Anaerobic Treatment as an Alternative for Municipal Wastewater Treatment at Sub-Optimal Temperature

This thesis includes both experimental and conceptual analysis of anaerobic treatment of municipal wastewater at sub-optimal temperature (optimal = 35°C). The purpose of this research was to evaluate the application of anaerobic biotechnology for full-scale treatment of mainstream liquid municipal wastewater under moderate climate conditions. Whereas anaerobic digestion of waste biosolids has been widely integrated into the waste management industry, there have been minimal advances in applying full-scale anaerobic reactors for treatment of municipal wastewater. Previous research suggests that anaerobically treating municipal wastewater is not yet gainful because the quality of treatment is not comparable to that of conventional wastewater treatment methods. In order to determine performance capabilities, the first phase used two laboratory-scale anaerobic reactors, the upflow anaerobic sludge blanket (UASB) and anaerobic baffled reactor (ABR), to treat synthetic wastewater with 306 mg/L COD for approximately one year. The primary findings of reactor operation were: · The UASB and ABR were able to achieve and maintain SCOD removal efficiencies of 94% and 89%, respectively, at a temperature of 20°C and hydraulic retention time of 12 hours. · Effluent SCOD was consistently at or below U.S. secondary discharge BOD limits, with the majority of residual SCOD believed to be the result of soluble microbial products. · At 20°C, 57% - 68% of produced methane was lost to reactor effluent as dissolved methane. The UASB and ABR both exhibited supersaturation at more than 2 times the expected saturation limit of methane, respectively. The results indicate that percent COD removal and methane gas production could affect methane solubility in anaerobic reactors. · It was determined that at temperatures equal to or above 14°C, aceticlastic methanogenesis is not the limiting factor of the overall anaerobic process. The second phase of this research investigated the energy and greenhouse gas (GHG) implications of applying anaerobic biotechnology for full-scale treatment of municipal wastewater. In order to successfully apply a full-scale anaerobic treatment system, it was believed that pairing anaerobic biotechnology with an aerobic post-treatment process offered the best alternative. Several conceptual comparisons were performed between conventional aerobic treatment systems and hybrid anaerobic-aerobic systems, with respect to energy requirements and GHG emissions. Although previous studies have been performed in a similar nature, none of them have analyzed the energy requirements and GHG emissions of treatment systems capable of removing BOD, nitrogen, and dissolved methane from municipal wastewater. Principal findings from the conceptual comparisons include: · Due to the anaerobic reactor, dissolved oxygen requirements were significantly reduced in the hybrid systems, subsequently reducing energy consumption and GHG emissions. · Applying anaerobic treatment as the primary biological process enabled the majority of influent BOD to be converted to methane, thereby relegating aerobic processes to a supportive role in effluent polishing. This resulted in significant reductions in excess biosolids with related decreases in energy demand and GHG emissions without compromising effluent quality. · Removing and oxidizing dissolved methane from anaerobic effluent imposes minimal energy demands in comparison to those for nitrogen removal. It was concluded that at 20°C, a full-scale treatment system employing an anaerobic reactor followed by aerobic post-treatment could exhibit both net-energy neutrality and net-zero GHG emissions