Study Of Endogenous Inhibition Effects On Solid-State Anaerobic Digestion From Food Wastes

Generation of food wastes has been increasing intensively as results of acceleratingurbanisation and improving level of consumption. This leads to wide attentions of efficienttreatment on food wastes. Traditional treatment methods, which are only focus on theamounts reduction, have many disadvantages, such as the ignorance of resource and energyutilization, the risk of secondary pollution and biological safety. Anaerobic digestion is goingto be an increasingly discussion because of its low disposal cost, little risk of secondarypollution, reliable biological safety and efficient utilization as resource and energy.Solid-state anaerobic digestion is an optional method on food wastes treatment, whichhas high treatment efficiency, low discharge and high resource utilization. However, theincomplete mixing of reaction system and low separation rate of discharge would lead to localaccumulation of metabolites intensively. Volatile fatty acid (VFA) and ammonia inhibitionwill be more intensive during this process, which hydrolyze from food wastes. This study wasbased on the starting up performance of solid state anaerobic digestion reactor. Twoendogenous inhibition factors were found on anaerobic digestion. After that, twoconcentration gradient experiments were set to analyze the inhibition effects on change ofanaerobic digestion pathways via the trends of anaerobic digestion efficiency, concentrationsof main metabolites, and active substances. Furthermore, the microbial community structurewas investigated using PCR-DGGE to explain the inhibition effect on microbial activity inanaerobic digestion system. Finally, a strategy was presented to mitigate the VFA inhibitionand improve the anaerobic digestion efficiency though ammonia control. The main results aredescribed as follows:(1) It is indicated that VFA and ammonia were the two main endogenous inhibitionfactors, which was concluded by the starting up of solid-state anaerobic digestion reactor.VFA could be accumulated easily during the performance of solid state anaerobicdigestion reactor. The concentration of VFA was3,000–4,000mg·L-1under the organic loadof8,000mg TS·L-1·d-1. And anaerobic digestion activity was inhibited, which the methaneyield decreased to40%–50%. When the organic loading raised up to12,000mg TS·L-1·d-1,VFA concentration increased by7,000mg·L-1and anaerobic digestion activity was severelyinhibited, which methane production decreased to about70%. Ammonia accumulationincreased slowly, and the final concentration was only about500mg·L-1after stoppingfeeding. Ammonia will inhibit the anaerobic digestion process via food wastes when thereactor was operated with a long time.(2) The mechanism of endogenous inhibition effects on solid-state anaerobic digestionvia food wastes was analzyzed from the aspect of metabolites, bioactivator and distinctivecoenzymes.There were significant changes of anaerobic digestion activity among different organicloads. The maximum methane production reached up to328.3ml·g-1TS under optimalorganic load. Excessive organic load led to intensive accumulation of VFA, which causedVFA inhibition on reaction system. And acetic acid was found to be the main component of VFA. Polysaccharide/protein ratio of extracellular polymeric substances (EPS) changedsignificantly with the increase of organic loading, which was mainly caused by the change ofprotein content. The obvious amplitude of variation on protein content, which was about50%,was probably a result of response to high acetic acid concentration inhibition. There weredifferent trends among three EPS types. Compared with the stability of tightly bound type(TB-EPS) content, soluble microbial products (SMP) and lossely bound type (LB-EPS)changed significantly with the increase of organic loading, which was probably the result ofintensified VFA inhibition. The aceticlastic methanogenesis was more sensitive to VFAinhibition than the hydrogenotrophic type. The maximum concentration of acetyl-CoA underdifferent organic loads, which was increased by88.9%, was more intensive than that of thecoenzyme F420, which was improved only by65.4%. It indicated that the intensive VFAinhibition on aceticlastic methanogenesis was the key factor of decrease of anaerobicdigestion activity.The decrease of maximum biogas accumulation rate and delay of biogas prduction werethe main results of ammonia inhibition on food wastes anaerobic digestion. Compared withcontrol, the maximum biogas production rates (BPR) of two biogas production phases undermaximum ammonia concentration were44.8%and45%. And the timepoints when reached tothe BPR6h and42h delayed, respectively. Biogas accumulation declined significantly whenammonia concentration of system was6000mg·L-1, which decreased by14.8%of control.The IC50ammonia inhibition of food waste digestion was7,860mg·L-1. It was speculatedfrom the fact that the increase of ammonia concentration in the reaction system led to thelowering of total organic carbon removal. The increase of ammonia concentration led to theexcessive accumulation of volatile fatty acids, which indicated that methanogenesis was moresensitive to the ammonia inhibition than hydrolysis and acidification. The production of EPSdecreased with the increase of ammonia concentration, which was from27.3%increment ofgroup C (0mg·L-1) to–4.5%of group N4(6,000mg·L-1). The intensive decline of SMP andLB-EPS indicated that the inhibition enhanced by the increase of ammonia concentration.Aceticlastic methanogenesis was inhibited by ammonia more intensively thanhydrogenotrophic methanogenesis, the change of acetyl-CoA concentration was higher thancoenzyme F420, which were35.4%and13.5%, respectively.(3) The changes of structure and fuction on main microbial communities, which wereeffected by endogenous inhibition factors, were studied from the aspect of molecular ecology.The replacement of dominant populations was the main result of VFA inhibition effecton the two microbial community structures. Compared with the archea community, thebiological diversity index (Shannon index) and species richness index (Margalef index) werehigher in bacteria community, which was1.1and142, respectively. This could keep astability of structure though change of dominant species, which would resist impact fromVFA inhibition effectively. Shannon index and Margalef index of archea were low, whichwere only0.63and25, respectively. It would lead to a significant impact of VFA inhibitionon methanogenesis, which might be an important reason of decrease of anaerobic digestionactivity. The change of dominant populations was the result of ammonia inhibition on bacteriacommunity of anaerobic digestion system. The inhibition could be mitigated because of thehigher biological diversity index and species richness index. Archea community could hardlytolerant the high ammonia concentration, which was caused by its low biological diversityindex and species richness index. The species which can product methane via hydrogen anddioxide carbon are more stable under ammonia inhibition than the species using acetic acidonly.(4) The stretagy of mitigating VFA inhibition and improving anaerobic digestion activitywas presented, which was based on the character of complementary effect between the twoendogenous inhibition factors.VFA inhibition was still significant in optimal organic load system without pH control.The inhibition could be mitigated observably through ammonia control. Under theconcentration of1,500mg·L-1, acetic acid content dropped from5.6g·L-1to2.8g·L-1,methane production increase by7.3%, and pH value was stable between6.8and7.2. However,excessive ammonia concentration could inhibit the reaction system as the type of freeammonia.