practical techniques to assess the dewaterability of … · 2019-03-22 · the market leading...

19th European Biosolids & Organic Resources Conference & Exhibition

www.european-biosolids.com

Organised by Aqua Enviro

PRACTICAL TECHNIQUES TO ASSESS THE DEWATERABILITY OF SLUDGE

TO OPTIMISE CURRENT AND FUTURE PERFORMANCE

Minall, R1, Smyth, M1 & Horan, N2 1Aqua Enviro, 2The University of Leeds

Corresponding Author Tel: 01924 242255, email; [email protected]

Abstract

Sludge dewatering is integral to the cost effective recycling of Municipal Mesophilic

Anaerobic Digestate to land, as such it is currently well understood. Any mass balance or

cost model for a plant in the municipal MAD market will make reasonable assumptions

based upon a historic database for; i) the dry solids content (%DS) of the cake ii) the mass of

polymer or conditioning agent and dry solids (kg poly/tds); and iii) the quality of the return

liquors.

A dewatering asset is designed to achieve performance guarantees for these components

and this is critical to the long term financial and operational success of any given

project. Process guarantees are held by the manufacturer and installer of the equipment

using a knowledge base based upon municipal MAD, however recent and potential future

developments in organic waste Anaerobic Digestion and Thermal Hydrolysis have produced

sludges with different dewatering characteristics (dewaterability).

Through Research & Development, Aqua Enviro have developed a practical method that

can be used to assess, compare and evaluate sludge dewaterability in a repeatable

manner to optimise current and future performance. The tests can either be undertaken in

the laboratory or on site.

This paper focuses on the dewaterability of anaerobic digestate produced from sludge

treatment processes incorporating Thermal Hydrolysis, Municipal Sewage Sludge and from

the Anaerobic Digestion of Organic Wastes.

Keywords

dewatering, dewaterability, polymer consumption, cake dry solids

Introduction

The UK landbank for sludge and biosolids is finite yet the quantity of organic resources

recycled is increasing. Historically, effective volume reduction of material recycled to land is

achieved through sludge dewatering (often of digestate) to produce fibre and liquor

fractions. The incorporation of Thermal Hydrolysis into the digestion process flow chain has

resulted in an increase in saleable, high quality sludge compliant with the Safe Sludge Matrix

in the market. Thermal hydrolysis includes pre hydrolysis and post digestion dewatering

stages, whilst the technologies employed are well known the sludges processed have

different characteristics and are not fully understood. Thickening and Dewatering costs are

rising significantly and in the AMP6 Totex environment are more likely to impact upon final

choice of process selection. To achieve business case deliverables, stringent asset standards

have been applied based upon polymer consumption, cake dry solids and return liquor

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quality, however the real values that can be applied to site specific sludges have not been

effectively quantified.

In the food waste and other sludge treatment industries, detailed knowledge and control of

operational costs associated with processing sludge is vital to effective development of a

business model. There is a dearth of information in this area and it is challenging for operators

with dewatering technologies to operate them cost effectively.

Sludge Type and Dewaterability

Biotic sludges such as surplus activated or digested sludge from waste water treatment, are

known to be difficult to dewater, due to their high compressibility and their gel-like water

retention capacity. These properties are partly attributed to the presence of surface charges,

which are due to the biological nature and the presence of weakly charged extra-cellular

polymeric substances (Curversa et al., 2009). To aid the dewatering process the use of

chemical agents is common and is referred to as conditioning. Much of the achievements in

higher dry solids from dewatering plant have come from the development of polymers and

polymer dosing systems for conditioning of sewage sludge.

Food waste digestate dewaters poorly compared to sewage sludges from where

technologies have been adopted. The reason for the reduced performance is because

food waste is predominantly an organic material and after homogenisation and dilution

contains almost no settleable solids (unlike a primary or secondary sludge). Within an

anaerobic digester, the anaerobic biomass does not produce exopolysaccharides therefore

the whole digestate is uncharged and thus requires larger poly doses (than municipal

sewage sludge digestate) and potentially an additional source of cations (e.g. iron) to aid

flocculation. During the digestion process the particle size is further reduced due to hydrolysis

and the fibrous material swells and largely resists biodegradation. It is largely a colloidal

suspension of organic material with the fibres associated with the feedstock.

The liquor fraction from food waste digestate (to be treated by a secondary process)

contains a large but difficult to quantify amount of digested solids, which makes process

control challenging and on occasions impossible (Smyth & Horan, 2014). Furthermore

digestate produced from food waste digesters (where the measured hydraulic retention time

is < ~35-40 days and organic loading rate >4kg.VS/m3/d) usually contains much higher levels

of volatile fatty acids and ammonia-N (relative to sewage sludge digesters). Increased

protein levels make sludges more difficult to dewater, and return liquor quality degrades,

effectively increasing the COD and ammonia loads to secondary treatment, especially on

sites where digestate recycling for dilution is employed.

Poor quality return liquors can impact an existing wastewater treatment plant, increasing the

cost of the secondary treatment of wastewater or even risking consent failure if insufficient

capacity is not available. Modelling of the levels to be anticipated and/or upfront bench

scale trials is essential to determine the carbonaceous and nitrogen loads as well as the

dewaterability potential.

Sludge from alternative sources (for example MBT) is readily dewatered but cannot go to

agricultural land, often requiring thermal destruction of the organic material. Alternatively,

material can be composted producing "Compost Like Outputs" (CLO). To achieve

conditions at which drying and incineration become cost effective, or even such that

material can be composted, it must first be dewatered to produce a cake.





Effective dewatering relies heavily upon the integration of dewatering equipment, sludge

type and characteristics (including initial solids concentrations and pre-treatment) and

polymer selection and dosing. Each component of the supply chain is sourced from different

suppliers and manufacturers with little integration of the systems except at point of use. In a

TOTEX AMP6 environment where operating costs significantly affect materials and process

selection, effective, optimised dewatering is increasingly important.

A typical sludge comprises solid and liquor fractions. Dewatering is the process through

which the liquor fraction is separated from the mother liquid to produce a cake/fibre

fraction. Through conventional dewatering processes, only free water is removed.

Figure 1: The sludge particle

Dewatering Technologies

The market leading technologies for the removal of water from a thickened or unthickened

sludge are Decanting Centrifuges, Dewatering Belts or Filter Presses. All three are able to

achieve sludge cake dry solids of 15-30% and are still in use throughout the Municipal

Biosolids Treatment industry, although alternative technologies have become available on

the UK market.

Centrifuge dewatering involves the application of a centrifugal force of between 500 and

3,000 times that of gravity, to accelerate the separation of the solid fraction from the liquid.

Centrifugation produces a dewatered sludge cake and a clarified liquid which is known as

centrate. Three types of centrifuge are in general use, namely: disc-nozzle, basket and solid

bowl, with the final option being the most popular in the UK wastewater industry for sites over

20,000 population equivalence.

A solid-bowl centrifuge has a continuous feed and discharge and it requires a base, case,

bowl, scroll conveyor, feed pipe, main bearings, gear unit, back drive and motor. The bowl

diameter dictates the capacity of the centrifuge and this typically ranges from 230 to 1800

mm, permitting a feed rate in the range 2.2 to 158 m3/h. The scroll comprises a helix or screw-

conveyor assembly. It fits concentrically into the bowl and has a central hub with a feed

compartment and feed ports. The conveyor speed is controlled by a gear unit and back

drive. It is usual to dose a polymer either to the feed compartment or into a separate

injection point. Occasionally it can be added to the thickened sludge prior to centrifugation.





Figure 2: Detail of the Centriquip decanter centrifuge illustrating the cylindrical conical

bowl with the screw conveyor assembly (Centriquip, 2014)

Dewatering belt presses operate by squeezing polymer dosed and flocculated sludge

through polyester belts on rollers spaced to achieve successively higher operating pressure.

Wet sludge (>6%DS) is applied to an open belt and is initially gravity thickened until applied

into a series of press rollers of ever decreasing diameter.

The effective area pressure rises:

Area pressure p (N/mm2) = Belt tension S (N/mm) / Roller radius (mm)

The process is low shear and low pressure compared to a decanting centrifuge and applies

pressures in the region of 1.2 to 1.5 bar, dependent upon design and belt tension.

Dewatering belts must be continually washed during operation to ensure that filtrate is able

to permeate through the belt and drain. Shear is applied through the tortuous path that the

belt takes between rollers squeezing the sludge tighter within the belt.

Product sludge cake at the end of a belt is pressed flat in appearance and is sufficiently

inelastic as to retain the imprint of the weave of the belt, commonly known as "golfballing".

Dewatering belts do require dedicated skilled operators to monitor and operate them on a

continual basis and cannot be left to operate without skilled supervisors being present,

however they operate at lower power and polymer consumptions than centrifuges and

additional advantages have been claimed associated with reduced rates of pathogen

‘regrowth’.





Figure 3: Dewatering Belt Cutaway (Vulgaire 2014)

Filter presses operate by filling a series or permeable sacks of thickened sludge which are

then dewatered through the application of hydraulic or air pressure to ~6 bar.

Figure 4: Filter Press Operational Cycle, Filling, Pressurising and Discharging (Laughton

2014)

Filter presses are batch processes and therefore tend to be on sites with low sludge

production, however the presses typically produce both a high %DS cake and good quality

filtrate in operation. Filter presses rely on a combination of cake filtration, through which

solids captured on the filter wall, act as a progressively fine filter until the pressure required to

drive the liquor out of the sludge exceeds a certain set point (notionally 6-8bar).

This type of press is still widely used within the food and beverage sectors for liquor recovery

due to the typically excellent filtrate that it produces.

The Bucher press is a dewatering technology derived from fruit juice processing. The

technology consists of press cylinder lines with permeable membranes (socks). Over a two

hour cycle, the cylinder opens (drawing tight the sock), fills with sludge (hour 1) rotates and

then closes forcing released water through the socks, this pressing lasts for one hour. The

process operates in batches of 2 hour cycles until all of the sludge is of a consistent dry solids

concentration.





Through the operation of multiple open/close cycles the press extracts a high shear force,

without the energy demand required by centrifuges. Polymer requirements are understood

to be higher than expected, with medium weight polymers preferred due to the high shear

associated with multiple open/close cycles.

Thames water have had extremely encouraging results in the region of 45%DS with the

application of 12kg Polymer/tonne Dry Solids. The Bucher press has yet to be operated long

term on sewage sludge in the UK, however Thames have recently installed them at Oxford

and Crossness.

Achieving 45%DS exceeds values typically associated with free water alone, indicating that

the Bucher Press action may be removing capillary or colloidal water as well as free water

typically associated with dewatering alone.

Figure 5: Pilot Bucher Press In Operation (Fountain 2013)

The Importance of Polymer

With the exception of energy (operational) costs, the consumption of polymer is one of the

largest on site operational costs associated with both thickening and dewatering.

Depending upon the sludge type and composition different asset standard requirements

(table 1) may be expected throughout the UK. Different polymer types and weights will

affect the dry solids and return liquor quality. The majority of polymers used as conditioning

agents are high molecular weight, long chain, water soluble synthetic organic polymers or

flocculants.





Table 1: Typical Asset Standard Values for Polymer Dosing

The ranges are broad to make up for variation in feedstock quality, however, to illustrate the

effect upon Operational Cost, a base case comparison for a large digestion site

incorporating thermal hydrolysis has been developed.

Table 2: Assumed Annual Polymer Operational Cost

Assuming a polymer purchase cost of £2.00 per kg and a design horizon of 330tDS/d (Smyth

2014), a net annual operational cost of £2.4M is forecasted. When the polymer consumption

increases or decreases within the bands expected for ‘typical’ sludges the operating cost

range could vary from £2.0-3.4 million per annum.





Figure 6: Polymer Consumption Cost Sensitivity Analysis

Polymer selection and makeup facilities are important considerations when designing and

optimising a dewatering process. There are two main forms of polymer. Solution polymers

are highly viscous. The viscosity of the solution is a function of the molecular weight of the

polymer. Products available as solutions are usually coagulant types which have very low

molecular weight and can be up to 50% active product, or low molecular weight

flocculants, which can be up to 20% active product. The chemical types for these solution

polymers are either polyamines or polyacrylamides for the coagulant types, or

polyacrylamide based products for flocculant types. For dosing into sludges, polymers of this

grade are readily made up in small dosing packages and require little ageing or maturation

time. Equipment is readily built off site and when correctly planned, can be installed onto a

pre-prepared area in less than a day.

The converse of the above applies to solid (or dry) polymers. The solutions of monomers are

mixed and when the reaction is complete what is left is a solid, jelly like substance, which has

to be processed and dried prior to arriving at the final product. The type of equipment used

to process the jelly will vary between manufacturers, but as a general rule solid grade

products are of a high molecular weight. Once transported to site, these dry materials need

to be rewetted and activated prior to dosing into sludge. Effective wetting and ageing

times for optimum polymer consumption are One to Two hours dissolution and ageing time,

with a shelf life of 4 hours before a new batch should be used.

The ultimate effectiveness of any dewatering technology, in terms of cake %DS is defined by

the proportion of free water within the sludge (figure 1) and how much of this can be

removed. There are many factors which influence free water release including, inter alia: the

choice of technology, site specific sludge characteristics, sludge pre-treatments, volatile

solids content, polymer type and polymer dose rate and location.





Quantifying the dewaterability potential of sludges

Polymer preparation, sludge flocculation and mixing are all vital in the effective operation on

site and are set according to manufacturer's recommendations during installation and

commissioning.

Proprietary tests involving laboratory centrifuges are available and can be used to provide

an indicative result with regards to a sludge propensity for dewatering, however few

guarantees can be based upon such tests. Process guarantees are typically set against

polymer consumption, cake and liquors quality however these are often set against asset

standard requirements without site specific sludge testing for which only three main tests are

available.

Established tests, Specific Resistance to Filtration and Capillary Suction Time, are available

but are not comprehensive in their behaviour in describing dewaterability of sludge and

cannot be directly compared to one another (Smollen 1986). A third test, thermogravimetric

analysis will determine the ultimate Free Water within a sludge, and hence ultimate cake dry

solids concentration but does not describe the practically achievable dry solids with polymer

additive. The CST test can be used to rapidly and effectively determine optimum polymer

doses, whilst the SRF test can be used to compare the characteristics of one sludge against

another, the two tests (CST and SRF) cannot be used for comparison purposes on the same

sludge.

The capillary suction time (CST) test measures rate of water release from sludge, the test itself

facilitates rapid comparisons between polymer doses and concentrations into sludges.

Capillary suction time testing is available using standardised test kit, marketed by Triton

Electronics.

The test uses chromatography paper onto which a sample of sludge is placed within a

capillary tube or reservoir. The time elapsed between the tube being placed onto the paper

and the time required to travel a set distance is recorded. Shorter CST times indicate that

water is released faster and can therefore be used to optimise chemical doses.





Figure 7: CST Testing Apparatus (Clesceri et al, 1998)

Capillary suction time will provide the optimum polymer dose of any trialled polymer for each

individual sludge type, however it cannot be used to determine the cake dry solids. The test

is fast and several tests can be undertaken on an hourly basis, facilitating rapid testing of

multiple polymers to determine the most effective dose rates.

The Specific resistance to Filtration test can be completed using standard laboratory ‘Quick

Fit’ filtration apparatus. A known volume of sludge is placed into a Buchner funnel and

filtered through a Filter Paper with an applied suction pressure of 49kPa. The amount of

filtrate is recorded at fixed time intervals (30 seconds).

Figure 8: SRF Testing Apparatus (Clesceri et al, 1998)





Using the method described by Smollen (1986), assuming that the filter paper has negligible

hydraulic resistance and that the sludge cake forming on the filter paper is not compressible,

then Darcy’s law can be applied to the results such that (in a batch test):

Equation 1: Darcy's Law and Its Application to Batch Processes (Smollen, 1986)

Where;

To determine r, experimental data must be plotted to determine the constant relationship

between time and Volume of filtrate released during the test. This function can be used to

determine the specific resistance to filtration in units of m/kg. Although this test provides a

value for dewatering effectiveness of the sludge it only measures small volumes and is not

effective with thickened sludges which foul the filter surface, reducing measurement

accuracy.

Thermogravimetric analysis is used to determine the ultimate free water of a sample of

sludge. The test procedure involves heating a sludge sample at a low temperature and

logging the sample mass until all of the water within the sample (including capillary and

intracellular water) has been removed. Since free water is most easily removed, then a

steady consistent mass loss can be expected until all of the free water has been removed.

Since capillary and intracellular water is more difficult to remove, the drying rate reduces,

creating a turning point in the mass loss curve. At this point, the free water mass within

sludge can be developed from which the ultimate dewaterability can be determined.





Figure 9: Thermogravimetric Analysis Typical Graph (Kopp 2014)

The test uses slow, low temperature drying of sludge and each sample must be analysed

over an extended time period. The test is unable to determine the effect of polymer

selected upon free water release for dewatering, nor will it compare specific technologies.

Since free water determination is used to determine the maximum dry solids, the actual dry

solids produced by any one treatment technology can only be estimated to be a proportion

of the ultimate value.

Site Process Changes

New schemes, for example installation of Cake Import Facilities, Thermal Hydrolysis or

wastewater treatment process changes will affect he makeup (and characteristic) of any

sludge to digestion or dewatering. Although CST, SRF and Thermogravimetry can provide

some information regard “current” sludge characteristics, they offer no evaluation of sludge

characteristics post process change in site.

Changes to the sludge dewatering characteristics may manifest in terms of polymer

consumption to achieve adequate dry solids type, return liquor quality and final cake dry

solids. With the current tests available there is no way of evaluating how all of these

parameters will be affected.

Aqua Enviro are able to offer a batch thermal hydrolysis service in which site specific sludge

and sludge blends can be thermally hydrolysed and anaerobically digested within our

bench scale digesters to evaluate VS destruction, gas yields and produce a digestate which

can have dewatering testing undertaken upon it.





Figure 10: Aqua Enviro THP Unit

Aqua Enviro Approach

Aqua Enviro are developing a new dewatering testing and optimisation unit which uses the

principles of the specific resistance to filtration test and thermogravimetric analysis but allows

the testing of much larger volumes of sludge than a Specific Resistance to Filtration test, in a

faster time than Thermogravimetric analysis.

The Aqua Enviro dewaterability optimisation test (DOT) uses a dewatering membrane which

can be pressurised internally up to 5 bar, forcing solids into the membrane fabric and

separating liquor through the membrane walls. During testing, the operational pressure and

mass of water generated are measured and logged using online data logging systems.

Figure 11: Aqua Enviro DOT Unit





Logging the volume of water produced through the DOT equipment over time allows us to

complete an equivalent of the Specifc Resistance to Filtration test, at a larger scale that that

provided by a simple filter paper test. The size and nature of this test also allows the user to

determine Cake Dry solids and return liquor quality for each individual sludge blend. The test

is rapid being completed within minutes facilitating quick comparison between sludges,

sludge blends and polymer types.

Combined with the CST tests, indicative optimum polymer types and doses can be trialled to

determine their effect upon both achievable cake dry solids and return liquor quality.

This novel approach to dewatering testing facilitates the testing of existing sludges on site. In

conjunction with Aqua Enviro’s pilot scale Thermal Hydrolysis unit and Anaerobic Digestion

reactors it is possible to model the effect of process changes, the implementation of new

processes or even changes to the sludge feedstock blends upon the dewaterability of the

resultant product sludge.

Conclusions

Sludge processing technologies have not changed significantly over the past 15 years,

however recent innovation in pressing technologies mean that the market place is changing

and evaluation of technologies is more important than ever.

With new innovations both in sludge treatment and sludge processing and a wider variety of

feed sludges, dewatering performance is less certain now than a decade ago. In a

wastewater, capital driven environment, guarantees are made by suppliers and main

contractors based upon having to treat a sludge which may not yet have been created with

no reference characteristics. The level of risk that the contractor and end client must

withstand is high and the potential for litigation greater than ever.

There are a number of available tests (Specific Resistance to Filtration, Thermogravimetric

Analysis and Capillary Suction time tests) which can be used in a limited number of

circumstances to help define what may be achievable in terms of cake dry solids, polymer

consumption, liquor quality. However, there is no individual test that can quantify all of these

factors.

Aqua Enviro's new dewaterability optimisation test is set to revolutionise this process,

providing a scalable, benchmark test against which realistic quality standards can be set

and measured.

References

Centriquip (2014). [Online] Available from: http://www.centriquip.co.uk/products.aspx?id=56

[Accessed 10/10/14]

Clesceri, Greenberg and Eaton (1998). Standard Methods for the examination of water and

wastewater 20th Edition. 2-83 to 2-85

Curversa, D., Usherb, S.P., Kilcullenb, A.R., Scalesb, P.J, Saveyna, H. and Vander Meerena. P.

(2009) The influence of ionic strength and osmotic pressure on the dewatering behaviour of


http://www.centriquip.co.uk/products.aspx?id=56




sewage sludge, Chemical Engineering Science, 64 , 2448 -- 2454 Manual of Practice No FD-

14 Sludge Conditioning.

Fountain (2013). Control of pre-THP dewatering and feed to THP using sludge bypass and

back-mixing Fountain, P. and Strange, G., Thames Water, UK, 19th European Biosolids &

Organic Resources Conference. Aqua Enviro, Wakefield.

Kopp (2014) Optimizing Polymer Preparation to Enhance Sludge Dewatering. Making

Dewatering Work Conference. Aqua Enviro, Wakefield

Laughton (2014) Selecting the right Equipment for your Sludge, Testing in the Lab Euroby

Limited. Making Dewatering Work Conference. Aqua Enviro, Wakefield

Smyth, M. (2014). Dewatering past, present and future. Conference presentation at Making

Dewatering Work, 23rd October 2014. Aqua Enviro.

Smollen (1986). Dewaterability of municipal sludges: A comparative study of specific

resistance to filtration and capillary suction time as dewaterability parameters. Water South

Africa Vol 12, 3, pp 123-132

Smyth & Horan, (2014). Operating experiences from organic waste digestion plants. In ed.

Horan, NJ, 19th European Biosolids & Organic Resources Conference. Aqua Enviro,

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http://vulgaire.com/4/belt-press/

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