anaerobic digestion in canada

ANAEROBIC DIGESTION IN CANADA Anna M. Crolla, M.A.Sc. Christopher B. Kinsley, M.Eng., P.Eng. Collège d’Alfred - University of Guelph, Canada Kevin Kennedy, Ph.D. University of Ottawa, Canada

1.0 Introduction Around the world, pollution of air and water from municipal, industrial and agricultural operations continues to grow. Governments and industries are constantly on the lookout for technologies that will allow more efficient and cost-effective waste treatment. One technology that can successfully treat the organic fraction of wastes and wastewaters is anaerobic digestion (AD). When used in a fully-engineered system, AD not only provides pollution prevention, but also allows for energy, compost and nutrient recovery. AD is growing to become a key method for both waste reduction and recovery of a renewable fuel and other valuable co-products. This chapter will describe:

• Various feedstocks for anaerobic digestion • Pre-treatment options for sewage sludge treatment • Anaerobic digester options for agricultural wastes with Canadian examples • Methane production from organic solid waste treatment using landfill and

designated anaerobic digestion systems • Anaerobic digestion options for industrial wastewaters

ATAU Course Notes – Anaerobic Digestion in Canada 1

2.0 Feedstock for Anaerobic Digestion 2.1 Sewage Sludge Digestion of sewage sludge provides significant benefits when recycling the sludge back to land. The digestion process provides sanitisation and also reduces the odour potential from the sludge. Typically between 30 and 70 % of sewage sludge is treated by AD depending on national legislation and priorities (IEA, 2001). The energy generated from the AD process is usually used to power the sewage treatment works and where applicable (larger facilities) the excess biogas is exported from the plant. 2.2 Agricultural Wastes Farm scale digestion plants treating principally animal wastes have seen widespread use throughout the world, with plants in both developing and technically advanced countries. In rural communities small scale units are typical; Nepal has some 47,000 digesters, while China estimates they have 6 million digesters (IEA, 2001). These digesters are generally used for providing gas for cooking and lighting for a single household. In more developed countries, farm scale AD plants are generally larger and the biogas produced is used to generate heat and electricity to run the farm and for off-farm export. The small farm scale digestion plants are based on simple stirred tank designs that use long retention times to provide the treatment required. Whereas modern developments in agricultural waste digestion have developed the concept of centralised anaerobic digestion (CAD) where many farms co-operate to feed a single larger digestion plant. 2.3 Municipal Solid Wastes Organic wastes from households and municipalities provide potential feedstocks for anaerobic digestion. Wastes can be treated to gain the biogas from the waste as well as stabilising it to prevent further problems in the landfill. 2.4 Industrial Wastes Organic solid wastes from industry are increasingly being controlled by environmental legislation. Breweries, vegetable and meat processing industries are increasingly using anaerobic digestion in their waste management strategies.


3.0 Treatment of Sewage Sludge in Canada 3.1 Introduction Municipal wastewater treatment plants (MWWTP) produce primary and secondary sludge streams that are high in organic content. Mesophilic anaerobic digestion of these sludges is often employed to reduce the mass of solids for disposal, reduce their pathogen content and to generate biogas for energy recovery. Presently in Canada there is a dual focus pertaining to the treatment of MWWTP sludges. Reducing green house gas emissions and utilization of alternative energy sources has created an interest in evaluating options that are available for maximizing the generation of biogas from anaerobic digestion for energy recovery. Additionally, pre-treatment options that increase the potential of producing Class A biosolids have a tendency to have more public support than those that only increase biogas production. The centre of Disease Control (USA) has recommended “all sludges be treated to class A standard because of the risk that disease could be transmitted through Class B sludge”. Recently, in Ottawa, Canada a biosolids management program that included spreading of mesophilically anaerobic digested municipal sludges on agricultural land was stopped as a result of public health concerns. The various emerging sludge treatment technologies are either reduction processes or digestion processes (Kelly, 2003) that can be coupled with some type of pre-treatment. Generally, sludge reduction technologies such as incineration, gasification, pyrolyis, wet air oxidation, fuel from sludge and supercritical water oxidation tend to operate at high temperatures, low pressures and short retention times. On the other hand sludge digestion processes and associated pre-treatment processes such as Microsludge and high pressure hydrothermal tend to operate at lower temperatures but higher pressures. On the macro-scale ultrasound seems to operate at low pressure and low temperature. However at the micro-scale the opposite is in fact true (discussed below). This chapter focuses on high powered ultrasonication, chemical and pressure cell disruption (Microsludge) and thermal and pressure (SUBBOR) pre-treatment that are now available commercially. More experimental pre-treatment options such as electropulse are not addressed. Additionally, these options do not include modifications to operating practices or implementation of alternative digestion technologies that also may lead to enhanced sludge stabilization and biogas production. Examples of other options not described include enhanced primary settling, submerged combustion, sludge


thickening, improved digester mixing (pancake versus eggshape design), increase solids residence times incorporating sludge recycle, temperature phased anaerobic digestion (TPAD) or aerobic thermophilic pre-treatment and dual digestion. 3.2 Sludge Production Production of biogas at municipal wastewater treatment plants will be a function of many factors including the sources of the sludge. Sources of sludge in most wastewater treatment plants consist of the primary clarifier sludge and waste sludge from secondary biological treatment processes. The generation and characteristics of primary sludges are influenced by the sources of wastewater and other waters (i.e. infiltration) to the sewer system, by climatic conditions (i.e. dry weather versus wet weather) and also operation of the primary settler (i.e. surface loadings, use of precipitants). Secondary sludges are influenced by the type of the secondary process employed (i.e. activated sludge versus trickling filters or extended aeration activated sludge versus conventional activated sludge) and by the operation of the secondary processes (i.e. solids residence time (SRT) in activated sludge). These will impact on the quantity of sludges produced and their biodegradability. For example, short SRTs generally produce more sludge that is more readily degradable.

It must therefore be recognized that the subsequently described pre-treatment options for enhancing biogas production at municipal wastewater treatment plants will be impacted by the sludge properties and direct comparison of options may be difficult. Hence, the impact of implementing a pre-treatment option on biogas generation will vary from plant to plant and may also vary temporally. 3.3 Pre-treatment Options 3.3.1 Microsludge Process Microsludge (US Patent No. 6,013,183, international patents pending) is a chemical and pressure pre-treatment process that significantly changes both the rate and the extent that waste activated sludge (WAS) is degraded in a conventional mesophilic anaerobic digester. The patented process uses alkaline pre-treatment and an industrial scale homogenizer to provide an enormous and sudden pressure change to burst the cells. The resulting liquefied WAS is readily degraded in an anaerobic digester to form methane and carbon dioxide.


Conventional municipal wastewater treatment typically involves mesophilic anaerobic digestion of both primary solids and secondary solids (WAS) from aerobic biological treatment to produce methane and carbon dioxide. The rate limiting step is digestion of the WAS. The rate-limiting step for anaerobic digestion of WAS is the destruction of the cell membrane of each microbe (Parkin and Owen, 1986). Anaerobic digestion of WAS is both slow and incomplete because the individual cell membranes are not significantly degraded in conventional mesophilic (35 °C) anaerobic digesters that rely on enzymes to promote cell lysis.

Anaerobic digestion of WAS without pre-treatment falls short of an ideal biosolids management system for the following reasons:

1. Large quantities of undigested sludge still require disposal. 2. Partially digested sludge generates offensive odours and greenhouse gases. 3. Incomplete pathogen kill necessitates additional sludge processing before biosolids

are safe to use as a fertilizer. 4. Undigested sludge is a wasted resource since the methane generation potential is not

fully realized. Description of Microsludge Process Operations The Microsludge process (Stephenson and Dhaliwal, 2000) utilizes alkaline pre-treatment to weaken cell membranes, mechanical shear to reduce particle size, a self-cleaning screen to remove oversize debris, and an industrial scale homogenizer to provide an enormous and sudden pressure change to burst or “lyse” the cells. Figure 1 illustrates how Microsludge can be integrated into a WWTP. The heart of the Microsludge process is an industrial scale homogenizer that provides a large and abrupt pressure drop. At 12,000 psig (82,700 kPag), WAS in the cell disruption homogenizing valve is accelerated up to 305 meters per second in about 2 microseconds. This high velocity flow then impinges on an impact ring, disrupting the cell membranes and producing a liquefied WAS homogenate.


Figure 1: The Microsludge process (Stephenson and Dhaliwal, 2000)

Table 1: Optimum Microsludge Operating Conditions

PROCESSING STEP OPTIMAL SETTING

Chemical Pre-treatment 1. <300 mg/L Na (added as sodium hydroxide) in anaerobic digester

2. WAS pH ≥ 10 to promote cell lysis

3. Holding time of 1 hour or more to promote cell lysis

Homogenizer Pressure 12,000 to 14,000 psig (83,000 to 96,000 kPag) for maximum cell lysis

Anaerobic Digester pH ≈ 7.0 to minimize ammonia toxicity

Table 2 compares no pre-treatment and Microsludge pre-treatment on digestion of a 40:60 primary:WAS sludge mixture at a municipal wastewater treatment plant in Chilliwack (1 hr from the City of Vancouver), Canada. Comparison of soluble COD (sCOD) remaining for conventional 15 day digestion vs Microsludge pretreated sludge digested at 5,10 or 15 days indicated that the pre-treatment enhanced the anaerobic digestibility of biosolids. Soluble COD was reduced by 90% after 5 days mesophilic anaerobic digestion. Additional anaerobic digestion after 10 and 15 days resulted in 95% and 97% sCOD destruction respectively. In contrast, conventional anaerobic digestion resulted in just 17% destruction after 15 days.


Table 2: Effect of Microsludge processing on 40:60 primary:secondary solids

Pre-Treatment Anaerobic Digester

HRT (days)

sCOD (mg/L)

VS (mg/L)

Total VFAs (mg/L)

BOD (mg/L)

NH3 (mg/L)

40:60 Primary:Secondary Solids Feed

- 9,490 22,640 2,723 4,230 527

Microsludge 5 920 4,940 195 415 1,160

Microsludge 10 498 4,780 74 135 1,210

Microsludge 15 305 6,460 159 230 1,180

Conventional Feed - 8,330 34,269 1,601 3,420 280

Conventional Digestion 15 7,870 20,330 2,415 3,200 1,180 Similar improvements in volatile solids (VS) reductions, 78% after 5 d digestion with concomitant increased biogas production were reported for Microsludge pretreated sludge versus 41% VS reduction after 15 d conventional mesophilic anaerobic digestion (CMAD) with no pre-treatment. In terms of VS particulate remaining for disposal the Microsludge pre-treatment resulted in less particulate sludge remaining for disposal. Only about 12-14 % particulate sludge was remaining with Microsludge pre-treatment followed by CMAD compared with about 44% with no pre-treatment followed by CMAD. Microsludge pre-treatment was also reported to produce US EPA Class A Biosolids (Table 3). High pathogen kills and production of a Class A sludge is not commonly attained with mesophilic anaerobic digesters.

Table 3: Pathogen destruction for heat treated primary solids and Microsludge

processed secondary solids (40:60 mix) Pre-Treatment Anaerobic

Digester HRT (days)

Faecal Coliform (MPN/g)

Salmonella sp. (Presence/absence)

Raw Feed - 1.0*108 Present

Microsludge Homogenized Feed - 2.5*104 Absent

Microsludge 5 7.1*103 Absent



Conventional Digestion 15 8.6*107 Present


Advantages of Microsludge 1. Enhanced WAS biodegradability makes anaerobic digesters more effective within

WWTPs’ limited available footprint to destroy biosolids, thus avoiding large capital expenditures for new sludge management infrastructure that is capacity constrained.

2. It reduces the nuisance aspect of biosolids because it destroys pathogens, reduces odours, and yields lower quantities of stabilized biosolids for disposal.

3. It reduces net operating costs by significantly reducing the de-watered solids requiring disposal, generating higher volumes of methane for heat/power generation, and by operating the digesters more efficiently, thus lowering input costs for heating and mixing.

3.3.2 Ultrasound Process Ultrasound for sewage sludge treatment has started to become an established sludge pre-treatment method for anaerobic digestion with several full-scale treatment plants using this technology. As with the Microsludge process the most economical use of the process is for treatment of WAS. Ultrasound plants consist of a single or series of high power ultrasonic probes (Dirk European Holdings) or ultrasonic horns (Sonico Ltd) which emit frequencies above the audible range (20 kHz or above) which come into intimate contact with the sludge to be treated. Ultrasonic waves generated from the probes pass into the sludge thus altering its structure and making it more biodegradable. Disintegration of sewage sludge by means of ultrasound is based on the effects of acoustic cavitations in the liquid sewage sludge. Cavitation is the formation, growth and implosive collapse of bubbles in a liquid. Cavitational collapse produces intense local heating (5000K), high pressures (1000 atm), large heating and cooling rates and very fast jet streams in the order of 400km/h (Clark, 1998, Suslick, 1998). These intense localized “hotspots” result in the destruction of cell walls and membranes in sewage sludge releasing cellular contents into the mixed liquor as biodegradable COD and making cell components more susceptible to biodegradation. Typical exposure times for ultrasound pre-treatment is in the order of seconds and space requirements for the units is small and can be easily incorporated into existing designs (Figure 2).


Figure 2: Standard Sonix flow cell (left) designed to accommodate 5 horns and installed on TWAS line on digester wall (Sonico Ltd)

The largest ultrasonic probes presently available can deliver 16 kW power each, and are capable of treating a population equivalent of approximately 150 – 200,000 as secondary sludge. Typical lifespan of an ultrasonic probe is about 2 years. Depending on sludge thickness, 1 kW of ultrasound power can treat sludge with a population equivalent of between 10 and 32 thousand. Using ultrasound as a pre-treatment process increases hydrolysis of WAS. Hydrolysis rates were improved by 25-50%. Ultrasound has also shown to improved biogas production with increases from 25-50% (Table 4, Figure 3). Energy balances and organic loading calculations indicated that application of Ultrasound pre-treatment could result in a 25-30% decrease in digester retention time due to improved digestion. Additionally, Ultrasound pre-treatment has resulted in a production of about 7 kW of power (after losses) per kW of energy used for ultrasound. This would of course vary with operational factors such as age of sludge and sludge concentration and so on. It has also been reported that enhanced dewaterability of digested sludge resulted from ultrasound pre-treatment (Barber, 2003). Benefits of ultrasound treatment to operation of activated sludge operations were also reported including elimination of sludge bulking, enhanced biological nutrient removal, reduction in polymer and other flocculants and increased dry solids with dewatering (Barber, 2003)


Table 4: Enhanced Performance of WWTP with Ultrasound Pre-treatment Feature Guarantee Achieved Data

Biogas Production 20% 22% Increase of 90 litres/kg VS Electricity Production

20% 22% Increase of 269000 kWh/year

VS Destruction 20% 22% From 50% VS destroyed to 62% Dewatering 5 % points 5 – 7 % points From 16 – 23 % DS Cake Production – 33% – 31% Reduction of 2218 tons cake per

yr Polymer consumption

– 30% – 31% Reduction of 40000 kg/yr

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ioga

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crea

se in

bio

gas

Figure 3: Effect of applying ultrasound on gas production in full-scale digesters

Figure 4 shows a biogas production increase of about 50% in an anaerobic digester. The figure illustrates that when the sonication was stopped and TWAS feeding returned to about 20%, concomitantly biogas production in both the test and control reactors returned to identical production levels (08/17/2002). Specific biogas production increases from about 5.2-5.5 ft3/lbVS fed in the control to about 8.0-8.2 ft3/lbVS fed in the test digester


indicating the positive effect of ultrasound on sludge stabilization and VS destruction (Figure 5).

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Control - New Gas Meter Test - New Gas MeterContol - Old Gas Meter Test - Old Gas Meter

Ultrasonics on

Power turned upEnd of trial; TWAS feed reduced to ~20%

Meter change

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Figure 4: Mean daily gas production

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Control Test Figure 5: Specific gas production based on solids fed to the digester


3.3.3 Super Blue Box Recycling (SUBBOR) Process The Super Blue Box Recycling (SUBBOR) process is a patented (Vogt. et al. 1998) enhanced, multi-stage anaerobic digestion process for mixed municipal solid waste (MSW) and other biomass feedstock materials. The technology centers on enhanced high solids, thermophilic digestion after steam-pressure disruption of the ligno-cellulosic fibre components that are recalcitrant to conventional anaerobic digestion. Mixed MSW, rich in organic components but also containing inorganic materials, such as glass, aluminum and steel, as well as non-digestible plastic materials, has been pilot tested with a fully integrated process train designed to treat and recycle all of the MSW components. Methane yields from the MSW were 0.36 m3 CH4/kg VS representing a 40% increase over the yield obtained from conventional single stage digestion. The secondary digestion step after steam pressure disruption also shows a 40% improvement in total solids and volatile solids reduction. The residual organic fraction following two-stage digestion is fine in texture and is recovered as a clean Class A peat fraction with reduced contents of heavy metal and other fugitive non-digested contaminants. Mass and energy balance determinations indicate a high degree of MSW diversion from landfill disposal (greater than 80%) can be achievable by the SUBBOR process as well as substantial net electrical and thermal energy production. Continuous long-term trials of the SUBBOR process at 25,000 tonnes/year are underway.

The SUBBOR process has been primarily used for MSW in the past, but current applications of the process to sludge treatment are underway. The following sections will describe the process for MSW conditions. The heart of the SUBBOR process is the sequential two stage anaerobic digestion (Unit II, Figure 6) that employs steam pressure disruption of the primary digestate prior to its re-digestion in the secondary stage. Anaerobic digestion is carried out at medium to high solids (15-30% (w/w)) content and under thermophilic conditions (55oC).

The primary digestate after approximately 25 days digestion is removed from the primary digester and processed through a steam pressure disruption circuit. This step causes a “steam-explosion” of the internal water of the remaining non-digested fibers, causing fiber disruption (Liu et al., 2002). The extent of disruption is affected both by cooking time and cooking temperature which range from 5-20 minutes and 190- 270 oC respectively. The disrupted material is subsequently re-inoculated and re-digested in the secondary digestion stage to provide additional digestion and biogas production from the


substrate made accessible from pressure disruption. Digestate from the secondary digester is then further processed to provide secondary recovery of non-digested materials (metals, plastic, glass) and heavy metal removal, providing a recovered cleaned organic peat by-product fraction. Biogas produced during the primary and secondary digestion stages is routed to an energy recovery circuit where biogas is the energy source to produce electrical power and steam/heat co-generated energy products. A portion of the recovered energy is utilized for internal process needs while the balance is exported as product energy.

MSW

Inerts

Shredding/ Primary Products Recovery/

Milling/ Conditioning

Primary Digestion

Steam Pressure Disruption

Secondary Digestion

Secondary Products Recovery

Biogas (Energy)Recovery

Steel

Aluminum

Plastics

Peat

Plastics

Metals

UNIT I

UNIT II

Figure 6: SUBBOR two stage enhanced digestion process for mixed MSW


The 35-day primary digestion biogas yield is summarized in Table 5. The 60% CH4 in the biogas and overall CH4 yield of 0.25 m3/kg VS (provided for digestion) are average yields when using MSW as a feedstock (Owens and Chynoweth, 1993, Oleszkiewicz and Poggi-Varaldo, 1997, Mata-Alvarez et al., 1993).

Table 5: Methane yields and mass reductions from enhanced two-stage digestion of

MSW Biogas Mass Reduction % Digestion

Stage

Time of

Digestion

(days)

CH4 content

%

Yield

m3CH4/kgVS

TS VS

Primary 35 60 0.25 40 48

Secondary 15 65 0.11 16 19

Total 50 60-65 0.36 56 67

Typically, digestion in commercial anaerobic facilities is terminated after 15-20 days and processing is then completed by aerobic composting (DeBaere, 1999), presumably because biogas production beyond 20 days of digestion is not justified economically.

SUBBOR has also reported that further enhanced secondary digestion yields are achievable by shortening the course of primary digestion, followed by steam pressure disruption of the digestate and its secondary digestion. Thus, a correspondingly larger portion of the overall biogas yield can be obtained in the secondary digestion stage through an earlier termination of the primary stage followed by steam pressure disruption and re-digestion. This aspect is being further investigated as it holds potential for achieving similar overall yields with further reduced overall digestion times. The substantial boost to biogas production kinetics provided by the secondary digestion stage following disruption therefore provides process flexibility for adjusting the length of the overall digestion time needed for optimum gas yields. The SUBBOR process has successfully been shown to enhance digestion of MSW, however the application to municipal wastewater treatment plant sludges is just beginning. It is believed that as steam explosion technology can disrupt lingo-cellulose bonds in MSW it should be able to disrupt cell walls and membranes of municipal sludges. Studies using the SUBBOR process to enhance digestion of 6% thickened waste activated sludge (TWAS) and anaerobic biosolids cake after steam pressure disruption has shown an average of 75% increase in biogas production (Hamzawi, 1998 a&b).


4.0 Use of Anaerobic Manure Digestion in Canada 4.1 Background of Anaerobic Manure Digestion in Canada

The adoption of manure digesters is much more advanced in Europe than in North America, and especially more so than in Canada. This practical long-term European experience indicates that manure digesters do operate well and are of economic value in cooler climates, similar to that of Canada. There are an estimated 760 manure based biogas plants in Europe (IEA, 2001; Bo Holm-Nielson et al., 1997). Table 6 identifies the approximate number of biogas plants in various European and North American countries distributed among different digester feedstocks. Bo Holm-Nielson et al. (1997) identifies Germany as having the largest number of plants in Europe, dominated by small scale farm plants, while Denmark has the largest biogas production of 1.05 Peta Joules (PJ), originating mainly from co-digesting slurry with an average of 25 percent agro-industrial wastes. Sweden has also been identified as having a large manure based biogas production of about 0.4 PJ. Canada and the U.S.A. are beginning to grow their numbers of manure based biogas plants and currently stand at approximately 10 and 28 plants, respectively.

A study conducted by Agriculture and Agri-food Canada from 1973 to 1986 showed that sufficient biogas could be produced to provide supplemental heat to maintain the digestion process in the winter and for a farm to be almost self sufficient in the production of electricity. However, it was also concluded that for small to medium sized Canadian farms anaerobic digestion was not economically feasible because it was too labour and capital demanding. The results indicated that even when the technology was fully integrated into a farming operation involving energy production and recovery of protein, the energy and feed cost savings could not justify in economic terms the large capital investment, operating costs and management time that the farmer had to supply (Van Die, 1987). Increasing volatile energy costs and, increasing concerns about greenhouse gas emissions, odour and pathogens in manure has lead to increased interest in anaerobic digestion for the agricultural industry in Canada. Improved digester designs with lower capital and operating costs, and the successful implementation of manure digesters in Europe has also justified this second look into digesters for the Canadian agricultural industry (ManureNet, 2004).


Table 6: Number of Anaerobic Digester Plants in Various European and North American Countries (IEA, 2001)

Country Sewage Sludge

Landfill Gas Production

Biowaste or Industrial

Solid Waste

Agricultural Industrial Wastewater1

Austria 100 31 3 100 25 Canada 50 33 1 10 2 13 Czech Republic N/A 3 N/A N/A 10 4 Denmark 64 10 N/A 20 5 Finland N/A N/A 1 N/A 3 Germany N/A 170 MW of

electricity 4 49 380 91

Greece 2 1 N/A 1 2 Italy N/A N/A 4 50 38 Netherlands N/A N/A 2 3 5 84 Norway 17 40 N/A 2 5 Portugal N/A N/A N/A 94 3 Spain N/A N/A 1 6 27 Sweden 134 73 4 3 8 Switzerland 70 15 11 69 20 United Kingdom 200 160 1 25 26 U.S.A. 1600 270 N/A 28 92 Note: Data presented in table is not complete. Often data was not available because the information was not centrally located. 1 AD used for industrial wastewater pre-treatment 2 (ManureNet, 2004) 3 N/A: Not Available 4 Germany has a large landfill gas programme producing 170 MW of electricity, but the number of plants

is not available 5 (Bo Holm-Nielson et al., 1997) 4.2 Benefits of Anaerobic Manure Digestion for Canada The renewed interest in anaerobic manure digestion in Canada has been driven by the technology’s potential to accomplish the following: 1. Greatly reduce odour levels during manure processing, creating a relatively odour-

free end product. 2. Reduce pathogen levels in the final products - Additional post-digester technologies

can ensure pathogen-free end products.

3. Conserve nutrients - more than 90% of nutrients entering anaerobic digesters are conserved through the digestion process. By conserving nitrogen during digestion, the N:P ratio of the treated manure is more favourable for plant growth.


4. Reduce greenhouse gas (GHG) emissions - Since anaerobic digestion operates in a closed system, substantial reductions in greenhouse gas emissions (methane, nitrous oxide) are achieved.

5. Co-generation and Energy Independence - Anaerobic digesters produce methane

which can be captured for supplying energy (heat, electricity) for the operation. 6. The final products of anaerobic digestion are quite homogenous and are more

predictable as sources of plant nutrients since they are in a more mineral form (50% of carbon is converted to methane).

In Canada, the digester option is likely to be more attractive to larger operations, or to well-established existing operations wanting to expand and modernize their manure management systems. When considering manure digesters as an option, it may enhance the chances of success if local municipalities or other agricultural industries (vegetable or fruit processing, slaughter houses) or commercial industries (distillers, bio-fuel production) are considered as potential partners. This appears to be the trend of several European digesters. The addition of off-farm fatty wastes, animal rendering wastes or vegetable/cooking oils can act as an accelerant for methane production, increasing outputs by up to 4-fold. 4.3 Characteristics of Manure Organic waste management in livestock operations has long been an area of concern. Tables 7 and 8 illustrate typical annual manure production quantities and characteristics from various animal types in Canada, respectively.

Table 7: Typical Annual Manure Production per Animal (Semmler, 2002; OMAF, 2000) Livestock Manure Production

(x 1000kg/animal/year) Animal Units

Dairy Cow 18.25 1 Beef Cattle 9.13 0.5 Calf 3.65 0.2 Swine 2.92 0.16 Poultry 0.073 0.004 Note: Amounts of manure produced may vary depending on farming practices


Table 8: Typical Manure Characteristics As Excreted (CH2MHill et al., 1997; Fulhage et al., 1993)

Manure Type

(Animal Weight)

Total Nitrogen(kg/day)

Total Phosphorus (kg/day)

BOD5 (kg/day)

Volatile Solids

(kg/day) Dairy Cow (545 kg) 0.20 0.032 0.73 4.31 Beef Cattle (454 kg) 0.15 0.054 0.54 2.27 Swine (68 kg) 0.19 0.07 0.94 0.32 Poultry (1.8 kg) 0.38 0.14 1.68 0.02 Note: Manure concentrations will vary depending on feed composition Anaerobic digestion will treat manure by converting organic materials to carbon dioxide and methane gas (biogas). The conversion of solids to biogas results in a much smaller quantity of solids that must be disposed of after digestion. During the anaerobic treatment process, organic nitrogen compounds are converted to ammonia, sulphur compounds are converted to hydrogen sulphide, phosphorus is converted to orthophosphates, and calcium, magnesium, and sodium are converted to a variety of salts (Burke, 2001). All waste constituents are not equally degraded or converted to gas through anaerobic digestion. Anaerobic bacteria do not degrade lignin and some other hydrocarbons. The digestion of manure containing high nitrogen and sulphur concentrations, like swine manure, can produce toxic concentrations of ammonia and hydrogen sulphide. Waste constituents that are not particularly water soluble will breakdown more slowly. For example, dairy manure has been reported to degrade slower than swine or poultry manure. Approximately 10 % of volatile solids in dairy manure is lignin, thereby reducing the percentage of volatile solids available to be converted to biogas. 4.4 Overview of Manure Digester Types and Reactor Design for Manure

Treatment in Canada An array of anaerobic digesters have been developed and placed in operation in North America over the past fifty years. The main objective associated with an anaerobic manure digester is to convert solids to biogas while meeting the goals of anaerobic digestion. The goals of manure anaerobic digestion are as follows: 1. Reduce the mass of solids 2. Reduce the odours associated with the waste products 3. Produce clean effluent for recycle and irrigation


4. Concentrate the nutrients in a solid product for storage or export 5. Generate biogas for energy 6. Reduce pathogens The processes, either pilot or full scale, that have been used in Canada and the U.S.A. for the digestion of manure can be subdivided into high rate and low rate processes. Low rate processes consist of covered anaerobic lagoons, plug flow digesters, and mesophilic completely mixed digesters. High rate reactors include the thermophilic completely mixed digesters, anaerobic contact digesters and hybrid contact/fixed film reactors. 4.4.1 Factors controlling Anaerobic Digestion and Reactor Design The Solids Retention Time (SRT) is the most important factor controlling the conversion of solids to gas. It is also the most important factor in maintaining digester stability. The solids retention time is defined as:

))((

))((

ww

d

CQCV

SRT = [1] (Burke, 2001)

where V = digester volume, m3 Cd = solids concentration in digester, kg/m3 Qw = volume wasted each day, m3/day Cw = solids concentration of waste, kg/m3 In a conventional completely mixed, or plug flow digester, the hydraulic retention time (HRT) equals the SRT. However, in a variety of retained biomass reactors the SRT exceeds the HRT. As a result, the retained biomass digesters can be much smaller while achieving the same solids conversion to biogas. Figure 7 illustrates the percentage volatile solids destruction of manure as a function of SRT. The goal of process engineers over the last twenty years has been to develop anaerobic processes that retain biomass in a variety of forms such that the SRT can be increased while the HRT decreased. Effective retention systems will have SRT/HRT rations exceeding 3 (Burke, 2001). At an SRT/HRT ratio of 3 the digester will be 1/3 the size of a conventional digester (SRT/HRT =1).


Figure 7: Manure Volatile Solids Destruction with increasing SRT (Burke, 2001) The digester loading rate (kg/m3/day) is designed to maintain the necessary bacterial balance and prevent ammonia toxicity from occurring. The loading rate is the most appropriate measure of the waste on the digester’s size and performance. The loading rate is often reported as the mass of waste per digester volume. The digester loading can be calculated if the HRT and the influent waste concentration is known. The loading rate can simply be defined as:

)(1IC

HRTL

= [2] (Burke, 2001)

where HRT = volume of tank (V)/daily flow (Q), day CI = influent waste concentration, kg/m3 Digester loading rates (based on volatile solids) and hydraulic retention times for various manure types are presented in Table 9.


Table 9: Typical Loading Rates, Detention Times and Digester Volumes for Dairy, Beef, Swine and Poultry Manure (Fulhage et al., 1993)

Dairy Beef Swine Poultry Loading Rates *, lbs solids/ft3/day (kg solids/m3/day)

0.37 (5.9)

0.37 (5.9)

0.14 (2.2)

0.12 (1.9)

Hydraulic Retention Time, days

17.5

12.5 12.5 10.0

Digester Volume, ft3/animal (m3/animal)

26.0 (0.74)

13.5 (0.38)

5.0 (0.14)

0.37 (0.01)

Digester Volume for a Typical Livestock Operation, ft3 (m3)

75 cows: 1950 (55)

300 cows: 4050 (115)

500 hogs: 2500 (71)

15000 birds: 5550 (150)

* Loading Rates are based on mass of volatile solids per digester volume per day The Food to Microorganism (F/M) ratio is the key factor controlling anaerobic digestion. At a given temperature, the bacterial population can only consume a limited amount of food each day. The F/M ratio is the mass of waste supplied to the mass of bacteria available to consume the waste. This ratio is the controlling factor in all biological treatment processes.

UPD

VS

VSVSL

MF−

= [3] (Burke, 2001)

where LVS = volatile solids loading rate, kg/m3/day VSD = concentration of volatile solids in digester, kg/m3 VSUP = concentration of unprocessed volatile solids, kg/m3 For any given loading, efficiency can be improved by lowering the F/M ratio by increasing the concentration of biomass in the digester. Also, for any given biomass concentration within the digester, the efficiency can be improved by decreasing the loading. 4.4.2 Anaerobic Lagoons (Very Low Rate) Anaerobic lagoons are covered ponds. Manure enters at one end and the effluent is removed at the other. The lagoons operate at psychrophilic (< 20 oC), or ground temperatures. Consequently, the reaction rate is affected by seasonal variations. Since the reaction temperature is quite low, the rate of conversion of solids to gas is also low.


In addition, solids tend to settle to the bottom where decomposition occurs in a sludge bed. Little contact of bacteria with the bulk liquid occurs.

Figure 8: Covered Anaerobic Lagoon System (Burke, 2001) The biomass concentration is low, resulting in very low solids conversion to gas. In anaerobic lagoons there is a high F/M ratio with poor growth rates at low temperatures. There is little or no mixing thereby lagoon utilization is poor. Periodically the covered lagoons must be cleaned out due to solids accumulation. Solids may be screened and removed prior to manure entering the lagoon to minimize solids accumulation. However, a considerable amount of energy potential is lost with the removal of particulate solids. The advantage of anaerobic lagoons is the low cost. The low cost is offset by the lower energy production and poor effluent quality. A review by Burke conducted by (2001) determined that on average lagoon systems accepting a screened manure feed would achieve a 35% conversion of volatile solids to biogas at a loading of 0.04 kg/m3/day. 4.4.3 Completely Mixed Digesters (Low Rate) The most common type of anaerobic digester is the completely mixed reactor. Most sewage treatment plants and many industrial treatment plants use completely mixed reactors to convert solids to biogas. The completely mixed digester is heated and mixed. Most completely mixed digesters operate in the mesophilic range. Most of the initial anaerobic digesters installed to treat manure in Canada and the U.S.A. were completely mixed mesophilic digesters (Burke, 2001). The cost of mixing is high, especially if sand, silt and floating materials present in the waste stream must be suspended throughout the digestion period. Some completely mixed reactors operate in a thermophilic range where sufficient energy is available to heat the reactor.


Figure 9: Completely Mixed Manure Digester (Burke, 2001) Most completely mixed reactors are heated with spiral flow heat exchangers. These heat exchangers apply hot water to one side of the spiral and the anaerobic slurry to the other. The spiral heat exchangers have proven to be a successful method of efficiently transferring heat to the digesters. Completely mixed reactors can have fixed covers, floating covers, or gas holding covers. Floating covers are more expensive than fixed covers. Mixing can be accomplished with a variety of gas mixers, mechanical mixers, and draft tubes with mechanical mixers or simply recirculation pumps. The most efficient mixing device in terms of power consumed per gallon mixed is the mechanical mixer (Burke, 2001; Fulhage et al., 1993). The advantage of the completely mixed digester is that it is a proven technology that achieves reasonable conversion of solids to gas. It can be applied to the treatment of slurry wastes such as manure. The disadvantage of the completely mixed reactor is the high cost of installation and the energy costs associated with mixing. The completely mixed conventional anaerobic digester is a biomass growth based system. The process requires a constant conversion of a portion of the feed solids to anaerobic bacteria rather than biogas. Since anaerobic bacteria are constantly wasted from the process, new bacteria must be produced to replace the lost bacteria. Completely mixed thermophilic anaerobic digesters have a rapid conversion of solids to gas and biomass. It has been shown that the rate of conversion can be three times greater with thermophilic reactors (Burke, 2001).


Generally mesophilic completely stirred digesters will achieve a 40% conversion of volatile solids to biogas at a loading of 5.7 kg/m3/day of dairy and swine manure (Burke, 2001). Better conversions could be achieved at lower loadings. Thermophilic reactors appear to achieve greater conversions at higher loadings while mesophilic reactors appear to achieve greater conversions at lower loadings (Fulhage et al., 1993). 4.4.4 Plug Flow Digesters (Low Rate) The plug flow anaerobic digester is the simplest and least expensive form of digester. The plug flow digester can be a horizontal or vertical reactor. The horizontal reactor is the most common configuration. The waste enters on one side of the reactor and exits on the other. Since bacteria are not conserved, a portion of the waste must be converted to new bacteria, which are subsequently wasted with the effluent. Since the plug flow digester is a growth based system, it is less efficient than a retained biomass system and converts less waste to biogas.

Figure 10: Plug Flow Reactor (Burke, 2001) Plug flow system are subject to stratification wherein the sands and silts settle to the bottom and the organic fibers migrate to the surface. The stratification can be partially inhibited by maintaining a relatively high solids concentration in the digester. Periodically, solids must be removed from the plug flow digester. Sincere there is not easy way of removing the solids, the reactor must be shut down during the cleaning period. Cleaning costs can be considerable. Plug flow reactors are normally heated by a hot water piping system within the reactor. The hot water piping system can complicate the periodic cleaning of the reactor.


The plug flow reactor is a simple and economical system. Applications are limited to concentrated liquid dairy manure containing minor amounts of sand and silt. If stratification occurs because of a dilute waste or excess sand, significant operating costs will be incurred. Generally, a mesophilic plug flow reactor will achieve a 32% conversion of volatile solids to biogas at a loading rate of 2.5 kg/m3/day of dairy and swine manure (Burke, 2001). 4.4.5 Contact Digesters (High Rate) The contact digester is a high rate process that retains bacterial biomass by separating a concentrating the solids in a separate reactor and returning the solids to the influent. More of the degradable waste can be converted to biogas since a substantial portion of the bacterial mass is conserved. The contact digester can be either completely mixed or plug flow, thermophilic or mesophilic. The contact digester can treat both dilute and concentrated waste.

Figure 11: Contact Anaerobic Digester System (Burke, 2001)

Gravity separators (settling tanks) and solids thickeners have been used in the past for the solids separation. It was discovered that the solids could not be sufficiently concentrated in a gravity separator without degassing to remove the gas bubbles attached to the solids. Gravity separation techniques are only effective with dilute waste following a completely mixed reactor. Separation requires several days of detention and the digester solids concentration should be less than 2.5 % for gravity separation (Burke, 2001). Mechanical separation devices have been tested to reduce the detention time required by gravity separation. Centrifuges, gravity belts, membranes, and other mechanical separators have been used with limited success. The mechanical separators have been shown to inhibit the recirculated bacteria population and thus limiting the effectiveness of the contact process.


During the contact process, refractory organic and inorganic solids accumulate within the system. The accumulated sands, silts and non-degradable organic fibers dictate the rate of solids wasting. Wasting the non biodegradable solids causes the loss of bacterial mass and reduced process efficiency. 4.4.6 Sequencing Batch Digesters (High Rate) A sequencing batch reactor is a contact digester, which utilizes the same tank for digestion as well as separation. Generally, two or more tanks are used. The tanks are operated in a fill and draw mode. The separation is accomplished by gravity. Consequently, a more dilute, screened waste is treated. 4.4.7 Contact Stabilization Digesters (High Rate) The anaerobic contact stabilization process is a more efficient contact process. The process has the advantage of efficiently converting slowly degradable materials as cellulose in a highly concentrated reactor (Burke, 2001). Organic materials, which can be degraded rapidly, are digested in the contact reactor. The bacteria and slowly degradable organics are removed and degraded in a highly concentrated reactor. A contact mesophilic stabilization anaerobic digester can achieve a 78% conversion of volatile solids to biogas at a solids loading of 3.7 kg/m3/day of dairy manure (Burke, 2001).

Figure 12: Contact Stabilization Anaerobic Digester (Burke, 2001)


4.4.8 Phased Digesters Both acid phased and temperature phased digestion have been used to convert municipal sludge to biogas. Acid phased digestion takes advantage of the fact that the acid forming bacteria have a much higher growth rate than the methanogens. Consequently, the initial reactor can be much smaller than the subsequent methane producing digester. Acid phased digestion offers greater efficiency in the size of the anaerobic digesters.

Figure 13: Acid Phased Anaerobic Digester (Burke, 2001)

Temperature phased digestion has been used to digest dairy manure in the U.S.A. (Burke, 2001). The temperature phased digestion takes advantage of the pre-thermophilic digester to reduce pathogens from the manure.

Figure 14: Temperature Phased Digester (Burke, 2001)

A temperature phased sequencing batch reactor when treating dairy manure can achieve a 30 to 41% conversion of volatile solids to biogas, while producing a biogas with 62 to 66 % methane (Burke, 2001). These values are based on a system with a SRT/HRT ratio of 3 to 4 and a three-day HRT.


Table 10 summarizes the expected performance of each the various types of anaerobic digesters. Table 11 presents a summary of parameters of the anaerobic processes that can be used to convert all or a fraction of manure to biogas. Table 10: Expected Percentage of Volatile Solids Conversion to Biogas in Manure

(Burke, 2001) Process Load Conversion of Volatile Solids

to Biogas Entire Waste Stream Completely Mixed Mesophilic High 35 to 45 % Completely Mixed Thermophilic High 45 to 65 % Contact High 50 to 65 % Partial Waste Stream Plug Flow Mesophilic High 35 to 45 % Fixed Film High 55 to 65 % Lagoon Low 35 to 45 %

Table 11: Summary of Anaerobic Digestion Parameters to Convert All or Farction of Manure to Biogas (Burke, 2001)

Parameter

Com

plet

e M

ix -

Mes

ophi

lic

Com

plet

e M

ix -

The

rmop

hilic

Con

tact

–

Mes

ophi

lic

Plug

Flo

w –

M

esop

hilic

Lag

oon

Fixe

d Fi

lm

Not limited by Solids Concentration X X X Not limited by Foreign Material X X X Digest Entire Manure Waste Stream X X X Sand & Floating Solids Processing X X X Highly Effective at Odour Control X X Concentrate Nutrients in Solids X X Treat Additional Substrate X X X Stability X X X X Simplicity X X Flexibility X Net Energy Production X X


4.5 Qualitative Analysis of Anaerobic Digestion of Manure Professionally designed and implemented anaerobic manure digestion plants typically have the following results: 1. Odour reduction is usually in the order of 80% (Semmler, 2002) 2. Production of virtually odourless high grade organic liquid or marketable solid

fertilizer (depending on digester configuration), which can even be applied to growing crops without damage.

3. Production of biogas, with a methane content of approximately 60% (Semmler 2002), which can be stored and used on demand

4. Reduction of pathogens of up to 100% depending on configuration (Semmler, 2002).

5. Reduction of greenhouse gas emissions. 6. Reduces land base requirements for manure applications. 7. Provides for the possibility of reclaiming water. 8. Permits the addition of various substrates to increase biogas production, known as

co-digestion. 4.5.1 Nutrient Concentration and Retention The process of anaerobic digestion will convert nutrients from an organic form to an inorganic form. In plug flow, completely mixed and thermophilic digesters the quantity of nutrients entering the reactor equals the quantity of nutrients exiting the digester. However, retained biomass digesters such as the contact process, sequencing batch reactors and fixed film reactors, nutrients may be concentrated in a separate waste solids stream. In manure anaerobic digesters 90% of the phosphorus and 43% of the total nitrogen can be concentrated in the waste solids (Burke, 2001). Often the waste solids volume is only 1/5 of the influent volume. The ability to concentrate nutrients is an important characteristic of the anaerobic process as it proves the livestock producer with the control necessary to manage nutrient application to land. 4.5.2 Energy Production The quantity of energy produced from each cubic meter of manure processed is strictly a function of the percentage conversion of volatile solids to gas. Each pound (454 g) of volatile solids destroyed will produce 5.62 ft3 (0.16 m3) of methane (Burke, 2001). Each cubic foot (0.028 m3) of methane will contain 1000 Btu’s of energy. Therefore each pound (454g) of volatile solids converted will produce 5620 Btu’s of energy. At a 35 percent typical conversion efficiency, each pound (454 g) of volatile solids destroyed will produce 0.58 kWh of energy (Burke, 2001).


The conversion of volatile solids to gas is a function of the organic loading to the digester. Higher percentage conversions to gas are achieved at lower organic loadings. Low loadings however, translate into larger digestion facilities. However, it is possible to achieve a higher volatile solids conversion to gas by increasing the digester loading while maintaining a higher biomass concentration in the digester. In other words, the F/M ration remains low resulting in a higher rate of conversion (Burke, 2001; Fulhage et al., 1993). Conventional completely mixed and plug flow digesters, which do not retain biomass, will have comparable volatile solids destructions, while high rate retained biomass reactors will have higher rates of solids conversion to gas. Table 12 describes the potential gas production of swine, dairy, beef and poultry manure.

Table 12: Potential Gas Production of Swine, Dairy, Beef and Poultry Manure (Fulhage et al., 1993)

Dairy (545 kg cow)

Beef (454 kg cow)

Swine (68 kg hog)

Poultry (1.8 kg bird)

Gas Yield, ft3/lb (m3/kg)

7.7 (0.48)

15 (0.93)

12 (0.75)

8.6 (0.54)

Volatile Solids Produced, lb/day (kg/day)

9.5 (4.32)

5 (2.27)

0.7 (0.32)

0.044 (0.02)

% Reduction of Volatile Solids

31 41 49 56

Potential Gas Production, ft3/animal/day(m3/animal/day)

22.7 (0.64)

31 (0.87)

4.1 (0.12)

0.21 (0.006)

Energy Production Rate, Btu/hr/animal

568

775

103

5.25

Net Available Energy (after heating digester), Btu/hr

380

520

70

3.5

4.6 Examples of Manure Digesters in North America 4.6.1 BioGem Power Systems, Inc., Alberta, Canada (Centralized digester) BioGem Power Systems Inc. is a company from Alberta, Canada. The company has developed a biogas technology that utilizes organic waste from intensive livestock operations, and through anaerobic digestion, generates: electricity and thermal energy, reusable water and a dry nutrient rich organic material.


BioGem built the first commercial biogas facility in Alberta at the Hutterite colony that sells power to the Alberta Power Pool. The facility has been successful at providing power to the digester facility and selling excess power to the grid. Hutterite colony consists of a 1500 farrow to finish sow operation. The BioGem process consists of taking organic wastes (manure, sewage, dead stock, and plant matter) that has been made into a slurry and feeding it to the digester. Under controlled anaerobic conditions at 32oC, methane gas is produced. The methane is used to fuel an engine which drives a generator; producing electricity. Heat from the engine is captured through a hydronic heating system and returned to the facility for heating purposes. The digestion process takes approximately 35 days. At the completion of the digestion, the “spent liquor” is drawn out of the digester into a holding a tank. The liquor is similar in nutrient content to the raw feedstock, with the exception that 95% of the odours have been reduced and the material is in a non-settling form (Henteleff, 2004). The spent liquor can then be used directly as fertilizer or further treated in a wastewater treatment plant. This Hutterite facility has chosen to install a wastewater treatment plant to recover and reuse the water as washwater and livestock water. The sludge from the anaerobic digestion process is further dried through composting, where the end product is similar to peatmoss. This dried material represents 4% volume of the original raw volume of waste slurry (Henteleff, 2004). The Hutterite digester produces 350 kW/h of electricity. The expected payback period for this facility is 6 years. Figure 15 illustrates the Hutterite digester system.

Figure 15: Hutterite Organic Waste

(swine manure, sewage, plant material) Digester


4.6.2 Lethbridge Bioreactor, Alberta, Canada (Centralized digester) The Lethbridge system is currently under construction and is scheduled to be in operation in the fall of 2004. The Lethbridge system is an Integrated Waste Management System (IWMS) that involves a combination of solutions: anaerobic digestion (biogas), aerobic treatment (composting) and wastewater treatment. The IWMS concept is quite similar to that of the BioGem system. The Lethbridge system is strategically situated in an area known as “feedlot alley”, which produces between 60 to 65% of the slaughter beef in Canada. The ECB Enviro Berlin AG digester will accept a 60:40 split of beef manure and other organic waste (sewage, food processing waste, slaughter house waste, etc.). The digester has been designed to treat 100 million kg (100,000 tonnes) of manure per year. The system promises to generate 15 Gigawatt-hour of electricity while reducing equivalent CO2 emissions by 15 kilotonnes (Loh, 2003). This IWMS will produce methane that will be used to fuel a combined heat and power plant and/or a fuel cell to produce thermal and electrical energy. Treated solid effluent will be recycled into an aerobic digestion (composting). The liquid effluents will be purified through a biofiltration process and recycled. A portion of these effluents will be used as wash waters for barns, farm machinery and vehicles. The construction costs of this IWMS system is estimated at $5.5 million (CAN$).

Figure 16: Lethbridge Bioreactor, Alberta, Canada


4.6.3 Klaesi Digester, Ontario, Canada (On-farm digester) The Klaesi farm is a 200 cow dairy operation in Ontario, Canada. The digester is built into an existing concrete manure pit. The digester is an off-the-shelf Bohni design from Switzerland. A rubber membrane on top expands or contracts depending on the amount of gas that is collected. The manure mixture is heated to 40 oC and, when the gas is drawn off, the liquid left is moved to a second holding tank and from there is spread onto the farm’s 500 acres of farmland. The biogas runs a piston generator that produces power which is "net metered" to Hydro One (Ontario's major generating company). Net metering measures the energy used against the energy generated, resulting in a "net" energy total from which your bill is calculated. The Bohni digester cost between $170,000 (CAN$), with a payback of 10 years with electricity savings. The digester produces about 450 kW per day metered into the grid. Excess heat collected from the generator does two things: keeps the digester running at an optimal temperature and heats water which is connected to an outdoor furnace and heats two farmhouses as well. Maintenance on the system takes about 10 minutes a day, mostly on the generator.

Figure 17: Klaesi Dairy Manure Digester (Paul Klaesi forefront), Ontario, Canada


5.0 Treatment of Solid Waste in Canada 5.1 Composition of Organic Solid Waste Table 13 identifies the average composition of various components typically found in municipal solid waste in Canada; average was taken from 8 different landfills.

Table 13: Average Composition of Municipal Solid Waste from 8 Canadian Landfills (Georgia Basin, 2002)

Components % of Total Standard Deviation Organics 37.41 11.11 Paper 32.29 10.58 Plastic 13.31 5.37 Household Hygiene 3.80 3.27 Metals 3.36 1.50 Glass 3.11 2.30 Inorganic 2.92 3.81 Household Hazardous 2.15 2.07 Fines 1.19 1.70 Small Appliances 0.45 1.41

5.2 Organic Solid Waste Management Options The two common options for the disposal of organic solid waste in Canada is in a landfill or through decomposition in a dedicated reactor vessel. 5.3 Energy Recovery from Landfill Gas 5.3.1 The Production of Landfill Gas Landfill gas (LFG) is produced by the decomposition of organic materials in municipal solid waste (MSW). Typically, methane (CH4) and carbon dioxide (CO2) comprise 99 per cent of LFG, with trace gases including carbon monoxide, hydrogen, nitrogen and oxygen. The precise composition of LFG depends on the age of filled waste, and its exposure to water.


Table 14: Formation of landfill gas occurs in five stages (AGO, 1997):

1. Initial adjustment: The biodegradable portion of MSW is decomposed by microbial activity, under mostly aerobic conditions. Aerobic decomposition is sustained by air trapped within the landfill. Daily soil cover on the MSW supplies the necessary microorganisms for this stage.

2. Transition stage: The trapped air is depleted, and anaerobic conditions begin to prevail. Nitrate and sulphate become active in the biological degradation process. As a consequence, some nitrogen gas and hydrogen sulfide gases are emitted during this stage.

3. Acid stage: A different set of micro-organisms becomes active during this stage Acidogens, or acid formers, are now the principal microorganisms responsible for biodegradation. Their activity is characterised by a three-step process: hydrolysis, acidogenesis and carbon dioxide generation.

4. Methane fermentation stage: During this stage, methanogens or methane formers, become active. These organisms convert the acetic acid and hydrogen gas formed by the acid formers into methane and carbon dioxide. Most LFG is generated during this stage.

5. Maturation stage: Maturation occurs after the readily available biodegradable material has been converted into methane and carbon dioxide in the previous stage. The biodegradation process is now slow, because the nutrients available and easily biodegradable organic material was exhausted in earlier phases. LFGs are generated at a very slow rate.

Figure 18: Concepts of a Bioreactor Landfill (AGO, 1997)

.3.2 Potential Yield and Release of LFG Over Time

ms in

5 The volume of LFG generated from a landfill depends on the proportion of materials in the MSW deposits with some organic content, which is the food for microorganisthe anaerobic digestion process. Organic materials are not decomposed in equal


proportions, because the degradable fraction varies from product to product. For exama larger proportion of food wastes degrade slower than yard wastes. The amount odegradable materials in MSW is determined by the composition of waste, and its exposure to moisture in the landfill. Exposure to moi

ple, f

sture is particularly important, since can be managed as part of the landfill operations.

, decomposition of the remaining degradable matter extends over long periods of time.

half

it The total yield of LFG is not released as soon as decomposition commences. LFG isgenerated over time, and the degradation rate varies across types of waste. Table 15 shows the length of time over which half of the degradable fraction is transformed in LFG. In all cases

Table 15: Time taken for decomposition of

of the degradable content (AGO, 1997) Type of waste Years

Food 1

Garden 5

Cardboard 15

The basic pattern is that LFG generation from a particular quantity of MSW is highest in the two years after waste has been filled. During this time, anaerobic digestion of most othe degradable content of food wastes occurs. LFG generation continues after this time but at slowly decreasing rates. While gas generation can extend for periods of up to fifty years, in most cases LFG release occurs within five years, because food and garden w

f

aste pically comprise a large proportion of all organic materials in MSW (AGO, 1997).

er.

f LFG from a landfill. Table 16 lists five factors that determine the flow of landfill as.

ty The annual rate of methane generation is the proportion yielded in one year of the total amount of LFG that the landfill has the potential to yield over its lifetime. The annual rateof methane generation is higher if more of the MSW is food waste or exposed to optimal amounts of water. If a landfill comprises a relatively high proportion of paperboard, or is located in a dry climate, then the annual rate of methane generation will tend to be lowThe total yield of LFG and the annual rate of generation are key factors in the annual flow og


Table 16: Five factors that determine the annua f LF , 1997)

nits Label

l flow o G (AGO

Factor U

1 Total LFG yield per kg of MSW 3m /kg Lo

2 Filling rate kg/yr R

3 Time since landfill opened (years) number of years t

4 Time since landfill closure (years) of yearsnumber c*

5 Annual rate of LFG generation 1/years k

The US EPA has set 0.05 as the default value for k in their model of LFG generation (AGO, 1997). The US EPA model (first-order decay model) is used to estimate LFG:

LFGt = LoR[e-k(c) - e-k(t)]

r tions

about climate, composition of waste and landfill management, will not apply.

Table 17: Typical M from Landfill Gas (Environme

ne waste processed

The model can be used to get an idea of the LFG generated at a landfill with particulacharacteristics. It does not produce estimates of LFG flow rates. Model assump

ethane and Energy Production

nt Canada, 2001) CH captured in LFG4 48 m3/tonPercent CH in LFG 4 50-53 % Annual Energy generation rocessed 143 kWh/tonne of waste pGreenhouse gas emission reductions (LFG combustion) 128,000 tonnes eCO2/yr

Leachate management and filling practices comprise a set of important decisions thinfluence LFG generation and collection. Moisture content fundamentally affects anaerobic digestion in landfills. Filling practices affect the anaerobic process. The more

at

equently waste is covered, the more quickly anaerobic decomposition commences.

ity to groundwater are vital the examination of potential LFG recovery.

fr Some landfill characteristics are beyond the direct influence of the landfill owner. For example, the site’s area and depth is typically set by the local government, and may notbe able to be increased. Proximity to groundwater, and exposure to rainfall, are fixed. Thorough assessments of the landfill’s area, depth and proximto


5.4 Dedicated Anaerobic Digestion of Organic Solid Waste

rom e biogas and compost from the sludge residue in a more controlled environment.

have a number of advantages over modern sanitary landfills in that ana

•

ill can assist in the breakdown of other wastes.

. • tioner as one of the end products (energy from methane being the

other end product).

t

100 per cent of gas generated to be recovered and over a much orter time period.

.4.1 Anaerobic Digester Technologies MSW

d processes, wet continuous digestion, and multi-stage wet

igestion.

only balance favourable for operation at

ermophilic digestion temperatures (50-55oC).

Unlike energy recovery from landfill gas which can be unreliable, anaerobic digestion of organic solid waste in a dedicated facility provides the opportunity to obtain energy fth Anaerobic digestion may

erobic digestion: Makes landfills easier to manage by removing potentially problematic organic wastes, although the microbial activity in the landf

• Avoids the generation of gas in landfills. • Contributes to recycling targets (or enables recyclable material to be reclaimed). • Provides an enclosed system that enables all of the gas produced to be collected for use

Provides a soil condi

Modern sanitary landfills sites that are designed for methane recovery yield only 30-40per cent of the amount of gas actually generated. They are designed as tightly sealed units slowing the degradation process, with some biodegradable materials that mighcontinue to degrade for 50-100 years (AGO, 1997). By comparison a closed vessel bioreactor can enablesh 5 The leading anaerobic digestion concepts for MSW are dry continuous digestion, drybatch digestion, leach bed Dry continuous digestion involves a continuously-fed digestion vessel with a digestate dry matter content of 20-40 per cent. Both completely-mixed and plug-flow systems areavailable. Plug flow systems rely on external recycling of a proportion of the outgoing digestate to inoculate the incoming raw feedstock. In both cases, the requirement forminimal water additions makes the overall heat th


Dry batch digestion involves loading a vessel with MSW and digestate from another reactor. The vessel is sealed and left to digest naturally. Leachate is tapped from the base

f the vessel and recirculated to maintain a uniform moisture content and redistribute

s to facilitate start

p, inoculation and removal of volatile acids in the active reactor. The concept has also

xed r farm slurries. Effective

moval of glass and stones is required in the feed preparation stages to prevent their

cycled e

s which are then converted to biogas in a specialist high-rate industrial naerobic digester, usually an anaerobic filter or an upflow anaerobic sludge blanket

. The more anaerobic the process, the more of this carbon is

onverted to methane. The amount of carbon is expressed in terms of the percentage of fr

Tab ethane and Energy P Digestion of SW (Environme a, 2001)

osoluble substrates and methane-producing bacteria. The leach-bed process is similar to the dry continuous process except that leachate fromthe base of the vessel is exchanged between established and new batcheubeen described as "sequential batch anaerobic composting" (SEBAC). Wet continuous digestion involves the MSW feedstock being slurried to about 10 per cent dry solids to provide a feedstock that can be fed to a conventional completely-midigester similar to those commonly used for sewage sludges orerapid accumulation in the bottom of the main digester tank. Multi-stage wet digestion involves making a slurry of the MSW with water or reliquor. It is then fermented with hydrolytic and fermentative bacteria to release volatilfatty acidareactor. 5.4.2 Methane Production from Designated Anaerobic Digestion of MSW The main determinant of the amount of biogas is the amount of carbon in the organic waste. When the waste degrades some of the carbon becomes part of the cellular material of the microbes (assimilated carbon) and the rest of the carbon forms methane and carbondioxide (dissimilated carbon)c

esh weight (AGO, 1997):

le 18: Typical M roduction from AnaerobicM nt Canad

Biogas generation 110 m3/tonne waste processed Percent CH4 in biogas 52-54% Annual Compost produced 665 kg/tonne of waste processed Annual Electricity generation ocessed 320 kWh/tonne of waste prGreenhouse Gas Emission Reduction


(Biogas combustion) 161,500 (tonnes eCO2/yr) 6.0 Use of AD in the Treatment of Industrial Wastewaters in Canada

Factors which should be considered when assessing the suitability of anaerobic treatment of i u

spended solids toxic compounds

• Nutrient requirements

l factors impede treatment. High strength and fluctuations at occur in the type and quantity of wastes to be treated along with cleaning aids and

c

the rinks

strength although they are periodic and usually hot. The strength of the ffluents means that discharge costs are high and can be reduced through anaerobic

the

controlled the determinant of the rate of digestion is the concentration of bacteria.

6.1 Anaerobic digestion of industrial wastewater

nd strial wastewater include:

• The nature of the wastewater• The concentration of organic pollutants• Temperature • The concentration of su• Presence of

• Economics Most of the larger industrial anaerobic treatment plants are in the food, drinks and fermentation sectors, and the pulp & paper industry. The high organic content of food industry waste means that in principle they should be easily treated with anaerobic digesters. In practice, severathsanitisers create problems. Dairy product effluents are warm, strong and ideal for anaerobic digestion if the process is well controlled. Starch effluents are high strength but with a high proportion of colloidal solids which reduces biodegradability. The high strength means that anaerobidigestion using a solids tolerant process is an efficient treatment process. Sugar industryeffluent is suitable for anaerobic digestion although over a period of time the effluent becomes highly acidic. Lime is used to offset acidity but this accumulates reducing space for active biomass. Effluent from the manufacture of confectionery and soft dcan be highedigestion. 6.2 Digestion technologies The choice of digestion process is driven by the type of wastewater. The more rapidtreatment process the more likely it is to be viable. Provided that the process is properly


Industrial processes have been designed to retain the bacteria or to recycle the bacteria. The three main processes are the contact stirred tank reactor, the upflow anaerobic sludge lanket, and fluidised or expanded bed reactors.

.2.1 The contact stirred tank reactor (CSTR)

ration

s float. Settlement has be assisted by degassing, cooling, filtration or inclined plates.

b 6 In the CSTR bacteria are physically separated from the effluent by settlement or filtand recycled back into the reactor. The warmth of the effluent (relative to ambient temperatures) and the methane gas being given off make the solidto

.2.2 Upflow anaerobic sludge blanket reactor (UASB)

entous

retain

s a function of the gas roduction and the upward flow velocity of the influent.

.2.3 The fluidised bed

Significant further development appears to be required before it is adopted more idely.

.3 Industries Utilizing Anaerobic Digestion

e

in

effluents are high strength even where some by-products have already been recovered.

6 Key elements in the feed substrate for successful formation of the sludge blanket are calcium, phosphorus, aluminium and silicon, along with a large population of filambacteria and the generation of bacterial polymers. To reduce the time required for acclimatisation and adaptation it is now usual to start with a large inoculum of an alreadygranulated sludge. The reactor baffles are used to promote gas/solid separation tothe granules. The degree of mixing in the sludge blanket ip 6 The fluidised bed is designed to overcome the difficulties of biomass separation in completely mixed reactors and the loss of granulation or blockage in plug flow reactors.This is a relatively new technology and has acquired the reputation of being difficult to operate. w 6 6.3.1 Brewing Industry Brewing produces a cold relatively weak effluent from which several by-products havalready been recovered such as, malt residues, spent hops, yeast, and carbon dioxide. Reductions in water-use leading to a more highly concentrated effluent increases the opportunities for anaerobic digestion. Water charges and effluent charges are decisiveforcing changes to waste management practices. Distillery and fermentation industry


Anaerobic digestion is a sound solution to effluent problems in an industry that has few alternatives. Main process - Contact Stirred Tank Reactors 6.3.2 Vegetable Processing Industry Most vegetable processing produces a weak and cold effluent that favours aerobic treatment. Exceptions are pea processing and snack foods such as potato chips. These processes produce a higher strength waste with soluble starches that are suitable for anaerobic processes. A high degree of seasonality in pea processing militates against the high capital costs of anaerobic treatment. Main processes - Upflow Anaerobic Sludge Blanket and Contact Stirred Tank Reactors 6.3.3 Meat Processing Industry Meat processing wastes are difficult to handle irrespective of the treatment process used. They contain blood, faecal matter, grease, bone fragments, and hair along with biocides and disinfectants. Long retention time processes are considered to have the greatest potential. Chemical contamination from organic and inorganic sources poses unique problems as does the slow degradation of cellulosic material. The effluent is high strength and warm which favours anaerobic treatment despite the difficulties. References AGO (Australian Greenhouse Office), 1997. Methane Capture and Use – Waste Management Workbook, Environment Australia, Kingston, Australia. Barber William P. 2000. Effects of ultrasound on anaerobic digestion of sludge, CIWEM 7th European Biosolids and Organic Residuals Conference. Bo Holm-Nielson, J. and T. Al Seadi, 1997. Biogas in Europe: A general overview, South Jutland University Centre, Bioenergy Department, Esbjerg, Denmark. Burke, Dennis A., 2001. Dairy Waste Anaerobic Digestion Handbook, Environmental Energy Company, Publication No. 360-923-2000, Olympia, Washington, U.S.A., June 2001. CH2MHill and Payne Engineering, 1997. Constructed Wetlands for Livestock Management, Gulf of Mexico Program, Nutrient Enrichment Committee, January 1997. Clark, P.B. (1998) Ultrasound in sludge processing: the technology of the future? Innovations 2000 Conference, Cambridge, 1998. De Baere, L. 1999. Anaerobic digestion of solid waste: state-of-the-art, in Mata-Alvarez, J., Tilche, A., Cecchi, F. Proceedings of the second international symposium on anaerobic digestion of solid wastes, Barcelona


Environment Canada, 2001. Technical Bulletins on Landfill-Gas-to- Energy from the National Office of Pollution Prevention, Ottawa, Ontario, Canada. Fulhage, Charles D., Dennis Sievers and James R. Fischer, 1993. Generating Methane Gas from Manure, Department of Agricultural Engineering, University of Missouri-Columbia, U.S.A., October 1, 1993. Georgia Basin-Paget Sound, 2002. Technical Ecosystem Indicator Report - Solid Waste, Working Group on Environment, British Columbia, Spring 2002. Hamzawi, N., Kenndey, K.J., McLean D.D. 1998a. Anaerobic digestion of co-mingled municipal solid waste and sewage-sludge. Water Sci. Technol. 38:127-132. Hamzawi, N., Kenndey, K, J., McLean, D.D. 1998b. Technical feasibility of anaerobic co-digestion of sewage-sludge and municipal solid-waste. Environ.Technol. 19:993-1003. Henteleff, M. 2004. BioGem Power Systems Working Process. Presented at Integrated Solutions to Manure Management II Conference, London, Ontario, Canada, March 8-9, 2004. IEA Bioenergy, 2001. Biogas and More! Systems and Markets Overview of Anaerobic Digestion, AEA Technology Environment, Oxfordshire, United Kingdom, July 2001. Kelly, H.Emerging, 2003. Processes in Biosolids Treatment. 2nd Canadian Organic Residual Recycling Conference Pentiction B.C. April 24-25 Liu, H.W., Walter, H.K., Vogt, G.M., Vogt, H.S., Holbein, B.E. 2002. Steam pressure disruption of municipal solid waste enhances anaerobic digestion kinetics and biogas yield. Biotechnol. Bioeng. 77:121-130. Loh, L. 2003. Lethbridge Integrated Waste Management Pilot Project, Cement Association of Canada, May 2003. ManureNet, 2004. Manure Digesters, Agriculture and Agri-food Canada, http://res2.agr.ca/initiatives/manurenet/en/man_digesters.html, last updated April 7, 2004. Mata-Alvarez, J., Cecchi F., Pavan, P., Bassetti, A. 1993. Semi-dry thermophilic anaerobic digestion of fresh and pre-composted fraction of municipal solid waste (MSW): digester performance. Water Sci. Technol. 27:87-96. OMAF (Ontario Ministry of Agriculture and Food), 2000, Agricultural Pollution Control Manual, Ontario Government, Guelph, Ontario, March 2000. Oleszkiewicz, J.A., Poggi-Varaldo, H.M. 1997. High solids anaerobic digestion of mixed municipal and industrial waste. J. Environ. Eng. 123:1087-1092. Owens, J.M., Chynoweth, D.P. 1993. Biochemical methane potential of municipal solid waste (MSW) components. Water Sci. Techol. 27:1-14. Parkin, G.F. and Owen, W.F., 1986. Fundamentals of Anaerobic Digestion of Wastewater Sludges, Journal of Environmental Engineering, 112, (5).


http://res2.agr.ca/initiatives/manurenet/en/man_digesters.html

Semmler, N., 2002. Modern Biogas Technology: Environmentally sound & energy efficient advanced technology for organic waste management, Renewable Energy Technologies Inc., Trenton, Ontario, Canada, March 17, 2002. Stephenson, R.J. and Dhaliwal, H., 2000. Method of Liquefying Microorganisms Derived From Wastewater Treatment Processes, US Patent No. 6,013,183. Suslick K.S. 1998. in Kirk-Othmer Encyclopedia of Chemical Technology; 4th Ed. J. Wiley and Sons: New York, 26, 517-541 Van Die, P., 1987. An Assessment of Agriculture Canada’s Anaerobic Digestion Program, Engineering and Statistical Research Branch, Agriculture Canada Research Branch, Report No. 1-933. Vogt, G.M., Vogt, H.S., Walter, H.K. 1998. Apparatus and method for waste reduction and conversion. United States Patent no. 5,795,479.


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