Effect of gradually raising feeding total solid on methane production during continuous mesophilic fermentation of chicken manures

Abstract: Along with the rapid development of large-scale and intensive poultry breeding industry in China, the output of chicken manure shows a rapid increasing trend. And a total of 130 million tons was produced in 2010. This huge number of chicken manure will cause serious harms to the environment and human beings if no effective treatment. Due to the highly degradable organic matter in the chicken manure, methane fermentation is considered as one of the best method to minimize waste and recover energy. Organic nitrogen and urate in chicken manure are high, which leads to a toxic and inhibitory effect on anaerobic microbial activity, and it failures to produce more methane from chicken with high solids. Some studies had confirmed that long-term domestication of ammonia nitrogen can improve the ability of microorganisms to adapt ammonia nitrogen. For this purpose, a long-term experiment of chicken manure continuous fermentation under the mesophilic condition was carried out to explore its fermentation characteristics and evaluate the stability of the process. A 12 L continuous stirring tank reactor (CSTR) was operated for 330 days. Performance included volume methane production, methane yield, ammonia, volatilized fatty acids（VFAs）and pH value were investigated under different feeding concentration (5%, 7.5%, 10% and 15% based on total solids(TS)) and organic load rate (2.5, 3.75, 5.0 and 7.5 g/(L·d)). Sludge special methane production activity was evaluated by batch test with sodium acetate as substrates. During the 330 days operation, feeding concentration increased from 5% to 7.5%, 10% and 15%, ammonia and volatile fatty acids (VFAs) were increased from (2 110±370) to (6 890±220) mg/L and from (360±100) to (8 800±500) mg/L, when methane yield dropped from (253±9) to (173±22) mL/g. A high ammonia concentration of (6 510±300) mg/L was achieved with TS 10%. When TS changed to 10%, the methane production decreased to (203±13) mg/L, 20% lower than that with TS 5%. The biogas production would be decreased when the feeding TS was greater than 10%. The effect of ammonia accumulation on biogas production rate and VFAs accumulation was analyzed using the method of linear cumulative effect. The initial ammonia concentration of 5 500 mg/L will inhibit the mesophilic digestion of chicken manure. Ammonia accumulation resulted in the decrease of the ability for using acetic acid to produce methane. Compared with that under 2 500 mg/L ammonia concentration, the sludge methane production activity was decreased with a reduction efficiency of 59% and 98% when the ammonia concentration was increase to 6 500 and 7 000 mg/L, respectively. Chicken manure mesophilic methane fermentation temperature can stable operation under feeding TS of 5% and 7.5%, when the feeding TS reached 10%, hydrolysis and acidification and methane-producing process will be suppressed, which reduced the conversion rate of organic matter. This study suggests that feeding TS of chicken manure mesophilic methane fermentation is not more than 10%.