from the

anaerobic digestion of source-segregated biodegradable materials. This, coupled with the Anaerobic Digestion

Quality Protocol (ADQP), defines the point at which the waste becomes a product and is no longer considered a

waste in Wales, Northern Ireland and England. A similar approach has been adopted in Scotland, but this differs

from the ADQP in some details (SEPA, 2010). The Biofertiliser Certification Scheme3 has adopted the name

„biofertiliser‟ for Quality Digestate that complies with PAS110, the ADQP (where appropriate) and (again, where

appropriate) the Additional Scheme Rules for Scotland. This scheme is overseen by Renewable Energy Assurance

Ltd (REAL – a subsidiary of the Renewable Energy Association), but sites are audited and certified by two

independent certification bodies.

1.1 Biofertilisers – quality and certification

The PAS110:20104 specification for digestate quality has two aims for AD operators: (1) to ensure that digested

materials (biofertilisers) are made using suitable inputs (feedstocks) and processed sufficiently to pass the set

stability threshold, and (2) to ensure that the process has been well managed and monitored to produce quality

biofertiliser that meets market needs and protects the environment when used in accordance with good

agricultural practice. PAS110 does not cover production or quality of biogas.

The ADQP5 sets out additional criteria for the production of biofertilisers from specific (listed) waste types and

imposes various other constraints (for example, biofertilisers must comply with PAS110 and may only be used in

identified markets if specific records of such are kept). Compliance with these criteria is considered sufficient to

ensure that the material is fully recovered (complies with European „End of Waste‟ criteria) and can be used

without the need for further waste management controls. In addition, the ADQP indicates how compliance may

be demonstrated and points to best practice for the use of the fully recovered product. The ADQP aims to provide

increased market confidence in the quality of products made from wastes and reduce the financial burden of

compliance with waste regulation, encouraging greater recovery and recycling. Neither PAS110 nor the ADQP

exempt sites processing wastes from Environmental Permitting or other regulatory controls such as the Animal

By-Products Regulations.

If an AD operation complies with both PAS110:2010 and the ADQP then the digestate produced is not legally

considered a waste in England, Wales or Northern Ireland, and the Environmental Permitting Regulations (Waste

Management Licensing Regulations in Northern Ireland) no longer apply to the spreading of the biofertiliser to

land. A similar approach has been adopted in Scotland, but this differs from the ADQP in some details SEPA

(2010).

1.2 Digestate separation

Digestate can be utilised as is from the digester (whole) or it can be separated into „liquor‟ and „fibre‟ fractions

prior to land application. It should be noted that certification under the Biofertiliser Certification Scheme relates

to specific digestate fractions, which must be used in the form as supplied – separation of whole quality digestate

on farm would invalidate the „product‟ status of that material. If digestate is delivered to the farm as separate

fractions, then each should be accompanied by the agronomic data required by PAS110 to maximise crop benefit

from the different nutrient fractions present in the different digestate fractions.

3 http://www.biofertiliser.org.uk/ 4 Specification for whole digestate, separated liquor and separated fibre derived from the anaerobic digestion of source-segregated biodegradable materials. Available from: http://www.biofertiliser.org.uk/pdf/PAS-110.pdf 5 Quality Protocol. Anaerobic Digestate. End of waste criteria for the production and use of quality outputs from the anaerobic digestion of source segregated biodegradable waste. Waste Protocols Project (Environment Agency and WRAP), September 2009. (Abbreviated title Anaerobic Digestate Quality Protocol (ADQP). Available from: http://www.environment-agency.gov.uk/static/documents/Business/AD_Quality_Protocol_GEHO0610BSVD-E-E.pdf

http://www.biofertiliser.org.uk/

http://www.biofertiliser.org.uk/pdf/PAS-110.pdf

http://www.environment-agency.gov.uk/static/documents/Business/AD_Quality_Protocol_GEHO0610BSVD-E-E.pdf

http://www.biofertiliser.org.uk/pdf/PAS-110.pdf




1.3 Whole digestate

Whole digestate is unprocessed (in terms of physical properties) digestate delivered from the AD plant. The

physical and chemical properties of the whole digestate will vary according to feedstock input, AD process and

operation but typically the whole digestate will be between 6 – 12% dry matter (DM), and from some AD

processes („dry‟ processes) even higher. The whole digestate may also contain a fraction of a percentage of some

larger particles of plastics as well as materials such as small stones. When considering land applications of whole

digestate, the selection of spreading machinery should consider these physical attributes as the whole digestate

may be approaching the upper limits for effective distribution through conventional slurry application systems.

The whole digestate can be physically separated (e.g. via a belt press or centrifuge) to produce two fractions

called the liquor and the solid fraction (or fibre). These treatment processes are similar to the processes used for

the separation of livestock and other slurries into liquid and solid fractions.

1.4 Liquor

The coarse fibres will (to some extent) have been removed in this fraction. The liquor produced is generally more

easily applied through conventional band and injection spreading equipment. Liquor tends to contain up to 6%

DM, but this will be subject to the type of post-process separation completed.

1.5 Solid fraction/fibre

This fraction can be used effectively as a soil conditioner. The fibre may be subjected to aerobic composting prior

to application, which improves its stability, reduces its biological activity and lowers its ammonium nitrogen

content.

1.6 Digestate testing

Within PAS110 there are tests that have to be conducted on the products. These include:

nutrient content (total N, P, K and ammoniacal-N, sodium and chloride),

stability as the residual biogas potential (RBP) test,

volatile fatty acids (VFAs),

dry matter,

pH,

organic matter,

potentially toxic elements (PTEs) and

physical contaminants.

1.7 Project aims

The digestate stability test (Walker et al., 2010) determines the residual biogas potential of the sample under

test. The digestate has to pass with a result below the upper limit of 0.25 l / g Volatile Solids (VS). This limit

was based on a study of the RBP values of a limited number of samples of organic materials already commonly

applied to agricultural land (cattle slurry, pig slurry and anaerobically digested municipal wastewater biosolids).

This project was designed to test a greater range and number of materials to demonstrate that this upper limit

remains valid. Materials examined included samples of farm manures and slurries, biosolids and the digestates

that are now being produced from facilities treating source-segregated wastes.

Whilst some operators of commercial AD facilities may believe that the RBP is limit is too low, there are also those

who believe that the RBP limit is too high and that, at this limit, the digestates may be potentially sufficiently

odorous to routinely cause a nuisance when spread (as are some of the other (undigested) materials already

commonly spread on farmland). The available nitrogen in digestates is usually in the ammonium form. When

spread, this can be transformed through volatilisation into ammonia, which is one of the compounds that

commonly causes odour when at sufficiently high concentrations. Through secondary treatment, the ammonium

in digestates can be biologically converted to nitrate, reducing both its odour potential and opportunities for

ammonia volatilisation, potentially increasing the crop benefit of this valuable renewable fertiliser. Appropriate

spreading techniques might also be used to achieve similar ends.


Anaerobic digestion is also a reasonably conservative process, with little (~10%) of the input mass lost during

digestion. Whilst the resulting whole digestate has considerable agronomic value, the low dry matter content can

make it uneconomic to transport the whole digestate sufficiently far to fully realise its agronomic value. There is

therefore considerable interest in understanding how digestate volumes can be decreased, and the impacts that

separate approaches have on the agronomic value of the separated fractions.

The aims of this project were:

to investigate relationships between digestate odour and digestate stability (as determined by the PAS110

Residual Biogas Potential test), and stability and odour management through the use of appropriate spreading techniques;

to find out if there are alternative processing systems or process additives that could reduce the odour

potential of digestates;

to assess alternatives to the current PAS110 RBP test;

to investigate the impact of physical separation on the nutrient properties of different digestate fractions;

to formulate best practice for the spreading of digestates on agricultural land to minimise odours, to maximise nutrient benefits and to minimise the potential for environmental pollution; and

to produce a document outlining guidelines for the best practice in digestate spreading, including information

on sources of spreading equipment.


2.0 Methodology

This project was managed by SKM Enviros in collaboration with the Open University. Silsoe Odours Ltd undertook

the gas stripping and odour testing of the digestates.

2.1 Types of materials

Digestates were collected from nine AD facilities in Great Britain. Subsamples of two of these digestates were

processed through the research Sequencing Batch Reactor (SBR) units at SKM Enviros to produce a further two

post-treatment samples for „before‟ and „after‟ reactor testing. Five cow manure slurries, one cow/pig manure

slurry mix, and one digested sewage sludge were also collected for testing and analysis (Table 2-1). To maintain

the anonymity of those sites that were visited during this project, limited detail on input materials and typical AD

operating parameters (such as mean hydraulic retention times or the use of process additives) can be provided.

However, an indication of the types of digestate collected is given in Table 2-2.

Some digestates were derived from commercial and municipal food wastes, and the whole digestates were

approximately 2 to 6% dry matter contents. Some digestates were made from purpose-grown crops, such as

silage, mixed with farm slurry. Typical hydraulic retention times varied.

Table 2-1 Numbers of materials collected

Material Number collected Notes

Digestates 9

7 digestates were received as whole

digestates and 2 digestates as

separated liquids

4 whole digestates were

subsequently separated into liquid

and fibre fractions

2 digestates were also processed

through the SBR unit

Cow manure slurries 5

Pig manure slurries 4 1 separated into liquid and fibre

fractions

Cow/pig manure slurry mix 1

Biosolids (digested sewage) 1


2.2 Separation

A range of digestates was collected. Separation of four of the whole digestates (into fibre and liquor fractions)

and one pig manure slurry was carried out at the Open University (for method see Appendix 3). The digestate

samples processed were samples W29/13, 14, 16 and 20. The whole digestate was coded „w‟ and the liquor and

fibre as „l‟ and „f‟, respectively. Where separated liquors were produced, these were less than 2% dry matter.

Table 2-2 List of materials collected

OU Code Description Report Code Input type during study

period

W29/08 Digestate (separated liquor) Digestate A cattle slurry

W29/13 Digestate (whole) Digestate B food waste

W29/14 Digestate (whole) Digestate C cattle slurry with some maize

silage

W29/02 Digestate (whole) Digestate D (used in SBR2) mainly food waste

W29/03 Digestate (whole) Digestate E mainly food waste

W29/01 Digestate (separated liquor) Digestate F (used in SBR1) food waste with some cattle slurry

W29/20 Digestate (whole) Digestate G food waste

W29/16 Digestate (whole) Digestate H food waste

W29/12 Digestate (whole) Digestate I mainly food waste

W29/21 Treated Digestate Digestate SBR 1

W29/22 Treated Digestate Digestate SBR 2

W29/05 Whole cow slurry Cow slurry 1





W29/10 Pig and cow mix slurry Cow/Pig slurry

W29/15 Whole pig slurry Pig slurry 1




W29/18 Digested sewage Digested sewage


2.3 Chemical analyses

In addition to the odour and SBR tests described below, the following chemical and stability tests were performed

on the digestates: (for Open University methods, see Appendix 3).

Tests on whole samples:

Residual Biogas potential (RBP) in triplicate as per Walker et al. (2010)

pH (diluted to 5% dry solids where required)

Electrical Conductivity (EC) (diluted to 5% dry solids where required)

Total Nitrogen (N), Phosphorus (P), Potassium (K), Magnesium (Mg) and Sulphur (S).

Biological Oxygen Demand (BOD)

Chemical Oxygen Demand (COD)

Potentially Toxic Elements (PTEs): Zinc (Zn), Copper (Cu), Nickel (Ni), Mercury (Hg), Cadmium (Cd), Chromium (Cr) and Lead (Pb)

Tests on filtrates:

Volatile Fatty Acids (VFA)

Ammonium in solution plus other ions: Calcium, Magnesium, Potassium, Sodium, Chloride, Nitrate, Phosphate,

Sulphate.

Total dissolved carbon as purgeable dissolved carbon and non-purgeable organic carbon (NPOC).

Dry Matter (DM), Loss on Ignition at 550°C (LoI) and Volatile Solids (VS)

2.4 Odour tests

The olfactometry and hedonic tone tests were carried out by Silsoe Odours Ltd. The Silsoe Odour Laboratory is

accredited by UKAS (Testing Laboratory 0604) for odour determination by dynamic olfactometry. The odorous

gases were firstly stripped from the digestates using a standard technique (adapted from Hobson, 2002) , “Odour

Control in Wastewater Treatment – A Technical Reference Document” UKWIR Report Ref No.01/WW/13/3). The

odours were then tested by an odour panel with a sample retained for gas chromatography-mass spectrometry

(GCMS) characterisation by the Open University.

2.5 Odour potential – apparatus and procedure

The apparatus included:

A 20-litre plastic or stainless steel aeration cylinder;

A supply of odour free air;

A sampling arrangement, allowing both instantaneous and volumetric flow measurements.

The procedures for odour potential measurement are:

1 Measure the temperature of the liquid.

2 Fill the aeration cylinder with the 15litre liquid sample (digestate / slurry etc). If necessary, for sludges, dilute

the liquid sample to a final suspended solid concentration of <1%. Record the dilution factor.

3 Sparge air through the cylinder at a flow rate of 5 litres per minutes for 2 minutes to purge the headspace.

4 Collect 15 litres of the subsequent offgas using a Nalophan NA sample bag.

5 Measure the H2S concentration in the odour sample using a Jerome 631X monitor (0-50 ppm), or a Drager

tube (>50 ppm) depending on the range of concentrations detected.

6 Empty and wash the aeration cylinder.

7 The odour potential of the liquid sample is equal to the odour strength of the collected air multiplied by the

dilution factor. The equivalent H2S concentration was also adjusted to account for the pre-dilution of the sample.

2.6 Odour concentration measurements

Odour concentration measurements are made following the protocols described in BSEN13725:2003 'Air quality -

determination and odour concentration by dynamic dilution olfactometry‟. Olfactometry employs a panel of

human noses as sensors. A human nose can detect odour at concentrations well below the sensitivity levels of

chemical analytical methods. This measurement of odour concentration based on dilution of an odour sample to

the odour threshold is the most widespread method to quantify odours.

To establish the odor concentration, an olfactometer is used which employs a group of six panelists. A diluted

odorous mixture and an odour-free air are presented from sniffing ports to the six panelists. (The panelists must


fulfill certain requirements regarding their sensitivity of odour perception. The panel calibration gas used is Butan-

1-ol., which at a concentration of 40ppb gives an odour concentration of 1 ouE/m³). In comparing the odour

emitted from each port, the panelists are asked to report if they can detect a difference between the ports. The

gas-diluting ratio is then decreased by a factor of 1.6 (i.e. the concentration is increased accordingly). The

panelists are asked to repeat their judgment. This continues until each of the panelists respond „certain‟ and

correct twice in a row. The numerical value of the odour concentration is equal to the dilution factor that is

necessary to reach the odour threshold. Its unit is the European Odour Unit, ouE. Therefore, the odour

concentration at the odour threshold is 1 ouE by definition.

2.7 Hedonic Tone Assessment

In addition to odour, hedonic tone was determined. Hedonic assessment is the process of scaling odours on a

scale ranging from extremely unpleasant via neutral up to extremely pleasant. It is important to note that

intensity and hedonic tone, whilst similar, refer to different things. That is, the strength of the odour (intensity)

and the pleasantness of an odour (hedonic tone). Moreover, it is important to note that perception of an odour

may change from pleasant to unpleasant with increasing concentration and intensity.

The German guidelines „VDI 3882 Olfactometry; Determination of Hedonic Odour Tone‟ was used as a basis for

the assessment of hedonic tone and adapted for use with the olfactometer when making odour concentration

measurements following the protocols described in BSEN13725:2003. When the panelists responded with a

„certain‟ response to the odour presented during determination of the odour concentration, they were asked to

rank the hedonic tone on a +4 to -4 scale. Extremely unpleasant is -4, zero is neither pleasant nor unpleasant

and +4 is extremely pleasant. Panelists were asked to rank the hedonic tone until all the panelists had responded

to at least four supra threshold concentrations.

The results are presented graphically in Appendix 2, which show the mean panel responses to the odour

concentration presented.

2.8 SBR tests

2.8.1 Background

Digestates may exhibit relatively high concentrations of ammoniacal nitrogen (NH4-N). Also, depending upon the

degree of anaerobic degradation that has taken place, they can contain high concentrations of volatile fatty acids

(VFAs), which impact upon chemical oxygen demand (COD) and odour. Likewise, in landfills, liquids (termed

leachates) with high NH4-N and VFAs also occur. These liquids are routinely treated prior to discharge to sewer or

watercourse, and most reported successful treatment of landfill leachates has been carried out using aerobic

biological processes within a Sequencing Batch Reactor (SBR) system.

SBR treatment systems not only readily reduce high concentrations of COD, but also high concentrations of

ammoniacal-nitrogen by a process known as nitrification. Nitrification is the aerobic biological conversion of

ammoniacal nitrogen, (NH4-N), to nitrate nitrogen, (NO3-N).

The concentration of NH4-N contained in digestates will depend on the feedstocks. High protein input materials

such as food wastes are likely to lead to higher concentrations of ammonium. If the ammoniacal nitrogen was to

be converted to nitrate nitrogen prior to farm spreading, then there would be a lower odour potential as the latter

has no smell, and the agronomic value would be enhanced because less nitrogen would be lost through

volatilization to the atmosphere during spreading.

Given the success of Sequencing Batch Reactor approaches in reducing odours and ammoniacal loadings in

leachates, it was considered worthwhile trialling this approach for digestate management. Therefore two of the

digestates collected for analysis at the Open University were also processed through the SBR treatability trial

units based at SKM Enviros‟ Shrewsbury site.


2.8.2 Objectives for SBR trials

The objectives of the SBR trials were as follows:

(i) Digestate treatability: To confirm whether there are any fundamental features of digestate quality that

are likely to impair successful treatment by standard aerobic biological processes in a Sequencing Batch

Reactor (SBR) system.

(ii) To investigate nitrification of ammoniacal nitrogen to nitrate nitrogen;

(iii) To investigate reduction of odours;

(iv) To investigate reduction of VFAs;

(v) To determine the effect of treatment on RBP.

In order to test these objectives, two digestates were subject to SBR treatment, to enable „before‟ and „after‟

comparisons of ammoniacal nitrogen, odour, VFAs and RBP to be made.

2.8.3 The Treatment Units

The pilot-scale treatability units were constructed as shown in Figure 2-1, and consisted of a polyethylene

wheeled bin, modified to act as an aeration unit. A 20mm pipe was inserted to act as a small bellmouth overflow

weir, at a measured level within the tank. The discharge end of this pipe was connected via a solenoid valve to

an effluent storage container. Air was supplied from a compressor, enabling both oxygenation and mixing to

occur. A dosing pump was used to deliver digestate feed from the storage tank into the aeration unit.

Figure 2-1 General laboratory setup of the SBR trials

The treatment cycle followed an automatic 24 hour cycle comprising the following phases – feed and aeration,

settlement, discharge. After a quiet period, the 24 hour cycle repeats.

2.8.4 Operation

The trial units were seeded with sludge that was already acclimatised to concentrations of ammoniacal nitrogen

similar to those in the digestates under test, and the trials conducted between January and March 2011.

The 24 hour cycle of feed, aeration, settle, discharge was operated every day throughout the trial period. Prior to

discharge each day, samples were tested for ammoniacal nitrogen, nitrite, nitrate and pH. The volume of

digestate to be fed for the next cycle was determined, and pH adjusted as necessary to maintain it within the

optimum range for nitrification. The temperature of each unit was maintained within the range 240C – 270C

throughout the trial period.


2.8.5 Digestates treated

Two digestates were treated as follows:

Digestate D – After 2.8 SBR container volumes had been passed through the trial unit, a 25 litre sample was delivered to the Open University for comparison testing.

Digestate F – After 4 SBR container volumes had been passed through the trial unit, a 25 litre sample was

again delivered to the Open University for comparison testing.

The „before‟ and „after‟ results are presented and discussed at Section 3.1.3

3.0 Results and discussion

Results are discussed in the sections below. Raw data tables are provided in the Appendix 1 and hedonic tone

graphs in Appendix 2.

3.1 Chemical characteristics of digestate

3.1.1 Basics

Table A 1 and Table A 2 in Appendix 1 show data on the basic properties of the materials. The pHs are generally

slightly alkaline (6.8 to 8.4). The electrical conductivity (EC) represents the salt concentration (made up by the

cations e.g. ammonium, potassium, calcium, magnesium and sodium, and the anions e.g. chloride, sulphate,

nitrate and phosphate, plus organic ions) and ranged from 7.9 to 23.6 mS/cm. The range of ECs of the

digestates was similar to that of the farm slurries.

The dry matter contents of the dairy and pig slurries were typical of these materials, ranging from 7 to 10 % and

1 to 5%, respectively. The digestates were from wet digestion processes and their dry matter contents ranged

from 1 to 5%. The digested sewage had a dry matter content of 5%.

The organic matter contents of the materials were measured by loss on ignition of the dry matter, and volatile

solids (VS) in the materials calculated. The dry matter itself comprised over 50% organic matter in all materials,

and up to around 82% in the cow slurries. The VS contents ranged from 0.6% to 9%, being higher in the cow

slurries. The higher the VS content, the greater the amount of organic matter that will be applied to the soil per

tonne of material.

3.1.2 Nutrients and typical loading rates

Table A 15 and Table A 17 (Appendix 1) provide the data for the principal nutrient elements in the materials

(total N, P, K, Mg, Ca and S). These data have been transformed into farm fertiliser equivalents (Table A 16 and

Table A 18).

Table 3-1 Nutrient contents of whole materials

units Whole digestate

(n=9)

Dairy slurries

(n=6)

Pig slurries

(n=4) Sewage

Dry matter (DM) % 3.1 9.2 2.4 5.5

pH 8.0 7.2 7.3 7.9

Total N kg/m3 (n=8) 4.3 4.0 3.1 4.3

Ammonium N kg/m3 1.55 0.75 1.19 0.77

Total P2O5 kg/m3 0.8 1.4 1.0 3.8

Total K2O kg/m3 2.4 4.3 2.5 0.3

Total MgO kg/m3 0.2 0.9 0.5 0.3

Total SO3 kg/m3 0.6 3.2 0.6 1.9

The nitrogen availability depends on the proportion of ammonium-N to total N, how and when it is spread, and if

it is retained in the soil or lost to the atmosphere as ammonia. Almost all of the potassium (90%) can be taken


as available as the majority will be in solution. Phosphate availability is dependent on soil pH, but 50% can

usually be used as a release figure over the crop rotation (Fertiliser Manual (RB2096)).

The ions in solution were measured (see Appendix 1 Table A 11 and Table A 12). Due to the generally alkaline

pH of the materials, phosphate in solution was very low. During the AD process, nitrogen is made more available

as ammonium. As a proportion of the total N, a maximum of 70% ammonium was found in one digestate with

the others being between approximately 20 to 40%, generally greater than the farm slurries.

Taking the average data of the whole digestates as an example, the maximum amount of material that could be

applied to achieve 250 kg N/ha in an NVZ area would be 58 m3/ha and the amounts of total nutrients that would

be applied are shown in Table 3-2.

Table 3-2 Nutrients applied per hectare (from whole digestate @ 58 m3/ha)

kg/ha

Total N 250

Ammonium N 90

Total P2O5 47

Total K2O 140

Total MgO 12

Total SO3 35

A rough guide to crude fertiliser values of various materials is available at

http://www.wrap.org.uk/farming_growing_and_landscaping/compost_calculator.html. This guide takes the

approach (as per the Fertiliser Manual (RB20935)) that all of the phosphate and potash will become available to

crops over the following rotations and are as effective as artificial fertiliser.

3.1.3 Impacts of separation on nutrients, dry matter and Volatile Solids in digestate

Five of the materials were physically separated by screening through a sieve. The five materials that were

separated into liquid and fibre fractions were two digestates that passed the RBP test (B and C), two digestates

that failed the RBP test (G and H) and one pig slurry as a comparison with non-digestates. The volumes of each

fraction were on average 90.9 % liquor and 9.1 % fibre of the whole, resulting in average dry matters of 3.7 %

and 9.2 % for the liquor and fibre fractions, respectively, compared with the whole material at 4.0% DM (Table

3-3).

6 http://www.defra.gov.uk/publications/files/rb209-fertiliser-manual-110412.pdf


http://www.wrap.org.uk/farming_growing_and_landscaping/compost_calculator.html



Table 3-3 Separation percentage, dry matter, LOI and VS

% of whole DM % LOI % DM VS %

Pig slurry

Whole 4.81 71.7 3.42

Liquor 93.2 4.56 72.9 3.32

Fibre 6.80 9.68 84.4 8.16

Digestate B

Whole 4.50 66.8 3.01

Liquor 90.1 4.35 65.4 2.84

Fibre 9.90 5.91 75.4 4.46

Digestate C

Whole 5.24 74.7 3.92

Liquor 78.2 4.41 70.0 3.08

Fibre 21.8 9.36 83.0 7.76

Digestate H

Whole 2.05 63.9 1.31

Liquor 98.4 1.90 63.6 1.21

Fibre 1.60 9.80 87.9 8.61

Digestate G

Whole 3.57 76.8 2.74

Liquor 94.8 3.45 76.3 2.64

Fibre 5.20 11.3 77.9 8.84

Average

Whole 4.00 70.8 2.88

Liquor 90.9 3.70 69.6 2.62

Fibre 9.10 9.20 81.7 7.57

The organic matter content of the dry matter in the fibre was greater (81.7%) than in that of the liquor (69.6%)

leading to three times more volatile solids in the fibre material. The fibre, when spread, will therefore lead to

more organic matter being applied per tonne.

Separation of the five digestates resulted in an increase in the DM contents of the fibre materials to 9.2% and a

decrease in the liquor fraction to 3.7%; the organic matter contents changed to 7.6 and 2.6%, respectively. This

organic matter, when applied at the rates possible governed by the total nitrogen content of the digestate, will

provide soil conditioning effects.

The nutrient contents of the various digestates and their fractions are shown in Table 3-1. The nutrient profiles of

the separated fractions are reasonably similar, but of most interest is the relative proportions of the different

fractions resulting from the separation (Table 3-4). Given the very large liquor proportion, the majority of

nutrients will be present in this liquid fraction.

Table 3-4 Nutrient contents of separated materials kg/m3

Total N P2O5 K2O SO3 Ca MgO

Pig slurry

Whole 4.1 2.5 3.5 1.1 2.1 1.3

Liquor 4.0 2.4 3.3 0.9 2.0 1.2

Fibre i/s 2.8 3.4 1.5 2.6 1.3

Digestate B

Whole 4.1 1.2 3.2 1.2 2.3 0.2

Liquor 4.0 1.2 3.3 1.2 2.2 0.2

Fibre 4.3 1.3 3.5 1.3 2.4 0.2

Digestate C

Whole 3.7 1.3 4.2 1.0 1.3 0.7

Liquor 3.3 1.2 3.8 0.8 1.2 0.7

Fibre 2.8 1.1 3.9 1.2 1.4 0.6

Digestate H

Whole i/s 0.6 1.5 0.3 0.7 0.2

Liquor 3.6 0.6 1.4 0.2 0.6 0.2

Fibre i/s 1.3 1.8 1.0 2.1 0.5

Digestate G

Whole 4.2 1.0 2.3 0.4 0.7 0.3

Liquor 4.3 0.9 2.3 0.4 0.6 0.3

Fibre 13 3.7 2.3 1.0 4.7 1.2

Average

Whole 4.0 1.3 2.9 0.8 1.4 0.5

Liquor 3.9 1.2 2.8 0.7 1.3 0.5

Fibre 6.7 2.1 3.0 1.2 2.6 0.8

i/s=insufficient sample


3.1.4 Potentially Toxic Elements (PTEs)

Limited PTE analysis was carried out on the digestates and slurries (see Appendix 1 Table A 17 and Table A 18).

Biofertiliser applications must adhere to the maximum permissible annual rate of PTE addition over a 10-year

period as per the Code of Practice for the Agricultural Use of Sewage Sludge7 (the „Sludge Code‟). Copper and

zinc supplements are often added to pig feeds, which can lead to relatively high levels in pig slurries. All of the

pig slurries were above the 400 mg/kg PAS110 limit for zinc on a dry matter basis and two were above the

copper limit. One cow slurry exceeded the chromium limit (100 mg/kg dry matter).

Four digestates passed the PTE limits on a dry matter basis, three of these were mainly food waste derived.

Occasional exceedences of chromium, copper, nickel and zinc were found for other digestate samples. However,

the loading rates (on a dry matter basis) in these cases would still be extremely low and would not present a risk

to humans, animals, crops or the environment. It should also be considered that, when applying biofertilisers to

land, the receiving soils must first be analysed to baseline their PTE contents. The PTE loading from the applied

biofertilisers must then be calculated and compared with soil limits according to the ADQP requirements to ensure

that future risks are avoided.

3.2 Stability

3.2.1 RBP stability values

The Residual Biogas Potential (RBP) test aims to estimate the stability of digestates by recording the potential

volume of biogas (methane and carbon dioxide) produced by samples under ideal anaerobic test conditions. The

RBP result is presented as litres of biogas produced per g of volatile solids of the sample being tested (Walker,

M). Materials containing a high proportion of easily biodegradable carbon will produce more biogas and will have

a higher RBP value compared with a material containing a low proportion of easily biodegradable carbon. The

material containing a low proportion of easily biodegradable carbon (low RBP value) would be considered to be

more stable as it would be less biodegradable under anaerobic conditions.

Walker et al. (2010) recommended that the pass/fail stability threshold for PAS110 be set at the approximate

maximum level of RBP (0.25 litres biogas/g vs) that was found for cattle and pig slurries (n=4) and sewage

sludges (n=6) which were investigated during their study. It was argued that it was appropriate to set the

threshold at this level as slurries and sewage sludges are commonly permitted to be spread to land. However, it

was acknowledged by Walker et al., (2010) that setting the RBP threshold in this way was a relatively arbitrary

measure, and further research into the environmental impact of the threshold levels was recommended, as was

identification of further criteria for setting the threshold.

The research findings presented here (Figure 3-1) show that of the nine whole digestates tested, six passed the

RBP stability test. Of these six passes, three had RBP values much lower than the threshold (0.04, 0.06 and 0.09

litres biogas/g vs) and of these, two digestates were derived exclusively or mainly from cattle slurries (A and C)

while digestate B was derived from an AD plant treating food waste but employing a very long hydraulic retention

time. The three digestates which had values approaching the RPB threshold (0.18, 0.19 and 0.22 litres biogas/g

vs) were derived mainly from food waste.

Of the three digestates that failed the test, one digestate only just failed, with a high acetic acid concentration

suggesting it was quite active but not fully stabilised. The plant from which this was derived was still undergoing

commissioning. Two further digestates had very high RBP values which greatly exceeded the threshold level and

these also had relatively high digestate propanoic acid concentrations, indicating that the anaerobic digestion

processes from which the digestates had been derived may have been inhibited.

Figure 3-2 shows RBP values for the cattle and pig slurries. In general, the RBP values for the cattle slurries and

the mixed cattle and pig slurry tested were broadly comparable to the RBP threshold. However, the RBP values

for the pig slurries greatly exceeded the RBP threshold. It would be expected that the cattle slurries would have

RBP values lower than those for the pig slurries since the cattle slurries have already gone through a process of

anaerobic digestion in the guts of the cattle from which the slurries were derived. This anaerobic digestion

process would selectively digest the fraction of the organic matter in the cattle feed which would be amenable to

7 http://archive.defra.gov.uk/environment/quality/water/waterquality/sewage/documents/sludge-cop.pdf


anaerobic digestion leaving the anaerobic RBP test to respond to the partially stabilised residue only. The pig

slurry, by contrast, would be expected to contain material which would be highly biodegradable under the

anaerobic conditions of the RBP test.

Based on the data from this study, it might be observed that high-stability (i.e. low RBP value) digestates can be

produced from food-based inputs using very long retention times and from inputs based on cattle slurries utilising

typical retention times in the range 25-30 days. Also, it would appear that it is possible to achieve the PAS110

RBP stability threshold for digestates derived from mainly food-based inputs using retention times within the

range 20 to 60 days.

Figure 3-1 RBP values for the nine digestates (A to I) tested. The PAS110 RBP threshold is 0.25 L/g VS as shown

Figure 3-2 RBP values for the five cattle slurries (C1 to C5), cattle plus pig slurry (CP) and pig slurries (P1 to P4)

tested. The PAS110 RBP threshold is 0.25 L/g VS as shown


3.3 Impacts of separation on stability and odour potential

For the whole and separated digestates as presented in Table 3-5, it can be seen that the RBP results for whole

digestates and their separated liquor and fibre fractions can differ. This is because the RBP test is responding to

the relative amounts of biodegradable carbon found in each of the fractions, and also to the degree of

biodegradability of the carbon found in each fraction. For example, while whole digestate B easily passed the

PAS110 RBP test as did the separated liquor fraction, the fibre fraction showed an elevated RBP result which

approached the RBP threshold of 0.25 L/g VS. In this case it is likely that VFA concentrations were very low in the

whole digestate and separated liquor fraction, while the carbon in the fibre material was moderately

biodegradable. Digestate C was similar, except that the fibre fraction for this digestate appeared to contain

carbon which was less readily biodegradable under anaerobic conditions, resulting in a low RBP value.

Overall, it is important to recognise the fact that whole digestate and respective liquor and fibre fractions can

have different RBP values. Furthermore, since RBP values for separated liquor and fibre fractions will depend on

the concentration of VFAs in solution and the biodegradability of the solid carbon in the fibre, it is difficult to

predict how separation of the whole digestate will affect the RBP values for the resulting fractions.

For the highly odorous pig slurry, it would appear that separating the whole slurry greatly increased the odour

potential of the separated liquor fraction compared with the original whole slurry. However, for the digestates

which were characterised as having relatively little odour, it was found that the fibre fractions usually had slightly

higher odour potentials compared with the liquor fractions.

Table 3-5 Stability

COD g/L BOD g/L COD/BOD RPB

L/g VS

Odour

Potential

Pig slurry

Whole 44.7 28.3 1.60 0.638 3010

Liquor 81.8 38.2 2.10 0.546 10,600

Fibre 0.338 22.0

Digestate B

Whole 20.9 8.20 2.50 0.061 8.00

Liquor 50.0 8.90 5.60 0.062 6.00

Fibre 0.185 30.0

Digestate C

Whole 46.3 42.1 1.10 0.086 575

Liquor 41.9 11.6 3.60 0.097 249

Fibre 0.039 10.0

Digestate H

Whole 31.8 17.1 1.90 0.531 37.0

Liquor 29.0 17.7 1.60 0.406 12.0

Fibre 0.224 21.0

Digestate G

Whole 37.2 18.6 2.00 0.311 3.00

Liquor 40.9 20.7 2.00 0.410 13.0

Fibre 0.418 46.0

Average

Whole 36.2 22.9 1.82 0.325

Liquor 48.7 19.4 2.98 0.304

Fibre 0.241

3.3.1 Odour Potentials

Determination of Odour Potential for the digestates and slurries involved two distinct stages. Odours were first

stripped from each of the digestates and slurries using a controlled volume of odour-free air, and the resulting

gases were then pre-diluted before being presented to a testing panel during an assessment process called

dynamic olfactometry. The off-gas for each sample under test was further diluted in a series of steps until

panellists were confident that the odour threshold had been established (i.e. the point at which odour is just

detected; the odour concentration at the odour threshold is 1 ouE by definition). The degree of dilution for each

sample represents the Odour Potential and is expressed as an odour concentration (ouE m-3).

It should be appreciated that the odour stripping technique and the use of dynamic olfactometry to determine

Odour Potential is a laboratory-based method that is appropriate to ranking relatively similar materials in terms of

their potential to generate odour. Findings from this exercise should not be taken to reflect the actual odour

impact of those materials in the environment.


In general, the Odour Potentials for the untreated pig and cattle slurries were much higher than those for the

digestates. That is to say, the odours that were stripped from the slurries had much higher odour concentrations

compared with the digestates, as the gases stripped from the slurries required much more dilution to reach the

odour threshold.

The mean odour potentials for the three main types of materials tested were:

digestates 178 x 103 ouE m-3

cattle slurries 2,542 x 103 ouE m-3

pig slurries 15,125 x 103 ouE m-3

The odour potentials for the digestates selected for the study and for the comparison slurries are presented in

Table 3-6.

Table 3-6 Odour potentials for digestates and comparator slurries

Digestates

Odour

potential

103 ouE/m3

Comparator slurries

Odour

potential

103 ouE/m3

Digestate A (liquor) 5 Cow slurry 1 639

Digestate B 8 Cow slurry 2 3570

Digestate C 575 Cow slurry 3 6550

Digestate D 133 Cow slurry 4 1840

Digestate E 4 Cow slurry 5 113

Digestate F (liquor) 888 Cow/Pig slurry 5730

Digestate G 3 Pig slurry 1 45000

Digestate H 37 Pig slurry 2 3940

Digestate I 5 Pig slurry 3 3010

Digestate SBR 1 2 Pig slurry 4 8550

Digestate SBR 2 0.3 Digested sewage 30

3.3.2 Relationship between stability (RBP value) and Odour Potential

In terms of odour emissions from slurries and digestates it might be expected that anaerobic conditions during

AD treatment or storage would encourage the generation of odorous metabolic products such as reduced sulphur

compounds and hydrogen sulphide, ammonia, amines and thiols. A three-stage scheme for complete anaerobic

degradation of organic matter is usually proposed (Mackie et al., 1998). The first stage, or acid-forming stage,

involves a complex range of bacteria that hydrolyze the primary substrate polymers such as polysaccharides,

proteins, and lipids and ferment them, producing fatty and other organic acids, alcohol, NH3, sulphur compounds,

CO2, and H2. Propionate and longer-chain fatty acids, some organic acids, and alcohols are subsequently

degraded by a second intermediate group of bacteria called the obligate H2-producing acetogenic bacteria.

Finally, methanogens rapidly utilize the H2 produced by other bacteria to reduce CO2 to CH4, while the aceticlastic

methanogens cleave acetate to CH4 and CO2.

Efficiency of methane fermentation during complete anaerobic digestion is related to two important operational

factors: the hydraulic retention time and the volumetric organic matter loading rate. The rate-limiting step in

most fermentations is degradation of fatty acids. This clearly suggests that odorous metabolic carbon compounds

such as volatile fatty acids and reduced sulphur compounds which are readily produced during the anaerobic

digestion process can accumulate with high digester loading rates accompanied by low rates of conversion to

biogas. Odorizzi et al., (2003) contend that odour from waste treatment plants is associated with the stability (i.e.

degree of decomposition) of the waste being treated. Hence, it is likely that odour is associated mainly with the

early stages of biological waste treatment, often due to anaerobic conditions prevailing, when most readily

biodegradable material is available and when odorous metabolic products have yet to be effectively decomposed.

High levels of readily biodegradable carbon being present during digestion and in digestate or in stored slurries is


likely to give rise to high RBP values (i.e. showing high levels of biodegradability or low stability) and possible

odour problems. Equally, it might be expected that optimum hydraulic retention times and loading rates should

produce the necessary conditions during anaerobic digestion to fully digest the entrained biodegradable

compounds leading to low RBP values, reduced odour in the digestate – and maximum biogas yield.

This hypothesis is largely borne-out by the findings in this study for the livestock slurries, but not the data

determined for digestates. Most of the pig and cow slurries that were tested had high or very high odour

potentials and in general these were also associated with high or very high RBP values. The four pig slurries that

were tested were characterised as having very high RBP values which greatly exceeded the PAS110 RBP

threshold and they all had high or very high Odour Potentials (3,010, 3,940, 8,550 and 45,000x 103 ouE m-3). All

five cattle slurries had RBP values which were comparable to the RBP threshold and three of these had high

Odour Potentials (1840, 3570 and 6550 x 103 ouE m-3).

Unfortunately, this relationship between stability and odour potential was not demonstrated for the digestates

(see Figure 3-3). Three of the six digestates that passed the RBP test had very low Odour Potentials (4, 5 and 8 x

103 ouE m-3) while all of the three digestates that exceeded (i.e. failed) the RBP test threshold also had very low

Odour Potentials (3, 5, and 37 x 103 ouE m-3). In contrast, three of the digestates that passed the RBP test had

elevated Odour Potentials (133, 575 and 888 x 103ouE m-3).

Figure 3-3 Digestate RBP values with numerical values of Odour Potentials expressed as x 103 ouE m-3

As noted above, while odour potential is a useful tool, these findings should not be taken to reflect the actual

impact of those materials in the environment. Thus, both digestate stability and digestate odour should be

adequately managed within AD systems. It should be noted that some sites providing digestates for this study did

employ odour control methods separately from digestate stability management. To maintain the anonymity of

sites providing sample materials, the details of specific odour control methods cannot be provided in this report.

Many odorous compounds are detected by the human nose at very low concentrations. Therefore even emissions

from low odour potential digestates may be perceived as being unpleasant due to the presence of specific

compounds in relatively low concentrations, such as hydrogen sulphide or dimethyl disulphide. An analysis of the

compounds present in the gases stripped from the digestates with elevated odour potentials (C, D and F) showed

elevated levels of hydrogen sulphide. It is possible that the presence of elevated concentrations of hydrogen

sulphide may have been principally responsible for the elevated odour potentials for these digestates, which in

other respects appeared to be well stabilised.

In terms of identifying the origins of the hydrogen sulphide, the two digestates with the highest levels of

hydrogen sulphide in the off-gases (C and F) also used cattle slurry as input wastes to the digestion process and,

as shown in this report, some untreated cattle slurries can contain very high levels of hydrogen sulphide which

may have contributed significantly to the elevated levels found in the digestates. However, this effect was not

evident with Digestate A, which was derived exclusively from cattle slurry and was characterised as having a low


odour potential, with the off-gases containing very low concentrations of hydrogen sulphide. Digestates derived

from food-based inputs only contained very low concentrations of hydrogen sulphide in the off-gases with the

exception of Digestate D. It would appear that individual digestion processes and input materials can affect the

properties of the resulting digestates and further research is needed to more fully understand the nature of this

relationship.

From an odour perspective, decreasing the RBP threshold significantly to match the RBP values for highly

stabilised digestates (<0.1 L biogas/gVS) would seem to be inappropriate, as there is no current evidence that

this will guarantee low odour potentials. The method previously used for setting the RBP threshold is based on

RBP values for cattle and pig slurries that are applied to land untreated. On this basis the current threshold is set

at 0.25 L biogas/gVS, but using data from this study, it could be argued that the RBP threshold could be

increased to take account of the new RBP values for pig slurries. This would mean increasing the threshold to

approximately 0.8 L biogas/gVS, and further evidence from this study suggests that digestates with RBP values

up to this level also had low odour potentials.

However, given the uncertain linkage between stability and odour, together with the clear evidence that high RBP

values are linked to inhibited digestion processes (that are not maximising their biogas (and hence, revenue)

potential), it is recommended that the current RBP threshold is retained at the level of 0.25 litres biogas/g VS.

3.3.3 Characterisation of odour

A number of authors have identified odorous compounds associated with anaerobic processes. Adalberto Noyola,

studied odours associated with anaerobic sewage treatment works and cited odour sources as:

(a) inorganic gases such as hydrogen sulphide and ammonia;

(b) organic acidic compounds such as acetic, propanoic, butyric and lactic acids; and

(c) amines such as cadaverine and putrescine.

Other odorous organic compounds found in wastewater treatment plants cited by Adalberto Noyola, were:

skatole, phenols, oxysulfide, carbon disulfide, mercaptans of low molecular weight, thiophenes, dimethylsulfide,

dimethyldisulfide and dimethyltrisulfide, aldehydes and ketones. According to Smet & Van Langenhove (1998) in

the environs of AD and wastewater treatment facilities, hydrogen sulphide possesses such characteristic odour

that it generally masks the scent of other organic sulphide compounds. For this reason, hydrogen sulphide is

considered to be the most characteristic bad odour constituent in biogas (Metcalf & Eddy, 2003) , and (bio)

chemical steps are often deployed to strip hydrogen sulphide from solution during or after digestion.

Parker et al., (2002) identified a number of odorous compounds in landfill gas with the most odorous five

compounds being hydrogen sulphide, methanethiol, butanoic acid, ethanal (acetaldehyde) and carbon disulphide.

Similarly Susaya (2011) identified the dominant odorous compounds in lake sediment resulting from anaerobic

decomposition as hydrogen sulphide, ammonia, butanoic acid, pentanoic acid, ethanal, butanal, and dimethyl

sulphide.

The preliminary instrumental analyses of odorous gasses presented here focused on a selected set of odorous

compounds which were found to be typically associated with anaerobic decomposition. Digestates were stripped

of odour using the method detailed by Hobson (2002) and odour compounds were determined by Fourier

Transform Infrared Spectroscopy (FTIR) and Jerome hydrogen sulphide analysis. Total VOCs were measured by

Flame Ionisation Detection (FID), Photo Ionisation Detection (PID), and gas chromatography-mass spectrometry

(GC-MS).

As noted above, hydrogen sulphide is often associated with odours from anaerobic digestion processes. In this

study, hydrogen sulphide was found to be consistently present in the collected digestate gases and Figure 3-4

suggests that there was a reasonably good relationship between Odour Potential and the concentration of

hydrogen sulphide for all samples tested. From Appendix 1, Table A 3, it can be seen that the gasses stripped

from digestates C (138 ppm), D (22.7 ppm) and F (150 ppm) contained elevated concentrations of hydrogen

sulphide compared with other digestates. These elevated levels of hydrogen sulphide were also associated with

elevated odour potentials for these digestates.

Pentanoic acid was not detected. Butanoic acid and ethanal were each detected in only one sample.

Methanethiol, carbon disulphide, dimethyl sulphide and dimethyl disulphide were detected mainly in the cattle

and pig slurries. Ammonia concentrations were consistently low for all samples tested and it is possible that the


odour stripping method failed to adequately volatilise ammonia from the materials under test. Ammonia is known

to be highly soluble at ambient temperatures.

Figure 3-4 Concentration of hydrogen sulphide in gas collected during Odour Potential test plotted against Odour

potential results. One very high value for odour potential has been excluded. Hydrogen sulphide concentration

has been adjusted to account for dilution

3.3.4 Alternative metrics for determining digestate stability or degree of digestion

The Residual Biogas Potential (RBP) test is the current method used to determine digestate stability or degree of

digestion. The test is based on digesting samples in small-scale, batch anaerobic digesters over the 28 day period

of test. For digestates, the RBP test gives a direct measure of the capacity of the residual carbon in digestates to

produce biogas (methane and carbon dioxide) which in turn indicates how well the feedstock has been digested.

For digestates, the current RBP test will respond principally to the VFAs in solution, with low concentrations giving

a low RBP result and high concentrations giving a high RBP result, up to a limiting concentration which is known

to cause test inhibition. For this reason, the PAS110 RBP test is currently preceded by a VFA screen, which aims

to screen out samples with high VFA concentrations in solution lest they deliver subsequent false negatives in the

RBP test.

As noted above, the RBP test gives a direct measure of the capacity of the residual carbon in digestates to

produce biogas and to some degree the RBP test could be regarded as a test for assessing the effectiveness of

the AD process rather than for assessing the suitability of digestates for land spreading. Indeed, the RBP test is

included within the PAS110 specification (partly) to ensure that input materials have genuinely been submitted to

an anaerobic digestion process, limiting the potential for „sham‟ recovery of input materials. The main advantages

are that the RBP test is an anaerobic test and therefore reflects the anaerobic characteristics of the samples. It is

also capable of testing all fractions of digestate (liquid, slurry and fibre). Inhibition of the RBP test at high VFA

concentrations, and the relatively long test duration (28 days) are two disadvantages.

Hydrogen sulphide vs. Odour potential for digestates and

slurries

y = 0.1656x

R2 = 0.7637

0

200

400

600

800

1000

1200

1400

1600

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

Odour potential 10^3 ouE/m3

H2

S, p

pm

v/v


It should be noted that the RBP test gives a direct measure of digestate stability under anaerobic conditions and

as such provides the “benchmark” of stability against which other metrics or surrogate methods are compared.

This project investigated three digestate metrics as a possible alternative to the RBP test:

Total volatile fatty acid concentration

BOD and COD/BOD ratio

Dissolved carbon (DC)

The metrics above were assessed as direct alternatives to the RBP test rather than as possible pre-RBP

screening tests.

3.3.5 Total volatile fatty acid (VFA) concentration

This metric is currently used as the pre-RBP screen to identify possible test inhibition. The RBP value will reflect

the total VFA level in the digestate as well as reflecting other forms of carbon, such as partially decomposed

waste in solid particulate form. It should be noted VFA concentrations for slurry and liquid digestate samples are

determined using the filtered eluates from samples, rather than using the unfiltered liquids – which would also

contain carbon in solid form.

It would be expected that the total VFA concentration was positively related to the RBP value and this was found

as shown in Figure 3-5. However, total VFA concentration cannot be used with certainty to predict stability or

degree of digestion on its own, as carbon in solid particulate form in the digestate „as received‟ may also make a

significant contribution to RBP value (and is currently discarded prior to VFA testing). Hence, the VFA

concentration in the sample eluents may underestimate the full level of stability – which is given by the RBP test.

This is shown in Figure 3-5, where the VFA concentration was found to be less than the RBP value for many of

the digestate samples. Also, VFA determination cannot normally be carried out on separated fibre samples or dry

digestates since the test relies upon a liquid sample fraction.

Figure 3-5 Total volatile fatty acids (sum C2 to C6) as mg/L acetic acid against RBP L/g(VS), for digestates only


3.3.6 BOD and COD/BOD ratio

From Figure 3-6 it can be seen that a good relationship was found between BOD and respective RBP values for

the digestates that were tested – once a very high BOD outlier had been removed from the data set. BOD is an

aerobic biological test which uses diluted but unfiltered sample liquids and this tends to reflect all forms of carbon

present in the original samples. However, the BOD test is an aerobic test, and it is possible for it to produce very

different results from the anaerobic RBP test, depending on the composition of the digestates. Digestates

containing high levels of ligno-cellulose material, which will be resistant to decomposition under anaerobic

conditions, may produce low RBP values but high BOD values. This might be the case with the one outlier in

Figure 3-6 which was removed. This effect needs to be further investigated before the BOD test could be

recommended for testing the stability of anaerobic digestates. Furthermore, the BOD test, while it has the

capacity to test the whole sample, is only suitable for relatively dilute digestates (approximately <5% dry matter)

and is therefore not suitable for testing separated fibre samples or dry digestates. Because of this, the BOD test

may not be suitable as a direct replacement for the RBP test, but it may have merit as a separate screening test

for dilute digestates in terms of stability and suitability assessment for land application.

Figure 3-6 BOD values against RBP L/g(VS), for digestates only

No clear relationship can be discerned for COD/BOD ratios when plotted against respective RBP values (Figure

3-7). In this case, it can be seen that six values of COD:BOD ratio falling within the narrow range 2:1 to 3:1 are

associated with a range of RBP values extending from 0.05 to 0.55 L/g VS.


Figure 3-7 COD/BOD ratios against RBP L/g(VS), for digestates only

3.3.7 Dissolved carbon (DC)

As noted previously, the current RBP test incorporates a VFA screening test to identify high concentrations of

VFAs which may inhibit the subsequent RBP test. The VFA determination is carried out on filtered samples

extracted directly from digestate samples.

Instead of determining the VFA concentration using a gas chromatograph, the total dissolved carbon content may

be determined directly on the filtrate. In digestate samples, total dissolved carbon can take the form of organic

carbon (such as ethanioc acid) and inorganic carbon (such as carbonates). Purgable dissolved carbon is the

standard method for determining inorganic carbon, i.e. purging with carbon-free air after pre-treatment with acid,

though this may also liberate some volatile organic species. The remaining organic carbon fraction is known as

non-purgable organic carbon (NPOC).

Figure 3-8 shows the relationship between dissolved non-purgable organic carbon (NPOC) and RBP. A slightly

weaker correlation is found between total dissolved carbon and RBP. Compared with total VFA concentration,

NPOC concentrations reflect a wider range of carbon forms, though (obviously) exclude inorganic forms such as

carbonate, and possibly some volatile organic species.

It is recommended that further research is conducted to fully assess the potential of using concentrations of

dissolved carbon as a surrogate for RBP values. Most dissolved carbon equipment, such as the Shimadzy TOC

ASI-V used here, requires a filtered sample for analysis. However, alterations can be made to include

determination of suspended particulate carbon as well as dissolved carbon and this would be necessary if this test

is to be considered as a direct substitute for the RBP test. In addition, sample introduction and furnace modules

capable of handling sludges directly are now available. Further investigation of these methods is recommended.

With all indirect or surrogate tests, it is important to note that they may not be able to identify how

biodegradable the organic carbon might be under aerobic or anaerobic conditions. Until other tests are

investigated, the RBP remains the most fit-for-purpose methodology for determining the stability of anaerobic

digestates.


Figure 3-8 Non-purgable organic carbon against RBP L/g(VS), for digestates only

3.3.8 Hedonic tone

As previously discussed, olfactometry testing was used to assess the odour potential of the gasses stripped from

each digestate and each animal slurry. In addition, the stripped gasses were assessed for pleasantness of odour

using the Hedonic tone method. Panellists were subjected to increasing concentrations of each of the stripped

gasses and asked to rate their degree of pleasantness on a scale of +4 (most pleasant) to -4 (most unpleasant).

Hedonic tone data are presented in graphical format in Appendix 2. All of the gases stripped from all of the

organic materials under test were considered unpleasant in odour. The odour concentrations at, for example, a

hedonic tone of -2 (moderately unpleasant) was, on average 15 ouE m-3 for pig, 36 ouE m

-3 for cow and 12 ouE m-

3 for the digestates (Table 3-7). This indicates that the cow slurries required a greater concentration of odour to

be deemed unpleasant at a scale of -2 compared with the pig slurries and digestates. Equally, panellists rated

both the digestates and pig slurries to be moderately unpleasant at broadly similar odour concentrations,

suggesting that the odour from digestates at relatively low odour concentrations was considered to be as

offensive as pig slurry.

Odour will be dispersed from source and diluted by wind. The initial odour concentration of the material and

weather conditions will therefore determine the effect on any sensitive receptor. While digestates on average had

odour potentials which were some 14 times lower than the cow slurries that were tested, the hedonic tone rating

for digestates was on average, three times greater than for the cow slurries. This again suggests that digestates,

although having lower odour potentials compared with untreated cow slurries, can be considered to be

unpleasant even at low concentrations.

The separation of the five materials into liquid and fibre did affect the hedonic tone score of two materials, with

the fibres of pig slurry 3 and digestate C (slurry/crops) being unpleasant at lower odour concentrations than the

liquids or un-separated material. The hedonic tone ratings of the separated fibre and liquid for digestates B, G

and H were little affected.

Treatment by SBR did reduce the odour concentration slightly for a hedonic tone score of 2 but the odour

concentration (Table 3-7) had been greatly reduced.

Non-purgable organic carbon vs. RBP for digestates

y = 15628x

R2 = 0.8016

0

2000

4000

6000

8000

10000

12000

14000

16000

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

RBP, L/g(VS)

NP

OC

, m

g/L


Table 3-7 Odour concentration at hedonic tone score 2 (estimated from Appendix 2 graphs)

OU Code Report Code Input waste type during

study

Whole

Digestate

Separated

Liquid

Separated

Fibre

W29/08 Digestate A cattle slurry 10

W29/13 Digestate B food waste 7 2.5 5

W29/14 Digestate C cattle slurry with some

maize silage 43 35 3

W29/02 Digestate D (used

in SBR2) mainly food waste 18

W29/03 Digestate E mainly food waste 4

W29/01 Digestate F (used

in SBR1)

food waste with some cattle

slurry 12

W29/20 Digestate G food waste 5 5 3

W29/16 Digestate H food waste 4 3 5

W29/12 Digestate I mainly food waste 9

W29/21 Digestate SBR 1 After treatment 4

W29/22 Digestate SBR 2 After treatment 3

W29/05 Cow slurry 1 26





W29/10 Cow/Pig slurry 20

W29/15 Pig slurry 1 6


W29/07 Pig slurry 3 22 22 2


W29/18 Digested sewage 5


3.4 Treatment options/additives

3.4.1 SBR

One of the main objectives of the SBR trials was to investigate nitrification of ammoniacal nitrogen to nitrate as a

mechanism for improving the agronomic value and ease of management of digestates. Ammoniacal nitrogen in

sample D reduced from 3182 mg/l to 99 mg/l and in sample F from 1795 mg/l to 112 mg/l. The corresponding

increase in nitrate was from 2 mg/l to 1542 mg/l in sample D and from 13 mg/l to 768 mg/l in sample F. Some

nitrogen would have been taken up by sludge growth within the trial units (sludge growth is discussed further in

Section 3.4.3).

Results for potential biodegradability parameters and other parameters are tabulated and summarised below.

KEY

„D before‟ = Digestate D prior to SBR treatment.

„D after‟ = Digestate D after SBR treatment.

„F before‟ = Digestate F prior to SBR treatment.

„F after‟ = Digestate F after SBR treatment.

Table 3-8 Potential biodegradability parameters

RBP

L/g(VS)

COD

g/L

BOD

g/L COD/BOD

Total VFAs

g/L as

acetic

Total

dissolved

carbon

g/L

D before 0.181 20.4 7.9 2.6 0.90 5.33

D after <0 1.52 0.225 6.9 0.33 0.37

F before 0.220 20.9 7.9 2.7 2.11 4.41

F after <0 10.0 1.9 5.3 0.07 0.75

All measures of biodegradability were substantially reduced.

Of note, the RBP for the digestates were below zero.

COD:BOD ratio for both was higher.

Table 3-9 Odour potentials and associated gases

Odour

potential

103 ouE/m3

H2S

ppm

FID

ppm (CH4)

PID

ppm

(isobutane)

NH3

ppm

D before 133 22.7 5611 21.3 20.5

D after 0 0.0 10 0.9 1.9

F before 888 150 i/s* i/s* i/s*

F after 2 0.0 29 1.3 1.3

*i/s = insufficient gas sample.

Odour potential was virtually eliminated.

Complete removed of H2S.

Substantial reduction in CH4, isobutane and NH3 for D. *Insufficient sample. Gas volume produced in odour-stripping test was required for olfactometry testing in F

before treatment, so no direct comparison can be made. However, F after are similar in magnitude to D after.


Table 3-10 Compounds identified by GC-MS analysis

Me

tha

ne

thio

l

Bu

tan

oic

acid

Eth

an

al

Ca

rbo

n

Dis

ulp

hid

e

Dim

eth

yl

Su

lph

ide

Dim

eth

yl

Dis

ulp

hid

e

D before n.d. n.d. n.d. n.d. V Low Low

D after n.d. n.d. n.d. V Low V Low V Low

F before V Low Med n.d. Med Low Low

F after n.d. n.d. n.d. V Low V Low V Low

Table 3-11 Volatile fatty acids in solution (mg/L)

ace

tic

pro

pa

no

ic

iso

-bu

tyri

c

n-

bu

tyri

c

iso

-va

leri

c

n-

va

leri

c

he

xa

no

ic

he

pta

no

ic

octa

no

ic

D before 739 132 59 0 7 0 12 28 0

D after 258 36 0 18 8 21 25 36 123

F before 1366 619 202 55 47 37 28 59 85

F after 55 13 5 0 0 10 0 0 195

D after – Decrease in acetic, propanoic and iso-butyric acids. Slight increase in n-butyric, iso-valeric, hexanoic,

heptanoic and octanoic.

F after – Decrease in all except the last, octanoic, which increased.

Table 3-12 Carbon in solution

Purgeable dissolved

carbon

mg/L

NPOC*

mg/L

Total dissolved carbon

mg/L

D before 1710 3618 5328

D after 106 259 365

F before 1645 2770 4414

F after 242 505 747

*Non-Purgable Organic Carbon

All reduced substantially, with a greater percentage reduction in D than in F.


3.4.2 Sewer discharge consents

If it is not possible for the biofertiliser to be applied to land (for example there being insufficient agricultural land

available within economic distance) then an option for disposal of the liquid fraction might be discharge to sewer

or to watercourse. However, in order to control loadings on the sewage works, or to protect the quality of the

receiving watercourse, either the utility operators or the Environment Agency will impose conditions on the

quality of liquor discharged.

Parameters of relevance to digestate that are routinely controlled include COD, BOD, suspended solids and

ammoniacal nitrogen. The exact consent conditions will vary according to location, but will be related to the

capacity of the receiving works or watercourse to receive the liquor without adverse impact.

Consent conditions are usually stipulated as a maximum concentration that may not be exceeded, and sometimes

as a maximum daily load that may be discharged. The charges levied (by the utilities) for the receipt and

treatment of trade effluents correspond to the volume and strength of the effluent, with „strength‟ typically being

defined and measured by COD, suspended solids and ammoniacal nitrogen. Therefore, the lower the strength,

the lower the charges will be for a given volume of discharge.

As discussed in 3.4.1 above, aerobic treatment of digestates will reduce the concentration of these parameters

prior to discharge, particularly ammoniacal nitrogen.

SBR practicalities (including likely costs)

During the SBR trials, relatively rapid sludge growth within the trials units required sludge to be discarded on two

occasions in order to maintain nitrification, to prevent foaming and to aid settlement. This sludge growth was

probably due to the solids content and COD of the digestates being treated.

In a full scale treatment system, rapid sludge growth and the need for relatively frequent de-sludging is also likely

to occur. However, whenever de-sludging takes place, some of the nitrifying bacteria are also removed with the

sludge. This could lead to problems due to the differing growth rates of the various bacterial populations;

nitrifiers grow more slowly than the organisms that consume carbonaceous matter, so if the de-sludging

frequency is too great, there is the risk that the nitrifying population will be reduced at a faster rate than it can

re-grow. This would result in a loss of nitrifying capability over time. A balance needs to be struck between the

need to de-sludge and the need to retain nitrifying bacteria.

If SBR processes such as the one tested here are to be utilised for post-digestion transformation of digestates,

then more research is required, but the microbiological balance could be addressed by reducing the COD before

the nitrification stage in order to limit sludge growth. This could be achieved by:

a) Physical removal of solids by means of a suitable dewatering system, such as belt press or centrifuge.

b) A two stage biological system; the first stage being used for reduction of COD, and the second stage for

nitrification. Thus when the first stage is de-sludged, no nitrifiers would be lost.

Costs of a digestate treatment system will vary according to the volume and strength to be treated, and the

discharge consent parameters to be met. The typical throughput of current commercial SBR systems treating

leachate is 20,000 to 75,000m3/annum, which is similar to the typical range of throughputs for AD plants. A

reasonable budget estimate for incorporating a system into an AD plant would be around 5% of the overall

capital cost of the AD plant. Economies of scale at larger plants, and duplication of common equipment such as

dewatering plant and control systems might reduce the overall cost.

Operating costs would primarily be related to the electricity used for aeration, which might typically be in the

range 3 – 6 kWh per m3 of digestate treated, and which could be obtained from a CHP unit on site.


3.4.3 Additives

Two types of additives are particularly relevant to this report. Firstly, the addition of trace elements to the

digestion process to reduce the effects of inhibition associated with food waste inputs. Inhibition is known to

increase digestate VFA concentrations and this in turn could increase odour. Secondly, arising out of the

water/sewage anaerobic digestion treatment sector, it has been found that additions of various chemicals and

micro-organisms can reduce odour problems typically associated with reduced sulphur compounds (such as H2S).

These are briefly discussed below.

3.4.3.1 Addition of trace elements

Research by Yue Zhang carried out at the University of Southampton highlighted potential problems with the

stable digestion of source segregated food waste in commercial digesters. When operating on this substrate, the

digestion process typically has a high ammonia concentration and elevated levels of volatile fatty acids (VFA),

which can bring about acidification and (ultimately) failure of the digester if the buffering capacity is overcome.

They postulated that the relationship between ammonia and the elevated VFA levels resulted from toxicity to the

methanogen population, further complicated by impairment of the function of methanogens by trace element

deficiency. The University of Southampton concluded that selenium and cobalt are the key trace elements that

are needed for the long-term stability of anaerobic food waste digestion, but that are likely to be deficient in food

waste. The minimum concentrations recommended for selenium and cobalt in anaerobic food waste digesters at

moderate organic loading rates (~2-3 kg VS m3 day-1) are around 0.16 and 0.22 mg l-1, respectively.

Molybdenum, tungsten and nickel (Mo, W, and Ni) were found to be present in food waste in sufficient quantities

for moderate organic loading rates, but may also have to be supplemented in anaerobic digestion systems

operating with a higher rates (~5 kg VS m3 day-1and above). Prevention of VFA accumulation in the digester by

trace element supplementation is seen as an essential precaution, as microbiological recovery of a severely VFA-

laden digester is not a rapid process even after supplements are added – reducing potential biogas yields (and

site revenue). Some commercial anaerobic digester operators are currently experimenting with selenium and

cobalt additions to reduce ammonia inhibition.

3.4.3.2 Addition of various chemicals and micro-organisms to control odour

According to Curtin Water Quality Research Centre (CWQRC) Report No: Curtin Water Quality Research Centre, a

study of one particular waste water treatment plant showed that a biosolids product with a 15% lime dose still

produced substantial odours after four days of storage and the odours increased over time, while biosolids

products stabilised with 30% and 40% lime doses were very stable and the odours appeared to decrease as

storage time increased.

In general, an increase in iron concentration in the sludge or biosolids resulted in higher Total Volatile Organic

Sulphur Compounds (TVOSC) concentrations in the dewatered biosolids headspace, especially if iron was added

prior to or during digestion. It was found that addition of ferric chloride (FeCl3) to anaerobically digested solids

before dewatering did not reduce TVOSC emissions from cake until FeCl3 dose was at least 8% on a dry mass-

mass basis and this also required additional lime addition to maintain near neutral pH. However, a full-scale trial

at a different waste water treatment plant (WWTP) showed that iron addition at lower dosage rates may be

effective in reducing odours in some biosolids. Thus, it was concluded that the effectiveness of iron addition in

reducing odours seemed to be dependent on the site-specific characteristics of the biosolids.

Tepe, N bioaugmented a biosolids digester with Bacillus, Pseudomonas and Actinomycetes. Resulting digestates

were stored for ten days and odour from the bioaugmented digestate was compared with a control digestate.

The bioaugmented digestate (negligible) achieved significant reductions in methyl mercaptan emission compared

to control (300 ppm) while peak dimethyl sulphide emission was only 37% of that from the control.

Spreading best practice to reduce odour and maximise agronomic benefit

The nitrogen availability within digestate (of the total nitrogen) is typically higher than in farm slurries8 because of

the relatively high percentage of ammonium nitrogen within the digestate. It is estimated that for digestate

applications approximately 80% of applied nitrogen uptake by arable crops and grassland can be achieved if good

agricultural practices are followed. However, due to the large ammonium content within the digestate losses

8 Utilisation of digestate from biogas plants as biofertiliser – IEA Bioenergy 2010.


through volatilisation (odour release), nutrient leaching and runoff can be high if application and management

techniques are not adequate.

PAS110 includes a comprehensive suite of tests and maximum thresholds which must be applied to each of the

fractions of the digestate (whole digestate, separated liquor and separated fibre) which are proposed for

accreditation to the PAS. Farmers should ensure that the digestate properties match those that are

allowed/required for their farm applications and that the digestate is physically suitable for use. A FACTS advisor

(http://www.factsinfo.org.uk/facts/) must be used to determine the correct application rate and timing of the

digestate spreading.

3.4.4 Previous research and support systems

A great deal of research has been carried out by MAFF/Defra to improve spreading practices and to reduce

ammonia losses from farm manures and slurries. Defra project AM0120 (2002) reported that ammonia emissions

from slurries could be reduced by up to 90% using injection techniques, but that band spreading was less

effective at 30%. Incorporation of broadcast slurries reduced ammonia losses by 71% compared with not

cultivating them into the soil (Defra project WA0632, 2001). Incorporation techniques (Defra project WA0716,

2005) showed that losses of ammonia could be reduced by 65% or 85% by ploughing manures in within 24

hours or within 4 hours, respectively; discing was less effective than ploughing.

MANNER9 is a decision support system that can be used to accurately predict the fertiliser nitrogen value of

organic manures on a field specific basis. MANNER has been developed using results from the research, funded

by Defra, on organic manure utilisation on agricultural land and incorporates ammonia losses depending on the

management system for spreading/incorporation employed. A slurry type that most closely resembles digestate,

such as pig slurry, can currently be used for planning purposes.

Assistance for nutrient planning is available through free software at http://www.planet4farmers.co.uk/ and

practical advice is available at http://www.nutrientmanagement.org/.

3.4.5 Application

A number of factors should be considered when looking to apply digestate to land. Digestate, particularly when

produced from food waste feedstocks, can have chemical properties comparable to or stronger than raw farm

slurries. Therefore, the potential for odour, nutrient loadings and run-off or leaching should be fully assessed and

understood before spreading.

Application systems should be fit for purpose and be suitable according to Good Agricultural Practice. Before

application, as well as PAS110 and ADQP, the implications of each of the following should be considered:

Farm nutrient management plans10

NVZ legislation11 (currently approximately 4% of Wales in within an NVZ) in relation to total nutrient

application (250kg total nitrogen per hectare per year)

Previous results of soil sampling and nitrogen loadings

Agricultural and other Best Practice Codes

RB209 Fertiliser Manual

The application of the digestate should form the primary fertiliser application for the receiving crop, and other

fertilisers should be used to top up the required loading according to crop requirements and previous field

cropping history. It may be that when planning to use digestate as the primary fertiliser application that the

application requirements/rates may be limited by the phosphorus content of the digestate in relation to the soil

phosphorus index and the crop rotation requirements, rather than by the total nitrogen content of the digestate.

9 http://www.adas.co.uk/MANNER/FurtherInfo/tabid/274/Default.aspx 10 http://www.nutrientmanagement.org/Support-and-advice/NVZs/Grassland-derogation/ 11 http://www.netregs.gov.uk/netregs/businesses/agriculture/118732.aspx

http://www.factsinfo.org.uk/facts/

http://www.planet4farmers.co.uk/

http://www.nutrientmanagement.org/

http://www.adas.co.uk/MANNER/FurtherInfo/tabid/274/Default.aspx

http://www.nutrientmanagement.org/Support-and-advice/NVZs/Grassland-derogation/

http://www.netregs.gov.uk/netregs/businesses/agriculture/118732.aspx


The systems and methods for application of digestate are very similar to those for farm slurries and organic

manures (such as biosolids). Reference should therefore be made to the available guidance handbooks for

manure applications, for example:

Making better use of livestock manures on arable land (2001)12

Making better use of livestock manures on grassland (2001)13

Spreading systems for slurries and solid manures (2001)14

Managing manure on organic farms (2002)15

Application of slurries will correspond to whole digestate and the liquid fraction, while application of solid manure

corresponds to the fibre/solid fraction of digestate.

The following sections indicate where further technical consideration should be given.

3.4.5.1 Liquor and Whole Digestate application

Digestate can be spread to land either as whole digestate or the separate fractions of liquor and solid/fibre. The

choice of slurry handling, pumping and spreading system should be based on the properties of the digestate

fraction to be applied. Environmentally friendly slurry application requires the slurry to be evenly applied near or

under the soil surface. It is much more complicated to fulfil this requirement when the slurry has a high dry

matter content than when it has a low viscosity and can easily flow through band spreading hoses (Amon et al.,

2006).

Table 3-13 provides a general indication of the suitability of farm slurry application systems to cope with whole

and liquor digestate fractions.

As well as considering dry matter, it is worth noting that for selection of suitable machinery the following limits

are specified for PAS110. Bear in mind that these limits are based on dry matter contents, so total loadings of any

contraries should be very low indeed:

Total glass, metal, plastic and any other non-stone man-made fragments = <0.5%m/m dry matter

Stones >5mm = <8% m/m dry matter

Stones are known to be a particular issue for digestates derived from manures and slurries.

Table 3-13 Application systems

Digestate Fraction Application

Typical range of

dry matter

Dig

esta

te

Tra

nsp

ort

Syste

ms

Vacuum tanker Liquor - yes, Whole Digestate – possible if

DM% suitable Up to 12%

Pumped Tanker Liquor - yes, Whole Digestate – possible if

DM% suitable Up to 12%

Umbilical hose Liquor - yes, Whole Digestate may be

possible if DM% and particulates suitable. Up to 8%

Irrigator

Liquor – yes if DM% allows, Whole

Digestate unlikely to be suitable due to

DM% and particulates.

Up to 3%

Dig

esta

te

Dis

trib

uti

on

Syste

ms

Dual purpose

Spreader (or side

impeller discharge

spreader).

Whole Digestate and Liquor – no, unless

odour potential low -

Broadcast

Spreader

Whole Digestate and Liquor – no, unless

odour potential low Up to 12%

12 http://www.defra.gov.uk/foodfarm/landmanage/land-soil/nutrient/documents/manure/livemanure1.pdf 13 http://www.defra.gov.uk/foodfarm/landmanage/land-soil/nutrient/documents/manure/livemanure2.pdf 14 http://www.defra.gov.uk/foodfarm/landmanage/land-soil/nutrient/documents/manure/livemanure3pt1.pdf and http://www.defra.gov.uk/foodfarm/landmanage/land-soil/nutrient/documents/manure/livemanure3pt2.pdf 15 http://www.defra.gov.uk/foodfarm/landmanage/land-soil/nutrient/documents/manure/livemanure4.pdf

http://www.defra.gov.uk/foodfarm/landmanage/land-soil/nutrient/documents/manure/livemanure1.pdf


http://www.defra.gov.uk/foodfarm/landmanage/land-soil/nutrient/documents/manure/livemanure3pt1.pdf

http://www.defra.gov.uk/foodfarm/landmanage/land-soil/nutrient/documents/manure/livemanure3pt2.pdf



Digestate Fraction Application

Typical range of

dry matter

Band Spreader Liquor – yes, Whole Digestate if DM% and

particulates allow Up to 9%

Trailing Hose/Shoe Liquor - yes, Whole Digestate unlikely to

be suitable due to DM% and particulates. Up to 6%

Shallow injector Liquor - yes, Whole Digestate unlikely to


Deep Injector Liquor - yes, Whole Digestate unlikely to


3.4.5.2 Application – solid fraction/fibre

Application of the solid fraction/fibre can be achieved by machinery typically used for solid manure spreading:

Rotaspreader

Rear discharge spreader

Dual purpose spreader or side impeller discharge spreader

If using „fresh‟ rather than the composted solid fraction/fibre, its properties should be checked to ensure it will not have a negative effect on crops as if used fresh in some cases it can cause reduction in seed germination

rates and, in the short-term, lock up nitrogen within the soil.

3.4.6 Other factors for consideration regarding application

3.4.6.1 Application at correct rate

It is important that the equipment chosen for the application can apply the digestate fraction at the required flow

rate to achieve the application rate as identified by the farm Nutrient Management Plan and cropping

requirements. Equipment should be calibrated before use in order to achieve a uniform spread across the full

pass width to avoid uneven crop growth.

3.4.6.2 Odour and other emissions

Ammonia and VFAs are lost to the atmosphere on spreading and dispersed by air movements, which can cause

odours and offense to neighbours.

Around 40% of the readily available nitrogen content of manures is often lost following surface application to

land. Ammonia loss and odour nuisance can be reduced by ensuring that manures are rapidly incorporated into

soils (within 6 hours of application for slurries and 24 hours for solid manures to tillage land). For slurries, shallow

injection and band spreading techniques are effective application methods that reduce ammonia emission

(typically by 30 – 70%) compared with broadcast application. Also, slurry band spreading (trailing shoe and

trailing hose) and shallow injection application techniques increase the number of spreading days, and cause less

sward contamination than surface broadcast applications. These practices will also increase the amount of

nitrogen available for crop uptake. Ammonia losses are generally smaller from low dry matter slurries because

they more rapidly infiltrate into the soil. Higher dry matter slurries remain on the soil/crop surface for longer

leading to greater losses. Losses are also higher when slurries are applied to dry soils under warm weather

conditions (Fertiliser Manual (RB209) 16).

The use of broadcast spreaders and dual purpose spreaders should be avoided if the liquor fraction has a high

odour potential as this type of application is likely to cause odour issues. In this case application systems which

apply the liquor under the crop or grass canopy should be used, i.e. band spreaders, trailing hoses and injectors.

On arable rotations, digestate should be incorporated into the soil as soon as possible following application to

limit ammonia losses and odour, or below the crop canopy in a standing crop using trailing hoses.

16 http://www.defra.gov.uk/foodfarm/landmanage/land-soil/nutrient/nmu01.htm


http://www.defra.gov.uk/foodfarm/landmanage/land-soil/nutrient/nmu01.htm


3.4.6.3 Timing of Applications and Cropping

Within NVZs, there are closed periods during which manures containing more than 30% readily available nitrogen

(cattle or pig slurry, poultry manures and most biofertilisers) must not be applied, although limited applications

may be made by registered organic farmers.

Table 3-14 Closed periods

Grassland Tillage land

Sandy or shallow soil 1 September to 31 December 1 August to 31 December*

All other soils 15 October to 15 January 1 October to 15 January

Manufactured fertiliser 15 September to 15 January 1 September to 15 January

*If crop sown before 15 Sept - applications permitted between 1 Aug → 15 Sept

Consideration should also be given to the timing of the applications in relation to cropping cycles and grazing.

Where digestate applications are made to grazed grassland, the pasture should not be grazed for at least four

weeks following application, or until all visible signs of slurry solids have disappeared.

Where digestate applications are made before “ready to eat crops” i.e. crops that are generally not cooked before

eating, relevant industry guidance should be followed to minimise the risks of pathogen transfer.

Applications should only be considered when soil moisture content is at a level where there is negligible risk of

significant leaching or run-off. Spring applications should be avoided if the moisture content is not suitable as

this will increase the risk of leaching and run-off incidents particularly on heavy soils.

Sensible, practical precautions to reduce the likelihood of causing odour problems when spreading are to:

Check the weather forecast prior to spreading.

Check the wind direction and try to spread only when wind is blowing away from neighbouring properties.

Avoid spreading at weekends, bank holidays or evenings.

Do not spread close to houses.

Plough in immediately after application or use injection to minimise odour.

In addition to this, use of certain slurry application systems e.g. large slurry tankers could, under wet/heavy

conditions, cause considerable compaction which could negatively impact claims for Single Farm Payment.

Consider the application system being used in relation to compaction. A number of AD operators report the use of umbilical systems to avoid the use of tankers. Suitable irrigation systems and using a smaller boom size

to decrease the tanker size required could also be considered as potential options, although this might mean

crop-trampling, with shorter booms meaning that the whole crop cannot be covered if sticking to tramlines

3.4.7 Spreading equipment sources

Lack of available suitable low trajectory spreading machinery and associated services within Wales has previously

been cited as a potential issue for AD digestate application. However, recently a survey conducted by WAG‟s

Dairy Development Centre “Making allowances for nutrients in slurry”, indicated this was no longer the case17.

This report considers slurry handling and application and states that: “The key barrier of lack of available

machinery is no longer valid, with all groups acknowledging injectors/trailing shoes available in their area.”

When considering the installation of AD facilities farmers/operators should investigate the availability of local

contractors and the services they can offer. Many contractors currently own and operate suitable spreading

machinery which can be used for both farm slurries and digestates. A list of local contractors can be generated

using on-line search tools such as www.Yell.com. This link18 provides an example search showing agricultural

contractors in Wales.

This type of search is not likely to provide an exhaustive list, and services offered by local contractors will change

over time and in response to market demands.

17 http://www.ddc-wales.co.uk/client_files/nutrient_planning_-_making_allowances.pdf 18

http://www.yell.com/map/#mapSearchType=yellSearch&keywords=agicultural+contractors&companyName=&location=wales

&workflow=&scrambleSeed=90013014

http://www.yell.com/map/#mapSearchType=yellSearch&keywords=agicultural+contractors&companyName=&location=wales&workflow=&scrambleSeed=90013014




If suitable local contractor services are not available then prior to investment in AD, discussions should be held

with local contractors to see if there is willingness to invest in spreading equipment. Operators of AD facilities

have reported that local contractors have responded to the introduction facilities in their area, setting up services

and purchasing equipment in response. Typically this is facilitated by contractual arrangements for exclusive

rights for spreading of the digestate. Offering contractual arrangements of this nature provides confidence in

longer-term markets and profitability, and therefore increases the contractor‟s readiness to invest the significant

capital required for the equipment.

If machinery and contractor services are not available locally or if the farmer/contractor wishes to invest in the

required machinery this will be able to be sourced through most local agricultural equipment merchants or direct

from the manufacturers.

When considering the installation of AD facilities farmers/operators should also keep in mind:

A FACTS advisor must be used to determine the correct application rate and timing of the digestate

spreading. If neither the farmer nor contractor is FACTS qualified then FACTS consultancy services will also be required.

A significant landbank may be required for the spreading of the digestate. The landbank requirements should

be calculated at the early stages of planning and if required other local farmers may need to be engaged as outlets for the digestate.

Appendix 4 gives examples of spreading contractors in Wales and the Borders, as well as a more

comprehensive list including UK and further afield.


4.0 Conclusions and recommendations

The PAS110 RBP test has been shown to be a robust and cost-effective method for determining the stability and

degree of digestion for digestates. The PAS110 stability test needs to be capable of testing both solid and liquid

samples and because of this there is currently no single suitable alternative test – although a range of possible

alternatives was investigated.

A wider range of organic materials was also examined than had been possible when the RBP test was originally

developed. Based on the findings of this work, it is recommended that the current RBP threshold is retained at

the level of 0.25 L/g VS, which is comparable with the range of RBP values found for cattle slurries and which

seems to be associated with digestates with low to medium odour potentials.

Other findings from this study suggest that anaerobic treatment of biodegradable wastes can produce digestates

with low odour potentials. However, there were also anomalies with stabilised digestates passing the RBP test,

but having elevated odour potentials, possibly related to hydrogen sulphide. One hypothesis is that some high

digestate odour potentials may be associated with cattle slurry inputs as these can contain very high

concentrations of hydrogen sulphide – but this hypothesis needs to be tested. Certainly it is the case that

individual anaerobic digestion processes and input materials can affect the properties of the resulting digestates,

and this relationship is not yet fully understood.

It should be noted that Odour Potential is a laboratory-based method that is appropriate to ranking materials in

terms of their potential to generate odour. Odour Potential findings in this study should not be taken to reflect

the actual odour impact of those materials in the environment

The sequencing batch reactor (SBR) tests successfully demonstrated that the ammoniacal-nitrogen could be

converted to nitrate-nitrogen. Digestate stability was greatly increased, and the BOD, COD, total VFAs and

dissolved carbon lowered. The odour potential and hydrogen sulphide were eliminated. If it was intended that

the liquid digestates be discharged to sewer (for instance, if suitable land bank was not economically available),

then the loading of BOD, COD, suspended solids and ammoniacal nitrogen would have to be suitably low to

enable the sewage treatment works to cope with the extra demand on the system. This could be achieved

through an SBR approach – although costs would not be insignificant.

Separating the digestate into liquor and fibre fractions generally had little impact on the nutrient profiles of the

different fractions. However, since the liquor fraction comprised by far the majority of the total digestate mass,

this represents the biggest reserve of nutrients that could be recovered for agronomic benefit.

Currently, there are no agreed protocols for monitoring odour at AD sites and related areas. However, the degree

to which odour is a problem for AD sites, and the nature of those odour problems is unknown – although

anecdotal evidence suggest that odours at the time of spreading have proven problematic for some operators.

Good spreading practice should limit odours and maximise agronomic benefit from digestates, but the extent to

which this is the case can only be determined through fieldwork.

Various other aspects could also be investigated in more detail, to provide AD operators with a suite of tools from

which to select solutions appropriate to their own circumstances:

The role and performance of odour control additives, such as various iron compounds;

The possible use of hydrogen sulphide as a cost-effective marker compound for odour at AD sites;

The potential for new analytical equipment to determine total organic carbon in both solid and liquid samples

– and whether this offers a cheaper and equally robust alternative testing route than the RBP test.

Finally, a set of validated protocols could be established for monitoring odours at AD facilities.


5.0 References

Adalberto Noyola, Juan Manuel Morgan-Sagastume & Jorge E. Lopez-Hernandez (2006). Treatment of biogas

produced in anaerobic reactors for domestic wastewater: odor control and energy/resource recovery

Amon B, Kryvoruchko V, Amon T, Zechmeister-Boltenstern S. (2006). Methane, nitrous oxide and ammonia

emissions during storage and after application of dairy cattle slurry and influence of slurry treatment. Agriculture,

Ecosystems and Environment 112, 153–162.

Curtin Water Quality Research Centre. Literature Review on Reduction of Odour in Biosolids. CWQRC Report No:

CWQRC-2011-003.

http://www.watercorporation.com.au/_files/InfrastructureProjects/Biosolids_Reduction_of_Odour_Literature_Revi

ew_V2_2010.pdf Accessed 23 July 2011

Hobson, Yang. (2002). Odour Control in Wastewater Treatment – A Technical Reference Document. UKWIR

Report Ref No. 01/ww/13/3.

Mackie R.I., P. G. Stroot and V. H. Varel. (1998). Biochemical identification and biological origin of key odor

components in livestock waste J Anim Sci 1998. 76:1331-1342.

Metcalf & Eddy Inc. (2003). Wastewater Engineering: Treatment and Reuse. 4th edition, McGraw Hill, New York.

Odorizzi G., L. Paradisi and S. Silvestri (2003). Odour Impact Assessment from Waste Treatment Plants by

Olfactometry. Proceedings Sardinia 2003, Ninth International Waste Management and Landfill Symposium. S.

Margherita di Pula, Cagliari, Italy; 6 - 10 October 2003.

Parker, T., Dottridge, J., Kelly, S. (2002). Investigation of the composition and emissions of trace components in

landfill gas. Environment Agency R&D Technical Report P1-438/TR.

Tepe, N., Yurtsever D., Mehta R.J., Bruno C., Punzi V.L. and Duran M. (2008). Water Science and Technology, 57

(4), 589-594.

Yue Zhang and Mark Walker (September 2010). Optimising Processes for the Stable Operation of Food Waste

Digestion. Technical Report Defra project Code WR1208.

http://randd.defra.gov.uk/Document.aspx?Document=WR1208_9922_TRP.pdf Accessed July 23 2011

SEPA (2010). SEPA Regulatory Position Statement: the Use of PAS 110 certified Digestate from Anaerobic

Digestion. www.sepa.org.uk/waste/waste_regulation/idoc.ashx?docid=0bc42e43-c846-4b31-adfc-

599542358a7b&version=-1 Accessed July 23 2011

Smet E & Van Langenhove H (1998). Abatement of volatile organic sulfur compounds in odorous emissions from

the bioindustry. Biodegradation 9: 273–284

Steen, I. (1998). Phosphorus availability in the 21st Century: management of a non-renewable resource.

Phosphorus and Potassium 217: 25–31

Susaya, J., Kim, K., and Chang, Y. (2011). Characterisation of major offensive odorants released from lake

sediment. Atmospheric Environment 45, 1236-1241.

Walker, M., Banks, C., Heaven, S., and Frederickson, J. (2010). Development and evaluation of a method for

testing the residual biogas potential of digestates. Published by WRAP OFW004-005. ISBN: 1-84405-421-7.

http://www.watercorporation.com.au/_files/InfrastructureProjects/Biosolids_Reduction_of_Odour_Literature_Review_V2_2010.pdf

http://www.watercorporation.com.au/_files/InfrastructureProjects/Biosolids_Reduction_of_Odour_Literature_Review_V2_2010.pdf

http://randd.defra.gov.uk/Document.aspx?Document=WR1208_9922_TRP.pdf

http://www.sepa.org.uk/waste/waste_regulation/idoc.ashx?docid=0bc42e43-c846-4b31-adfc-599542358a7b&version=-1

http://www.sepa.org.uk/waste/waste_regulation/idoc.ashx?docid=0bc42e43-c846-4b31-adfc-599542358a7b&version=-1


Appendix 1 Data tables

Table A 1 Properties of digestates, potential biodegradability parameters

Digestates RBP

L/g(VS)

COD

g/L

BOD

g/L COD/BOD

Total

VFAs*

g/L as

acetic

Total

dissolved

carbon

g/L

digestate A 0.039 18.3 4.1 4.5 n/a 2.32

digestate B 0.061 20.9 8.2 2.5 0.50 2.64

digestate C 0.086 46.3 42.1 1.1 1.04 5.16

digestate D 0.181 20.4 7.9 2.6 0.90 5.33

digestate E 0.192 40.2 15.5 2.6 0.29 4.52

digestate F 0.220 20.9 7.9 2.7 2.11 4.41

digestate G 0.311 37.2 18.6 2.0 11.17 6.78

digestate H 0.531 31.8 17.1 1.9 9.53 6.07

digestate I 0.747 55.5 39.6 1.4 23.99 14.50

Digestate SBR 1 <0 10.0 1.9 5.3 0.07 0.75

Digestate SBR 2 <0 1.6 0.2 6.9 0.33 0.37

*Total VFAs is calculated as sum of C2 to C6 species from GC analysis, reported as acetic acid equivalent.

Table A 2 Properties of whole comparison slurries, potential biodegradability parameters

Comparison

slurries

RBP

L/g(VS)

COD

g/L

BOD

g/L COD/BOD

Total

VFAs*

g/L as

acetic

Total

dissolved

carbon

g/L

Cow slurry 1 0.203 65.8 15.3 4.3 4.49 5.35

Cow slurry 2 0.238 78.6 71.8 1.1 5.90 5.46

Cow slurry 3 0.238 56.3 47.5 1.2 6.12 5.98

Cow slurry 4 0.242 67.1 48.2 1.4 6.72 9.18

Cow slurry 5 0.275 86.6 28.6 3.0 7.72 8.47

Cow/Pig slurry 0.294 68.7 47.6 1.4 5.47 8.10

Pig slurry 1 0.501 24.8 15.9 1.6 11.17 8.26

Pig slurry 2 0.637 11.9 4.1 2.9 2.81 2.88

Pig slurry 3 0.638 44.7 28.3 1.6 9.07 6.47

Pig slurry 4 0.876 21.8 3.4 6.4 7.16 5.47

Digested sewage 0.099 55.6 45.4 1.2 0.43 2.29

*Total VFAs is calculated as sum of C2 to C6 species from GC analysis, reported as acetic acid equivalent.


Table A 3 Odour potentials and associated gases from digestates

Digestates

Odour

potential

103 ouE/m

3

H2S

ppm

FID

ppm (CH4)

PID

ppm

(isobut.)

NH3

ppm

digestate A 5 0.1 2330 0.7 0.4

digestate B 8 1.7 2305 66.1 13.5

digestate C 575 138 9092 43.9 5.5

digestate D 133 22.7 5611 21.3 20.5

digestate E 4 0.4 6964 6.8 17.7

digestate F 888 150 0 0.0 n/a

digestate G 3 1.8 5465 5.6 1.6

digestate H 37 0.7 731 9.5 2.0

digestate I 5 0.4 1344 57.5 19.7

Digestate SBR 1 2 0.0 29 1.3 1.3

Digestate SBR 2 0.3 0.0 10 0.9 1.9

Table A 4 Odour potentials and associated gases from whole comparison slurries

Comparison slurries

Odour

potential

103 ouE/m

3

H2S

ppm

FID

ppm (CH4)

PID

ppm

(isobut.)

NH3

ppm

Cow slurry 1 639 187 4478 45.4 12.1

Cow slurry 2 3570 451 8391 119.7 3.3

Cow slurry 3 6550 1258 3779 288.4 4.5

Cow slurry 4 1840 40.8 988 59.9 13.1

Cow slurry 5 113 24.0 7197 23.1 15.3

Cow/Pig slurry 5730 1161 5123 369.5 10.4

Pig slurry 1 45000 3809 2539 672.8 4.1

Pig slurry 2 3940 1360 2187 298.1 0.0

Pig slurry 3 3010 692 2439 147.6 4.1

Pig slurry 4 8550 852 1615 257.1 0.1

Digested sewage 30 1.0 2774 14.4 3.0


Table A 5 Digestates: Compounds identified by GC-MS analysis

Digestates

Meth

an

eth

iol

Bu

tan

oic

acid

Eth

an

al

Carb

on

Dis

ulp

hid

e

Dim

eth

yl

Su

lph

ide

Dim

eth

yl

Dis

ulp

hid

e

digestate A n.d. V Low n.d. V Low V Low n.d.

digestate B n.d. n.d. n.d. n.d. n.d. n.d.

digestate C n.d. n.d. n.d. n.d. n.d. n.d.

digestate D n.d. n.d. n.d. n.d. V Low Low

digestate E n.d. n.d. n.d. n.d. n.d. n.d.

digestate F V Low Med n.d. Med Low Low

digestate G V Low n.d. n.d. V Low n.d. V Low

digestate H n.d. n.d. n.d. Low Low Low

digestate I n.d. n.d. n.d. V Low V Low V Low

Digestate SBR 1 n.d. n.d. n.d. V Low V Low V Low

Digestate SBR 2 n.d. n.d. n.d. V Low V Low n.d.

n.d. = not detected

Table A 6 Whole comparison slurries, Compounds identified by GC-MS analysis

Comparison

slurries

Meth

an

eth

iol

Bu

tan

oic

acid

Eth

an

al

Carb

on

Dis

ulp

hid

e

Dim

eth

yl

Su

lph

ide

Dim

eth

yl

Dis

ulp

hid

e

Cow slurry 1 n.d. n.d. n.d. V Low V Low V Low

Cow slurry 2 n.d. n.d. n.d. V Low V Low V Low

Cow slurry 3 n.d. n.d. n.d. Med Low Low

Cow slurry 4 n.d. n.d. n.d. Low High Med

Cow slurry 5 n.d. n.d. n.d. V Low n.d. n.d.

Cow/Pig slurry n.d. n.d. n.d. Low High Med

Pig slurry 1 V Low n.d. n.d. V Low V Low V Low

Pig slurry 2 Low n.d. Low High High V High

Pig slurry 3 n.d. n.d. n.d. Low V Low V Low

Pig slurry 4 V Low n.d. n.d. Low Low Med

Digested sewage V Low n.d. n.d. V Low n.d. V Low

n.d. = not detected


Table A 7 Properties of digestates

Digestates pH EC

mS/cm

Dry Matter

%

Loss on

Ignition

%DM

Volatile

Solids

%

digestate A 7.9 9.5 1.74 62.2 1.08

digestate B 7.7 12.6 4.50 66.8 3.01

digestate C 7.9 11.4 5.24 74.7 3.92

digestate D 8.4 21.2 2.81 61.5 1.73

digestate E 8.0 23.6 3.39 65.3 2.21

digestate F 8.3 18.7 1.70 59.4 1.01

digestate G 7.5 14.8 3.57 76.8 2.74

digestate H 7.8 15.7 2.05 63.9 1.31

digestate I 8.2 22.6 2.81 63.5 1.79

Digestate SBR 1 8.1 16.9 3.36 34.1 1.15

Digestate SBR 2 7.8 19.8 2.67 11.6 0.31

Table A 8 Properties of whole comparison slurries

Comparison slurries pH EC

mS/cm

Dry Matter

%

Loss on

Ignition

%DM

Volatile

Solids

%

Cow slurry 1 7.3 7.9 9.84 81.3 8.00

Cow slurry 2 7.2 8.1 7.35 79.3 5.83

Cow slurry 3 6.9 8.0 7.96 82.8 6.58

Cow slurry 4 7.5 10.3 9.57 81.9 7.83

Cow slurry 5 6.8 7.4 10.41 86.4 9.00

Cow/Pig slurry 7.5 11.1 10.03 52.2 5.23

Pig slurry 1 7.8 20.5 2.20 53.1 1.17

Pig slurry 2 7.3 7.0 1.05 63.5 0.66

Pig slurry 3 7.1 12.7 4.81 71.1 3.42

Pig slurry 4 7.2 11.8 1.57 57.4 0.90

Digested sewage 7.9 7.5 5.46 63.7 3.47


Table A 9 digestates: Volatile fatty acids in solution (mg/L)

Digestates

aceti

c

pro

pan

oic

iso

-

bu

tyri

c

n-

bu

tyri

c

iso

-vale

ric

n-

vale

ric

hexan

oic

hep

tan

oic

octa

no

ic

digestate A n/a n/a n/a n/a n/a n/a n/a n/a n/a

digestate B 190 329 20 8 42 6 0 0 32

digestate C 960 69 10 7 12 0 18 0 117

digestate D 739 132 59 0 7 0 12 28 0

digestate E 275 0 16 0 0 0 0 0 909

digestate F 1366 619 202 55 47 37 28 59 85

digestate G 9169 819 248 440 1363 76 40 5 32

digestate H 6159 2657 642 244 812 199 34 6 232

digestate I 6609 17542 1502 568 2617 295 81 22 923

Digestate SBR 1 55 13 5 0 0 10 0 0 195

Digestate SBR 2 258 36 0 18 8 21 25 36 123

Table A 10 Whole comparison slurries, Volatile fatty acids in solution (mg/L)

Comparison

slurries

aceti

c

pro

pan

oic

ios

-

bu

tyri

c

n-

bu

tyri

c

iso

-

vale

ric

n-

vale

ric

hexan

oic

hep

tan

oic

octa

no

ic

Cow slurry 1 3377 845 128 253 192 61 35 24 5039

Cow slurry 2 4475 1014 184 423 210 71 39 9 528

Cow slurry 3 4593 1135 139 447 233 91 28 20 2726

Cow slurry 4 5315 1141 107 408 149 66 0 28 2382

Cow slurry 5 5168 2127 313 468 253 162 104 127 5146

Cow/Pig slurry 4607 582 96 265 197 47 0 59 9840

Pig slurry 1 8339 2257 339 503 590 106 39 7 247

Pig slurry 2 1809 609 102 363 184 98 41 22 822

Pig slurry 3 5605 2198 366 1093 645 297 271 61 2096

Pig slurry 4 4423 1499 244 993 447 294 473 34 125

Digested sewage 412 14 0 0 0 0 6 4 10


Table A 11 Digestates: Ions in solution (mg/L)

Digestates Cl

mg/L

PO4

mg/L

NO3

mg/L

SO4

mg/L

Na

mg/L

NH4

mg/L

K

mg/L

Mg

mg/L

Ca

mg/L

digestate A 257 8 2 50 131 619 505 75 38

digestate B 972 10 2 <5 205 1058 904 <10 24

digestate C 542 27 2 13 308 971 1124 53 28

digestate D 488 71 2 10 980 3182 2217 <50 73

digestate E 938 11 <1 <5 444 2103 176 <50 <10

digestate F 263 86 13 8 <50 1795 322 82 170

digestate G 969 13 2 <5 310 1569 445 <10 21

digestate H 438 7 <1 <5 361 1223 546 <50 19

digestate I 1642 40 <1 14 242 1421 223 <50 12

Digestate SBR 1 564 52 768 105 2431 112 409 12 26

Digestate SBR 2 731 22 1542 118 1875 99 422 <10 49

Table A 12 Whole comparison slurries, Ions in solution (mg/L)

Comparison

slurries

Cl

mg/L

PO4

mg/L

NO3

mg/L

SO4

mg/L

Na

mg/L

NH4

mg/L

K

mg/L

Mg

mg/L

Ca

mg/L

Cow slurry 1 889 18 12 151 153 661 951 85 89

Cow slurry 2 275 4 11 48 142 688 913 75 10

Cow slurry 3 469 13 17 218 111 677 619 68 59

Cow slurry 4 569 12 24 625 260 990 1059 74 105

Cow slurry 5 674 16 19 269 <50 354 322 82 170

Cow/Pig slurry 475 18 22 1107 133 1156 1283 118 305

Pig slurry 1 1348 77 <1 <5 503 2115 892 <50 38

Pig slurry 2 360 36 13 126 70 454 285 <10 58

Pig slurry 3 278 40 11 28 170 1130 777 <50 70

Pig slurry 4 316 22 12 8 138 1044 541 14 43

Digested sewage 83 67 2 <5 85 768 363 <10 23


Table A 13 digestates: carbon in solution

Digestates

Purgeable dissolved

carbon

mg/L

NPOC*

mg/L

Total dissolved

carbon

mg/L

digestate A 1141 1179 2320

digestate B 1619 1018 2637

digestate C 1451 3709 5160

digestate D 1710 3618 5328

digestate E 2049 2468 4517

digestate F 1645 2770 4414

digestate G 776 6009 6785

digestate H 1068 5000 6068

digestate I 1115 13380 14495

Digestate SBR 1 242 505 747

Digestate SBR 2 106 259 365

Table A 14 Whole comparison slurries, carbon in solution

Comparison slurries

Purgeable dissolved

carbon

mg/L

NPOC*

mg/L

Total dissolved

carbon

mg/L

Cow slurry 1 575 4777 5352

Cow slurry 2 422 5035 5457

Cow slurry 3 495 5487 5982

Cow slurry 4 749 8435 9184

Cow slurry 5 524 7946 8470

Cow/Pig slurry 637 7465 8102

Pig slurry 1 1441 6822 8263

Pig slurry 2 313 2563 2876

Pig slurry 3 511 5958 6469

Pig slurry 4 400 5070 5470

Digested sewage 902 1392 2293


Table A 15 Nutrient content of digestates in dry matter

Digestates P

%DM

K

%DM

S

%DM

Ca

%DM

Mg

%DM

digestate A 1.01 11.35 0.83 2.78 1.48

digestate B 1.20 5.89 1.09 5.10 0.26

digestate C 1.05 6.57 0.74 2.53 0.80

digestate D 1.37 6.88 0.54 6.28 0.21

digestate E 1.82 2.43 1.23 2.42 0.32

digestate F 0.94 11.03 0.76 1.17 0.06

digestate G 1.18 5.34 0.49 2.66 0.58

digestate H 1.26 6.00 0.52 3.34 0.62

digestate I 0.75 6.68 0.40 2.30 0.09

Digestate SBR 1 0.42 5.71 0.35 0.56 0.06

Digestate SBR 2 0.09 8.60 0.20 0.26 0.05

i/s = insufficient sample

Table A 16 Nutrient content of digestates kg/m3

Digestates N

kg/m3

P2O5

kg/m3

K2O

kg/m3

SO3

kg/m3

Ca

kg/m3

MgO

kg/m3

digestate A 1.81 0.42 2.49 0.38 0.51 0.45

digestate B 4.11 1.27 3.28 1.26 2.36 0.20

digestate C 3.66 1.31 4.29 1.00 1.37 0.72

digestate D 4.64 0.89 2.37 0.39 1.79 0.10

digestate E 5.21 1.44 1.01 1.07 0.84 0.18

digestate F 4.32 0.37 2.28 0.33 0.20 0.02

digestate G 4.24 0.98 2.33 0.44 0.74 0.35

digestate H i/s 0.60 1.50 0.27 0.69 0.21

digestate I 6.16 0.61 2.86 0.36 0.82 0.05

Digestate SBR 1 0.79 0.33 2.36 0.30 0.15 0.05

Digestate SBR 2 0.74 0.05 2.81 0.13 0.06 0.02



Table A 17 Nutrient content of whole comparison slurries in dry matter

Comparison

slurries

P

%DM

K

%DM

S

%DM

Ca

%DM

Mg

%DM

Cow slurry 1 0.66 3.81 0.43 2.27 0.60

Cow slurry 2 0.57 3.76 0.34 1.87 0.50

Cow slurry 3 0.82 3.33 0.60 2.51 0.65

Cow slurry 4 0.61 4.14 0.63 2.35 0.58

Cow slurry 5 0.66 3.50 0.44 1.35 0.51

Cow/Pig slurry 0.67 4.40 5.54 13.92 0.75

Pig slurry 1 0.78 12.75 1.28 1.36 0.32

Pig slurry 2 1.45 8.54 1.00 3.70 0.69

Pig slurry 3 2.28 5.97 0.95 4.44 1.60

Pig slurry 4 2.20 10.83 0.77 5.20 1.63

Digested sewage 3.07 0.44 1.41 4.66 0.30


Table A 18 Nutrient content of whole comparison slurries kg/m3

Comparison

slurries

N

kg/m3

P2O5

kg/m3

K2O

kg/m3

SO3

kg/m3

Ca

kg/m3

MgO

kg/m3

Cow slurry 1 3.41 1.49 4.52 1.07 2.23 0.98

Cow slurry 2 3.10 0.96 3.33 0.63 1.07 0.61

Cow slurry 3 3.46 1.53 3.24 1.22 2.03 0.88

Cow slurry 4 4.59 1.39 4.98 1.58 2.34 0.96

Cow slurry 5 4.20 1.57 4.39 1.15 1.40 0.89

Cow/Pig slurry 4.98 1.58 5.47 14.28 14.37 1.28

Pig slurry 1 4.73 0.40 3.45 0.72 0.31 0.12

Pig slurry 2 1.21 0.35 1.07 0.26 0.39 0.12

Pig slurry 3 4.05 2.52 3.47 1.14 2.14 1.29

Pig slurry 4 2.28 0.80 2.06 0.30 0.64 0.43

Digested sewage 4.32 3.93 0.29 1.96 2.29 0.27



Table A 19 Elemental content of digestates: toxic elements

Digestates Cd

mg/kg

Cr

mg/kg

Cu

mg/kg

Pb

mg/kg

Ni

mg/kg

Zn

mg/kg

Hg

mg/kg

digestate A <3 27.8 139.6 <30 20.0 418 <0.1

digestate B <3 79.0 144.8 <30 48.8 218 0.1

digestate C <3 59.8 64.0 <30 33.5 225 <0.1

digestate D <3 19.4 56.7 <30 36.0 162 <0.1

digestate E <3 168.0 77.0 <30 32.9 416 <0.1

digestate F <3 17.9 101.2 <30 21.0 381 <0.1

digestate G <3 472.5 107.0 <30 397.4 301 <0.1

digestate H <3 14.0 282.0 <30 16.5 679 <0.1

digestate I <3 9.2 27.6 <30 25.5 186 <0.1

Digestate SBR 1 <3 4.3 28.8 <30 <7 144 <0.1

Digestate SBR 2 <3 1.6 4.9 <30 12.4 <7 <0.1


Table A 20 Elemental content of whole comparison slurries: toxic elements

Comparison

slurries

Cd

mg/kg

Cr

mg/kg

Cu

mg/kg

Pb

mg/kg

Ni

mg/kg

Zn

mg/kg

Hg

mg/kg

Cow slurry 1 <3 50.1 58.3 <30 33.3 146 <0.1

Cow slurry 2 <3 35.2 48.3 <30 18.7 155 <0.1

Cow slurry 3 <3 55.7 128.3 <30 35.8 179 <0.1

Cow slurry 4 <3 30.5 45.2 <30 20.7 155 <0.1

Cow slurry 5 <3 138.6 67.3 <30 55.6 143 <0.1

Cow/Pig slurry <3 82.7 62.8 <30 49.6 199 <0.1

Pig slurry 1 <3 18.1 78.2 <30 9.4 514 <0.1

Pig slurry 2 <3 9.1 549.8 <30 10.0 1361 <0.1

Pig slurry 3 <3 10.2 1006.3 <30 12.9 2435 <0.1

Pig slurry 4 <3 16.8 185.7 <30 9.3 899 <0.1

Digested sewage <3 135.4 365.5 <30 77.9 614 0.5



Appendix 2 Hedonic tone graphs

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Appendix 3

Open University methods

All samples were delivered to the laboratory within 24 hours and refrigerated stored at 0 to 4oC for the minimum storage time possible (BS12176). Five selected samples were separated into liquor and fibre fractions by passing through a 1mm sieve, and if feasible a 0.5mm sieve. The separated fractions were weighed. Odour measurement was carried out by Silsoe odours using a standard “odour potential” extraction. Additional analysis of the gas collected was carried out at the Open University. VOCs were collected from 1L gas on sorbent tubes using Tenax TA and analysed by thermal-desorption GC-MS. Also H2S was analysed by Jerome X731 for levels up to 50ppm, and using a Draeger X-am 7000 and dilution as necessary for higher levels. Ammonia was measured by Gasmet FTIR, which also allowed other gases to be quantified including NOx, CH4. Total VOC indices

by FIC and PID were tested on a Thermo TVA toxic gas analyser if sufficient gas remained. Biodegradability under anaerobic conditions was measured by production of biogas according to the PAS100 Residual Biogas Potential test (Walker et al., 2010). Dry matter was determined by drying at 103º C and organic carbon by loss on ignition at 550 ºC (BS12879). A Kjeldahl digestion modified to include nitrates was used for total N (BS13654-1). Aqua regia digests for total elements (BSI 2001/1) were analysed for major and trace elements by ion-coupled plasma atomic emission spectrometry (BSI, 1998) on a Leeman Prodigy ICP-AES. Mercury was analysed on the digest on a Leeman Hydra AF Gold+ mercury analyser. Biological and chemical oxygen demand (BOD, COD) were measured by Aqua Enviro Ltd using in house methods, see below. A clear solution of each whole sludge was prepared by centrifugation and filtration to <0.45 µm. Volatile fatty

acids were measured on a mixture of clear solution and phosphoric acid by GC-FID (Shimadzu News 3/2007). Ions in solution including NH4, K, Ca and Mg were analysed on a Dionex DX3000 ion chromatograph. Total dissolved organic carbon was measured on a Shimadzu TOC ASI-V total organic carbon analyser. A pre-treatment with acid and purging with carbon-free air was used as for inorganic carbon, though as this may include some volatile fatty acids, it is termed purgeable carbon. The remaining carbon in solution was measured as non-purgeable organic carbon. Reference: Walker, M., Banks, C., Heaven, S., and Frederickson, J. (2010). Development and evaluation of a method for testing the residual biogas potential of digestates. Published by WRAP OFW004-005. ISBN: 1-84405-421-7.


Aqua Enviro COD Method

Reagents 1.

Substance Hazard Protective measures

First Aid

Digestion reagent

TOXIC IRRITANT

CORROSIVE

May cause cancer May cause heritable genetic

damage Harmful by inhalation in contact with skin and

swallowed Danger of cumulative effects

Causes severe burns Harmful to aquatic

organisms, may cause long-term adverse effect on aquatic environment.

Wear gloves and eye

protection. Avoid

inhalation

SKIN: removed contaminated clothing, wash skin up to 10

mins. If severe obtain medical atttention

EYES: irrigate with water for 20 mins. OBTAIN MEDICAL

ATTENTION In case of accident or feeling

unwell seek medical advice with label from box is possible

Procedure 2. PUT ON GLOVES AND EYE PROTECTION a. Shake the sample well b. Carefully remove the cap from the COD digestion reagent vial for the appropriate range.

Range

Low 0-150mg/l

Medium 0-1500mg/l

High 0-15000mg/l

c. Hold the vial at a 45° angle d. Add 2ml of sample to the low and medium range vials or 0.2ml to the high range vials. e. Replace the cap tightly f. Hold the vial by the cap and invert several times to mix the contents g. Place in a heating block at 150°c for 2 hours h. Prepare a blank using distilled water in the same way, again using 2ml for the low and medium

range and 0.2ml for the high range and place in the heating block i. After 2 hours remove and allow to cool for 5 minutes before inverting again to mix j. Leave until room temperature.

To read the vials – Low Range 3.

k. Use program 16 on the DR890 for the low range vials. l. Press 7 (PGRM) m. Press 1 and 6 and Enter the display will show mg/l COD n. Insert the COD/TNT adapter o. Clean the outside of the blank vial with a paper towel p. Place blank in the adaptor and cover with the lid q. Press ZERO display will show 0mg/l COD r. Clean the outside of the sample with a paper towel s. Place sample in the adaptor and cover with the lid t. Press READ value in mg/l will be displayed (Press CONC to view in other units)

To read the vials – Medium and high range 4.

u. Use program 17 on the DR890 for the low range vials. v. Press 7 (PGRM) w. Press 1 and 7 and Enter the display will show mg/l COD x. Insert the COD/TNT adapter y. Clean the outside of the blank vial with a paper towel


z. Place blank in the adaptor and cover with the lid aa. Press ZERO display will show 0mg/l COD bb. Clean the outside of the sample with a paper towel cc. Place sample in the adaptor and cover with the lid dd. Press READ value in mg/l will be displayed (Press CONC to view in other units) ee. Multiply the value by 10 for the high range vials

Interferences 5. Chloride is the most common cause of interference and the vial can turn cloudy if too high a level of chloride is present. Mercuric sulphate is present in the vials and this can eliminate chloride up to the concentration shown in the table below, if this is exceeded it can be possible to add 0.50g mercuric sulphate and this will raise the maximum concentration to that shown in the second column

Maximum Cl- concentration

in sample

Maximum Cl- concentration, with addition of 0.50 mercuric

sulphate

Low range 2000mg/l 8000mg/l

Medium range 2000mg/l 4000mg/l

High range 20,000mg/l 40,000mg/l

Blanks 6.The blank may be repeatedly used for measurements using the same lot of vials. It should be stored in the dark. Monitor decomposition by measuring the absorbance at the appropriate wavelength (420 or 610nm). Zero the instrument in the absorbance mode, using a vial containing 5ml of deionised water and measure the absorbance of the blank. Record the value. Prepare a new blank when the absorbance has changed by about 0.01 absorbance units.


Aqua Enviro BOD Method

1. Reagents a. Phosphate buffer (BOD/a) b. Magnesium sulphate solution (BOD/b) c. Calcium chloride solution (BOD/c) d. Ferric chloride solution (BOD/d)

Substance Hazard Protective measures

First Aid Spillage Procedures

Phosphate Buffer

Solution

None assigned at this

concentration

Standard lab procedures

Standard lab procedures Dilute with water and run to waste

Magnesium sulphate solution

None assigned at this

concentration


Standard lab procedures Dilute with water and run to waste

Calcium chloride

Irritating to eyes


EYES: irrigate with plenty of water. If

severe obtain medical attention

Dilute with water and run to waste

Ferric chloride solution

Irritating to eyes


EYES: irrigate with plenty of water. If

severe obtain medical attention

Dilute with water and run to waste

Place sufficient distilled water in a container for the number of BOD bottles you are using and add 1ml of each nutrient solution for every 1l of water. Aerate for at least 1 hour; the temperature of water should be 20 ±3°c, if necessary the container should be placed in warmer or colder water to achieve this.

2. Calibrating dissolved oxygen meter. e. Ensure the DO probe tip is free of water. Place the probe tightly into a BOD bottle half full of water

and leave for 15minutes. During this time, the air above the water will saturate with water vapour, this is the equivalent of oxygen-saturated water, which has a fixed DO content at a given temperature.

f. Turn on the DO meter, press the right hand button on the top row to enter the calibration menu, then press the left hand button to automatically calibrate, it should then display DO calibration saved on the screen.

g. It is imperative that all equipment used in the test is clean and free from organics to avoid contamination.

h. The table shows concentration of dissolved oxygen at saturation for a range of temperatures. This

is a guide and the values alter according to the altitude. If the DO probe has a vastly different value then the calibration should be checked.

Temperature C

Dissolved oxygen (saturated) mg/l

15 10.1

16 9.8

17 9.6

18 9.4

19 9.2

20 9.1

21 8.9

22 8.7

23 8.6

24 8.4

25 8.3

26 8.1


3. Dilution of samples

i. The samples must be diluted in the bottle with the dilution water (check with COD test to find

approximate strength). Approximately for domestic wastewater BOD=COD/2. Industrial samples can be higher or lower than this carrying out the BOD test with couple of dilutions can eliminate any problems).

Expected BOD range (mg O2/l)

Volume of sample to be diluted to 275ml in BOD bottle (ml)

0-21 100

12-42 50

30-105 20

60-210 10

120-420 5

300-1050 2

600-2100 1

1200-4200 0.5

j. If the expected BOD is greater than this range, additional dilution should be carried out prior to the

BOD test. Record any additional dilution and use it in the final calculation. k. Samples should be warmed to room temperature if you are using 50ml of sample of more.

4. Procedure

l. You need 1 bottle per sample and an extra 1 for a blank. Label each of these appropriately and half fill them all with dilution water.

m. Add 1ml of ATU mixture to each bottle. n. Consulting the above table decide which dilutions are appropriate for the sample and pipette or use

a measuring cylinder to insert the required volume of sample into the bottle. Fill all the bottles to the top with dilution water. Tap the bottles to get trapped air bubbles to rise to the surface.

o. One by one take the bottles and measure the DO concentration of the contents as follows. i. If the probe does not have a stirrer then a magnetic one must be added to the bottle before

measurement, ii. Rinse the probe with distilled water and place it carefully in the bottle avoiding the entrapment

of air bubbles. Switch on the stirrer and wait for the display to equilibrate. Record the reading (this should be similar to the value you got when the calibration was carried out if it isn‟t there may be a problem with the calibration or the temperature of the water/sample).

p. Remove the probe and refill the bottle very carefully with dilution water and put the top on. Invert the bottle to see if there are any air bubbles; if so they must be removed by removing the stopper and topping the bottle up again.

q. Place all bottles in the BOD incubator, this is set at 20c and is dark to prevent the growth of algae.

Leave for 5 days. r. After 5 days, remove the samples from the incubator and measure the DO content of each by

removing the stopper and placing the probe into the sample without introducing any air bubbles.

5. Calculation of BOD5

BOD5 = dDODO 21

Where: DO1 = initial dissolved oxygen reading DO2 = final dissolved oxygen reading after 5 days

d = dilution factor = sampleofvolume

275

The blank should not have a BOD of >0.5mg/l if it does then the other results are not valid and the analysis should be repeated.

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