Baseline Performance Monitoring of Commercial Dairy Anaerobic Digester

Transcription

1 Baseline Performance Monitoring of Commercial Dairy Anaerobic Digester C. Frear, W. Liao, T. Ewing and S. Chen Background Manure is generally a low quality wastewater for biogas production as much of the organics have already been biologically converted during animal digestion. Because of this, the resulting manure is composed to a large degree of stabilized or recalcitrant organics, resulting in only limited biogas production from its subsequent digestion. Despite the low production, though, manure provide stability to the digestion process as its organic loading is relatively low and it is highly alkaline which controls the ph throughout the digestion process (Callaghan et al., 2002; Hartmann and Ahring, 2005). In contrast, other wastewaters such as food processing wastes, industrial greases and oils, and organic fraction municipal solids (OFMSW) are rich in biodegradable organics and correspondingly contain a large biogas production potential. Unfortunately, these wastes are often difficult to digest in practice as they are often too high in organic loads, contain high levels of inhibitory substances, and contain low levels of alkalinity or buffering capacity to control ph adequately throughout the digestion process (Gallert and Winter, 1997). Notably, the biogas industry has recognized the beneficial effects that can be achieved by combining the digestion of manures with other organically-rich wastewaters or waste solids, namely increased biogas production and enhanced stability. This practice is called co-digestion, with the non-manure waste streams being referred to as substrates, and although considerable research has been done at laboratory scale (Alatriste-Mondragon et al., 2006; Braun et al., 2003), little demonstration of co-digestion has been done at commercial scale (DEA, 1995; Kaparaju et al., 2002; Kumke and Langhans, 2000). The CFF project was instrumental in soliciting stakeholder involvement, assisting in grant writing for federal funds and providing direct funds for Washington State s first dairy anaerobic digester (AD). The digester was designed and operated from the start to co-digest dairy manure with substrates from locally available food processing facilities. The digester, which continues to serve as a test-bed and demonstration project for our AD work, was completed in 2004 and is located in Lynden, Washington on a 700 cow dairy farm. The digester, a patented modified plug-flow digester with axial dispersion and sludge recycling, was designed by GHD Incorporated of Chilton, WI, USA ( and installed through license agreement by Andgar Corporation of Ferndale, WA, USA ( To allow for possible future farm expansion, use of trucked-in manure from nearby farms, and utilization of other imported organic materials (known as substrates) for co-digestion, the design size was set at 1,500 cows or 2,025 animal units (AU). Scrape-collected manure was piped or trucked to a receiving pit and pumped directly into the mesophilic (37.8 o C) Ch. 3 Commercial AD Performance Page 1

2 anaerobic digester heated with reclaimed waste heat from a 450/500 kw Caterpillar G398 (Peoria, IL, USA) reciprocating engine and generator set. Codigestion substrates (egg breakage, ravioli sauce, fish breading and artificial crab) entered the farm gate via tipping fee contracts and were loaded in regular batches to a collection pit for mixing with the manure. After AD, coarse fibrous solids were separated from the digester effluent using a US Farm (Tulare, CA, USA) 0.30 cm slope screen with dewatering auger. Specific mean operating parameters for the commercial digester measured during monitoring are given in Table 3.1. Table 3.1: Operating Parameters for Commercial Digester Parameter Unit Mean a Cows AU b 938 ± 87 Effective Volume m 3 3,899 Manure Flow m 3 /day ± Percentage Substrate % ± 1.60 Total Flow m 3 /day ± HRT days ± 2.92 OLR kg VS/m 3 day b 2.01 ± 0.19 Temperature o C 37.8 ± 0.5 Design GHD Modified Plug Flow with Axial Mixing Manure Handling System Scrape Pit/AD/Screw Press/Storage Lagoon Engine Set Caterpillar G398 coupled to a 450 KW Generator a Data is the average of daily herd and flow recordings with mean standard deviations (n=198) at α=0.05 b AU= animal units; VS= volatile solids; HRT = hydraulic retention time; OLR = organic loading rate Separated solids were used onsite as bedding replacement. The liquid stream from the separator was stored in a lagoon until regional regulations allowed landapplication. Pilot-scale research units (struvite, ammonia, electro-coagulation, and phosphorous solids) were installed and connected to the liquid stream prior to lagoon entry for research on nutrient recovery technology. Digested liquids as well as non-digested controls were applied to fields and the effects on greenhouse gas emissions and crop growth were directly measured and modeled. Although commercial use of the resulting biogas was for electrical generation, pilot-scale systems were set up for testing of biogas scrubbing technologies and the development of compressed methane fuel. Results of the pilot-testing of various value-added technologies as well as the impacts of field application of liquid effluent are discussed in detail in later chapters. This chapter focuses on an extended performance evaluation completed during The baseline data collected from the monitoring were instrumental in generating overall farm and AD system mass, nutrient, energy and climate impact balances. They also pointed researchers and industry towards areas where improved technologies and processes would most benefit farm and digester economics. In particular, the results of this initial study highlighted the role of co- Ch. 3 Commercial AD Performance Page 2

3 digestion in improving reactor performance, farm economics and nutrient management. Performance Monitoring Monitoring Methods Samples were collected weekly for twelve months from six different locations around the digester: (1) manure inlet to collection pit; (2) substrate inlet to collection pit; (3) sampling port directly in front of digester inlet; (4) sampling port at effluent exit point from digester; (5) post-separation liquid inlet to lagoon; and (6) post-separation coarse fibrous solids. Biogas was sampled at a port just proximal to the engine/generator set using Tedlar bags (Smith Air Sample, Hillsborough, NC, USA). Manure and biogas flow measurements were logged using a Siemens Mag 8000 (Spring House, PA, USA) electromagnetic liquid flow meter and an Aaliant Target Mark V (Spartanburg, SC, USA) strain gage gas meter, respectively. Mean parameter values with standard deviations for the manure, substrate, and combined co-digestion influent to the digester are summarized in Table 3.2. All parameters were analyzed according to protocols established by Standard Methods (APHA, 2005) or by methods described within Frear et al. (2009). Commercial-scale operation of the digester did not allow for an opportunity to study the co-digestion process directly against a manure-only control. Instead, an indirect manure-only comparison was modeled using laboratory batch manure samples and additional literature data as reported in Frear et al. (2009). An enterprise budget spreadsheet was used to compare the two scenarios of co-digestion and manureonly. In order to simplify comparisons and maintain consistency, equal flow rates to the digester were assumed for both scenarios. Co-digestion was assumed to be a result of a mean flow rate of m 3 /day of mixed material consisting of a mean substrate (non-manure) volume representing 16.03% of total flow and scrape manure from 938 AU. The manure-only scenario was at an equal overall flow rate composed of just scrape manure from an assumed 1,098 AU. Ch. 3 Commercial AD Performance Page 3

4 Table 3.2: Manure, Substrate and Co-Digestion Influent Characteristics Parameter Units Scrape Manure a Substrate b Co-Digestion c TS g/l ± ± ± VS g/l ± ± ± 3.93 COD g/l ± ± ± TKN g/l 2.45 ± ± ± 0.93 TAN g/l 1.72 ± ± ± 0.45 TP g/l 0.39 ± ± ± 0.14 K g/l 2.44 ± ± ± 0.35 Fecal Coliform 1,000 cfu/g 356 ± ± 247 a Data is the average of (n=6) with mean standard deviations at α=0.05 b Data is the average of triplicates with mean standard deviations (n=3) at α=0.05 Individual substrates mixed according to flow percentage and analyzed as mixture c Data is the average of (n=24) with mean standard deviations at α=0.05 TS = total solids; VS = volatile solids; COD = chemical oxygen demand; TKN = total Kjeldahl nitrogen; TAN = total ammonia nitrogen; TP = total phosphorous; K = potassium Results and Discussion Synergistic Effects of Co-Digestion and Digester Stability Commercial-scale digester monitoring results from this study (Table 3.3) indicate that at the volumes practiced, mixing manure with substrates for co-digestion allowed for more preferred levels of key micronutrients, neutral ph, and additional alkalinity, while also producing C/N and C/N/P ratios of 28:1 and 112:4:0.5 respectively, very near the 25-32:1 and 115:4:1 ratios recommended for AD (Liu et al., 2009). Table 3.3: Parameter enhancements via co-digestion Substrate C/N Alkalinity ph Nutrients Micro-Nutrients Ratio g CaCO3/L N:P:K Element Dairy 11: ± ± 6:1:6 Fe, Mn, S, Mg, Ca, Ni, Manure 0.08 Substrate a 56: ± ± 10:1:1 Se, Ni 0.96 Co-Digestion 28: ± ± :1:4.5 All a Individual substrates mixed according to flow percentage and analyzed as mixture Over 6 months of monitoring, the Ripley Ratio (volatile acids over total alkalinity) and effluent ph were measured to determine whether co-digestion led to the expected improvements in biogas productivity and stability (Figure 3.1). Ripley et al., (1986) determined that a ratio below 0.25 was indicative of a stable digestion process with spikes and troughs deviating from the steady state indicating upset and potential failure. As can be seen from the figure, throughout the course of the monitoring, both values remained quite constant and well below threshold values, Ch. 3 Commercial AD Performance Page 4

5 pointing to strong reactor stability. Stability was further indicated by effluent volatile fatty acids (VFA) concentrations that were consistently below detection level and constant effluent ph levels. Stability increased slightly over time, as indicated by the fact that variation from baseline and the value of the Ripley Ratio decreased over the monitoring period Effluent Ripley Ratio Effluent ph Ripley Ratio Effluent ph /1/06 5/1/06 6/1/06 7/1/06 8/1/06 9/1/06 10/1/06 11/1/06 12/1/06 Date Figure 3.1: Effluent ph and Ripley Ratio for co-digestion The stability indicated by the Ripley Ratio and digester performance are in contrast to results that other AD and cow rumen researchers have obtained when investigating substrate versus manure levels. Koster and Lettinga (1984) showed that total ammonia nitrogen (TAN) concentrations above 1.7 g/l (at ph 8.0) inhibit methane-forming bacteria: however, mean influent and effluent TAN levels of 1.87 and 2.65 g/l, respectively in this study showed no adverse inhibition. Rumen fermentation (idealized plug-flow digester with solids retention and mixing) studies indicate that total fat and protein levels within a dairy cattle dry matter diet should not exceed 7 and 18%, respectively (Jenkins, 1993). Determination of dry matter intake for this commercial digester showed that the given type and amount of substrates utilized should have exceeded the suggested fat and protein intakes and led to fermentation inhibition if the digester was performing similar to a cow rumen. Factors contributing to the unexpected digester stability and for the ability of the digester to overcome the relatively high concentrations of potential inhibitors could include the high level of manure alkalinity (above the 2-5 g/l recommended by Metcalf and Eddy (2003)), availability of manure macro- and micro-nutrients (Callaghan et al., 2002), and bacterial acclimatization (Angelidaki and Ahring, 1994; Edelmann et al., 2000). Ch. 3 Commercial AD Performance Page 5

6 Biogas Production Direct comparison of the manure-only and co-digestion scenarios (Table 3.4) shows the positive impact of substrates on biogas production, digester productivity, and performance. Overall gas production more than doubled, while the methane productivity increased by 1.8 times from a manure-alone modeled baseline of 0.21 m 3 CH4/kg VSload to 0.38 m 3 CH4/kg VSload for co-digestion. Table 3.4: Biogas Production and Reactor Performance Parameter Units Co-Digestion Manure-Only a Total Biogas m 3 biogas/day 4,649 ± 377 2,216 CH4 Productivity m 3 CH4/kg VS Added 0.37 ± CH4 Productivity m 3 CH4 /kg VS D 0.66 ± Vol. Production m 3 biogas/m 3 day 1.19 ± Biogas % CH ± Composition a Simulation of manure-only production using described assumptions, literature data from (Hill, 1984) and (US-EPA, 2005), batch digestion data, and flow rate of m 3 /day While the co-digestion led to a notable increase in methane productivity, theoretical methane productivity as related to VS destruction (Bu) indicates only a slight change as determined through calculations using Bushwell s formula, as described by (Metcalf and Eddy, 2003): and using the following assumptions: VSlipid (C57H104O6); VSprotein (C5H7O2N); VScarb (C6H10O5); and VSVFA (C2H4O2) VS total = 64.0 kg/m 3 with respective VS fractions being 38.92%, 32.82%, 15.31%, and 12.95% for protein, carbohydrate, lipid and VFA respectively Theoretical values for Bu as calculated by the above Bushwell model using known values for protein, lipids, carbohydrates, and VFA are 0.57 m 3 CH4 /kg VS Destroyed at 38 o C and 1 atm for manure and 0.61 m 3 CH4 /kg VS Destroyed at 38 o C and 1 atm for codigestion. The slight increase in value is due to nearly 4-fold and 2-fold increases in fat and protein percentages, respectively for co-digestion material. The experimentally-derived Bu for this study was 0.66 ± 0.14 m 3 CH4 /kg VS Destroyed at 38 o C and 1 atm, above what the theoretical calculations suggest. However, the mean difference is well within the standard deviation. The ratio of Bo/Bu for the two scenarios becomes 0.56 and 0.42, respectively, for the co-digestion and manure-only scenarios. This summarizes the degree to which the biodegradability or conversion efficiency of the wastewater was increased (33%). Notably, this 33% increase in biodegradability as defined by Bo/Bu is roughly equivalent to the percent difference Ch. 3 Commercial AD Performance Page 6

7 in VS reduction for manure and substrate (36%). The co-digestion scenario produced a volumetric performance of 1.19 ± 0.10 m 3 biogas/m 3 day which was a doubling over the manure-only modeled scenario of 0.57 m 3 biogas/m 3 day. Despite the large improvement in performance, the co-digestion performance is much lower than could be expected (1.91 m 3 biogas/m 3 day) had the digester been designed for the actual flow rates being produced on the farm (1,098 AU) as opposed to a projected future size (2,025 AU). This over-sized digester negatively impacted volumetric performance and as such emphasizes the need for engineering balance in considering the tradeoffs between allowing future growth and effective sizing of a digester. Wastewater Stabilization Although commercial digester discussion often centers upon overall biogas production, a key function of the digester is to remediate air and water quality concerns. Previous manure-only digester studies (US-EPA, 2004; 2005; US-EPA, 2008) have shown that commercial dairy digesters are capable of total solids (TS), volatile solids (VS), chemical oxygen demand (COD), and VFA reductions in the range of 25-35%, 30-40%, 38-42%, and 86-88%, respectively. Table 3.5 lists the reduction results for this co-digestion commercial evaluation, showing reductions higher than the ranges given above for all parameters identified; 40.61%, 55.28%, 67.72%, and 99.87%, respectively. This performance is not surprising given that the substrates being co-digested were extremely high in TS, VS, COD, VFA and organic fraction (OF = VS/TS). The long HRT may also have contributed to the additional biodegradation and reduction. Ch. 3 Commercial AD Performance Page 7

8 Table 3.5: Influent and Effluent Parameters and Percentage Reduction Performance Parameter Influent Effluent Mean % Reduction (g/l) TS ± ± VS ± ± FS ± ± 3.96 NA COD ± ± VFA 7.71 ± ± TKN 4.12 ± ± 0.53 NA TAN 1.87 ± ± TP 0.51 ± ± 0.10 NA K 2.31 ± ± 0.27 NA ph 6.87 ± ± Alkalinity 8.96 ± ± FC (cfu/g) 339,031 ± 247,461 3,418 ± 7, NA refers to mean reduction parameters not statistically relevant as determined by General Linear Model (GLM) ANOVA analysis with Statistical Analysis System program 9.0 (SAS Institute Inc. NC) at α=0.05 with n=24 samples. All reductions were with calculated p-values < except for FS (0.2121), TKN (0.2355), TP (0.0417), and K (0.4567). TS = total solids; VS = volatile solids; FS = fixed solids; COD = chemical oxygen demand; VFA = volatile fatty acids; TKN = total Kjeldahl nitrogen; TAN = total ammonia nitrogen; TP = total phosphorous; K = potassium; FC = fecal coliform reported in colony forming units (cfu) per dry gram solid Pathogen reduction capabilities of AD are also an important concern, as both manure and substrates are capable of transferring zoonotic agents to crops grown for animal or human consumption (Guan and Holley, 2003). AD has been identified as a management practice capable of producing effective reductions in pathogens. Since many pathogens are present in low concentrations and are therefore difficult or time consuming to detect, indicator organisms which are present in large numbers within feces and which are readily counted are often used as an analysis tool. Fecal coliform is one such indicator organism. Reductions in fecal coliform populations during this mesophilic digestion process were nearly 99% or 2.0 log10 which is comparative to the 2.3 log10 reduction noted by US EPA (2005) but considerably lower than the 3.1 log10 reduction reported by Wright et al., (2003). The lower pathogen reduction estimate for this study might be explained by the fact that a significant portion of the digester influent was composed of pre-consumer substrates which contained a low initial fecal coliform count. An operational concern of commercial digesters is the potential for solids build-up in the digester due to improper mixing. Although the biological and chemical processes occurring in the digester will cause a certain degree of mineralization of organic N and P, both TP and TKN should stay constant throughout the digestion process. Differences between influent and effluent concentrations could indicate an unwanted organic accumulation, thus TP and TKN values can be indirect indicators for effective mixing and fluid/solids flow. In addition, analysis of fixed solids (FS = TS-VS) can point to accumulation of inert solids such as sand and grit that might Ch. 3 Commercial AD Performance Page 8

9 develop from farm operations. As shown in Table 3.5, values for FS, TP and TKN show no statistical difference between the influent and effluent concentrations. This indicates that the axial mixing within the digester was not experiencing stratification and was maintaining plug-flow properties even though the studied TS and flow rates were well below that normally identified for use with traditional plug-flow systems (US-EPA, 2006). The apparent ability of this patented GHD modified plug flow technology to maintain axial mixing may explain the previously noted increased biogas production when compared to similar but traditional nonmixed studies (US-EPA, 2005). Effect on Nutrient Balance One concern regarding on-farm co-digestion is the potential for nutrient overload. Many dairy CAFOs operate under nutrient management plans designed to ensure that only nutrients expected to be used by crops are applied to the limited land area of the farm. Figure 3.2 shows that a farm digesting the volume of manure and substrates assumed by our modeling scenario did experience a significant nutrient load increase, particularly in regard to nitrogen which increased by 56.7%. Figure 3.2: Changes to farm-level nutrient loads experienced by test dairy (Lynden, WA) when they digesting dairy manure only, or when they co-digest manure and organic food waste substrates This is not surprising as the protein content of the manure was 1.5% while the protein content of the co-digestion substrates were nearly double at 2.8%. Corresponding to the increase in nitrogen levels is the increase in ammonia (TAN) levels due to the mineralization of a significant portion of the organic fraction of the nitrogen to TAN. Thus, the 36% and 55% of dairy producers who already overload N and P nutrients on their farms (USDA-APHIS, 2004) will have to examine whether the economic benefits of co-digestion can offset possible changes in their nutrient Ch. 3 Commercial AD Performance Page 9

10 management plans unless a nutrient recovery system that integrates with the AD unit is implemented. Climate Impacts An important environmental outcome of AD is the potential for mitigation of GHG emissions, particularly since the use of liquid manure handling systems within dairy CAFOs has led to an estimated 34% increase in manure methane emissions from (US-EPA, 2008). Presently, an estimated 8% of all anthropogenic methane emissions in the U.S. results from animal manure management with dairy manure representing 43.3% of the total (US-EPA, 2008). Commercial digester operation allows for a controlled anaerobic process and subsequent methane production and capture that otherwise would occur, albeit at reduced kinetics, in the usual open-air anaerobic storage lagoons. The monetary value placed on the reduction from a baseline open-air release to the environment can benefit the dairy producer through the sale of carbon credits. In addition, use of the combined heat and power (CHP) produced by the AD process or the production of bio-methane can be reported as a fossil-fuel offset or green tag. The result is two digester-related mechanisms for controlling and reducing GHG emissions, each with an assigned protocol and monetary attachment (CCX, 2008; US-EPA, 2008). In addition, there is the potential for a third credit if one assumes that substrates used in co-digestion would normally be treated in landfill operations where (like anaerobic lagoons) they would produce non-captured methane emissions to the open-air. Although presently no such substrate protocol exist, data available from this research in conjunction with landfill assumptions presented by Murphy and McKeough (2004) can be used to infer a potential carbon credit for digesters using substrates in lieu of landfill disposal. Carbon reductions and offsets from each of the three categories at the flow rates utilized in the commercial digester studied are summarized in Table 3.6. Table 3.6: Greenhouse credits and revenues from two scenarios at commercial digester Co- Digestion Manure Only Manure Credit OFMSW Credit Offset Total MT CO2 e/yr a, b MT CO2 e/yr c MT CO2 e/yr d MT CO2 e/yr 3,397 6,066 1,128 10,591 3,977 NA 472 4,449 a (US-EPA, 2008) b MT = metric tons (1 MT = 1 Mg); MMT = million metric tons (1 MMT = 1 Tg) c (Murphy and McKeogh, 2004) d 0.4 MT CO 2e/MWh less 15 kwh and assumed 90% runtime (CCX, 2008) These numbers can be generalized to both a per cow basis and on a Washington State basis for both manure-only and co-digestion scenarios assuming: (1) all co- Ch. 3 Commercial AD Performance Page 10

11 digestion on AD farms occurs at a volumetric flow rate of 15-20% and consists of food processing waste with average biogas production capabilities determined in this study; (2) that of Washington State s 135 CAFO dairies larger than 500 cows, representing 192,000 WEC, 40 of these can have AD installed during the next 10 years; and (3) that of those 40 installations, all are scrape facilities and, ten each will be on 1,000, 1,500, 2,000 and 2,5000 WEC farms respectively, totaling 70,000 WEC or a little over a 1/3 of the CAFO population in the State. Table 3.7: Generalization of greenhouse credits and revenues Co- Digestion Manure Only Manure Credit MT CO 2e /cow yr a, b OFMSW Credit c Offset Total for Washington State MT CO 2e /cow yr MT CO 2e /wet MT MT CO 2e /cow yr d MT CO 2e /cow yr MMT CO 2e / yr NA NA a (US-EPA, 2008) b MT = metric tons (1 MT = 1 Mg); MMT = million metric tons (1 MMT = 1 Tg) c (Murphy and McKeogh, 2004) d 0.4 MT CO 2e/MWh less 6% parasitic load and assumed 90% runtime (CCX, 2008) Thus, for planning purposes given the assumptions stated, CAFO dairies in Washington State could expect to mitigate and receive carbon credits totaling 5.57 MT CO2 e/cow yr if they install an AD unit for manure-only digestion similar to that modeled in this study. 1 If they co-digest and a protocol for calculating carbon credits for landfill displacement similar to that calculated here can be agreed upon then the GHG mitigation and corresponding carbon credit rises 2.74 times to a value of MT CO2e/cow yr. Expressed across a statewide plan as proposed over the next ten years, the state could plan, in manure-only and co-digestion scenarios, respectively, for roughly 0.4 and 1.1MMT CO2e/yr in carbon reductions. Comparing these figures to the whole farm GHG emission estimates discussed in Chapter 1, incorporating manure-only AD units for methane capture and use reduce the potential whole farm GHG release by nearly 50%. If co-digestion is considered then there is a net sink of nearly 40%, though the farm boundaries are different in the pre- and post- digestion scenarios in this case, so that a true GHG calculation would require adjustment. A more comprehensive analysis would also capture additional benefits such as transportation fuel offsets for reduced hauling distances. Conclusion The performance monitoring of the commercial digester and its comparison of codigestion practice to an assumed manure-only baseline have resulted in important 1 MT = metric tons (1 MT = 1 Mg); MMT = million metric tons (1 MMT = 1 Tg) Ch. 3 Commercial AD Performance Page 11

12 mass, nutrient, energy and climate balances that were used throughout the project to develop new systems, processes and technologies, all aimed at improved economic viability, technology adoption and climate mitigation. Specifically, the performance monitoring added to overall scientific knowledge in that it: Made available commercial-scale performance data for a U.S. dairy anaerobic digester practicing co-digestion, data which is lacking in the available literature; Validated laboratory-based assumptions about co-digestion (regarding chemical environment changes and effects on stability and biogas performance) on a commercial scale; Emphasized the impact co-digestion can have on farm nutrient balances and the corresponding need that arises for nutrient recovery systems that work in concert with AD units, especially when co-digestion is being practiced. Key Project References Related to Chapter The majority of the work presented in this chapter has been previously published as: Frear, C., Liao, W., Ewing, T., Chen, S., Evaluation of co-digestion at a commercial dairy anaerobic digester Bioresource Technology, Submitted. References Alatriste-Mondragon, F., Samar, P., Cox, H.H.J., Ahring, B.K., Iranpour, R., Anaerobic codigestion of municipal, farm, and industrial organic wastes. A survey of recent literature. Water Environment Research, 78, Angelidaki, I., Ahring, B.K., Anaerobic thermophilic digestion of manure at different ammonia loads: effect of temperature. Water Research, 28, APHA, Standard Methods for the examination of water and wastewater. 21st ed. American Public Health Association, Washington DC. Braun, R., Brachtl, E., Grasmug, M., Codigestion of proteinaceous industrial waste. Applied Biochemistry and Biotechnology, 109, Callaghan, F.J., Wase, D.A.J., Thayanithy, K., Forster, C.F., Continuous codigestion of cattle slurry with fruit and vegetable wastes and chicken manure. Biomass and Bioenergy, 22, CCX, Agricultural methane emission offsets and renewable energy emission offsets. Chicago Climate Exchange, Chicago, IL. DEA, Overview report on biogas plants in Denmark. Danish Energy Agency, Copenhagen, Denmark. Ch. 3 Commercial AD Performance Page 12

13 Edelmann, W., Engeli, H., Gradenecker, M., Co-digestion of organic solid waste and sludge from sewage treatment. Water Science and Technology, 41, Frear, C., Liao, W., Ewing, T., Chen, S., Evaluation of co-digestion at a commercial dairy anaerobic digester Bioresource Technology, Submitted. Gallert, C., Winter, J., Mesophilic and thermophilic anaerobic digestion of sourcesorted organic wastes: effect of ammonia on glucose degradation and methane production. Applied Microbiology and Biotechnology, 48, Guan, T.Y., Holley, R.A., Pathogen survival in swine manure environments and transmission of human enteric illness-a review. Journal of Environmental Quality, 32, Hartmann, H., Ahring, B.K., Anaerobic digestion of the organic fraction of municipal solid waste: influence of co-digestion with manure. Water Research, 39, Hill, D.T., Methane productivity of the major animal waste types. Transactions of the ASAE, 27, Jenkins, T.C., Lipid metabolism in the rumen. Journal of Dairy Science, 76, Kaparaju, P., Luostarinen, S., Kalmari, E., Kalmari, J., Rintala, J., Co-digestion of energy crops and industrial confectionery by-products with cow manure: batchscale and farm-scale evaluation. Water science and technology : a journal of the International Association on Water Pollution Research, 45, Koster, I.W., Lettinga, G., The influence of ammonium-nitrogen on the specific activity of pelletized methanogenic sludge. Agricultural Wastes, 9, Kumke, G.W., Langhans, G., Plant scale co-fermentation of farm manure and industrial organic wastes. Annual Residuals and Biosolids Management Conference, 14th, Boston, MA, United States, Feb. 27-Mar. 1, 2000, Liu, Y., Miller, S.A., Safferman, S.I., Screening co-digestion of food waste water with manure for biogas production. Biofuels, Bioproducts & Biorefining, 3, Metcalf, Eddy, Wastewater engineering: treatment and reuse. 4th ed. McGraw Hill, Boston, MA. Murphy, J.D., McKeogh, E., Technical, economic and environmental analysis of energy production from municipal solid waste. Renewable Energy, 29, Ripley, L.E., Boyle, W.C., Converse, J.C., Improved alkalimetric monitoring for anaerobic digestion of high-strength wastes. Journal - Water Pollution Control Federation, 58, Ch. 3 Commercial AD Performance Page 13

14 US-EPA, A comparison of dairy cattle manure management with and without anaerobic digestion and biogas utilization. United States Environmental Protection Agency, Washington DC. US-EPA, An evaluation of a mesophilic, modified plug-flow anarobic digester for dairy cattle manure. United States Environmental Protection Agency. US-EPA, Market Opportunities for Biogas Recovery Systems A Guide to Identifying Candidates for On-Farm and Centralized Systems. United States Environmental Protection Agency, Washington DC. US-EPA, An evaluation of a covered anaerobic lagoon for flushed dairy cattle manure stabilization and biogas production. United States Environmental Protection Agency. US-EPA, Inventory of US greenhouse gas emissions and sinks: United States Environmental Protection Agency, Washington DC. USDA-APHIS, Dairy 20002: Nutrient management and the US dairy industry in United States Department of Agriculture Animal and Plant Health Inspection Service. Wright, P.E., Inglis, S.F., Stehman, S.M., Bonhotal, J., Reduction of selected pathogens in anaerobic digestion Proceedings of the Ninth International Symposium, Animal, Agricultural and Food Processing Wastes IX, Raleigh, NC, pp. pp Ch. 3 Commercial AD Performance Page 14