| With the promotion and popularization of modern standardized livestock and poultry farms in scale,standardized disposal of excrement has been one of the unavoidable problems for sustainable development of livestock and poultry house.In resent years,fermentation for biogas production has received an universal attention for its ability of degradation of organic waste and renewable biogas production for reduction of the pressure on energy demand.However,ammonia inhibition appeared within fermentation of chicken manure(CM)can reduce biogas yield,which in turn,limits the promotion of this technology to CM.Ammonia inhibition is universal in fermentation of substrates which are rich in organic nitrogen.Therefore,CM was chosed as the research object in this study.In mesothermal condition,batch and continuous fermentation was tested with variables of initial total solids(TS)concentrations and organic loadings.Laws of generation and accumulation of free ammonia,and the influence of free ammonia on methane production were studied for the feasibility of high-load monofermenation of CM.Modified Gompertz equation and Cone equation were used to analyze kinetics of substrate degradation.Finally,technical decompressive stripping was performed to remove ammonia from the liquid fraction of digestate,and the entire product was recycled to the fermenter to control total ammonia nitrogen(TAN)concentration in the fermenter.Water consumption was reduced and zero release of biogas slurry was realized.At the same time,long-term and continuously operated monofermentation of CM was monitored with organic loading rates(OLR)between 5.3~6.0 gVS/(L d)and concentration of TAN between 4.00~8.00 g/L.The main conclusions are as follows:(1)In fermenters with volume of 15 L,fermentation of CM was steadily operated without ammonia inhibition when initial TS was 15% and OLR was 4.0 gVS/(L d).At that period,concentration of free ammonia nitrogen(FAN)of digestate was 0.17 g/L,and methane yield was 82% of the theoretical yield which was obtained from the Biochemical Methane Potential test.The ability of hydrolysis and acidification of this system was better than that of the system with initial TS of 10%.(2)Volume of fermenters was 1 L in batch dry fermentation of CM.Cumulative methane yield decreased with the increase of initial concentration of TS(those were 20%,25%,30% and 35%).Ammonia inhibition was obversed in fermentation with initial TS of 20%.After fermentation lasted for 40 days,cumulative methane yield was 54% of the theoretical yield,concentration of FAN of digestate was 0.66 g/L and concentration of volatile fatty acid(VFA)was 5.85 g/L.In order to distinguish inhibition caused by free ammonia and mass-transfer,fermentation of chicken manure with initial TS of 20% was operated in 15-L fermenters.Stirring operation was applied at different time stages.Results found that gas production increased obviously since strring operation for the fermenter which strring began from day 8,while serious foaming was found in the fermenter 2 days later.Cumulative methane yield was 65% of the theoretical yield after 29 days’ fermentation,concentration of FAN of digestate was 0.76 g/L and VFA was 7.94 g/L.For the fermenter which strring began since day 24,cumulative methane yield was 70% of the theoretical yield,FAN of digesate was 0.60 g/L and VFA was 4.93 g/L.(3)Digestate discharged from the fermenter was daily collected,and ammmonia was removed from liquid fraction of digestate by decompressive stripping.Concentration of TAN of the reflux was technically controlled by adjusting operating parameters of the stripping and then concentration of TAN of fermentation liquor was steadily controlled within certain concentration without the addition of chemical agents.In a 15-L fermenter,feedstock with initial TS of 15% was fed in.OLR of 5.3 gVS/(L?d)was achieved with an average FAN concentration of 0.77 g/L and a biogas yield of 0.39 L/gVS.When OLR was increased to 6.0 gVS/(L?d),stable operation could be obtained with an average FAN concentration of 0.86 g/L and a biogas yield of 0.27 L/gVS.Results show that stable operation of high-load monofermentation of CM in slightly inhibited state is possible.Controlled recirculation of ammonia-depleted liquid fraction of digestate could be a solution for conquering both ammonia inhibition and managing digestate in large-scale applications.Thresholds for inhibition onset seem to be in accordance with other studies around 0.60 g/L(expressed as concentration of FAN). |
