Increased energy recovery and enhanced pathogen inactivation through anaerobic digestion of thickened wastewater sludge

This study had two objectives: to investigate the possibility of increased energy recovery from anaerobic digestion of sewage sludge and to inactivate pathogens in wastewater sludge by means other than thermophilic temperature. All research was done through numerous lab experiments using wastewater sludge from the North End Water Pollution Control Centre in Winnipeg.; It was proposed that toxicity from un-ionized form of VFA could be used in acid mesophilic (38°C) and low-mesophilic (24°C) conditions. The toxicity from the un-ionized form of VFA is based on cytoplasm acidification from a proton released after acid ionization inside the bacterial cell and anion accumulation inside the cell which may lead to osmotic stress. Batch acid digesters were able to achieve Class A biosolids standards with respect to fecal coliforms concentrations under both operating conditions.; When acid digesters were operated in semi-continuous mode, three acid digesters operated in series were needed to achieve Class A level. This was possible in mesophilic conditions only. Effluent from the three mesophilic acid digesters operated in series also showed highest inactivation of Ascaris suum ova from all systems tested, achieving complete inactivation of eggs enclosed in sentinels and 2.8% viability of free floating eggs.; Sludge thickening, which results in sludge flow reduction, was chosen as a method to achieve enhanced energy recovery. To establish optimum solids content in sludge, a series of hydrolysis tests were performed. The tests were to assess the impact of environmental conditions, such as operating temperature and sludge thickness on digestion. Optimum sludge solids content was defined as solids content that does not inhibit hydrolysis rates at any given temperature. This should result in uninhibited VS destruction and biogas production. Increase in solids content in raw sludge (from initial concentration of 3.6% TS) that would be accompanied by proportional increase in hydrolysis rate was found to be approximately 50% or up to 5.4% TS at 24°C, and 70% or up to 6.1% TS at 38°C.; Sludge volume reduction after thickening could be used for extension of SRT in existing digestion system for additional VS destruction and biogas production or operational and construction savings at smaller digestion volumes. Both scenarios were investigated and it was concluded that additional digestion time would be the most cost effective solution where infrastructure is available. Extension of SRT depends mainly on the degree of sludge thickening. For instance, by thickening the sludge from 3.9% TS (long term average at NEWPCC) at 22 days SRT, it is possible to achieve at 5.7% TS 32.2 days SRT. Increasing the solids content by 0.9% TS (from 4.8% TS to 5.7% TS---laboratory experiment) or 1.7% TS (from 3.9% TS to 5.7% TS---simulated data) would increase digestion SRT by an additional 4 to 10 days, and it could potentially increase methane production by 7% to 15%, respectively.; A new proposed digestion system that would enable enhanced pathogen inactivation and increased energy recovery consisted of three mesophilic acid digesters followed by a mesophilic gas digester. Depending on the initial solids content in raw, un-thickened sludge, the new digestion system could provide an additional 13% energy for recovery (including costs of gravity thickening) over conventional mesophilic anaerobic digestion (currently employed in Winnipeg). Also, the quality of biogas from this multi-phase digestion system showed up to 72% methane content in biogas, which was statistically higher than from a conventional digestion system.