Study On Ammonia Tolerance Response Mechanism During Anaerobic Digestion Of Kitchen Waste

Kitchen waste is a typical municipal organic waste. The solid residual kitchen waste(SRKW) is rich in carbohydrate and protein. Thus anaerobic digestion(AD) is an efficient process for treatment of SRKW, which could simultaneously realize recycling, reducing and harmless disposal. However, AD of protein-rich organic waste is generally subjected to ammonia inhibition, even leading to process failure. The ammonia inhibition in AD is mainly due to free ammonia for its suppression the activity of methanogens. Free ammonia may passively diffuse into the cell, causing the change of intracellular pH, proton imbalance and potassium deficiency. The inhibition of methanogens could lead to the accumulation of volatile fatty acids(VFAs), and then the decline of pH. The dual inhibitions of ammonia and VFAs, and the decline of pH, would lead to the “inhibited pseudo-steady state” in AD system. Therefore, complete understanding the information about AD characteristics of SRKW, efficient strategy for alleviating ammonia inhibition, and the ammonia tolerance response(ATR) mechanism of the anaerobic system, are the foundation and precondition for large-scale treatment of SRKW.The SRKW was used as the substrate during AD in this dissertation. The AD characteristics of SRKW and methane production potential were determined by batch mode. Then an efficient acclimation strategy was developed to improve the ammonia tolerance of the continuous treatment system and alleviate the in situ ammonia inhibition. The high-throughput 16 S rDNA sequencing technology and selective pathway inhibition method were employed to expound the ATR mechanisms of microbial community and methanogenic pathway, and to explain the transforming of acetate to methane under inhibited acetotrophic methanogenesis(AM) pathway. The main research results of this dissertation are shown as follows:The batch AD of SRKW under mesophilic condition faced varying levels of acid inhibition, even the complete stagnation of methane production. Adding Na HCO3 to the system could partially prevent its acidification and improve its treatment capacity. The maximum specific methane production(SMP) of 604.32 mL/gTS was obtained at ISR 1:0.5 with high TS removal rates of 90.40%. The Logistic function, the Modified Gompertz model, and the Transfer function were all suitable for fitting the methane production process of SRKW. However, among these models, the Transfer function was the best because of its high R-squared valueThe ammonia produced from the decomposition of SRKW during continuous AD was the primary factor for reactor upset or failure. The ammonia first inhibited the activity of methanogens, and it then induced the accumulation of VFAs followed by the decline in pH. Therefore, an acclimation strategy based on “gradient ammonia stress” was developed in the continuous AD of SRKW. The ammonia tolerance of the system was correspondingly strengthened during gradient increasing the ammonia concentration in the reactor. During the increase of ammonia concentration from approximately 2000 mg/L to 4500 mg/L, the reactor operated well with a relative daily biogas production of 58.93 L/d, methane percentage content of 62.31%, the soluble chemical oxygen demand concentration of 7238 mg/L, and VFA concentration of 578 mg/L. During acclimation, with increased ammonia stress, the activities of amylase and protease were relatively stable; by contrast, CoF420 increased with improved buffering capacity. Through acclimation, the half inhibition concentration(IC50) of ammonia to biogas production for anaerobic sludge increased from 3709 mg/L to 10072 mg/L with an increase rate of 171.56%. Similarly, the IC50 of ammonia to methane production increased from 3111 mg/L to 9180 mg/L with an increase rate of 195.08%. This finding indicates that the ammonia tolerance of the anaerobic system was significantly improved.The microbial community remarkably changed during acclimation with the gradient increase of in situ ammonia stress. The relative abundance of acetotrophic methanogens(i.e., mainly Methanosaetaceae and Methanosarcinaceae) gradually declined, whereas that of hydrogenotrophic methanogens was increased along with the increment of Firmicutes–Clostridia bacteria. Candidatus Cloacamonas, a special microorganism that belongs to the WWE1 candidate division according to the reference, was detected. Candidatus Cloacamonas is probably a syntrophic hydrogen-producing bacterium in anaerobic digester.The syntrophic acetate oxidation(SAO) process was indirectly proven to exist in AD under ammonia stress through the selective inhibition method, in which the methanogenic inhibitor 2-bromoethanesulphonate, the AM pathway inhibitor fluoroacetate, and the hydrogenotrophic methanogenesis(HM) pathway inhibitor lumazine were used as the selective inhibitors. The methanogenic pathway transformation from AM to SAO-HM was an ATR mechanism of the anaerobic system to ammonia stress. This event verifies that the acetate could be successfully metabolized to methane without any acid inhibition when the AM pathway is suppressed.The average SMP of SRKW in acclimated ammonia-tolerant system under high ammonia load was 448 m L/g TS, which was 31.37% higher than that in the un-acclimated system. The optimum ISR increased from 1:1.5 to 1:2, thereby improving the treatment capacity of the batch anaerobic system by 33.33%. Furthermore, the increase rate in methane production was enhanced with organic load(i.e., 6.12 times at ISR 1:2.5), and no acid acclimation was observed in the system.The VLR of a continuous stirred tank reactor(CSTR) for the anaerobic treatment of SRKW under the ammonia load of 4000 mg/L to 4500 mg/L reached 5 kgTS/m3/d with a volumetric biogas production rate of 2.65 m3/m3/d. Contrarily, the SMP of SRKW decreased with VLR. The ammonia tolerance threshold of the CSTR used in this study was 8156 mg/L. During the increase of ammonia load, Methanobacterium was the dominant methanogen, whereas the relative abundance of Methanosaeta and Methanosarcina declined.