Anaerobic digestion of dairy manure influenced by the waste milk from milking operations

Anaerobic digestion of dairy manure influenced by the waste milk from milking operations

J. Dairy Sci. 94:3778–3786 doi:10.3168/jds.2010-4129 © American Dairy Science Association®, 2011. Anaerobic digestion of dairy manure influenced by t...

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J. Dairy Sci. 94:3778–3786 doi:10.3168/jds.2010-4129 © American Dairy Science Association®, 2011.

Anaerobic digestion of dairy manure influenced by the waste milk from milking operations X. Wu,* C. Dong,† W. Yao,‡ and J. Zhu‡1 *Department of Chemical Engineering, Tshinghua University, 100000 Beijing, China †School of Environmental Science and Engineering, Zhejiang Gongshang University, 310018 Hangzhou, China ‡Southern Research and Outreach Center, University of Minnesota, 35838 120th Street, Waseca 56093


It is not uncommon that a significant amount of milk from milking operations is discharged to manure digesters on dairy farms. To understand the effect of milk on the digester performance, experiments using batch digesters (500-mL flasks) were carried out in this study to co-digest milk and dairy manure at different milk levels for biogas production and pollutant reduction, and a total of 8 treatments were examined [i.e., control (without milk) and 1, 3, 5, 7, 9, 14, and 19% milk additions]. The temperature for all digesters was maintained at 37 ± 0.5°C throughout the experimental period, which was 28 d. The results showed that co-digesting milk with dairy manure could increase biogas productivity, with the percent cumulative biogas volume increased by 5.6, 16.3, 26.5, 40.8, 50.2, 79.9, and 103.8%, as compared with the control, for milk addition of 1, 3, 5, 7, 9, 14, and 19% (vol/vol), respectively. However, the CH4 content in the biogas decreased slightly as the milk content increased (from 66.5% for the control to 63.5% for 19% milk treatment), implying that the added milk could promote CO2 production. To avoid that, the milk content in the manure should be controlled below 3%. A linear relationship for the total biogas volume produced with the milk content in the manure was revealed, with a correlation coefficient of 0.99. An improved removal efficiency of chemical oxygen demand was observed for milk-treated digesters. Good linear regressions between the total biogas production and the percent chemical oxygen demand decrease and the substrate carbon/nitrogen ratio were also obtained (correlation coefficients: 0.93 and 0.99, respectively). Besides, co-digestion of dairy manure and milk was found to improve substrate solids breakdown, but had little effect on percent volatile fatty acid decrease. In summary, the waste milk co-digested with dairy manure may not cause negative effects on anaerobic digester performance. Received December 25, 2010. Accepted April 18, 2011. 1 Corresponding author: [email protected]

Key words: co-digestion, dairy manure, milk, biogas production INTRODUCTION

Anaerobic digestion has long been considered an environmentally friendly process that offers many advantages in effectively treating a variety of organic waste streams. First, it produces renewable energy in the form of methane (Björnsson et al., 2000; Khanal, 2008). As the push for developing renewable energy sources is gaining momentum, the number of anaerobic manure digesters in the United States is steadily increasing. According to the report from the World Dairy Expo held in Madison, Wisconsin in 2009 (World Dairy Expo Inc., 2009), the US dairy industry is taking the lead in adopting anaerobic technology because the majority (over 75%) of operating US manure digesters are installed on dairy farms. It is anticipated that this trend will continue, as the country is determined to decrease its reliance on ever-diminishing fossil-based energy resources. Second, the technology can significantly decrease the polluting strength of the treated waste materials, such as chemical oxygen demand (COD), thus ameliorating their pollution potential to the environment when discharged. Because of the nature of dairy operations, a tangible amount of milk coming from the milking parlor wastewater is often discharged to the bulk manure, which can dramatically increase the COD level of such waste streams. Because of its high-COD material content (190,000 mg/L; Callaghan et al., 1997), the common practice of putting the milkcontaminated manure on cropland has the potential of causing severe contamination of surface and ground waters from runoff and leaching. Past research showed that even a small amount of milk, if entering the water courses, could be highly polluting and poses a treacherous threat to water quality (Wase and Thayanithy, 1993). Such practice is, therefore, drawing increased scrutiny from the public and environmental regulatory agencies (Samkutty et al., 1996; Cumby et al., 1999; Willers et al., 1999; Cannon et al., 2000). With environ-



mental regulations becoming more rigorous, regulatory compliance has become a matter of growing concern to dairy operators in the US because they will be held responsible for potential water contamination problems resulting from discharging untreated milk-containing waste streams to water courses. Fortunately, with the number of dairy producers willing to adopt anaerobic digesters on their farms continuing to grow, the concern for such pollution could be tempered. However, a remaining question of this remedy is whether the added milk has any effect on the overall digestion process in terms of biogas production and pollutant removal. Limited information on co-digestion of dairy manure with waste milk exists in the available literature. The only report was the work done by Callaghan et al. (1997), where 1-L digesters were used to investigate the digestion process performance under shock-loading conditions. In their study, milk was added to different digesters at 3, 6, and 9% levels after the digesters had entered the steady state to simulate shock loadings. This experimental design only examined the situation of sporadic milk discharges to the digester, without considering the batch lagoon digesters more commonly used in the Midwest, which continuously receive mixed substrates containing both manure and waste milk throughout the operation. Therefore, it is our belief that quantitative performance information of batch digesters related to biogas production and COD decrease through co-digestion of milk and dairy manure will be of great interest to the vast majority of dairy producers having lagoon digesters. The objective of this project was to investigate the overall response of co-digesting dairy manure with milk added at 7 different levels (i.e., 1, 3, 5, 7, 9, 14, and 19%), using laboratory-scale batch anaerobic digesters. The co-digestion performance was evaluated based on total biogas volume production, methane concentration, and its volume in the biogas generated; changes and decreases in COD and VFA, changes in TS, and total volatile solids (TVS); and the effect of substrate carbon-to-nitrogen ratios. MATERIALS AND METHODS Manure and Milk Sources

A one-time collection of both fresh and digested dairy manure was conducted from a dairy farm located in St. Peter, MN, which was equipped with a scraped manure-handling system. The on-site digester was a plug-flow unit running under mesophilic temperature. According to the farm management, the waste milk from the milking operation was directly discharged to a storage lagoon without going through the digester. The


same lagoon was also used as storage for the digester effluent. The raw milk used in this study was obtained from another dairy farm in the vicinity of the University of Minnesota Southern Research and Outreach Center in Waseca, MN. If not used immediately, the collected fresh manure was stored in 3.78-L bottles in a freezer to preserve the nutrients. The same substrate materials were used for all of the bench-scale digestion trials to minimize experimental errors associated with variations in manure characteristics potentially resulting from different collection times. Bench-Scale Digestion Experimental Design

In this study, the bench-scale digesters were constructed using 500-mL flasks (working volume: 300 mL) with rubber stoppers that contained a flushing port and a gas port that allowed biogas release and collection. Each flask was filled with a mixture of 50% (vol/ vol) fresh dairy manure, 50% (vol/vol) digested dairy manure, and a desired amount of raw milk to simulate the actual co-digestion substrate, making up to a total of 300 mL of liquid in the digester. The stopper was then installed and sealed at its contact with the flask and tubing using silicone sealant, and each flask was purged with 1 L of nitrogen gas to create and maintain a completely anaerobic environment inside the digester. Figure 1 represents the experimental setup of a typical batch digester. Seven milk addition levels in terms of percent liquid volume (vol/vol) in the digester were tested in this study (1, 3, 5, 7, 9, 14, and 19%) to determine the milk effect on the digester performance related to biogas productivity and COD removal. One digester without milk addition was also operating as the control side-by-side with the milk-treated ones. All digesters were placed in a water bath that maintained temperature at 37°C (mesophilic digestion) during experiments, with each treatment tested in triplicate. Sampling and Analysis

For all flask digesters, the biogas volume produced was measured by withdrawing the biogas from the gas collection bag with a 500-mL gas syringe and its contents of H2, CH4, and CO2 were analyzed by a gas chromatograph (Varian CP-3800; Varian Inc., Walnut Creek, CA) equipped with a thermal conductivity detector and a Select Permanent Gasses/CO2 solution-set column (Varian CP7429), using helium as the carrier gas at a flow rate of 30 mL/min. The oven, injector, and detector temperatures were kept at 50°C, 120°C, and 150°C, respectively. The biogas composition was determined every other day for each digester. In addition, liquid samples (50 mL) were also collected from Journal of Dairy Science Vol. 94 No. 8, 2011



Figure 1. Schematic of a laboratory-scale batch anaerobic digester.

each digester at the end of each run for characteristics determination. The parameters determined for the collected manure included TS, TVS, and total Kjeldahl nitrogen (TKN) contents and pH, following standard methods (APHA, 1998). The COD of manure and milk was determined using the US EPA-approved Hach sealed tube/colorimetric measurement with a 2-mL sample volume (Hach Co., 1993). RESULTS AND DISCUSSION Cumulative Biogas Production Affected by Different Milk Contents

Figure 2 presents the means of cumulative biogas volume produced for all of the milk concentrations tested over the entire experimental period. With reference to the control, several observations appear to be uniquely caused by milk additions. First, it was clearly shown that increasing the milk content could increase the cumulative biogas production during the operation, with the total volume of biogas produced being 5,260, 5,790, 6,300, 7,010, 7,480, 8,960, and 10,150 mL for the milk treatments of 1, 3, 5, 7, 9, 14, and 19%, respectively, Journal of Dairy Science Vol. 94 No. 8, 2011

as opposed to the control (4,980 mL). Second, higher milk content could significantly raise the initial biogas production rate (more discussions follow), as indicated by the slopes of all of the curves at the beginning of the experiment (see the inset graph in Figure 2), which started to climb steeply after only 6 h into the digestion process. This early increase in production rate was consistent with the pattern for batch digestion of animal manure as modeled by Hobson (1985), with higher milk content resulting in higher increases in the production rate. However, it differed from Hobson’s model in that the sudden increase in production rate was sustained only for a short period of time (6 h) before entering into a much slower stage characteristic of a relatively moderate increase. Third, the presence of milk appeared to have some influence on the stability of the digestion process, as evidenced by the fluctuation of biogas production curves at high milk concentrations. For instance, the treatments having milk contents up to 7% demonstrated similar trends. But for milk contents of 9, 14, and 19%, the fluctuation in biogas production volume became progressively conspicuous. Especially for the 19% milk treatment, the biogas volume produced first jumped from 190 mL at 6 h to 1,190 mL at 12 h after the digestion started, followed by a relatively moderate production period before it jumped again after 8 d of digestion. This observed trend in biogas production, increasing and then decreasing, followed by recovery to a much higher level, was somewhat similar to the trend observed by Peck et al. (1986), who were monitoring the effect of temperature shock loads on anaerobic digestion systems, although the response in biogas volume of the milk/manure system studied herein was much more pronounced. Another study by Callaghan et al. (1997) also reported that the biogas production rate could be elevated in digesters receiving a greater loading of waste milk. Because milk has a high level of COD (Table 1), copious VFA can be produced through anaerobic fermentation (Callaghan et al., 1997), making available ample substrates for biogas production. Therefore, the observed higher biogas volume produced at 19% milk addition in this project could be attributed to the introduction of a larger amount of milk into the digestion system. Considering the results from previous workers and from this study, it may be concluded that milk can increase biogas production when co-digested with dairy manure. Returning to the biogas production rates, it was observed that the biogas production rates (slopes of the curves) were, in general, similar for the treatments up to a milk content of 9% (Figure 2). For the last 2 milk concentrations (14 and 19%), the production rates increased sharply at the beginning and then dropped slightly before catching up with other treatments after



Table 1. The characteristics of fresh manure, digested manure, and milk used in this study Parameter1 pH TS (g/L) TVS (g/L) COD (mg/L) TKN (mg/L) VFA (mg/L)

Fresh manure

Digested manure


6.95 72.7 57.5 21,500 2,942.9 5,930

7.48 58.5 44.3 7,500 2,645.8 590

6.87 139.7 131.9 120,600 5,095.7 2,380


TVS = total volatile solids; COD = chemical oxygen demand; TKN = total Kjeldahl nitrogen.

d 5 (for 14%) and d 10 (for 19%). The maximum production rates for all treatments except 19% could be determined by obtaining the slopes in the first 10 d, which were 13.9, 14.9, 16.4, 18.1, 20.2, 21.0, and 22.3 mL/h for the control, 1, 3, 5, 7, 9, and 14% milk content, respectively. For the 19% milk content, the maximum rate (excluding the sudden increases at the beginning of the experiment) actually occurred between d 10 and 15, which was 23.2 mL/h. It is interesting to note that when most biogas production rates began to level off after d 10 or 15 (14% only), the 19% milk treatment continued to show a higher production rate all the way to the end of the experiment, indicating that the biogas

production could potentially last much longer because of the sufficient substrates provided by the added milk. Again, the data have verified that adding milk in dairy manure could increase volumetric biogas production. Increases in biogas generation rate by 7 and 18% were obtained when milk was added to the manure digesters at merely 1 and 3% (vol/vol), respectively. Cumulative CH4 Volumes Affected by Different Milk Contents

Figure 3 documents the performance of different treatments in cumulative CH4 production. In the first

Figure 2. Profile of cumulative biogas production from dairy manure for different milk contents of 0 (control), 1, 3, 5, 7, 9, 14, and 19%. Journal of Dairy Science Vol. 94 No. 8, 2011



10 or so days of the experiment, the 19% milk treatment produced the smallest cumulative volume of CH4. An interesting point here is that, when Figure 2 is consulted, it is not difficult to find that the cumulative volume of biogas produced by the 19% milk treatment is somewhat on par with the rest during the same period. It thus, suggests that the low productivity in pure CH4 volume could be a result of the low CH4 content in the biogas. This postulate is evidenced by the average CH4 concentration data presented in Figure 4, where the increased milk content caused a decrease in CH4 content in the biogas. A similar observation was also reported by Callaghan et al. (1997), in which a sharp drop in CH4 concentration was observed for digesters receiving loadings of waste milk, with the effect becoming more pronounced as the loading rate increased. In addition, Figure 4 reveals a scenario that was unknown before and merits further discussion. It appears that positive linear relationships exist for the milk content with the volumes of total biogas and CH4 produced, with correlation coefficients of 0.99 and 0.99, respectively. This observation indicates that adding milk to dairy manure digestion will promote the volumetric production of both biogas and CH4. However, as described earlier, the CH4 content in the produced biogas deteriorated as

the milk content increased (from 66.5% for the control to 63.5% for the 19% milk treatment; Figure 4). It can, thus, be inferred that although the volumes of total biogas and CH4 were increased by increasing the milk content in the digester, the increase in CH4 volume was not in tandem with that in the total biogas volume (as shown in Figure 4, where the 2 regression lines had different slopes), implying that a significant amount of CO2 was concurrently produced. Apparently, the presence of milk in the digestion substrate is the only legitimate cause for the increasing production of CO2, which is driven by the intensified conversion of the excess substrate present to acetic acid by acid-forming bacteria (Callaghan et al., 2002). This observation is also in conformance with the behavior of anaerobic digestion systems under shock organic loading conditions predicted by the model developed by Marsili-Libelli and Beni (1996). Also from Figure 4, although the effect of milk on lowering the CH4 content in biogas is observed for all milk treatments, the extent of such an effect is different. The milk effect on CH4 content in biogas was not significant for manure containing milk up to 3% (vol/ vol), but it turned significant at 5%. Summarizing the above discussions leads to an intuitive suggestion that

Figure 3. Total CH4 production from dairy manure versus milk content for different milk contents of 0 (control), 1, 3, 5, 7, 9, 14, and 19%. Journal of Dairy Science Vol. 94 No. 8, 2011



Figure 4. The relationships among the volumes of total biogas and CH4 produced, milk content, and CH4 content in the biogas from dairy manure.

to avoid production of a substantial amount of CO2 due to the spilled milk in the digestion process, dairy producers should manage to control the milk content in the digester liquid at ≤3%. Although this project presents data from batch operations, the observed information may provide some insights into the potential effect of manure/milk codigestion under continuous conditions (e.g., the continuous-flow dairy farm digesters). According to Figure 3, the effect of milk addition in terms of total CH4 volume production was not obvious until after d 6 for milk content of up to 14% (not for 19%), as compared with the control. This implies that if a continuous anaerobic digester is operated on a hydraulic retention time shorter than 6 d, little difference should exist in cumulative CH4 volume produced between dairy manure digestions with and without milk addition. This is good news for the operators of the most popular high-rate upflow anaerobic sludge blanket (UASB) digesters, which normally have hydraulic retention time shorter than 6 d. Therefore, milk addition at the levels tested in this study may have little influence on UASB digesters. Characteristics Changes in Digestates from the Digestion of Dairy Manure with Milk

Table 2 shows the average values of COD, TKN, and C/N ratios before and after digestion for all of

the treatments, including the percent decreases of COD and TKN. The added milk substantially increased the digester content COD as the amount of milk increased. However, at the end of the experiment, the final COD in most digester effluent samples reached a fairly similar level. Comparing the COD data in Table 2 with the COD of digested manure in Table 1 suggests that the digestion process for the majority of the treatments could be deemed as completed properly. In addition, because all of the experiments were run on the same time schedule, the COD degradation efficiency obviously increased with increasing milk addition from 49.7% for the control to 77.8% for the 19% milk treatment. The improved COD removal efficiency in company with the increasing milk content could be attributed to the gradually elevated C/N ratio due to the added milk (from 5.19 for the control to 10.7 for the 19% treatment) because it is recognized that the optimum C/N ratio for anaerobic digestion is around 20/1 to 30/1 (Parkin and Owen, 1986). This could explain the continuous increase in COD removal as the C/N ratio increased, as shown in Table 2. At the end of experiment, the effluent C/N ratio averaged 2.75, which was very close to the value for the digested dairy manure (2.83; Table 1). When the C/N ratio falls into this neighborhood, ammonia inhibition to anaerobic digestion normally becomes prominent (Hills and Roberts, 1981; Hashimoto, 1983) and the process is, thus, considered completed. As for TKN, the removal Journal of Dairy Science Vol. 94 No. 8, 2011



Table 2. The average values of chemical oxygen demand (COD) and total Kjeldahl nitrogen (TKN) for the batch digestion process with different milk contents COD (mg/L)

TKN (mg/L)

C/N ratio

Milk content (%)



Decrease (%)



Decrease (%)



0 (control) 1 3 5 7 9 14 19

14,500 15,561 17,683 19,805 21,927 24,049 29,354 34,659

7,300 7,700 7,300 8,100 8,300 7,600 7,800 7,700

49.7 50.5 58.7 59.1 62.1 68.4 73.4 77.8

2,794 2,817 2,863 2,909 2,955 3,002 3,117 3,232

2,742 2,792 2,840 2,794 2,908 2,970 3,080 3,206

1.9 0.9 0.8 4.0 1.6 1.0 1.1 0.8

5.19 5.52 6.18 6.81 7.42 8.01 9.42 10.7

2.66 2.76 2.57 2.89 2.85 2.56 2.53 2.40

efficiency was almost negligible, which is the typical behavior commonly observed for anaerobic digestion, indicating that the digestion operation was carried out successfully. Based on the information obtained from this study, it may be concluded that milk content up to 19% (vol/vol) in dairy manure may have little negative effect on the COD removal efficiency in the anaerobic digestion process. Table 3 lists other parameters monitored in this study, including pH, TS, TVS, and VFA. It can be seen that the changes in pH due to co-digestion of dairy manure with milk, regardless of milk level, are not significant because all of the final pH values before and after digestion are within ± 0.2 pH units for normal operations (Björnsson et al., 2000). Nonetheless, the added milk was clearly conducive to solids breakdown, as reflected by the percent TS content decrease that increased with increasing milk content in the digestion liquid (from 11% for the control to 34% for the 19% treatment). However, the percent TS content decrease was not linearly correlated with the added milk content because a similar TS content decrease was observed

for milk contents of 1, 3, 5, and 7%, although they all almost doubled that of the control, whereas an increase in TS content decrease was seen from 9 to 14 and 19% (25 to 34%). Similar scenarios were observed for the percent decrease of TVS. Therefore, it may be concluded that co-digestion of dairy manure with milk could be instrumental in breaking down the solid materials in the manure. Substantial decreases in VFA are observed across the treatments and little difference appears between the control and the milk-treated digesters, suggesting that the influence of milk levels added to dairy manure on VFA reduction via anaerobic digestion is minimal (Table 3). The final VFA levels of different treatments (except 7 and 9% milk treatments) are generally in conformance with the range for which the anaerobic digestion process is considered completed (Björnsson et al., 2000). The reason for the 2 higher VFA final concentrations in the digester effluent (7 and 9% milk treatments) is not readily known. Figure 5 shows the relationships among percent VFA and COD decreases, total biogas produced, and

Table 3. The average values of pH, TS, total volatile solids (TVS), and VFA before and after digestion Milk content Item pH Initial Final TS (g/L) Initial Final Decrease (%) TVS (g/L) Initial Final Decrease (%) VFA (mg/L) Initial Final Decrease (%)




7.66 7.51

7.69 7.47

7.70 7.51

65.6 58.15 11

66.3 52.97 20

67.8 55.93 18

50.9 41.79 18

51.7 38.29 26

53.3 40.76 24

3,260 820 75

3,251 870 73

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3,234 490 85

5% 7.68 7.50





7.72 7.47

7.74 7.49

7.75 7.58

7.72 7.63

69.3 54.4 22

70.8 55.09 22

72.23 53.82 25

76.0 50.33 34

79.7 52.92 34

55.0 39.34 28

56.6 40.2 29

58.2 38.84 33

62.2 36.7 41

66.3 38.81 41

3,216 830 74

3,198 1,250 61

3,181 1,750 45

3,137 670 79

3,093 450 85


the C/N ratios of the digestion substrates. Good linear relationships are observed between the total biogas production and the percent COD decrease and the substrate C/N ratio, with correlation coefficients of 0.93 and 0.99 obtained, respectively. As discussed earlier, the substrate C/N ratio was a critical factor for anaerobic digestion, with the optimum value falling between 20/1 and 30/1 (Parkin and Owen, 1986), and the C/N ratio in this study was 10.7, which is far below the lower limit of the reported optimum C/N ratio range, so it is not surprising to see that the volumetric biogas production continued to increase with the increasing substrate C/N ratio. With an almost perfect linear relationship, it may be concluded that, in the current range of C/N ratios examined, the percent COD decrease can be accurately estimated using the equation presented in Figure 5 based on the measurements of total biogas volumes produced. However, such a relationship was not observed between the percent VFA decrease and the total biogas production, which indicates that cautions have to be exercised when using VFA to gauge the performance of anaerobic digesters in terms of biogas productivity, although VFA is identified by some researchers as a potential indicator for monitoring the anaerobic digestion process (Ahring et


al., 1995; Kono and Himeno, 2000). Björnsson et al. (2000) evaluated parameters for monitoring an anaerobic co-digestion process of municipal wastewater sludge and food processing wastes and concluded that VFA could be a superior monitoring parameter only under overloading conditions, which was consistent with reports from several other researchers who extensively studied the influence of VFA on anaerobic digestion (Elefsiniotis and Oldham, 1994; Magbanua et al., 2001; Khanal, 2008; Tait et al., 2009). The results from this study are in agreement with the previous findings by confirming that VFA may not be a good indicator to monitor anaerobic digestion performance if its level is within normal operating concentrations. CONCLUSIONS

The data from this study indicated that milk could increase biogas volumetric production if co-digested with dairy manure. A linear correlation between the total biogas volume produced and the milk content in the manure was obtained, with a correlation coefficient of 0.9993. The added milk also increased the maximum biogas production rate as compared with the control (7 and 18% increases were observed when

Figure 5. Relationships between chemical oxygen demand (COD) and VFA reductions and the total biogas volume produced from dairy manure. Journal of Dairy Science Vol. 94 No. 8, 2011



milk was added to the digested manure at merely 1 and 3% levels, respectively). However, it was observed that milk addition could decrease the CH4 content in the biogas produced, implying a potential increase in CO2 production. To avoid that, the milk content in the digester liquid should be limited to below 3%. The COD degradation efficiency was not affected by the added milk, and rather, an increase in efficiency was observed with increasing milk content from 49.7% for the control to 77.8% for the digester with 19% milk treatment. Good linear correlations were obtained for the total biogas production with the percent COD decreases and the substrate C/N ratio, with the correlation coefficients of 0.93 and 0.99, respectively. The added milk was also found to be conducive to solids breakdown, indicated by the doubled percent TS content decrease for milk contents of 1, 3, 5, and 7%, as compared with the control (11% vs. average of 21%). No clear relationship between percent VFA decrease and total biogas volumetric production was observed. ACKNOWLEDGMENTS

The authors thank the Minnesota Legislature Rapid Agricultural Response Fund for providing financial support to this project. REFERENCES Ahring, B. K., M. Sandberg, and I. Angelidaki. 1995. Volatile fatty acids as indicators of process imbalance is anaerobic digesters. Appl. Microbiol. Biotechnol. 43:559–565. APHA (American Public Health Association). 1998. Standard Methods for the Examination of Water and Wastewater. 20th ed. L. S. Clesceri, A. E. Greenberg, and A. D. Eaton, ed. American Public Health Association, Washington, DC. Björnsson, L., M. Murto, and B. Mattiasson. 2000. Evaluation of parameters for monitoring an anaerobic co-digestion process. Appl. Microbiol. Biotechnol. 54:844–849. Callaghan, F. J., K. Luecke, D. A. J. Wase, K. Thayanithy, and C. F. Forster. 1997. Co-digestion of cattle slurry and waste milk under shock loading conditions. J. Chem. Technol. Biotechnol. 68:405– 410.

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