Enhanced anaerobic digestion of combined wastewater sludges through solubilization of waste activated sludge
Creators: name: Knezevic, Zorica
Publisher: name: University of British Columbia
Year: 2008

description: Pilot-scale research on enhancement of anaerobic co-digestion of combined primary and secondary sludges was performed, using a low-level alkaline solubilization process for the secondary sludge. Research was conducted in two phases: the first phase investigated chemical dosages of calcium hydroxide and sodium hydroxide and mechanical mixing times for waste activated sludge (WAS) solubilization; the second, pilot-scale phase monitored the performance of anaerobic digestion, using mixtures of solubilized WAS and primary sludge as a feed. The first phase results showed that solubilization of WAS can be effectively performed using both chemicals separately. However, for the same chemical dosage and anoxic mixing time, sodium hydroxide was more effective in WAS solubilization. Performance of three pilot-scale units, one control and two experimental (expCa(OH)₂ and exp NaOH) units was monitored in the second phase. Each anaerobic unit was 160 L in liquid volume and operated as a single-stage, high-rate system; operating temperature was 35°C. The ratio of WAS to primary sludge was 65/35 by volume; a chemical dose of 15 meq/L and anoxic mixing time of 5 hours was applied for WAS solubilization. All parameters were kept constant through the whole study except solids retention time (SRT); SRT was gradually decreased from 25 to 10 days, in 5 day increments, i.e. 4 runs. The second phase results indicated better digester performance of both experimental units, with exp NaOH outperforming lime. Volatile mass reduction was independent of SRT and the best overall VSS reduction was observed in the exp NaOHunit. However, the improvement in overall VSS reduction was attributed to the solubilization process itself, and not necessarily to better digester performance per se. The addition of sodium hydroxide to the WAS allowed for a decrease of SRT from 25to 10 days, without losing VSS reduction efficiency. Carbon removal (in terms of total and soluble COD removal) actually increased with a decrease in SRT and the highest carbon removal was achieved in the exp NaOHunit. Better carbon removal was also reflected in a higher methane content of the gas. The effect of solubilizing the WAS, prior to digestion, was also reflected in a higher unit gas production (L/kg VSS reduced) in both experimental units. Results obtained in the second phase proved that WAS pretreatment enhanced volatile mass reduction, total and soluble COD removal, gas quality and unit gas production in the anaerobic digestion of mixed wastewater sludges. Also, there was no major effect on effluent quality in terms of nitrogen, phosphorus (except for lower soluble phosphorus in the exp Ca(OH)₂ unit), VFA and TOC concentrations. Based on the results obtained in this study, it is strongly recommended that a full scale evaluation be undertaken of combining pretreated WAS with primary sludge in a high-rate, single-stage anaerobic digester and implement a complete cost-benefit analysis of such.