Domestic wastewater is the most abundant type of wastewater. Direct discharge of untreated domestic wastewater has environmental and public health risks due to the presence of organics, nutrients and pathogens. Application of anaerobic processes for the treatment of domestic sewage, which at present is largely treated by aerobic processes, has drawn considerable attention recently. Anaerobic processes can be applied for the removal of organic matter (methanogenesis) and nitrogen (anaerobic ammonium oxidation (Anammox)).The toxicity of fluoride to methanogenesis was investigated. The results indicate that acetoclastic were more susceptible to fluoride than hydrogenotrophic methanogens. The concentration of fluoride causing 50% inhibition (IC50) to acetoclastic ranged from 18.1 to 155.7 mg L-1 while for hydrogenotrophic methanogens was > 400.0 mg L-1.The feasibility of a combined system consisting of anaerobic up-flow anaerobic sludge blanket (UASB) followed by aerobic activated sludge (AS) reactor for removal of carbonaceous and nitrogenous contaminants from strong synthetic sewage (2.5 g chemical oxygen demand (COD) L-1) was also studied. The average combined removal of total COD, volatile fatty acids (VFA) and protein was higher than 89.0%, 99.0% and 97.0%; respectively. Extensive nitrification (96.0%) was observed when dissolved oxygen (DO) concentration was > 2.0 mg L-1. In contrast, only partial nitrification occurred when the AS received high organic loads and/or the DO level was below 2.0 mg L-1.The inhibitory effect of nitrite and nitrate on methanogenesis was evaluated. Methanogenic activity was inhibited by the presence of NOx- compounds (i.e., nitrite and nitrate). The inhibition imparted by nitrate was not due to the nitrate itself, but rather to its reduced intermediate, nitrite. The toxicity of NOx- to methanogens was found to be reversible after all the NOx- were reduced during denitrification.Moreover, the development of Anammox enrichment cultures was evaluated. Anammox cultures were successfully developed using sludge samples collected from municipal wastewater treatment plants (WWTPs) as inocula but not from methanogenic granular sludges. Return activated sludge (RAS) collected from WWTP operating for biological nitrogen removal had the highest intrinsic level of Anammox activity. RAS Anammox culture was developed rapidly within 40 days with a doubling time of 6.8 days.

A series of lab-scaled digestion studies including conventional mesophilic anaerobic digestion(MAD), thermophilic anaerobic digestion (TAD) at a range of treatment temperatures, and mesophilic high solids digestion of thermally pretreated wastewater sludge (THD) were carried out. Enhanced digestion performance in terms of solids destruction and methane generation by THD relative to MAD was achieved, and was largely attributable to the solubilization and subsequent biodegradation of energy-rich substrates within blended primary and secondary sludge. TAD was observed to underperform MAD, especially at elevated temperatures as methanogenic inhibition resulted in the accumulation of headspace hydrogen, thus resulting in poor removal of volatile fatty acids. The thermodynamics of fatty acid metabolism was favorable at each digestion temperature, thus it was concluded that microbial inhibition was the controlling factor in poor thermophilic performance. Inhibition by free unionized ammonia (NH₃) was characterized for THD and MAD biomass. Acetic acid degradation was equally affected over a range of NH₃ concentrations; however, methane generation by THD was less sensitive to ammonia inhibition, thus suggesting that methanogenesis by THD was less dependent on the NH₃-sensitive process of aceticlastic methanogenesis. Total ammonia nitrogen (TAN) and bicarbonate alkalinity were stoichiometrically produced from proteinaceous material during thermal hydrolytic pretreatment and subsequent high solids anaerobic digestion. Combined effects of TAN and high pH resulted in NH₃-inhibition during THD. Kinetic evaluations suggested that a growth rate reduction of approximately 65% was associated with in-situ NH₃ concentrations of the THD reactor. NH₃-inhibition was apparently responsible for a shift in dominant methanogenic community of the aceticlastic Methanosarcina barkeri in MAD to the hydrogenotrophic Methanoculleus bourgensis in THD. A similar shift in methanogenic community was observed between low temperature thermophilic digestion at 47°C, where the dominant order was Methanosarcinales, to high temperature thermophilic digestion at 59°C where the dominant order was Methanobacteriales. These findings support a process-driven pathway shift from aceticlastic to non-aceticlastic methanogenesis between 180 and 290 mg/L NH₃-N. Such a threshold is supported by previous literature related to ammonia tolerance of pure cultures of methanogens and has significant implications for the kinetic design of advanced anaerobic digestion processes. / Ph. D.