EENNEERRGGYY FFRROOMM WWAASSTTEEWWAATTEERR

AA FFEEAASSIIBBIILLIITTYY SSTTUUDDYY TTEECCHHNNIICCAALL RREEPPOORRTT

RReeppoorrtt ttoo tthhee

WWaatteerr RReesseeaarrcchh CCoommmmiissssiioonn

bbyy

S Burton, B Cohen, S Harrison, S Pather-Elias,

W Stafford, R van Hille & H von Blottnitz

Department of Chemical Engineering

University of Cape Town

WRC Report No. 1732/1/09 ISBN 978-1-77005-849-1 Set No. 978-1-77005-846-0

JULY 2009

ii

DISCLAIMER

This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the WRC, nor does mention of trade names or commercial products constitute

endorsement or recommendation for use.

iii

EEXXEECCUUTTIIVVEE SSUUMMMMAARRYY

The current view of wastewaters is that they generally represent a burden and necessarily

incur energy costs in processing before they can safely be released into the environment. The

opportunity exists to improve the current wastewater treatment processes by applying new

solutions and technologies that can also reduce energy inputs and/or generate energy for

other processes. This study explored the various waste streams and the appropriate

technologies that could be used to generate energy.

A survey of the quality and quantity of wastewaters in South Africa identified the top three

sectors having the greatest potential energy recovery as the formal and informal animal

husbandry sector (cows, pigs and chickens), fruit and beverage industries (distillery, brewery,

winery, fruit juicing and canning) and domestic blackwater (sewage). An estimated 10 000

MWth can be recovered from the wastewaters in the whole of South Africa, representing 7%

of the current Eskom electrical power supply*. However, since most of the waste streams are

widely distributed, the energy from wastewater is best viewed as on-site power.

The most appropriate technologies and their limitations are partly determined by the value of

the required energy product (heat, electricity, combined heat and power or fuel) and the

driving market forces that determine how this energy product can be used with our current

technology. Furthermore, the ease of separation of the energy product from water can be key

to the feasibility of the process (e.g. biogas separates easy from wastewater by natural

partitioning whereas bioethanol requires energy intensive distillation). Anaerobic digestion (AD) is the most commonly recognised technology and has been applied to wastewaters of

different characteristics at both small and large scales. AD is suitable for use with domestic

sewage (particularly since 40% of South Africans are currently not serviced with waterborne

sewage), as well as in the industrial and agricultural sectors. Bioethanol production by fermentation is suited to concentrated, high carbohydrate wastewaters and has potential in

the fruit industry where sugar-rich wastewaters are generated in large volumes. Similarly,

combustion and gasification are restricted to applications with concentrated waste streams

(containing

iv

infrastructure, but the technology is still in early development. A single waste stream or

technology may not be suitable for achieving efficient energy recovery and the integration of

technologies and/or waste streams may be required to realise the maximum energy from

wastewater potential. There are also frequently missed opportunities for reducing energy

needs by the recovery of waste heat. The greatest potentials are realised when an industrial

ecology approach is used to integrate process or waste heat from several industries, with pre-

planning and the formation of industrial parks.

The net energy generated, reduction in pollution (wastewater treatment), and water

reclamation are the main costs and benefits considered in assessing the feasibility of an

energy from wastewater project. However, additional benefits such as certified emission

reductions (CERs), fertiliser production, or the production of other secondary products could

tip the balance of economic feasibility. For the implementation of energy from wastewater

technologies, essential services (WWTP operation, schools, and hospitals) and the needs of

communities not serviced by sewage and electrical infrastructure should preferentially be

targeted.

Several risks, barriers and drivers to developing an energy from wastewater project were

identified. There is a general lack of research capacity and skills, and a greater need for

research collaboration and information-sharing between research groups, government

agencies and municipal practitioners. There is also no incentive for the generation of clean,

renewable energy such as feed-in tariffs, green energy tariffs or peak tariffs.

The use of wastewater as a renewable energy resource can improve energy security while

reducing the environmental burdens of waste disposal. Energy from wastewater therefore

facilitates the integration of water, waste and energy management within a model of

sustainable development.

v

CONTENTS: Page

1. TECHNICAL REPORT: Appropriate energy from wastewater technologies and a review of literature and national and international practice 1.1 Anaerobic Digestion (AD) to produce biogas 1

1.1.1 Introduction 1.1.2. Advantages and constraints associated with AD 1.1.3 Application and practice of AD technology

1.1.4. Implementation of AD 1.1.5 Costs and benefits

1.2 Combustion and gasification for the production of steam 20 1.2.1 Introduction 1.2.2 Application of combustion / gasification

1.2.3 Costs and benefits of combustion / gasification 1.3 Utilisation of waste heat 23

1.3.1 Introduction 1.3.2 Implementation of utilisation of waste heat 1.3.3 Costs and benefits in utilisation of waste heat

1.4 Fermentation to produce biomass and secondary products 28 1.4.1 Introduction 1.4.2 Implementation of fermentation of wastewaters 1.4.3 Cost and benefits in fermentation of wastewaters

1.5 Algal biomass for biodiesel production 31 1.5.1 Introduction 1.5.2 Implementation of biodiesel production 1.5.3 Costs and benefits in biodiesel production from wastewaters

1.6 Fermentation for production of bioethanol 36 1.6.1 Introduction 1.6.2 Implementation of bioethanol production 1.6.3 Costs and benefits in bioethanol production from wastewaters

1.7 Microbial fuel cells 40 1.7.1 Introduction 1.7.2 Implementation of MFCs 1.7.3 Costs and benefits in the application of MFCs to wastewaters

1.8 References 45 2. APPENDIX: Surveys, case studies and workshops 53

2.1 Survey of WWTP wastewaters 2.2 Survey of brewery wastewater 2.3 Survey of pulp and paper wastewaters 2.4 Survey of petrochemical wastewaters 2.5 Survey of Animal husbandry wastewaters 2.6 Case study of energy from fruit wastewaters 2.7 Case study of combustion versus gasification of waste stream 2.8 Case study of Cape Flats WWTP: Best practice? 2.9 Case study of the performance of household anaerobic digesters 2.10 Industrial stakeholder workshop

vi

Acknowledgements: Certain studies that have been incorporated into this project were undertaken by students at

UCT. These are acknowledged as follows:

Thermal gasification or anaerobic digestion of biomass for use in a distributed renewable energy network (Sarah Dowling)

Optimising algal growth and lipid production (Melinda Griffiths) Energy potential of the wastewaters from the paper and pulp and the brewing industry

(Andrew Krige)

Energy potential of wastewaters of the petroleum and animal husbandry industries (Deleki Zuko and Sehoana Kabelo Albert)

Deriving energy from fruit wastes (Andrew de Beer and Peter Dawson) Carbon dioxide sequestration and fixation by algae (Nick Langley and Bertus Louw) Cape Flats WWTP: Best practice? (Loice Badza)

1

1: APPROPRIATE ENERGY FROM WASTEWATER TECHNOLOGIES AND NATIONAL AND INTERNATIONAL PRACTICE

This report comprises a review of the literature with respect to energy produced from wastewater, and

a review of information available on the practice of energy generation from wastewater occurring in

South Africa and internationally.

This report constitutes the technical section of the final report for this research project, WRC

K5/1732. The report is divided into sections, each dealing with one technology, and providing, with

respect to the respective technology: Introduction on the theory of the technology; information on its

application and the energy potential.

SECTION 1.1 Anaerobic digestion to produce biogas 1.1.1 Introduction Anaerobic digestion leads to the ultimate gasification of organic carbon containing wastewaters and

slurries to carbon dioxide, methane and hydrogen. It has been successfully used in the wastewater

treatment industry to reduce both the volume and COD of waste sludge. Anaerobic digestion of

wastewaters and sludges is a complex process, involving a number of different microorganisms. A

schematic representation of the process, typically consisting of three metabolic stages, is shown in

the process flow diagram in the Figure 1, below (GATE, 1999).

Figure 1. Schematic showing the stages and microorganisms involved in anaerobic digestion for biogas production.

Initially, particulate organic matter such as cellulose, hemicellulose, pectin and lignin are solubilised in

a hydrolysis step catalysed by extracellular enzymes. The soluble products of this stage are converted

2

into organic acids, alcohol, hydrogen and carbon dioxide by acidogenic microorganisms. The simple

organic substrates are further metabolised by acetogenic microorganisms to produce acetate, the

primary organic substrate for methanogenesis, hydrogen and carbon dioxide. Aside from acetate, CO2

and hydrogen, methanogenic microorganisms may also directly ferment other substrates, of which

formic acid and methanol are the most important (Bouallagui et al., 2007).

The AD digester can be designed as a cylindrical, cube shaped, or egg-shaped vessel and is

composed of a waterproof material with an inlet into which the fuel is introduced, normally in slurry

form. In small to medium scale facilities, the gas holder is typically an airtight steel container which

covers the top of the digester, excluding air and collecting the gas produced (Amigum and Von

Blottnitz, 2007). For industrial scale plants or ponding digesters the gas storage component is

appropriately designed for the application. The simplest reactor type consists of a single digester,

operated as a fed-batch or continuously stirred tank reactor. All the processes associated with

anaerobic digestion occur within the single unit, which may lead to process inefficiency, given the

differences in metabolism of the microbes responsible for the different phases of digestion. The typical

influent substrate concentration is in the range of 3 to 8% total solids (Gunaseelan, 1997), although

the solids loading can be as high as 25%. Retention times within these reactors are typically in excess

of six days. To overcome the problem of biomass washout while reducing the residence time, a

number of reactor types have been developed including anaerobic filter or fluidised bed systems

(where biomass is encouraged to attach to solid support material) and the upflow anaerobic sludge

blanket (UASB) reactor (certain microorganisms induce floc formation thereby retaining biomass in

the absence of a solid support). The use of two-stage digesters, that physically separate the

hydrolysis, acidogensis and acetogenesis stage form the methanogenic stage (Ghosh et al., 1975;

Gunaseelan, 1997; Niishio et al., 2007), often result in significantly reduced total retention times and

increased biogas production as the conditions for both sets of organisms can be independently

optimised.

There are several sources of waste with different characteristics: municipal (sewage), agriculture

(waste from animals, crop residues, and food processing) and industries (paper and textiles,

petrochemicals). Anaerobic digesters have been successfully employed for the generation of energy

from a wide range of these wastes. The table below illustrates the range of substrates that have been

tested at laboratory or pilot scale. Typical biogas yields from the digestion of primary sewage sludge in

an egg-shaped digester, common on sewage treatment plants, range from 0.4-0.9 l/g organic dry

substrate.

The yields are typically expressed in terms of gas production per unit of COD or volatile solid (VS),

and are in the order of 0.2 to 0.45 m3/kg VS (Gunaseelan, 1997). The energy yield from methane is

calculated based from a value of 890.3 kJ per mole of methane (or 11.04 kWh/m3 of methane) (Duerr

et al., 2007).

3

Table 1: Methane and hydrogen production potential from agricultural wastes utilising a range of anaerobic digestion technologies

Feed Temp. (C)

HRT (days) Organic loading rate

Potential energy recovery

Reference

Bread waste (100 g/L) + SS (10% w/v)

55 - 29 g-wet wt/L.d 145 m3 H2/d (214 kwh)

514 m3 CH4/d

Nishio & Nakashimada, 2007

Beer waste (pressed filtrate from lauter tun)

50 (H2) 37 (CH4)

- - 2.2 l H2/L (24 kJ/L) 2.2 l CH4/L (79

kJ/L)

Nishio & Nakashimada, 2007

Fruit and vegetable waste 47 1.06 g VS/L.d 0.16 l CH4/g VS Bouallagui et al., 2005

Fruit and vegetable waste 32 0.9 g VS/L.d 0.26 l CH4/g VS Bouallagui et al., 2005

Fruit and vegetable waste 20 1.6 g VS/L.d 0.47 l CH4/g VS Bouallagui et al., 2005

Fruit and vegetable waste 23 3.6 g VS/L.d 0.37 l CH4/g VS Bouallagui et al., 2005

Fruit and vegetable waste 20 2.8 g VS/L.d 0.45 l CH4/g VS Bouallagui et al., 2005

Fruit and vegetable waste 2.5 6.8 g VS/L.d 0.35 l CH4/g VS Bouallagui et al., 2005

Fruit and vegetable waste 7 + 10 4.4 g VS/L.d 0.34 l CH4/g VS Bouallagui et al., 2005

Fruit and vegetable waste 2 + 2.3 5.65 g VS/L.d 0.42 l CH4/g VS Bouallagui et al., 2005

Brewery slurry 55 26 3.23 g COD/L.d 0.37 l CH4/g COD Zupancic et al., 2007

Brewery slurry 55 17.5 5.40 g COD/L.d 0.38 l CH4/g COD Zupancic et al., 2007

Brewery slurry 55 16 6.27 g COD/L.d 0.42 l CH4/g COD Zupancic et al., 2007

Brewery slurry 55 13.5 8.57 g COD/L.d 0.37 l CH4/g COD Zupancic et al., 2007

Artificial garbage slurry 55 0.5 + 0.54 75 g/L.d 199 mmol H2/L.d 442 mmol CH4/L.d

Ueno et al., 2007

Fruit and veg waste + manure

35 21 3.19 g VS/L.d 0.45 l CH4/g VS Callaghan et al., 2002

Barley waste + SS 37 - - 0.025 l CH4/g VS Neves et al., 2006

Hydrolysed barley waste + SS

37 - - 0.22 l CH4/g VS Neves et al., 2006

Cheese whey 37 1 + 4 19.78 g COD/L.d 0.30 l CH4/g COD Saddoud et al., 2007

Olive mill water + solids 55 36 3.62 g COD/L.d 46.2 l CH4/L OMW.d

Fezzani & Ben Cheikh, 2007

Olive mill water + solids 55 24 5.58 g COD/L.d 38.2 l CH4/L OMW.d

Fezzani & Ben Cheikh, 2007

Olive mill water + solids 55 24 1.79 g COD/L.d 29.6 l CH4/L OMW.d

Fezzani & Ben Cheikh, 2007

Distillery effluent 35 14 2.74 kg/Cu m 4.44 l CH4/L Banerjee & Biswas, 2004

Distillery effluent 55 14 2.74 kg/Cu m 5.40 l CH4/L Banerjee & Biswas, 2004

Molasses 35 7 7 g COD/L 0.23 l CH4/g COD Jimnez et al., 2004

Fermented molasses 35 3 7 g COD/L 0.31 l CH4/g COD Jimnez et al., 2004

Algal sludge 35 10 4 g VS/L.day 0.14 l CH4/g VS Yen & Brune, 2007

Algal sludge + waste- paper

35 10 4 g VS/L.day 0.29 l CH4/g VS Yen & Brune, 2007

Palm oil mill effluent

- 7

40 g COD/L

0.24 l H2/g COD

Vijayaraghavan et al.,

2006 Jackfruit peel - 12 33 g VS/L 0.40 l H2/g VS Vijayaraghavan et al.,

2006

4

1.1.2 Advantages and constraints associated with AD Anaerobic digestion offers a number of significant advantages over competing technologies for the

treatment of carbonaceous municipal, industrial and agricultural effluents. These include low energy

requirements, reduced sludge production and the economic recovery of energy. Thermophilic1

anaerobic digestion has the additional benefits of increased waste stabilisation and sludge

dewatering. This is particularly relevant to the treatment of sewage sludge, where the elevated

temperature leads to increased destruction of viral and bacterial pathogens (Chen et al., 2007 and the

study comparing combustion with AD and enhanced AD summarised in the K5/1732 Essence report),

but may incur large energy input costs.

In terms of constraints on the feasibility of anaerobic digestion technology, inhibition of the component

microbial processes and the resulting decrease in process efficiency can be a major concern.

Common reasons for poor performance are mixing problems, or the inability to maintain the correct

balance between acid forming and methane forming microbial communities. These two microbial

groups differ widely in their physiology, nutritional requirements, growth kinetics and susceptibility to

environmental perturbations (Chen et al., 2007). Inhibitory substances may be present at high

concentrations in wastewaters and sludges, or may be formed during the initial phases of anaerobic

digestion. Inhibition is typically identified by a decrease in steady state methane production and an

accumulation of organic acids. The most significant inhibitors are discussed briefly below.

Ammonia. Ammonia is produced by the biological degradation of nitrogen-containing compounds,

most commonly urea and proteins (Kayhanian, 1999). Its aqueous speciation is pH dependent, with

the ammonium ion (NH4+) dominating at low pH and free ammonia (NH3) under alkaline conditions.

The free form is freely membrane permeable and has been suggested to be the primary source of

inhibition, resulting in a proton imbalance or potassium deficiency (Gallert et al., 1998). Of the

organisms involved in anaerobic digestion, the methanogens are most susceptible to inhibition by

ammonia (Kayhanian, 1994). The extent of inhibition is mediated by a number of factors, such as pH,

temperature, the presence of antagonistic cations (Na+, Ca2+, Mg2+) and acclimation, to the extent that

literature values for 50% reduction in methane production vary from 1.7-14.0 g/L total ammonia

nitrogen (Chen et al., 2007).

Sulphide. Sulphate is a common constituent of many industrial and agricultural wastewaters. Under

anaerobic conditions it is reduced to sulphide by sulphate reducing bacteria (SRB) (Hilton and

Oleszkiewicz, 1988). Inhibition occurs at two levels, initially through competition for substrate and

secondarily as a result of sulphide toxicity. SRB are not able to utilise complex organic carbon sources

so do not compete with hydrolytic or acidogenic microbes. Competition between SRB and acetogenic

and methanogenic bacteria has been observed, but results are often contradictory. The extent of

1 Thermophilic microorganisms grow optimally at temperatures above 40C. Mesophilic microorganisms grow optimally at temperatures between 25 and 40C; and psychrophilic microorganisms grow optimally below 5C.

5

competition appears related to the initial microbial concentrations and the COD/SO42- ratio, with SRB

dominating at COD/SO42- ratios below 1.7, MPB above 2.7 and active competition occurring between

the two (Choi and Rim, 1991). Temperature has also been shown to have an effect with SRB more

likely to dominate at 37C and MPB at 55C (Colleran and Pender, 2002). With respect to sulphide

toxicity there are considerable discrepancies in the published literature with respect to the nature of

sulphide toxicity and the relative toxicity of dissociated and undissociated sulphide. Mechanistically,

sulphide inhibits cellular activity by denaturing native proteins, interfering with coenzyme sulphide

linkages and affecting assimilatory sulphide metabolism (Speece 1983; Vogels et al., 1988).

Unionised sulphide appears to play the dominant roles between pH 6.4 and 7.2, with total sulphide

concentrations becoming more important above pH 7.8. Fermentative microbes are less susceptible

to sulphide toxicity than acetogenic or SRB, with the MPB being the most susceptible (McCartnery

and Oleszkiewicz, 1991). For MPB the published IC50 values range between 50 and 250 mg/L,

depending on pH (Parkin et al., 1983; Oleszkiewicz et al., 1989; McCartnery and Oleszkiewicz, 1993).

Sulphide removal and acclimation of MPB have been shown to improve AD performance.

Light metal ions. The effect of aluminium, calcium, magnesium, potassium and sodium on

methanogenesis has been extensively studied (Chen et al., 2007). These ions may be present in the

wastewater, be released by digestion of organic matter or be added for pH control. In all cases

moderate amounts of the ions are required for growth, but excessive amounts are inhibitory.

Excessive amounts of Ca and Mg affect the system primarily through the precipitation of carbonate

and phosphate, resulting in the loss of essential nutrients and buffering capacity as well as scaling of

the microbes (Keenan et al., 1993; Van Langerak et al., 1998). Potassium and sodium are more

acutely toxic, given their role in maintaining membrane potential among other functions (Jarrell et al.,

1984; Soto et al., 1991). The IC50 values reported in literature differ significantly due to matrix

composition and degree of acclimation, but range between 0.15-0.74 m for potassium (Mouneimne et

al., 2003) and 5.6-53 g/L for sodium (Soto et al., 1993; Kim et al., 2000; Vallero et al., 2003a, b).

Heavy metal ions. Heavy metals may be present in significant concentrations in municipal sewage

and sludge, as well as a number of industrial effluents. While these elements form essential

components of enzymes that drive many anaerobic reactions they become acutely toxic at elevated

concentrations (Sterrit and Lester, 1980). Significantly, they are not biodegradable and although

present below toxic concentrations may be accumulated to inhibitory levels within the cells over time.

The toxicity of heavy metals is affected by a number of factors such as form of the metal, pH and

redox potential, but once again the methanogenic microbes are more susceptible than acetogens

(Zayed and Winter, 2000). Published IC50 values vary considerably and appear particularly affected by

solids content, which has a mitigating effect. However, the relative sensitivity of acidogenesis and

methanogensis to heavy metals is Cu>Zn>Cr>Cd>Ni>Pb and Cd>Cu>Cr>Zn>Pb>Ni respectively (Lin,

1992; 1993).

6

Organics. A wide range of organic compounds have been found to be inhibitory to anaerobic

processes. These are predominantly non-polar compounds which accumulate in bacterial

membranes, resulting in swelling of the membranes, leakage of cellular components, disruption of

ionic gradients and eventually cell lysis (Sikkema et al., 1994; Chen et al., 2007). The inhibition

concentrations vary for specific compounds and are affected by the concentration of the compound,

biomass concentration, exposure time, cell age, acclimation and temperature. Some of the major

classes of inhibitory organic compounds are chlorophenols, halogenated aliphatics, N-substituted

aromiatics, long chain fatty acids and lignin and lignin related compounds (Chen et al., 2007). A

number of these groups form important fractions of wastewaters and can have a significant impact on

efficiency of biogas formation.

1.1.3 Application and practice of AD technology The application of anaerobic digestion technology appears to fall into two distinct categories, based

largely on the development status of the location. In developing countries, particularly in Africa and

Asia, the technology is applied at small or institutional scale and the biogas generated is used

primarily for heating or cooking, particularly in areas not serviced by the electrical grid. In these

situations the level of technology employed is kept low, such that the plant is robust and does not

require careful monitoring or technically skilled maintenance staff. Ideally the construction of such

units can be achieved by relatively unskilled labour.

In developed countries the installations tend to larger and employ more sophisticated technology. The

drivers for implementing such technology are not the provision of basic services to isolated

communities, but rather generation of renewable, environmentally benign energy. Developed

countries, particularly in Europe, often have an existing natural gas infrastructure for heating,

electricity generation and motor vehicle fuel. For boilers and stationary combined heat and power

(CHP) engines the biogas can be utilised with little or no modification. However, prior to utilisation as

motor vehicle fuel or incorporation into the natural gas grid the biogas needs to be purified to a high (>

95%) methane content. This process is uncommon in developing countries.

Based on the distinctly different technologies and drivers, it is not possible to apply uniform criteria to

assess changes in cost and efficiency with changes in scale. However, a recent study by Amigun and

Von Blottnitz (2007) investigated scale economies of 21 biogas installations across the African

continent. The installations varied in capacity from four to 100 m3. Plant capital costs were converted

from local currency to US$ equivalents and a statistical assessment performed, correlating the plant

capacity to capital cost. The relationship between plant capacity and installation cost is not

proportional and has been represented by the empirical expression:

C = kQn

where k is the dimensioned proportionality factor, C is the cost, Q the capacity and the exponent n the

cost capacity factor. For chemical plants an n value of 0.6 is typical, although this has been shown to

increase to 0.85 where the process involves significant solids handling. The study found a statistically

7

significant cost capacity factor of 1.20 for small scale biogas installations (Amigun and Von Blottnitz,

2007). This is a significant finding and implies that rather than reducing capital costs (6/10 rule) scale

up of small scale biogas installations significantly increases capital investment costs. The outcome of

this study contrasts with a similar study performed in India by Singh and Sooch (2004). They

investigated three different models of small scale biogas plants and concluded that installation and

operating cost increased proportionally with capacity, implying an n value of 1. However, the capacity

of the plants used in their study did not exceed 6 m3. In addition, each model was assessed

individually as the construction costs varied. Despite these discrepancies, both studies indicate that

small scale biogas installation deviated significantly from the 6/10 rule.

1.1.4 Implementation of AD The application of biogas, generated by the anaerobic digestion of organic material, for heating,

cooking and energy generation has been widely employed for many years. China began a program of

mass adoption of household biogas in 1975 and within a few years units were being constructed at a

rate of 1.6 million per year. The units were poorly designed and of low quality and by the 1980s many

of these were no longer in use. By 1992, only 5 million units remained operational and many of these

had been redesigned due to leakage. This pattern of rapid introduction and failure of biogas units was

repeated in India, Nepal, Vietnam and Sri Lanka. However, as technology and research improved the

units became cheaper and more robust, particularly following the development of polyethylene

digesters (Ho, 2005). There are currently over 2 million family sized units in operation in India, and

over 200 000 families a year are switching from the traditional fireplace to biogas for cooking and

heating (Singh and Sooch, 2004; Ho, 2005).

Biogas utilisation in Nepal is increasing rapidly and the country has recently overtaken India and

China as the nation with the highest number of biogas plants per capita. Its program is regarded as a

model for successful application of alternative energy in the developing world. There are currently

over 120 000 biogas plants in Nepal, with the Biogas Support Program (BSP) targeting in excess of

300 000 plants by 2009 (Acharya et al., 2005). The BSP is facilitating this by ensuring uniform plant

design and quality, continued R & D and importantly through subsidisation (up to 50% of capital cost)

and stimulation of financial support mechanisms (Acharya et al., 2005).

The production of biogas using AD in a decentralised WWTP in South Africa is explored further as a

case study in the current project. (This will be described in the final report).

In developed countries, on-farm and centralised rural biogas facilities are still significant, but the

majority of biogas generation is performed at a larger scale and is driven by environmental and

economic concerns and the burgeoning carbon economy. Many of the applications of the biogas

generated in this manner require improvement of the biogas quality. The composition of biogas, the

applications and the techniques employed for upgrading biogas in the developed world are discussed

below. The biogas produced by anaerobic digestion is composed primarily of methane (CH4) with

8

significant amounts of carbon dioxide and nitrogen and a number of constituents at trace

concentrations. The typical biogas composition is shown in Table 2:

Table 2: Typical biogas composition

Component Typical range (%)

Methane (CH4) 50-80

Carbon dioxide (CO2) 25-50

Nitrogen (N2) 0-10

Hydrogen sulphide (H2S) 0-3

Hydrogen (H2) 0-1

Oxygen (O2) 0-2

Ammonia (NH3) 0-3

Siloxanes Trace

Methane, also known as natural gas, has a heat of combustion of 802 kJ.mol-1. This value is the

lowest of the hydrocarbon fuels, but on a per unit mass basis methane is more attractive than

complex hydrocarbons. Methane is considered to have an energy content of 39 MJ.m-3. Based on the

typical range of methane concentrations in biogas, a heating value of 18.6-26.04 MJ.m-3 is standard

(Amigun and Von Blottnitz, 2007). The characteristics of biogas are somewhere between natural gas

and town gas (derived from cracking of cokes). The Wobbe index is a measure of the

interchangeability of fuel gases and is defined as the higher heating value over the square root of the

specific gravity (Table 3).

Table 3: Characteristics of different fuel gases

Parameter Unit Natural gas Town gas Biogas(70% CH4, 30% CO2)

Calorific value (lower) MJ.m-3 36.14 16.1 21.48

Density kg.m-3 0.82 0.51 1.21

Wobbe index (lower) MJ.m-3 39.9 22.5 19.5

Max. ignition value m.s-1 0.39 0.70 0.25

Theor. Air requirement m3 air/m3 gas 9.53 3.83 5.71

Max CO2-conc in stack gas vol % 11.9 13.1 17.8

Dew point C 59 60 60-160

Biogas can be applied to all applications designed for natural gas, although for certain applications

the biogas needs to be upgraded to remove non-methane components. Table 4 indicates the gaseous

components which need to be removed in order to upgrade biogas for particular applications.

9

Table 4: Gaseous components requiring removal prior to application of biogas

Application H2S CO2 H2O

Gas heater (boiler) < 1000 ppm no no

Kitchen stove yes no no

Stationary engine (CHP) < 1000 ppm no no condensation

Vehicle fuel yes recommended yes

Natural gas grid yes yes yes

Gas boilers do not require high quality gas, although it is recommended that H2S be removed to below

1000 ppm in order to maintain the dew point around 150C. Where significant amounts of H2S are

present, the condensate will contain sulphurous acid which can corrode chimneys and heat

exchangers, particularly if they are composed of mild materials. Biogas has been used as a fuel for

combined heat and power (CHP) engines for many years, typically at sewage works, landfill sites and

biogas installations. Engine sizes range from 45 kW to several MW depending on the size of the

installation. The gas quality requirements are similar to those for a gas boiler, with the exception that

H2S concentration should be reduced to guarantee a reasonable operating life for the engine. Petrol

engines are more susceptible to H2S corrosion than diesel engines. As a result diesel engines are

almost exclusively used for large scale (> 60 kWel) applications.

The development of modern gas turbines for electricity generation is a significant new development

as these robust engines have comparable efficiency to internal combustion engines. The turbines

facilitate heat recovery in the form of steam. Efficient small-scale gas turbines have not been

developed, with the majority of engines being larger than 800 kWel.

The operation of motor vehicles on natural gas is a growing worldwide phenomenon. There are

currently in excess of 1 million gas powered vehicles worldwide. Biogas has been used to power

these vehicles, but the gas quality demands are high. Therefore, purification of the biogas is required

to increase the calorific value, remove corrosive components (H2S, NH3 and H2O) and particulates,

specifically siloxanes (Dewil et al., 2006). The upgraded biogas has a methane content in excess of

95%. A number of European studies have confirmed that upgraded biogas rated as the cleanest fuel

with respect to impact on the environment, climate change and human health.

A wide variety of technologies exist for the upgrading of biogas to vehicle fuel standards. These are

summarised in Table 5, below.

10

Table 5: Summary of current technologies for upgrading biogas

Component removed Technology Notes Carbon dioxide Water scrubbing Counter-current packed bed, can remove some H2S. Potable water not

needed. Polyethylene glycol

scrubbing (Selexol)

Similar to water scrubbing, but more efficient reduced solvent demand and pumping costs. Solvent stripped with steam or inert gas to minimise S0 formation.

Carbon molecular sieves

Loose adsorption of molecules in pores selectivity by pore size and gas pressure differences. Typically 4 units in series. Removes CO2 and water vapour.

Membranes systems

Two main technologies: High pressure (gas-gas). Operated at 36 bar. Good removal of CO2, H2S. Less efficient N2 removal. Membrane life 3 yrs. Hollow fibre membranes allow compact units. Low pressure (gas-liquid). Uses microporous hydrophopic membrane at 1 bar. Liquid adsorbent NaOH or amine. Efficient removal of H2S and CO2, both can be recovered as saleable products (S0 and industrial CO2)

Hydrogen sulphide Biodesulphurisation Typically done at digester stage. Use Acidithiobacillus sp. to form S0 and some SO42-. Stoichiomteric O2 dosing critical.

Biological filters Combined with water scrubbing in large applications. Provides increased surface area for reaction. 4-6% air added to biogas to facilitate oxidation.

FeCl3 dosing to digester slurry

Precipitation of insoluble iron sulphide. Reduces high H2S levels, but further reduction required for vehicle fuel quality.

Iron oxide H2S reacts with iron oxides or hydroxides to form iron sulphides. Endothermic reaction so TC > 12C required. Iron oxide regenerated by reoxidation, yielding S0. Regeneration in separate bed. Iron oxide surface area increased by coating wood chips or using pellets.

Impregnated activated carbon

In association with CO2 removal by molecular sieves. H2S oxidation catalysed by impregnating AC with potassium iodide. Best operation at 7-8 bar, 50-70C.

Solvent scrubbing As described above for CO2, using water, Selexol or NaOH. Siloxanes Solvent extraction Siloxanes (organic Si compounds) are oxidised to SiO2 in combustion

engines. Can cause major damage and significantly reduce engine life. Removed by absorption into specific hydrocarbon mixture.

Oxygen and nitrogen Membranes or molecular sieves

Introduced form air through inefficient harvesting of biogas. High O2 increases explosion risk. Removal is expensive prevention by efficient biogas collection advocated.

Biogas utilisation in EU countries is extensive, as can be seen Table 6. However, the majority is still derived from landfill sites rather than liquid wastes or sewage sludge. A relatively small portion of the gas is currently used in electricity generation.

11

Table 6: European Union energy from biogas data for 2006. Data includes substrate source for anaerobic digestion and the proportion of the energy applied to electricity generation (adapted from Biogas barometer, 2007).

Country Total biogas % for electricity Biogas per AD substrate source (GWh) (GWh) Landfill Sludge Other

Germany 22 370 33 6 670 4 300 11 400

UK 19 720 25 17 620 2 100 0

Italy 4 110 30 3 610 10 490

Spain 3 890 17 2 930 660 300

France 2 640 19 1 720 870 50

Netherlands 1 380 21 450 590 340

Austria 1 370 30 130 40 1 200

Denmark 1 100 26 170 270 660

Poland 1 090 26 320 770 0

Belgium 970 22 590 290 90

Greece 810 71 630 180 0

Finland 740 3 590 150 0

Czech Republic 700 25 300 360 40

Ireland 400 27 290 60 50

Sweden 390 14 130 250 10

Hungary 120 18 0 90 40

Portugal 110 30 0 0 110

Luxembourg 100 33 0 0 100

Slovenia 100 32 80 10 10

Slovakia 60 7 0 50 10

Estonia 10 70 10 0 0

Malta 0 0 0 0 0

In many European countries where intensive agriculture, particularly animal farming, is practiced eutrophication of aquatic environments due to influx of animal waste became a significant environmental issue. In Sweden, during the 1990s, there was a move towards regional biogas plants to deal with this issue. Two examples are in operation in the cities of Laholm and Linkping. The inputs and outputs to these plants are shown below:

Table 7: Input and output data from sample Swedish biogas plants, 2004 data (IEA Bioenergy, 2007) Input (tonnes/year) Laholm plant Linkping plant

Manure from pigs and cattle 28 000 2 000

Abattoir waste 10 000 30 000

Industrial organic waste 3 000 6 000

Household waste 1 000 250

Other 6 000 7 000

Total 48 000 45 000

Output (tonnes/year)

Biofertiliser for farming 28 000 52 000

Other 15 000 0

Total 43 000 52 0001

Both plants are operated as conventional stirred tank reactors (2 250 m3 and 3 700 m3 respectively)

with residence times of 25-30 days. In both cases the biogas is upgraded prior to use. In the case of

12

Laholm the upgraded biogas is supplemented with 5-10% propane to increase the Wobbe number to

that of natural gas. A portion (25% annual average) is directed to the district heating plant, while the

remainder (18 000 MWh/year) is added to the natural gas grid. The Linkping plant produces an

equivalent of 48 000 MWh/year of biogas, over 95% of which is upgraded to vehicle fuel quality. Since

2005 all public transport vehicles in the city (> 60 buses) have been converted to run on biomethane

and in 2005 the first biomethane powered train was unveiled. This has made it possible to reduce CO2

emissions from urban transport by over 9 000 tonnes per year, while reducing local emissions of dust,

sulphur and nitrous oxides (IEA Bioenergy, 2007).

Management of bioenergy facilities is an important factor in ensuring efficient energy recovery. A five

and a half year study on a bioelectricity plant in Greece, generating energy from biogas derived from

sewage sludge digestion, highlighted this issue. Technical issues and the fact that plant operation was

synchronised with the local supply network resulted in the unit only covering about 16% of the energy

requirements of the facility, despite having the capacity to cover almost 40%. Even at a highly

subsidised electricity price the payback on capital costs has been estimated at 16 years, based on

outputs of the last 2 000 days (Tsagarakis and Papadogiannis, 2006).

In South Africa the first biogas to electricity plant, funded by carbon credits generated under the Clean

Development Mechanism (CDM) of the Kyoto Protocol, was launched in October 2007. The plant is

situated near Mossel Bay and utilises process wastewater generated during the operation of

PetroSAs gas to liquid plant at Duinzicht as the substrate for anaerobic digestion. Historically, the

biogas generated was flared and has led to an estimated loss of at least 1 300 GWh of gross heat

value since the commissioning of the plant. The plant was developed and financed by MethCap, a

company owned by the international environmental and engineering firm WSP, and will make use of

three GE Jenbacher biogas generator sets, each with an output of 1.4 MW, giving the plant a total

output of 4.2 MW. The plant is expected to generate 33 000 certified emissions reductions (CEMs)

annually.

1.1.5 Costs and Benefits Capital costs for the construction of small scale (6 m3) biogas units have been shown to vary from

around US$300 to US$900, depending on location, plant design and construction material (Amigun

and Von Blottnitz, 2007; Singh and Sooch, 2004). The cost capacity factor governing increase in

construction cost appears to be regionally dependent, with a value of 1.2 being calculated in Africa,

but only 1.0 in Southeast Asia. Annual operating costs for such digesters are in the range of US$230,

with the significant majority being associated with the acquisition of substrate manure. This is

consistent with European studies (IEA Bioenergy, 2007) which highlight acquisition and transport of

substrate as the major operating expense.

The anaerobic digestion of organic wastes has a number of associated benefits, in addition to the

treatment of a waste source and the generation of energy. Methane is a significant greenhouse gas

13

with a global warming impact considerably greater than that of carbon dioxide according to the IPCC

(Ishikawa et al., 2006). Waste organic material degrades naturally, often under anaerobic conditions,

with the associated release of methane. The controlled digestion of this material with the collection

and utilisation of the biogas significantly reduces uncontrolled methane discharge with the associated

reduction of the greenhouse effect.

Upon completion of the anaerobic digestion process a number of by-products remain. The liquid

effluent resulting from the process typically requires an additional aerobic treatment prior to discharge,

in order to reduce COD and BOD to acceptable levels. In addition to the wastewater a solid residue,

termed digestate, remains. The composition of the digestate depends on the initial feedstock and

influences its potential applications (Singh and Sooch, 2004; Ho, 2005).The typical composition is

predominantly lignin, cellulose, biomass sludge and inorganic components, including ammonia,

nitrates and phosphates. In the absence of pathogens or potentially toxic elements the digestate

makes an ideal biofertiliser, often after a period of aerobic treatment to degrade lignin and oxidise

ammonia to nitrate. If the digestate is not suitable for fertiliser it can be incorporated into building

materials such as bricks or fibreboard (IEA Bioenergy, 2007).

As noted above, biogas is a particularly clean burning fuel. When used as a cooking and heating fuel

in rural households this results in associated health benefits, largely due to the reduction in particulate

material. A survey conducted by the BSP in Nepal indicated a significant reduction in occurrences of

respiratory problems, asthma, eye infections and chronic headaches among respondents who had

switched to biogas from dirtier fuels. Additional benefits were derived from no longer having to collect

and carry firewood (Acharya et al., 2005).

Table 8: Total suspended particle concentrations in flue gas of common cooking fuels (Acharya et al., 2005).

Fuel Total suspended particles (mg/m3)

Biogas 0.25

LPG 0.32

Kerosene 0.48

Crop residue 5.74

An additional monetary benefit derived from the combustion of biogas exists in the form of carbon

credits. For example, each of the functioning digesters in Nepal prevents five tonnes of carbon dioxide

equivalents from being pumped into the atmosphere annually. These credits can be sold to

developed countries to offset their excessive emissions and are worth in excess of US$ 5 million.

Despite the encouraging models, current anaerobic digestion technology is not yet sufficiently efficient

to compete with fossil fuel technologies on a purely economic level. For this to occur significant

improvements are required in process technology, or additional benefits such as waste reduction or

greenhouse gas mitigation need to be taken into consideration.

14

1.2 Combustion and gasification for the production of steam 1.2.1 Introduction Combustion: The direct combustion of biomass in the presence of excess air results in the formation of hot flue

gases that are typically used to produce steam to drive the electric turbine, with an efficiency of 33 to

45% (Bain and Craig, 1988). Two approaches for combustion are presented: mono-incineration,

where the sewage or biomass sludge is the only energy source and co-combustion, where these are

combusted in the presence of conventional fuels such as coal. In the combustion of sludges, the first

step focuses on dewatering and thermal decomposition with release of volatiles (devolatilisation or

pyrolysis) resulting in a gaseous stream containing H2 (2 to 5%), CO2 (7 to 24%), CO (28 to 66%) and

hydrocarbons (16 to 33%). As the operating temperature increases, the CO and H2 components

increase, and are subsequently oxidised in the second stage to carbon dioxide and water (Werther

and Ogada, 1999).

Gasification: Gasification is a thermal process yielding a combustible gas (or producer gas or fuel gas) as a

product. Gasification occurs in an atmosphere of sub-stoichiometric oxygen. Biomass particles are

heated up and release their volatile components in a process termed pyrolysis or devolatilisation and

when the oxygen concentration is limited, syngas rich in CO and H2 is produced.

Either sequentially or simultaneously, depending on the heating rate and the supply of oxygen, the

carbon skeleton is consumed via combustion and gasification reactions leaving residual ash and tar.

The reactions involved and the heat enthalpy (H0,rxn) are shown in the reaction scheme below (2):

Figure 2. Reaction scheme for the complete combustion of biomass.

The gas produced is characterised as having a low (5 MJ/Nm3) to medium (15 MJ/Nm3) energy

content, depending on whether air or steam is used as the gasifying agent (Scott, 2004; Prins et al.,

2007; McKendry, 2002). Bubbling bed or fluidised-bed gasifiers and fixed bed downdraft gasifiers are

two types which have found application in small scale production. They display good mixing properties

15

and can effectively crack tars if the bed is catalytically active (Min, 2005).

Table 9: Estimation of biomass resources in SA for conversion to energy. The amounts of biomass, coal fines or sewage required for a medium size power plant (2 mW electric output or 5 mW heat output) is tabulated.

a) Total biomass available = agriculture residues + forestry residues + 5% of available land dedicated to energy crops (invasive

plant excluded)

b) Total RSA coal production for 2003: 238 million tonnes (www.bullion.org.za).

c) Waste: 0.1 kg/person/day dry weight. South Africa population: 48.5 million.

d) Higher heating values or HHV does not account for the formation of water vapour in the flue gas stream. Hence the need for

adjusting the calc according to the moisture content of the feedstock

e) Adapted from reference (Kavalov and Peteves, 2005)

f) Coal fines moisture content as fired: 64%; Coal fines ash content: 47%; Coal fines HHV: 16 mJ/kg

g) Assuming a thermal efficiency of 40%

1.2.2 Application of combustion / gasification There are limited examples of this application in South Africa (some application has been reported for

the fruit industry e.g. juicers in Ceres and Grabouw, and for bagasse in the sugar industry). It is widely

used in Europe in order to meet the regulations on material sent to landfill (Lurgi and Philip Conoco

are main designers and suppliers of incinerators). BRI Energy (USA) has developed a unique process

in which a range of waste materials undergo thermal gasification at high temperature into CO, CO2

and H2 (http://techmonitor.net/techmon/05jul_aug/nce/nce_waste.htm).

1.2.3 Costs and benefits of combustion / gasification When considering the potential for combustion or gasification of biomass from wastewater, it is

essential to consider the potential operational problems of this approach. Specifically, moisture

content, tar formation, mineral content and bed agglomeration are highlighted. The feedstock to a

gasifier has to be relatively dry, with a moisture content of 50 to 60%. Several gasifier designs have

been proposed to include a dryer upstream of the combustor, but there is a clear trade off between

the amount of energy available in the feedstock to the amount of energy expended on drying. Tar

formation lowers the overall conversion of biomass to gas. To convert tars, either in-bed technology or

downstream reforming is required. Primary tars can be thermally cracked at 800C, while secondary

16

tars can be cracked at 1000C. In addition, the ash that remains may find application in the brickwork

industry or may need to be disposed of into the environment. There are several negative

environmental effects as a result of the gaseous and solid phase pollutants, (heavy metals, dioxin,

furans and N2O and NOx). However, evidence suggests that the heavy metals, dioxin and furan

emissions can be controlled and that NOx emissions remain below 5% while N2O and CO emissions

are prevented by high temperature and methods such as Activated Carbon Facilitated Oxidation (AC-

FOX) (Brain, 2007).

17

1.3 Utilisation of waste heat 1.3.1 Background Many processes give rise to liquid streams with elevated temperatures. Depending on the source and

fate of these streams (i.e. waste or product streams), the need may exist for reducing the temperature

of the streams prior to discharge or utilisation. This cooling can be achieved either by dedicated

cooling systems or through heat exchange with other streams. The use of the energy inherent in

these streams in other applications which require heat (rather than cooling or discharging without

energy recovery) has the potential to offset energy demand for other heating applications from other

(non-renewable) resources. The process industry has had decades of experience with heat

integration and the literature, analytical tools and practice are well established in this area. The main

motivation has been financial, but there are also growing incentives to improve energy efficiency to

lower greenhouse gas emissions. Therefore, neither on-site process heat recovery nor energy

recovery from hot gaseous streams are considered in this review. There are opportunities to recover

waste heat from processes where onsite recovery is not already widely practiced. The main

motivations for exploring such opportunities are the potential financial savings, and environmental and

sustainability considerations associated with the supply of energy from fossil fuels.

Here we discuss the heat contained in wastewaters which are to be discharged from industrial

processes. This represents one element of overall process heat integration where waste process

heat, including that contained in process streams which are at a certain temperature and require

cooling prior to being fed into another part of the process, is used elsewhere in the process. The

distinction is made in the context of the current study which is focused on energy from wastewater, in

reality this cannot be considered in isolation. An understanding of fundamental thermodynamics

suggests that the recovery of waste heat can be implemented at any scale. Providing there is a heat

gradient between the two streams (i.e. one is hotter than the other), and that they are separated by a

good heat conductor, energy will flow from the hot stream to the cooler stream. It is noted that if the

temperature difference between the two streams is not great enough the heat transfer will be

ineffective. In addition, the heat capacities of the liquid affect their suitability for heat exchange.

Specific heat capacity (expressed in J/g.K) measures the number of joules of energy required to

change the temperature of one gram of the substance by one Kelvin.

The range of applications for waste heat are a function of the quality of the heat high grade heat has

a wider range of industrial applications than lower grade heat, although the latter can be used

applications such as pre-heating of feed streams. Thus finding the correct match between available

energy supply and energy requirements is essential.

Given that heat exchange is usually conducted over an equipment boundary (such as a heat

exchanger), the chemical composition of the wastewater is relatively unimportant, providing the

construction materials of the heat exchanger are suitably matched to the wastewater composition.

18

The chemical composition will, however, affect the specific heat capacity of the waste stream and this

needs to be matched with the rate of flow of wastewater so that optimal heat transfer down the

temperature gradient can occur. This application may, however, be inappropriate for safety reasons

when a wastewater stream is used directly in space heating there may be risks of circulating

hazardous wastewaters through offices and warehouses.

The theory surrounding heat integration is well established, with various optimisation tools such as

pinch analysis being available to design energy recovery systems to maximise the amount of energy

recovered. Integrated heat recovery usually simultaneously considers recovery from both liquid and

gaseous streams. Heat recovery may also be integrated with other system optimisations. For example

a WRC funded study by Fraser et al (2006) explored simultaneous optimisation of recovery of heat

and water in industrial processes using thermal pinch analysis and water pinch analysis. One of the

key findings from the study was the significant benefits in doing heat exchanger network design early

in the design process.

Despite the theory being relatively well established, it is identified that the practice of on-site heat

integration in industry is not as extensive as would be expected. Apart from the impact of fundamental

stream properties, and the requirement for finding matching heat streams, other factors which may

limit the realisation of opportunities for heat integration include ensuring sufficient energy availability

on a site, the fact that heat cannot be transported over long distances, and the availability of suitably

qualified personnel (preferably with postgraduate experience in the field) for opportunity identification,

communication, design and implementation. Anecdotal evidence suggests that the latter is a

significant barrier to implementation of opportunities (PI, 1999).

In addition to on-site waste heat recovery, with the emergence of the meta-discipline of Industrial

Ecology in recent years there has been a regional sharing of process heat, outside of specific plant

boundaries. These applications suffer the similar limitations to implementation described from on-site

usage. Some examples are, however, presented below. 1.3.2 Implementation of utilisation of waste heat Although a priori planning for recovery of process heat is preferred, retrofitting of reticulation for use of

process heat is possible, depending on the particular application and plant configuration. Furthermore,

the financial viability of retrofitting heat integration is increased if an additional savings such as water

usage is simultaneously realised. Thus, if retrofitting is desired, its feasibility needs to be evaluated on

a case-by-case basis.

The level of monitoring and maintenance for applications of process heat are typically fairly low once

they have been optimised, although once again this will depend on the application. Many industrial

processes need a consistent temperature for operation, hence monitoring is required to ensure that

this is maintained by the supplied heat. At the same time, however, the heat source is likely to be

19

consistent (if it originates from another steady state process), and hence the need for constant re-

optimisation of the system is likely to be limited.

A wide variety of examples of overall process heat integration can be found from around the world,

although as mentioned previously experience suggests that these are not exploited as extensively as

they could be. It is further noted that within the context of this study it is difficult to isolate examples of

energy recovery from wastewater rather than process streams. Furthermore, isolation of case studies

which focus on liquid process heat streams only is difficult as total plant heat optimisation focuses on

a combination of liquid and gaseous streams or gaseous streams alone.

Natural Resources Canada (2004) identifies various projects which may be suitable for application of

process integration, and classifies them on the basis of payback periods. Some examples of these are

shown in Table 10.

Table 10: Example projects suitable for heat integration (Natural Resources Canada, 2004)

Quick Wins Short-to medium-payback projects, typically with a one-to three-year payback

Long-term payback projects, typically with a three-to six-year payback:

Operational improvements to refrigeration systems, e.g. adjusting compressor pressure to ambient temperature;

Hot water management improvements, e.g. balancing hot water supplies with demands;

Enhanced evaporator performance, e.g. improved non-condensable venting.

Instrumentation modifications, e.g. improved control loops;

Optimisation of refrigeration systems, e.g. increasing refrigerant pressures within operational constraints;

Evaporator feed preheat by improving heat recovery;

Rearranging existing heat exchangers to make better use of them, and addition of new heat exchangers;

Pasteuriser heat recovery, e.g. use of heat storage;

Recovering steam condensate to boiler house;

CIP water preheat by heat recovery;

Indirect heat recovery via a hot water loop;

Kettle vapour heat recovery; Cogeneration (CHP) via steam

turbine, gas turbine, or gas/diesel engine;

Boiler economiser (payback will be lower for large steam boilers);

Mechanical vapour recompression (heat pumps);

Increased number of evaporator effects.

The success of integrated applications for waste heat utilization has been varied since it depends

upon fostering collaborations for sharing energy and other resources. A good example of regional,

integrated waste heat utilisation is the Asns power station in Kalundborg, Denmark which was not

included in planning during construction of the power station (http://www.symbiosis. dk/, 2007). Asns

is Denmarks largest power station with an installed capacity of 1 037 MW. The excess heat from the

power station provides process steam for the local refinery, pharmaceutical and enzyme

manufacturing factories, and supplies heating for over four thousand households in the town. In

addition to energy integration, other benefits include reduced water usage, waste recovery, and

additional employment creation. At the Bruce Energy Centre in Canada some forward planning to

integrate energy usage was undertaken. The Centre is situated adjacent to a nuclear Ontario Power

station enabling a supply of steam and electricity at a low cost. The electricity and steam is used by

an alcohol distillation operation, a company which dehydrates crops to produce nutrient rich feeds for

livestock, a food manufacturer which concentrates fruit and vegetables, a plastic manufacturer and for

heating a greenhouse.

20

A number of further examples of the use of waste heat for district household heating in colder climates

can be found. The city of Gtheborg (Sweden) uses process heat from industries to supply a

proportion of the district heating system. The heat is from waste heat from oil refineries in the vicinity

of the city, and from a waste-fired CHP plant. Heat is also recovered via heat pumps that utilise

sewage water (Holmgren, 2007). Heat integration is also feasible for low-level industrial waste heat as

shown by a study in Delft (Netherlands). The heat generated by a pharmaceutical plant in the North of

the city at a temperature of between 25 and 35C is upgraded by heat pumps before being taken to

the central heating grid (Ajah et al., 2007). Similarly, low grade waste heat from power stations and

industrial applications has been used for water heating in aquaculture applications. The Asns trout

fish farm (Denmark) and the Happy Shrimp company (Netherlands) make use of waste heat from a

power plant to heat aquaculture ponds. Similar applications are seen the Czech Republic, Canada,

France, the US and others.

Other examples of applications include:

Various Exxon Mobils refineries have achieved significant savings through heat integration (Exxon Mobil, 2007),

Through heat integration, a pulp and paperboard mill in Canada achieved savings on purchased thermal energy of 15% and reduced the temperature of effluents by 3C, with a

simple payback period of less than 10 months for most projects (Natural Resources Canada,

2007).

The city of Vancouver has established heat recovery from municipal sewage. Raw sewage is passed through a heat exchanger where thermal energy is captured. Electrically-powered

heat pumps are used to boost temperatures from the 10 to 20o C sewage temperatures, to

the 65 to 90o C range for the hot water distribution system. The heat pump will produce

roughly three units of heat energy for every one unit of electricity consumed an efficiency

rating of 300 per cent. Southeast False Creek will be the first sewage wastewater heat

recovery plant in North America, and could set a precedent for the development of this

technology in other locations within Vancouver, and in other urban areas.

(http://vancouver.ca/sustainability/documents/sefc-factsheetheatplant-back.pdf)

Heavy industry in South Africa lends itself favourably to heat integration within and between industrial

plants. However, the limitations presented above, particularly those surrounding human resource

capacity and integrated (pre-) planning have played a role in not realising further opportunities thus

far. It is further identified that the potential for use of low grade heat for district heating is limited due to

the milder weather than in many parts of Europe where such opportunities are realised.

1.3.3 Assessment of costs and benefits in utilisation of waste heat The primary costs associated with heat integration relate to any additional piping, pumping, insulation

and equipment requirements for heat exchange. On a process plant these are likely lower than when

sharing of process heat is outside the plant boundaries. At the same time, use of process heat can

21

allow for the downsizing or elimination of the need for other energy sources such as boilers. The

financial benefit and payback periods on investment will vary widely depending on the situation. In

general a retrofit will be more expensive than incorporating integration into the original project design.

Operating expenditure will be relatively low, and will be limited to pumping costs and general

maintenance.

Unlike many of the other technologies presented in this report, use of process heat achieves no

wastewater treatment benefits (apart from removing the need for cooling prior to discharge which may

otherwise be required). Furthermore, there is no added benefit associated with the production of

secondary products or wastes which may require management elsewhere. Wastewater containing

pollutants from which heat energy is recovered will still require treatment prior to discharge.

22

1.4 Fermentation to produce biomass and secondary products 1.4.1 Background on fermentation of wastewater Wastewater streams often contain organic compounds which may be useable as carbon and nutrient

sources for growth of microorganisms. Such compounds would be referred to as fermentable

substrates, and these fermentations can generate fuels such as bioethanol and biogas (see sections

1.1 and 1.6) directly, but also:

Serve as pre-treatments for energy from wastewater technologies (such as enhancing bioethanol or biogas production from certain wastewaters).

Provide a nutrient medium to support the growth of biomass (microorganisms, algae or plant). This biomass can subsequently be combusted or used in bioethanol, biogas or biodiesel

production technologies.

The applications of fermentation include treating wastewaters from olive-mills dairies, breweries, wine

distilleries, whiskey distilleries, slaughter-houses, potato waste, and the tomato canning industry.

Examples of South African wastewaters containing potentially fermentable substrates with estimates

of volume and load are shown in Table 11, below.

Table 11: Examples of South African wastewaters containing fermentable substrates

Wastewater COD (g/L) VOLUME (ML per year)

Load (Mg/ year)

Sewage 0.8 -1.2 Av= 0.86

2 766 400 2 379 104

Dairy* 1.5-9.2 Av= 5.3

6346 33 637

Red meat and poultry abattoirs

11-21 Av= 16

11 000-31 000 336 000

Olive production 55 201 Av= 100

89 8 900

Fruit processing 5-15 Av=10

14 000 140 000

Distilleries: -Grain and grape

-Sugar cane molasses

25 45 Av=30

Grain: 63 Grape: 342

12 150

35 3500-4000 131 250 Winery 6 1000 6 000

Brewery 3 28 000 23 533 Textile Industry 0.1 -2.5

Av=1 25 000 25 000

Pulp and Paper 0.7 80 000 56 000 Petrochemical waste 0.2-0.9

Av= 0.7 1140 (crude);

3048 (synthetic); 2 to 11(re-refinery)

2 939

*Only the formal dairy industry sector considered here. Other animal husbandry sectors (cattle for beef, pigs and chicken are

not shown (case studies in the Appendix for further details).

Important issues which need to be considered in developing energy-yielding bioconversions from

wastewaters include:

The volume, variability and/or characteristics of the wastewater from any one industrial activity may be too low to provide suitable media for the fermentation. However, combining or

integrating of wastes may be feasible or necessary.

23

The concentration of carbonaceous components may be very low and need de-watering in order for efficient fermentation to be achieved. This dewatering process will have its own

energy input requirement.

Many industrial wastes may contain other components which are inhibitory to microbial growth. These may need to be removed by, for example, solvent extraction or precipitation

before the waster is used for fermentation. This extraction may yield economically valuable

secondary products (as in the case of olive wastewater).

1.4.2 Implementation of fermentation of wastewaters The basic fermentation technologies and knowledge base are well established. The requirements for

sterility, cooling/ heat transfer and extent of aeration are important issues in the scale-up of

operations. Wastes would be introduced into the bioreactor or separation/extraction equipment and products would undergo downstream processing that is separate from the main plant and therefore

there should be little problem with retrofitting into existing infrastructure. Bioconversion is a well-

established laboratory technology that generally requires constant monitoring of several parameters

for the control and optimisation for a particular waste stream. Many parameters (e.g. mixing, pH, dO2,

nutrient levels) can readily be automated and controlled, but in some cases may require skilled

operators.

The most abundant agricultural wastes are rich in complex polymeric molecules such as cellulose that

require significant energy for breakdown. In nature, several hydrolytic and oxidative enzymes

produced by a variety of fungi and bacteria work in synergy to degrade cellulose, hemicellulose and

lignin (Gosh and Gosh, 1992). Fermentations can provide biological methods of pre-treatment where

the addition of water may be required for processing of the waste. The pre-treatment of corn stover

with fungi such as Cyathus stercoreus and Phanerochaete chrysosporium has been shown to

effectively improve the cellulose digestibility three- to five-fold (Keller et al 2003), thereby increasing

the yields in the subsequent ethanol fermentation. The fungus Trichoderma reesei was shown to

improve the acid hydrolysis of cotton waste with a subsequent increase in the yield from ethanol

fermentation (Rajesh et al., 2008). Similarly, the biological conversion of wheat and rice straw to

ethanol has been show to be feasible with a pre-treatment of Aspergillus niger or Aspergillus awamori

followed by standard Saccharomyces cerrevisaeia fermentation to yield 2.5 g/L ethanol (Seema et al.,

2007).

The two-stage or multi-stage anaerobic digester systems also consist of fermentation pre-treatments

(see section 1.1). The different digestion vessels are optimised to bring maximum control over the

microbial communities so that the initial stages of hydrolysis, acetogenesis and acidogenesis occur in

separate bioreactor(s) to the methanogenesis that produces methane-rich biogas. The two stage

system has been shown to increase efficiency by reducing the residence time while increasing the

yields of biogas attained for a range of organic substrates (Schfer et al., 1999; Gunaseelan, 1997).

This has been successfully demonstrated for municipal waste and represents 10% of the anaerobic

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treatment capacity in Europe (de Baere 2000; Pavan et al., 2000). The two-stage or multi-stage

digestion systems have particular application to wastewaters rich in complex polysaccharides, lipids,

and proteins that are inaccessible for other energy from wastewater technologies such as bioethanol

production (Zeeman and Sanders 2001, Ahring 2001). For example, the biogas yields from cellulose-

rich wastes can be greatly improved through pre-treatment using a twostage system (Gijzen et al.,

1998) and this has been successfully been applied to the water hyacinth, Eichhornia crassipes, that

often invades watercourses in South Africa (Kivaisi and Mtila, 1997). Similarly, the two stage system

has been shown to be effective for biogas production from the lipid and protein rich wastewaters of

slaughterhouses and fish processing factories (Saddoud and Sayadi 2007; Ahring 2001).

Fermentations have also been used for the production of other valuable secondary products. These

include the cultivation of biomass for animal feed (Ugwuanyi et al., 2007), growth of the valuable

biocontrol fungi, Trichoderma viride, (Verma et al., 2006), and the production of valuable secondary

metabolites such as antibiotics and carotenoids (Ho, 2005; and http://www.ctre.iastate.edu/research).

1.4.3 Assessment of cost and benefits in fermentation of wastewaters An assessment of the costs and benefits is complex and specific to the waste stream and context.

Costs include the capital costs of the bioreactor and associated equipment, operational costs for

trained staff to maintain and optimise cultures, mixing, aeration (where required), sterilisation and

separation costs. The benefits include water bioremediation through removal of COD (waste disposal

reduction) the energy generated from wastewater and the value of any additional secondary products.

Fermentations can be used to directly produce energy products such as bioethanol and biogas

(discussed elsewhere section 1.1 and 1.2 and 1.6), as a pre-treatment step, or to generate biomass

from the nutrients in the waste stream. The generated biomass may later be used as an energy

resource for a secondary energy generating technology (i.e. combustion/gasification, bioethanol,

biogas or biodiesel production) and/or it may contain valuable secondary products (e.g. plant

fertilisers, animal feeds or neutraceuticals). The feasibility will depend on integration an assessment

of these complex factors, but will primarily depend on the characteristics of the feedstock and its

supply. In the chain of collection and transport, wastes often become complex wastewaters simply

because of dilution; thereby rendering the wastewaters unsuitable for generating energy using a

number technologies. Where this cannot be avoided, the use of dilute wastestreams to grow biomass

or the combining of waste streams may enable an energy from wastewater process to become

feasible.

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1.5 Algal biomass for the production of biodiesel 1.5.1 Background on production of biodiesel from wastewaters In order to utilise the dissolved solutes within wastewater for energy generation, it is valuable to

convert these to an energy rich raw material that can be removed from the dilute wastewater

environment through a physical separation of phases (i.e. to create the energy raw material in a

phase other than the liquid phase.) One approach is the formation of biomass for subsequent

processing. An extension of this approach is the accumulation of hydrocarbons of low oxidation state

as oils in the biomass, typically using algae. In this environment, the wastewater body provides

nutrients to support algal growth. The algae synthesise cellular components, including oils, using

energy from sunlight (photosynthesis). Additionally, the major carbon source,CO2, can be provided

from an industrial off-gas. This approach has been successfully demonstrated by using Botryococcus

braunii to produce hydrocarbons from a secondary sewage effluent (Banerjee, 2007). Due to their

simple cellular structure, microalgae have higher rates of growth, photosynthesis and productivity than

conventional crops. The cultivated algae could be used in several technologies such as anaerobic

digestion or the extraction of oil for biodiesel production.

Certain species of algae can produce large quantities of vegetable oil, up to 80% dry weight, as a

storage product (Becker 1994). Hence, algae can be up to 23 times more productive per unit area

than the best oil-seed crop (Table 12). Other advantages are that algae can be grown on wastewaters

(saline and other) and consume less fresh water than conventional crops since many species can

tolerate brackish or saline waters (Tsukahara and Sawayama 2005).

The conversion of the oil through transesterification is shown below (Figure 3). The transesterification

reaction converts triglycerides (oil) to alkyl esters (biodiesel) by the addition of an alcohol such as

methanol in the presence of a catalyst. Glycerol is a by-product (Harding, 2007).

Figure 3. Transesterification of oil to produce biodiesel

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Most algal species with high lipid contents considered for biodiesel production are usually either

green algae (Chlorophyceae) or diatoms (Bacillariophyceae) (http://www1.eere.energy.

gov/biomass/pdfs/biodiesel_from_algae.pdf.). The main components of algae are proteins (typical 40

to 60%), carbohydrates (typically 15 to 30 %), nucleic acids (

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production may make it economical to purify the glycerol by-product of biodiesel production.

A variety of waste streams can be used for algal oil production, but must have sufficient

concentrations of inorganic nitrogen and phosphate to enable the support of algal growth. The optimal

range of nutrients to support algal growth is 0.2 to1.0 g/L nitrogen (as ammonia or nitrate) and 0.05 to

20 mg/L phosphate (Banerjee et al., 2002; Tsukahara and Sawayama, 2005). Organic carbon often

promotes growth of algae, but high levels of organic carbon can be inhibitory to growth and promote

the growth of heterotrophic bacteria. Sewage wastewaters are a particularly good medium and algae

help to remove the COD, nitrogen and phosphorous load, as well as many odours. Further, the

photosynthetic algae generate enough oxygen to allow bacterial oxidation of organic waste. This

releases nutrients (CO2, ammonia, phosphate) for conversion into additional algal biomass

(Benemann et al., 1977). It should be noted that nitrogen limitation is often used to increase the oil

yield of a variety of species. This is only possible in a high-N content waste stream if algal culture

continues until the nitrate, ammonia and urea are depleted and then nitrogen limitation occurs. In

secondary sewage treatment, it took 9 days for B braunii to reduce nitrate levels to below 0.01 mg/L

(Tsukahara and Sawayama, 2005). Studies at UCT are underway to relate nitrogen availability to lipid

productivity.

Benemann et al. (1977) reported that almost all sewage and wastewater oxidation ponds are of the

facultative type. These are 1 to 3 m deep and arranged in cells of up to 50ha, mixed only by wind

action and recirculation of effluent. The bottom of these ponds is anoxic, resulting in fermentation of

settled sludge and algae. This design is optimal for waste-treatment but not algal growth. To maximise

algal biomass, ponds must be shallow (30-50cm) and have short retention times (2-4 days in

summer) and be mechanically mixed. These are called high-rate algal ponds and can produce over

100 mg/L algal dry weight. This requires a retrofitting of ponds for algal growth for either biomass

(linked to combustion or fermentation) or lipid production.

Harvesting involves concentrating the algae from their dilute suspension, which is a considerable

challenge (Benemann et al., 1977). Settling or chemical flocculation can be economical, but can also

take considerable pond-time (settling can take 2-3weeks). Oil extraction techniques are similar to

those used in the food and vegetable oil industry, but require a purpose-built extraction plant close to

the algal ponds, for integrated operation. Transesterification requires unique equipment, but does not

have to be on-site and extracted oil may be transport to a central transesterification facility.

The components are relatively simple and consist of ponds, pumps and some form of harvesting and

oil extraction process that can be operated by trained, but not highly skilled personnel.

Transesterification is fairly simple but strict production standards are required in order to meet

specifications. Further, chemical handling requires a level of skill, but this operation can be

centralised.

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1.5.2 Implementation of biodiesel production No large-scale production of biodiesel from algal oil has been reported. Biodiesel has been made

from algal oil at the lab scale and it has been shown to be technically feasible (Miao and Wu, 2006)

and there are several examples.

Piggery wastewater in rural Korea used to grow Botryococcus braunii for hydrocarbons (An et al., 2003). The growth rate of B braunii in secondarily treated sewage was 0.35 g/L/week and

algal concentration could be maintained at 400 mg/L after 11 days. (Tsukahara and

Sawayama, 2005)

A group of 7000 gallon ponds and one 1.6 acre pond in California showed that algae could probably be produced for anaerobic digestion at less than $0.01/pound (1960 $) in open,

shallow, sewage-fertilised ponds with recycle of digester residue and water and the recovery

of heat and CO2 from the power plant for use in the growth process (Oswald and Golueke,

1955). Used algal species found naturally in sewage water (mostly green algae such as

Chlorella and Scenedesmus (together constitute 95-99% of wild sewage algal population in

California), Chlorogonium, Chlamydomonas, Euglena and Microactinium). Chlorella

pyrenoidosa could be maintained at 250 mg/L. A 2-3 day retention time was optimal and a

mixing velocity of at least 1 foot/sec required.

Solix biofuels in Colorado (http://www.solixbiofuels.com/) builds and tests photobioreacors and a second generation prototype has been launched.

Some parts of South Africa (e.g. in the Upington area, N Cape and near Messina, Limpopo) have

ideal conditions for growth of high-oil content algae: long sunlight hours during summer, relatively high

temperatures. South Africa also has more open land (compared to Europe), and a relatively large

demand for transport fuels such as diesel (8.7 billion litres used in 2006). Several saline waste-water

streams relating to the mining and desalination industries could also potentially be used by salt-

tolerant algal species, supplemented by seawater in coastal areas.

South Africa does not have significant large-scale algal farming experience. The biofuels industry in

South Africa is also still in the formative stage. There are a number of fine chemicals and nutraceutical

algal production, including NCSA, Cape Carotene, BioDelta.

The most recent development is a New Zealand company called Aquaflow Bionomic producing

biodiesel from wild algae grown in sewage ponds. This is believed to be the world's first commercial

production of bio-diesel from "wild" algae. The company expects to produce biodiesel at the rate of at

least one million litres of the fuel each year. They propose to harvest algae directly from the settling or

stabilisation ponds of standard WWTP and other nutrient-rich wastewaters (http://www.nzherald.co.nz/

and http://www.aquaflowgroup.com/)

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1.5.3 Costs and benefits of biodiesel production from wastewaters The costs vary widely depending on whether open ponds or closed bioreactors are used, the source

of the water, power used, nutrients and CO2 and the use of wastewater, flue gas, recycled heat and

power. There have been multiple pilot-scale trials of algal growth for biodiesel. The Aquatic Species

Programme (funded from 1978-1996) was a DOE program focussed on production of biodiesel from

high-lipid content algae in open ponds, using waste CO2 from flue gas. Great advances were made in

algal culture engineering, but the process, although technically feasible, was found to be

uneconomical at the time due to high cost of algae production. This technology has not yet been

successfully implemented anywhere on a large scale, mostly due to low algal yields making the

process uneconomical. The main factors were (1996):

* Low cost of conventional fuel (diesel prices were hovering around $1.10 per gallon).

* No monetary value for carbon mitigation capability of biodiesel.

* Higher than expected cost of the production system.

* Lower than expected productivity of outdoor open pond system.

A decade later, the world is different. Diesel is selling at or above the $2.50 mark; while the cost of

petroleum oil continues to increase. There are also new incentive s from the Clean development

Mechanism and Certified Emission Reductions (carbon credits) for algal biodiesel. A robust market

for renewable biofuels is emerging.

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1.6 Fermentation for production of bioethanol 1.6.1 Background on bioethanol production from wastewaters The production of bioethanol as a renewable liquid fuel is well established. Bioethanol can either be

used on its own or blended with conventional liquid fuels to form either Gasohol or Diesohol (as an

alternative to butanol). Factors driving its position as a biofuel include ease of transport as it is a

liquid, a heating value of about 67% of gasolin