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This study used a batch experiment and a long-term experiment to investigate the effects of FeCl3 and FeCl2 on the high-solids anaerobic digestion (HSAD) of sludge for improving the methane productions. The results of batch experiment with an organic loading rate of 6.93 gVS/L suggested that the cumulative methane productions were enhanced by 28.9% and 6.4% with 200 mg/L FeCl3 and 250 mg/L FeCl2 in HSAD system, respectively. The higher methane productions were also obtained by 200 mg/L FeCl3 and 250 mg/L FeCl2 in the long-term experiment for 114d. An excessive FeCl3 could inhibit the HSAD process, which would be gradually recovered by the ammonia and the microbes acclimated to this environment. The organics degradation results indicated that the addition of 200 mg/L FeCl3 or 250 mg/L FeCl2 could show a positive effect on the substrate environment of a stable total volatile fatty acids (TVFA) concentration and pH, and promote the protease and dehydrogenase activities in HSAD system. In addition, the SRT played a significant role in the HSAD process by balancing hydrolysis-acidification and methanogenesis. An appropriate dosage of FeCl3 and FeCl2 improved the methane productions, and blocked the accumulation of TVFA caused by shortening SRT.

The effect of glucose on nitrogen removal and microbial communities in the anammox-denitrification systems was investigated. The optimal nitrogen removal could be achieved when the influent glucose concentration was 56.4mgL-1. The influent nitrite to ammonium ratio of 0.95-1.40 would not obviously affect the nitrogen removal due to the coexistence of anammox, denitrification and partial denitrification. The anammox activity was deteriorated with increasing glucose concentration. When the influent glucose concentration was increased to 374.9mgL-1, the average ammonium removal efficiency decreased from 97% to around 10% and anammox activity was seriously inhibited. The anammox activity quickly recovered with decreasing influent glucose and increasing influent nitrite. High-throughput sequencing analysis suggested that the predominant genus changed from Candidatus Kuenenia to Diaphorobacter with the addition of glucose and then changed to Hydrogenophaga with the decrease of glucose. It indicated that organics concentration had an effect on the microbial communities.

The performance of biogas generation and sludge degradation was studied under different zero-valent iron/activated carbon (ZVI/AC) ratios in detail in mesophilic anaerobic digestion of sludge. A good enhancement of methane production was obtained at the 10:1 ZVI/AC ratio, and the cumulative methane production was 132.1 mL/g VS, 37.6% higher than the blank. The methane content at the 10:1 ZVI/AC ratio reached 68.8%, which was higher than the blank (55.2%) and the sludge-added AC alone (59.6%). For sludge degradation, the removal efficiencies of total chemical oxygen demand (TCOD), proteins, and polysaccharides were all the highest at the 10:1 ZVI/AC ratio. The concentration of available phosphorus (AP) decreased after anaerobic digestion process. On the other hand, the concentrations of available nitrogen (AN) and available potassium (AK) increased after the anaerobic digestion process and showed a gradually decreasing trend with increasing ZVI/AC ratio. The concentrations of AN and AK were 2303.1-4200.3 and 274.7-388.3 mg/kg, showing a potential for land utilization.

A novel sulfide-based denitrification and Anammox process was established for simultaneous removal of nitrogen and sulfide in a UBF reactor. The effects of the N/S ratio on reactor performance were investigated under five N/S molar ratios (4.56, 2.38, 0.96, 0.73, and 0.51). The best total nitrogen removal efficiency was 82.8% at a N/S ratio of 2.38. When the N/S ratio exceeded 0.96, Anammox contributed to more than 90% of the N loss. Sulfide was completely removed during the full operational period and S0 accumulation occurred when N/S ratio was less than 1. Thiobacillus (6.1%) and Candidatus Kuenenia (18.8%) were the main functional microorganisms when nitrate was in excess on day 12. As nitrate became limited on day 50, Thiobacillus (21.0%), Sulfurimonas (3.9%), and Candidatus Kuenenia (19.7%) became dominated. In this study, Candidatus Kuenenia was not inhibited by the sulfide.