Study On Kinetics Characteristics And Microbial Mechanism Of Process Instability During Anaerobic Digestion Of Food Waste

To reveal the source and key link of process instability; investigate the key microorganisms steering the functioning of anaerobic digesters and suggest the potential early warning microbial indicators associated with process instability, controlled experiments focused on overloading and ammonia inhibition were carried out in laboratory scale anaerobic digesters. Techniques including physical-chemical analysis, specific methanogenic activity?SMA? tests and high throughput sequencing were performed to clarify the digestion kinetics and microbial mechanisms of process instability. The results showed that:?1? The sludge loading?i.e. Substrate-Inoculums ratio, S/I? is more suitable than organic loading rate?OLR? to delimit the loading threshold of digesters, and when the S/I>2, the digesters will be threatened by overload. However, total ammonia-biomass concentration ratio?TAN/BC? is not applicable to characterize the ammonia inhibition threshold of anaerobic digester. The promotion of BC to process efficiency is fluctuated and limited, TAN is the main factor to determine the efficiency of AD process in ammonia inhibition system.?2? In digesters suffering overloading shocks, the hydrolysis rate constant?k? and methanogenesis rate constant(R_max) changed non monotonously with the change of OLR, BC and S/I, thus the value of these two parameters are unable to determine the stability of the digesters. In contrast, the ratio of k/R_max? was negatively correlated with process efficiency, the ratio should be maintained at a level of 1.69³.44?p=0.05? in highly efficient digesters. The increase in OLR will weaken more in methanogenesis than hydrolysis, resulting the increase of the ratio, and thus exacerbate the mismatch between acid production and consumption, eventually resulting in process acidification and instability. On the contrary, the increased BC promote more in methanogenesis than hydrolysis, then tends to alleviate this imbalance, which is of significance for the efficiency and stability of the AD process.?3? In digesters suffering ammonia inhibition, k, R_max and the k/R_max? were all unable to determine the stability of the digesters. Because in the case of low TAN, the increased TAN can simultaneously reduce the k and R_max?, resulting the constant k/R_max?, and the digesters presented “inhibited steady state” characteristics, namely no accumulation of intermediate metabolites, but the retention time increased and the methane yield decreased; while in the case of high TAN, the TAN weaken more in methanogenesis than hydrolysis, then the obvious increase of k/R_max? ratio was observed, indicating the mismatch between acid production and consumption. At this time, the digesters have been exhibited ammonia poisoning phenomenon accompanied by drastic accumulation in intermediate metabolites and sharp decline in methane yield.?4? In the overload test, the reactor characterized with high specific methanogenic activity?SMA? at stable operation stage; though Methanosaeta was the dominant methanogens, the coexistence of acetoclastic and hydrogenotrophic groups was observed. Besides, other microbial communities including hydrolytic, acidogenic and acetogenic bacteria were balanced development. Under high OLR, the acidogenic bacteria?phyla Tenericutes and Actinobacteria? rapidly proliferated, and the resulting high VFA yield is the first reason of process deterioration. The high VFA yield induced an increase in the abundance of syntrophic acetogenic bacteria?class Clostridia? and resulting the increase of partial pressure of H2 in digester; while the abundance and activity of hydrogenotrophic methanogens decreased, which decreased the H2 consumption rate. The mismatch of metabolism between bacteria and methanogens is the second reason of process deterioration. Moreover, the accumulated inhibitors?VFA and TAN? directly lowered the metabolic activity of acetoclastic methanogens, and shifted the dominant hydrogenotrophic methanogens from Methanospirillum to Methanoculleus, then indirectly lowered the metabolic activity of hydrogenotrophic methanogens, which may be another reason of process deterioration.?5? In the ammonia inhibition test, the reactor also characterized with balanced microbial community at stable operation stage. With the increase of TAN, Methanosaeta was gradually eliminated because they are high sensitive to TAN. Then the acetate metabolism became the rate-limiting step in the reactor, which is the original reason of the process instability. The accumulated acetate affected the degradation of fatty acids in the upper stream, which further induced the propionate accumulation, at this time the process instability stepped into the second stage. The high concentration of VFA was then negatively correlated with the abundance of protective genus Actinomyces, the decrease of Actinomyces destroyed the microbial aggregates, and finally, only acid produced or tolerant hydrolytic and acidogenic bacteria remained in the digester. The decrease of protective genus Actinomyces and deletion of acetogenic bacteria broke the series of the AD process, which is the basic reason for the instability of the ammonia inhibition digester.?6? Judging the process stability of a digester according to general ecological parameters is not a sophisticated method; it is still controversial what level of community complexity a healthy, well-balanced, efficient microbial consortium should have for the production of biogas. While the abundance of genus Actinomyces and family Syntrophomonadaceae may be used as potential microbial warning indicators as they were correlated with process stability, and theoretically possible.This paper creatively proposed that methanogenesis is the key link of process instability, the process deterioration begin in the mismatch between hydrolysis kinetics and methanogenesis kinetics(k/R_max?); systematically identified how microbiome behave against process disturbances, stated that inefficient methanogens is the crux of process instability; suggested the potential early warning microbial indicators associated with process stability, proved the feasibility of microbial management. This results provide basic data for develop targeted microbial control technology, and provide theoretical support for develop microbiological diagnosis technology. Thus, it is important to ensure the efficient and stable operation of the anaerobic digesters.