Characterization And Ammonia Inhibition Of Anaerobic Digestion Treating Swine Manure At High Solids

China has a big share in the swine-breeding market,leading to the severe challenge that how to reasonably deal with the large amount of swine wastes.Anaerobic digestion(AD)is a widely applicated technology for the treatment and comprehensive utilization of livestock and poultry wastes.However,the conventional AD is normally conducted at a low total solid(TS)content between 1%and 5%,resulting in larger reactor with increased operating cost and too much residual biogas slurry with difficulty in local farmland utilization.Adjusting the initial TS content larger than 8%can decrease the biogas slurry immensely as well as save the space of anaerobic reactor that are drawing more and more attention.However,manure is a kind of nitrogen-rich substrate and may lead to ammonia inhibition during its treatment by high solid anaerobic digestion(HSAD).The inhibition will cause the decrease in biogas yield,which has been a bottleneck in the promotion of HSAD application.In this study,swine manure was chosen as the substrate and the mesophilic anaerobic digestion was conducted to investigate and compare the characteristic of AD process at different initial TS contents.Based on the condition of AD at low TS,further investigations were carried out with external ammonia or volatile fatty acids(VFAs)addition to reveal the formation mechanism of ammonia inhibition during the HSAD process.Finally,the effect of zero-valent iron addition(ZVI)addition on the relief of ammonia 'inhibited steady state' was investigated during the HSAD treatment of swine manure.The main results are as follows:1.The characteristic of ammonia release and the effect of ammonia on methane production were clarified during the HSAD treatment of swine manure.The main profiles of ternary pH buffer system were verified as ammonia-VFAs-carbonates,and their characteristic of evolution was also identified.The ammonia release met the regulation of first-order kinetic model with decreased release rate constant when the TS content increased.The rapid ammonia release promoted the concentration of total ammonia nitrogen(TAN)in the HSAD ranging from 3.5 to 6.6 g-N/L,leading to the methanogenesis stagnation with serious VFAs accumulation(21.4 to 32.5 g/L).The ternary pH buffer system with evolutive ammonia-VFAs-carbonates components maintained the pH fluctuation in the suitable range for stabilizing the performance of HSAD,and the buffer mechanism can be revealed by a simulated ternary contour.Ammonia has been an indispensable profile of the pH buffer system more than a potential inhibitor during the HSAD process.2.The characteristics of microbial community evolution were revealed to investigate the reason for the decrease of methane production in the HSAD at increased TS content.The bio-chemical methane potential of swine manure in the AD at 4%,8%,11%and 14%TS content was 271.0±0.5,303.1±23.0,266.3±24.5 and 207.0 ±14.9 NmL/g-VS,respectively.Meanwhile,the corresponding lag phase of methanogenesis was 2.8,55.0,61.9 and 42.0 days,respectively.The microbial community during the whole AD process was dominated by Firmicutes,Bacteroidetes and Proteobacteria.The archaeal community was dominant by Methanosphaera and Methanosaeta at the beginning of AD process for all groups,while dominant by Methanosarcina(44.4%)and Methanosaeta(18.2%)at 4%TS content,Methanoculleus(51.5%)and Methanosarcina(28.3%)at 8%TS content,Methanoculleus(87.7%)at 11%TS content and Methanoculleus(97.2%)at 14%TS content,respectively,after the AD treatment.According to the variation of relative abundance for the relevant microbes,the high concentration of free ammonia(FAN)probably not only induced the shifting of main methanogenesis from the aceticlastic pathway to the hydrogenotrophic pathway,but also depressed the propionate-oxidizing bacteria and acetate-oxidizing bacteria,which frustrated their syntrophic cooperation with hydrogenotrophic methanogens and thus the decrease of methane potential in HSAD at 11%and 14%TS content.3.The synergetic inhibition of ammonia and VFAs on methanogenesis was clarified during the HSAD treatment of swine manure.Based on the condition of AD at 4%TS content above investigated,the effect of ammonia,VFAs or ammonia&VFAs with different concentrations on the methane production was investigated.Results indicated that the synergetic effect of ammonia and VFAs was notable when compared with that of single ammonia or single VFAs.When the concentration of TAN was regulated to about 1.5(control group),3.5,5.5 and 6.0 g-N/L,the lag phase of methanogenesis in experimental group was 0.2,8.7 and 33.7 days and the corresponding methane potential of swine manure was decreased by 13.8%,36.4%and 55.2%when compared with the control group,respectively.The main reason for the decrease might be the propionate residual.When the concentration of VFAs was regulated to about 2.6(control group),21.0,28.0 and 32.0 g/L,the lag phase of methanogenesis in experimental group was 6.7,10.4 and 20.4 days which increased with the VFAs concentration.Despite of this,the cumulative methane production from both swine and added VFAs was not depressed according to the result of control experiment and calculation analysis.When the concentrations of TAN and VFAs were both regulated to the same level as above-mentioned,the lag phase of methanogenesis was 19.2,47.7 and 51.5 days,much longer than that with either single ammonia or single VFAs addition.This indicated that the synergetic inhibition of ammonia and VFAs emerged.Furthermore,the cumulative methane production was decreased by 3.5%,21.6%and 44.0%,respectively,when compared with the corresponding concentration of single VFAs addition.The characteristic of the co-inhibition on methane production and the cause for it were both similar to those in the HSAD.4.The evolution of methanogenic community differs under the different modes of inhibition as well as the increasing concentration of inhibitors.The methanogens in the group with single ammonia addition was dominant by Methanosaeta when TAN was lower than 3.5 g-N/L while dominant by Methanosarcina when TAN was higher than 5.5 g-N/L.The methanogens in the group with single VFAs addition was dominant by Methanosarcina and then replaced by Methanoculleus with the increasing VFAs concentration.In the group with low addition amount of TAN and VFAs,Methanosarcina dominated at the first half of AD and then replaced by Methanoculleus.While in the group with high addition amount of TAN and VFAs,Methanosaeta dominated in the prior period of AD and then Methanoculleus dominated until the end of AD.The dominance of Methanoculleus was closely related with the increasing level of FAN.5.The resolutive effect of zero-valent iron(ZVI)addition on the ammonia 'inhibited steady state' was clarified during the HSAD of swine manure.In the HSAD at 10%initial TS content,the concentration of TAN increased to 4.5 g-N/L in three days since the HSAD started,leading to the ammonia 'inhibited steady state'.ZVI accelerated the rate of methane production and propionate degradation via reinforcing the direct interspecies electron transfer(DIET)between the syntrophic bacteria and the methanogenic archaea,promoting the efficiency of methane production during the HSAD treating nitrogen-rich swine wastes.The 'inhibited steady state' was relieved on day 12 when the dosage of ZVI was increased to 160 mM,while until day 28 in the control group.The optimal dosage of ZVI was 320 mM which increased the cumulative methane production by about 21.2%and shortened the technical digestion time by about 51.9%when compared with those in the control group.The relative abundance of Methanosarcina increased from 25.5%at its initial stage to 78.3%after AD in the group with optimal ZVI dosage(320mM),while only increased to 41.2%in the control group.Meanwhile,the addition of ZVI(320mM)enhanced the relative abundance of Syntrophomonas,belonging to hydrogen-producing acetogenic bacteria,from 0.5%to 3.7%,while just to 1.7%in the control group.