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Anaerobic codigestion of hog and poultry waste
Bioresource Technology 76 (2001) 165±168
Short communication
Anaerobic codigestion of hog and poultry waste Benjamin S. Magbanua Jr. a,*, Thomas T. Adams b, Phillip Johnston c b
a Department of Civil Engineering, Mississippi State University, Mississippi State, MS 39762-9546, USA Department of Biological & Agricultural Engineering, University of Georgia, Driftmier Engineering Center, Athens, GA 30602-4435, USA c Alternative Fuels, Inc., 5335 Briarleigh Close, Atlanta, GA 30338, USA
Received 20 August 1999; received in revised form 10 July 2000; accepted 11 July 2000
Abstract Anaerobic batch tests were performed using hog and poultry wastes in various proportions. Treatments that received both wastes produced higher yields of biogas, up to 200 30 mL/g volatile solids (VS) destroyed, and methane, up to 130 20 mL/g VS destroyed, compared to either waste alone. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Animal waste; Anaerobic digestion; Biomethanation; Codigestion; Methanogenesis; Waste treatment
1. Introduction
2. Methods
The anaerobic digestion of manure entails the conversion of dissolved and particulate organic matter into methane and carbon dioxide via a series of interrelated microbial metabolisms (Hill, 1982). Due to the many complex interactions between the various constituent populations of the microbial consortium, a number of factors can upset the anaerobic digestion process. Excessive volatile fatty acids (VFA) accumulation can inhibit methanogenesis, while high hydrogen levels can inhibit propionate- and butyrate-degrading acetogens (Angelidaki et al., 1993). Manure also contains compounds, i.e., proteins and urea, which upon degradation release ammonia, a potent inhibitor of aceticlastic methanogens (Heinrichs et al., 1990). Ammonia toxicity has been reported in the digestion of cattle manure, which has approximately 2.5 g/L ammonia nitrogen (NH3 ±N) (Angelidaki and Ahring, 1994), and of hog and poultry manure, which contain more than 4 g/L NH3 ±N (Angelidaki and Ahring, 1993). This study was undertaken using euents from hog and layer operations with continuous or intermittent manure ¯ushing, to evaluate the feasibility of a service providing for the disposal of these relatively dilute wastes.
Hog waste was collected from the drainage sump of a continuously-¯ushed hog house at the University of Georgia (UGA, Athens, GA) Swine Research Facility. Layer manure was collected fresh at the UGA Poultry Research Facility and diluted with approximately three volumes of tap water to approximate the concentration obtained after washdown. Each waste was agitated vigorously and poured through a coarse (approximately 6 mm) plastic mesh to remove gross solids and stored at 4°C for up to 24 h until use. The screened hog waste contained 3380 120 mg/L soluble chemical oxygen demand (SCOD), 9750 mg/L total solids (TS), 9400 1000 mg/L volatile solids (VS), 6300 600 mg/L total suspended solids (TSS), 6100 600 mg/L volatile suspended solids (VSS), and 220 20 mg/L ammonia nitrogen (NH3 ±N). The screened poultry waste contained 7810100 mg/L SCOD, 17 400 mg/L TS, 14 600 1500 mg/L VS, 14 600 1500 mg/L TSS, 12 500 1300 mg/L VSS, and 1500 40 mg/L NH3 ±N. The carbon content and carbon:nitrogen (C:N) ratios were not measured, however a C:N ratio of 15 has been reported for poultry manure (Haug, 1993) and 10±16 for pig manure (van Fassen and van Dijk, 1987). The eect of using various ratios of hog to poultry waste on gas and methane production was evaluated using a factorial experiment. Five replicates of six treatments, containing 100 (H100), 80 (H80), 60 (H60), 40 (H40), 20 (H20), or 0 (H0) mL of hog waste, plus sucient poultry waste to make 100 mL, were prepared
* Corresponding author. Tel.: +1-662-325-3050; fax: +1-662-3257189.
0960-8524/01/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 0 8 7 - 0
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B.S. Magbanua Jr. et al. / Bioresource Technology 76 (2001) 165±168
in 125 mL serum bottles. The bottles were sealed using aluminum crimp closures with gray butyl rubber septa, and incubated at 35 2°C for up to 113 d. Biogas production was measured daily by water displacement. Headspace methane concentration was monitored in one series of bottles, while a second series was regularly sampled for NH3 ±N analysis and pH measurement. Periodically, single bottles from each treatment were sacri®ced for solids and SCOD analysis. Methane concentration was measured using a 5890 Series II Plus gas chromatograph (Hewlett-Packard, Palo Alto, CA) equipped with a thermal conductivity detector and a Hayesep A 120=140 400 1=800 0:08500 (12 m ´ 3 mm ´ 2 mm) stainless steel column (Alltech, Deer®eld, IL), with helium as the carrier gas. Total ammonia nitrogen (NH3 ±N) was quanti®ed based on the phenate method (Standard Methods Part 4500NH3 D, Greenberg et al., 1992) using a TRAACS 2000 analyzer (Braun + Luebbe, Norderstedt, Germany). SCOD was quanti®ed by the colororimetric dichromate closed re¯ux method (Standard Methods Part 5220D, Greenberg et al., 1992) using Hach (Loveland, CO) COD digestion vials. Samples for ammonia and SCOD analysis were acidi®ed to a pH of approximately 2 with
o-phosphoric acid, centrifuged at 12 000 rpm for 10 min, and the supernatant decanted. SCOD samples were purged with nitrogen for 2 min to remove residual dissolved methane (Noguera et al., 1994). Total (TS), volatile (VS), total suspended (TSS), and volatile suspended (VSS) solids were determined as described previously (Standard Methods Part 2540, Greenberg et al., 1992). The pH was estimated using pH paper (Micro Essential Laboratory, Brooklyn, NY), to within 0.5 pH units. Analysis of variance (ANOVA) was performed using the SAS software package (The SAS Institute, Cary, NC) to assess the eect of the ratio of hog waste to poultry waste on the dierent response variables. A value of p less than or equal to 0.05 was considered to indicate statistically signi®cant dierences. 3. Results and discussion VS (Fig. 1(a)) and VSS (Fig. 1(b)) decreased in all treatments. VS destruction (Fig. 2(a)) appeared comparable in all treatments except H100, where it was lower; VS (p 0:0077) and VSS (p 0:0003) destruction diered signi®cantly between treatments. SCOD
Fig. 1. Variation of (a) volatile solids concentration, (b) volatile suspended solids concentration, (c) soluble chemical oxygen demand, (d) cumulative gas volume, (e) cumulative methane volume, and (f) pH in each treatment during the anaerobic batch test. Error bars represent standard error on ®ve replicates.
B.S. Magbanua Jr. et al. / Bioresource Technology 76 (2001) 165±168
167
treatments (p 0:0012), but the yield of methane on VS did not (p > 0:05). The methane yields were below values typically reported for high-rate digester con®gurations, 300±660 mL/g VS for swine manure (Masse et al., 1997) and 115±390 mL/g VS for dairy manure (Sa¯ey and Westerman, 1992, 1994). The pH (Fig. 1(f)), initially 5:0 0:5 for the hog waste and 6:0 0:5 for the poultry waste, dropped over the ®rst 15±20 d to as low as 4:5 0:5, but eventually stabilized at 6:0 0:5 in H0 and at 7:0 0:5±7:5 0:5 in the remaining treatments. Hence the pH, which varied signi®cantly between treatments (p 0:0462), was in the range considered optimum for methanogenesis in all treatments except H0. Total NH3 ±N (Fig. 2(c)) within each treatment varied little over time, and ranged from 340 10 mg in H100 to 1660 80 mg/L in H0. Free ammonia was calculated using Hansen et al. (1998): NH3 10ÿpH 1 ; 10ÿ 0:09018 2729:92=273T NH3 NH 4
Fig. 2. (a) Fractional volatile solids destruction, (b) gas and methane yield, and (c) total and free ammonia concentration, plotted against the initial volatile solids fraction contributed by hog waste. Error bars represent standard error on ®ve replicates.
(Fig. 1(c)) initially increased as particulate organic matter was hydrolyzed and solubilized, but subsequently decreased in all treatments except H0 as the dissolved organic compounds were degraded. H100 exhibited the lowest initial gas production (Fig. 1(d)), but eventually attained 380 2 mL after 99 d. In contrast, gas production in H0 leveled o after approximately 10 d and totaled 260 10 mL after 99 d. Treatments combining the two wastes produced higher gas volumes, up to 1020 9 mL after 113 d (H40). Biogas yield (Fig. 2(b)) was highest for H80, 200 30 mL/g VS destroyed. The total gas production (p 0:001) and the yield of biogas on VS (p 0:0095) both varied signi®cantly between treatments. Methane production from H0 was extremely low (Fig. 1(e)), 14 1 mL after 99 d, while H100 produced 220 1 mL in the same period. Treatments that received both wastes produced the most methane, up to 700 6 mL after 113 d (H40). Methane yield was highest for H80, with a production of 130 20 mL/g VS destroyed. The total methane production diered signi®cantly between
where [NH3 ] is the free ammonia concentration, mol/L N, NH 4 the ammonium ion concentration, mol/L N, and T is the temperature, K. Calculated free NH3 ±N (Fig. 2(c)) ranged from 3:2 0:2 mg/L in H0 to 24:4 0:9 mg/L in H20. Both the total (p 0:0119) and free (p 0:0263) ammonia concentrations varied signi®cantly between treatments. Ammonia can inhibit anaerobic digestion, but the total ammonia concentration that can be tolerated is relatively high. Methanogens present in digested sewage sludge could tolerate up to 5 g/L NH3 ±N, and those in digested piggery manure from 0.6 to 3.1 g/L NH3 ±N (van Velsen, 1979). In the digestion of potato juice, methanogenesis occurred even at 11.8 g/L NH3 ±N but ceased at 16 g/L NH3 ±N, while VFA production was hardly aected by high ammonia concentrations (Koster and Lettinga, 1988). Ammonia inhibition in attached growth anaerobic reactors initially occurred at 8.1 g/L NH3 ±N, and 50% inhibition was observed at 9.4 mg/L NH3 ±N under shock load conditions and at 12.1 mg/L NH3 ±N with prior adaptation (De Baere et al., 1984). Anaerobic digesters exhibited ammonia inhibition at 3.0 g/L at any pH and at 1.5±3.0 g/L at pH 7.4 and above (McCarty, 1964). The pH dependence of ammonia inhibition is due to the equilibrium between free ammonia and the ammonium ion, where free ammonia is the inhibitory chemical species and aceticlastic methanogens are the trophic group most sensitive to it (Heinrichs et al., 1990). Anaerobic digestion was inhibited at a free ammonia concentration of 150 mg/L (McCarty and McKinney, 1961), while acetate uptake was reduced by 50% at 128 mg/L NH3 ±N (Heinrichs et al., 1990). The free NH3 ±N concentrations calculated in this study were far below those reported as inhibitory. Moreover, the wastes had undergone dilution, which reduces the volumetric ammonia loading and promotes stable digester
168
B.S. Magbanua Jr. et al. / Bioresource Technology 76 (2001) 165±168
operation (Webb and Hawkes, 1985; Pechan et al., 1987). The anaerobic digestibility of hog and poultry manure, despite their high NH3 ±N content, is well documented. Stable digester operation is possible provided the microbial consortium is suciently acclimated (Angelidaki and Ahring, 1993). With hog manure, although inhibition was observed at 1.1 g/L free NH3 ±N, digestion continued even at 6 g/L free NH3 ±N, albeit at a greatly reduced methane yield (Hansen et al., 1998). With poultry manure, up to 2.6 g/L total NH3 ±N had no eect on biogas or methane production, but reductions of 50±60% in biogas production and 80±90% in methane production were observed with 2.6±7.9 g/L total NH3 ±N (Krylova et al., 1997). The superior biogas and methane yields of treatments that combined hog and poultry waste demonstrates that codigestion of these wastes is viable. The apparently complementary nature of the two wastes, however, may be an artifact of the absence of an added inoculum. The hog waste was collected from a drainage sump and may have harbored a signi®cant quantity of methanogenic bacteria, while the poultry manure was collected from the ground after exposure to the air and may have had a low population of obligate anaerobes. At the same time, the hog waste had lower levels of solids, SCOD, and ammonia than the poultry waste. Hence, in the mixed waste treatments, the hog waste could have supplied methanogens, while the poultry waste may have provided additional substrate. Additional factors that may have inhibited methanogenic activity in H0 are the low pH, and/or the excessive VFA accumulation that this may have indicated. Acknowledgements Mark Eiteman, Matt Smith, and K.C. Das provided equipment and supplies used in this project, while Barbara Hanel and Jennifer Chandler assisted in the experimental and analytical work; their respective contributions are gratefully acknowledged. References Angelidaki, I., Ahring, B.K., 1993. Thermophilic anaerobic digestion of livestock waste: the eect of ammonia. Appl. Microbiol. Biotechnol. 38, 560±564.
Angelidaki, I., Ahring, B.K., 1994. Anaerobic thermophilic digestion of manure at dierent ammonia loads: eect of temperature. Water Res. 28, 727±731. Angelidaki, I., Ellegaard, L., Ahring, B.K., 1993. A mathematical model for dynamic simulation of anaerobic digestion of complex substrates: focusing on ammonia inhibition. Biotechnol. Bioengrg. 42, 159±166. De Baere, L.A., Devocht, M., Van Assche, P., Verstraete, W., 1984. In¯uence of high NaCl and high NH4 Cl salt levels on methanogenic associations. Water Res. 18, 543±548. Greenberg, A.E., Clesceri, L.S., Eaton, A.D., 1992. Standard Methods for the Examination of Water and Wastewater, 18th ed. American Public Health Association, Washington. Hansen, K.H., Angelidaki, I., Ahring, B.K., 1998. Anaerobic digestion of swine manure: inhibition by ammonia. Water Res. 32, 5±12. Haug, R.T., 1993. The Practical Handbook of Compost Engineering. Lewis, Boca Raton, FL. Heinrichs, D.M., Poggi-Varaldo, H.M., Olieskewicz, J.A., 1990. Eects of ammonia on anaerobic digestion of simple organic substrates. J. Environ. Engrg. 116, 698±710. Hill, D.T., 1982. A comprehensive dynamic model of animal waste methanogenesis. Trans. ASAE 25, 1374±1380. Koster, I.W., Lettinga, G., 1988. Anaerobic digestion at extreme ammonia concentrations. Biol. Wastes 25, 51±59. Krylova, N.I., Khabiboulline, R.E., Naumova, R.P., Nagel, M.A., 1997. The in¯uence of ammonia and methods for removal during the anaerobic treatment of poultry manure. J. Chem. Tech. Biotechnol. 70, 99±105. Masse, D.I., Droste, R.L., Kennedy, K.J., Patni, N.K., Munroe, J.A., 1997. Potential for the psychrophillic anaerobic treatment of swine manure using a sequencing batch reactor. Can. Agric. Engrg. 39, 25±33. McCarty, P.L., 1964. Anaerobic waste treatment fundamentals III. Publ. Wks. 95, 91±94. McCarty, P.L., McKinney, R.E., 1961. Salt toxicity in anaerobic digestion. J. Water Pollut. Contr. Fed. 33, 399±415. Noguera, D.R., Araki, N., Rittmann, B.E., 1994. Soluble microbial products (SMP) in anaerobic chemostats. Biotechnol. Bioengrg. 44, 1040±1047. Pechan, Z., Knappova, O., Petrovicova, B., Adamec, O., 1987. Anaerobic digestion of poultry manure at high ammonia nitrogen concentrations. Biol. Wastes 20, 117±131. Sa¯ey, L.M., Westerman, P.W., 1992. Performance of a low temperature lagoon digester. Bioresour. Technol. 41, 167±175. Sa¯ey, L.M., Westerman, P.W., 1994. Low-temperature digestion of dairy and swine manure. Bioresour. Technol. 47, 165±171. van Fassen, H.G., van Dijk, H., 1987. Manure as a source of nitrogen and phosphorus in soils. In: van der Meer H.G., Unwin, R.J., van Dijk T.A., Ennik G.C. (Eds.), Animal Manure on Grassland and Fodder Crops: Fertilizer or Waste? Martinus Nijho, Dordrecht, pp. 27±45. van Velsen, A.F.M., 1979. Adaptation of methanogenic sludge to high ammonia±nitrogen concentrations. Water Res. 13, 995±999. Webb, A.R., Hawkes, F.R., 1985. The anaerobic digestion of poultry manure: variation of gas yield with in¯uent concentration and ammonia nitrogen levels. Agric. Wastes 14, 135±156.
Short communication
Anaerobic codigestion of hog and poultry waste Benjamin S. Magbanua Jr. a,*, Thomas T. Adams b, Phillip Johnston c b
a Department of Civil Engineering, Mississippi State University, Mississippi State, MS 39762-9546, USA Department of Biological & Agricultural Engineering, University of Georgia, Driftmier Engineering Center, Athens, GA 30602-4435, USA c Alternative Fuels, Inc., 5335 Briarleigh Close, Atlanta, GA 30338, USA
Received 20 August 1999; received in revised form 10 July 2000; accepted 11 July 2000
Abstract Anaerobic batch tests were performed using hog and poultry wastes in various proportions. Treatments that received both wastes produced higher yields of biogas, up to 200 30 mL/g volatile solids (VS) destroyed, and methane, up to 130 20 mL/g VS destroyed, compared to either waste alone. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Animal waste; Anaerobic digestion; Biomethanation; Codigestion; Methanogenesis; Waste treatment
1. Introduction
2. Methods
The anaerobic digestion of manure entails the conversion of dissolved and particulate organic matter into methane and carbon dioxide via a series of interrelated microbial metabolisms (Hill, 1982). Due to the many complex interactions between the various constituent populations of the microbial consortium, a number of factors can upset the anaerobic digestion process. Excessive volatile fatty acids (VFA) accumulation can inhibit methanogenesis, while high hydrogen levels can inhibit propionate- and butyrate-degrading acetogens (Angelidaki et al., 1993). Manure also contains compounds, i.e., proteins and urea, which upon degradation release ammonia, a potent inhibitor of aceticlastic methanogens (Heinrichs et al., 1990). Ammonia toxicity has been reported in the digestion of cattle manure, which has approximately 2.5 g/L ammonia nitrogen (NH3 ±N) (Angelidaki and Ahring, 1994), and of hog and poultry manure, which contain more than 4 g/L NH3 ±N (Angelidaki and Ahring, 1993). This study was undertaken using euents from hog and layer operations with continuous or intermittent manure ¯ushing, to evaluate the feasibility of a service providing for the disposal of these relatively dilute wastes.
Hog waste was collected from the drainage sump of a continuously-¯ushed hog house at the University of Georgia (UGA, Athens, GA) Swine Research Facility. Layer manure was collected fresh at the UGA Poultry Research Facility and diluted with approximately three volumes of tap water to approximate the concentration obtained after washdown. Each waste was agitated vigorously and poured through a coarse (approximately 6 mm) plastic mesh to remove gross solids and stored at 4°C for up to 24 h until use. The screened hog waste contained 3380 120 mg/L soluble chemical oxygen demand (SCOD), 9750 mg/L total solids (TS), 9400 1000 mg/L volatile solids (VS), 6300 600 mg/L total suspended solids (TSS), 6100 600 mg/L volatile suspended solids (VSS), and 220 20 mg/L ammonia nitrogen (NH3 ±N). The screened poultry waste contained 7810100 mg/L SCOD, 17 400 mg/L TS, 14 600 1500 mg/L VS, 14 600 1500 mg/L TSS, 12 500 1300 mg/L VSS, and 1500 40 mg/L NH3 ±N. The carbon content and carbon:nitrogen (C:N) ratios were not measured, however a C:N ratio of 15 has been reported for poultry manure (Haug, 1993) and 10±16 for pig manure (van Fassen and van Dijk, 1987). The eect of using various ratios of hog to poultry waste on gas and methane production was evaluated using a factorial experiment. Five replicates of six treatments, containing 100 (H100), 80 (H80), 60 (H60), 40 (H40), 20 (H20), or 0 (H0) mL of hog waste, plus sucient poultry waste to make 100 mL, were prepared
* Corresponding author. Tel.: +1-662-325-3050; fax: +1-662-3257189.
0960-8524/01/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 0 8 7 - 0
166
B.S. Magbanua Jr. et al. / Bioresource Technology 76 (2001) 165±168
in 125 mL serum bottles. The bottles were sealed using aluminum crimp closures with gray butyl rubber septa, and incubated at 35 2°C for up to 113 d. Biogas production was measured daily by water displacement. Headspace methane concentration was monitored in one series of bottles, while a second series was regularly sampled for NH3 ±N analysis and pH measurement. Periodically, single bottles from each treatment were sacri®ced for solids and SCOD analysis. Methane concentration was measured using a 5890 Series II Plus gas chromatograph (Hewlett-Packard, Palo Alto, CA) equipped with a thermal conductivity detector and a Hayesep A 120=140 400 1=800 0:08500 (12 m ´ 3 mm ´ 2 mm) stainless steel column (Alltech, Deer®eld, IL), with helium as the carrier gas. Total ammonia nitrogen (NH3 ±N) was quanti®ed based on the phenate method (Standard Methods Part 4500NH3 D, Greenberg et al., 1992) using a TRAACS 2000 analyzer (Braun + Luebbe, Norderstedt, Germany). SCOD was quanti®ed by the colororimetric dichromate closed re¯ux method (Standard Methods Part 5220D, Greenberg et al., 1992) using Hach (Loveland, CO) COD digestion vials. Samples for ammonia and SCOD analysis were acidi®ed to a pH of approximately 2 with
o-phosphoric acid, centrifuged at 12 000 rpm for 10 min, and the supernatant decanted. SCOD samples were purged with nitrogen for 2 min to remove residual dissolved methane (Noguera et al., 1994). Total (TS), volatile (VS), total suspended (TSS), and volatile suspended (VSS) solids were determined as described previously (Standard Methods Part 2540, Greenberg et al., 1992). The pH was estimated using pH paper (Micro Essential Laboratory, Brooklyn, NY), to within 0.5 pH units. Analysis of variance (ANOVA) was performed using the SAS software package (The SAS Institute, Cary, NC) to assess the eect of the ratio of hog waste to poultry waste on the dierent response variables. A value of p less than or equal to 0.05 was considered to indicate statistically signi®cant dierences. 3. Results and discussion VS (Fig. 1(a)) and VSS (Fig. 1(b)) decreased in all treatments. VS destruction (Fig. 2(a)) appeared comparable in all treatments except H100, where it was lower; VS (p 0:0077) and VSS (p 0:0003) destruction diered signi®cantly between treatments. SCOD
Fig. 1. Variation of (a) volatile solids concentration, (b) volatile suspended solids concentration, (c) soluble chemical oxygen demand, (d) cumulative gas volume, (e) cumulative methane volume, and (f) pH in each treatment during the anaerobic batch test. Error bars represent standard error on ®ve replicates.
B.S. Magbanua Jr. et al. / Bioresource Technology 76 (2001) 165±168
167
treatments (p 0:0012), but the yield of methane on VS did not (p > 0:05). The methane yields were below values typically reported for high-rate digester con®gurations, 300±660 mL/g VS for swine manure (Masse et al., 1997) and 115±390 mL/g VS for dairy manure (Sa¯ey and Westerman, 1992, 1994). The pH (Fig. 1(f)), initially 5:0 0:5 for the hog waste and 6:0 0:5 for the poultry waste, dropped over the ®rst 15±20 d to as low as 4:5 0:5, but eventually stabilized at 6:0 0:5 in H0 and at 7:0 0:5±7:5 0:5 in the remaining treatments. Hence the pH, which varied signi®cantly between treatments (p 0:0462), was in the range considered optimum for methanogenesis in all treatments except H0. Total NH3 ±N (Fig. 2(c)) within each treatment varied little over time, and ranged from 340 10 mg in H100 to 1660 80 mg/L in H0. Free ammonia was calculated using Hansen et al. (1998): NH3 10ÿpH 1 ; 10ÿ 0:09018 2729:92=273T NH3 NH 4
Fig. 2. (a) Fractional volatile solids destruction, (b) gas and methane yield, and (c) total and free ammonia concentration, plotted against the initial volatile solids fraction contributed by hog waste. Error bars represent standard error on ®ve replicates.
(Fig. 1(c)) initially increased as particulate organic matter was hydrolyzed and solubilized, but subsequently decreased in all treatments except H0 as the dissolved organic compounds were degraded. H100 exhibited the lowest initial gas production (Fig. 1(d)), but eventually attained 380 2 mL after 99 d. In contrast, gas production in H0 leveled o after approximately 10 d and totaled 260 10 mL after 99 d. Treatments combining the two wastes produced higher gas volumes, up to 1020 9 mL after 113 d (H40). Biogas yield (Fig. 2(b)) was highest for H80, 200 30 mL/g VS destroyed. The total gas production (p 0:001) and the yield of biogas on VS (p 0:0095) both varied signi®cantly between treatments. Methane production from H0 was extremely low (Fig. 1(e)), 14 1 mL after 99 d, while H100 produced 220 1 mL in the same period. Treatments that received both wastes produced the most methane, up to 700 6 mL after 113 d (H40). Methane yield was highest for H80, with a production of 130 20 mL/g VS destroyed. The total methane production diered signi®cantly between
where [NH3 ] is the free ammonia concentration, mol/L N, NH 4 the ammonium ion concentration, mol/L N, and T is the temperature, K. Calculated free NH3 ±N (Fig. 2(c)) ranged from 3:2 0:2 mg/L in H0 to 24:4 0:9 mg/L in H20. Both the total (p 0:0119) and free (p 0:0263) ammonia concentrations varied signi®cantly between treatments. Ammonia can inhibit anaerobic digestion, but the total ammonia concentration that can be tolerated is relatively high. Methanogens present in digested sewage sludge could tolerate up to 5 g/L NH3 ±N, and those in digested piggery manure from 0.6 to 3.1 g/L NH3 ±N (van Velsen, 1979). In the digestion of potato juice, methanogenesis occurred even at 11.8 g/L NH3 ±N but ceased at 16 g/L NH3 ±N, while VFA production was hardly aected by high ammonia concentrations (Koster and Lettinga, 1988). Ammonia inhibition in attached growth anaerobic reactors initially occurred at 8.1 g/L NH3 ±N, and 50% inhibition was observed at 9.4 mg/L NH3 ±N under shock load conditions and at 12.1 mg/L NH3 ±N with prior adaptation (De Baere et al., 1984). Anaerobic digesters exhibited ammonia inhibition at 3.0 g/L at any pH and at 1.5±3.0 g/L at pH 7.4 and above (McCarty, 1964). The pH dependence of ammonia inhibition is due to the equilibrium between free ammonia and the ammonium ion, where free ammonia is the inhibitory chemical species and aceticlastic methanogens are the trophic group most sensitive to it (Heinrichs et al., 1990). Anaerobic digestion was inhibited at a free ammonia concentration of 150 mg/L (McCarty and McKinney, 1961), while acetate uptake was reduced by 50% at 128 mg/L NH3 ±N (Heinrichs et al., 1990). The free NH3 ±N concentrations calculated in this study were far below those reported as inhibitory. Moreover, the wastes had undergone dilution, which reduces the volumetric ammonia loading and promotes stable digester
168
B.S. Magbanua Jr. et al. / Bioresource Technology 76 (2001) 165±168
operation (Webb and Hawkes, 1985; Pechan et al., 1987). The anaerobic digestibility of hog and poultry manure, despite their high NH3 ±N content, is well documented. Stable digester operation is possible provided the microbial consortium is suciently acclimated (Angelidaki and Ahring, 1993). With hog manure, although inhibition was observed at 1.1 g/L free NH3 ±N, digestion continued even at 6 g/L free NH3 ±N, albeit at a greatly reduced methane yield (Hansen et al., 1998). With poultry manure, up to 2.6 g/L total NH3 ±N had no eect on biogas or methane production, but reductions of 50±60% in biogas production and 80±90% in methane production were observed with 2.6±7.9 g/L total NH3 ±N (Krylova et al., 1997). The superior biogas and methane yields of treatments that combined hog and poultry waste demonstrates that codigestion of these wastes is viable. The apparently complementary nature of the two wastes, however, may be an artifact of the absence of an added inoculum. The hog waste was collected from a drainage sump and may have harbored a signi®cant quantity of methanogenic bacteria, while the poultry manure was collected from the ground after exposure to the air and may have had a low population of obligate anaerobes. At the same time, the hog waste had lower levels of solids, SCOD, and ammonia than the poultry waste. Hence, in the mixed waste treatments, the hog waste could have supplied methanogens, while the poultry waste may have provided additional substrate. Additional factors that may have inhibited methanogenic activity in H0 are the low pH, and/or the excessive VFA accumulation that this may have indicated. Acknowledgements Mark Eiteman, Matt Smith, and K.C. Das provided equipment and supplies used in this project, while Barbara Hanel and Jennifer Chandler assisted in the experimental and analytical work; their respective contributions are gratefully acknowledged. References Angelidaki, I., Ahring, B.K., 1993. Thermophilic anaerobic digestion of livestock waste: the eect of ammonia. Appl. Microbiol. Biotechnol. 38, 560±564.
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