Sludge Characteristics that Impact Dewatering of Anaerobically Digested Biosolids at Multiple Facilities

摘要

The Region of Waterloo operates three major wastewater treatment plants (WWTPs) (Kitchener, Galt and Waterloo) that employ anaerobic digestion and dewatering. Each facility processes sludge with different characteristics and uses different primary coagulants for P control (iron-based at Kitchener WWTP and Waterloo WWTP, and alum at Galt WWTP) and polymers (dry or emulsion) for dewatering. Considerable differences in the polymer consumption and cake dryness after centrifugation have been observed at the differing plants. This study sought to assess the sludge/biosolids characteristics that affect dewatering at the three dewatering facilities with the intent of optimizing polymer use and minimizing biosolids haulage. An analysis of historical data on anaerobically digested biosolids (ADB) produced at the three facilities was carried out in order to establish a comparison between the locations. The historical results indicated that ADB from Galt do not dewater as well as ADB generated at both Kitchener and Waterloo, and this was attributed to the higher levels of extracellular polymeric substances (EPS) and higher mono to divalent cation ratio of the Galt biosolids. An assessment of the dewatering characteristics at different locations of the Kitchener WWTP suggested no significant impact of mixing and pumping on dewatering at Kitchener. The results indicated that better digestion and higher cake solids content was being obtained at facilities where an iron-based coagulant was used for phosphorus precipitation. Capillary suction time (CST), time to filter (TTF) and a modified centrifugal test were used to assess operational and process factors affecting dewatering and to determine an optimal polymer dosage for the ADB at each of the three facilities. CST results conducted with different polymer solution strength suggested that polymer dosing ranging from 15 to 20 kg/dry tonne might be optimum for all three dewatering facilities. These doses were confirmed using the modified centrifugal technique. Both CST and TTF results indicated that biosolids mixing and pumping operations at Kitchener prior to dewatering do not significantly impact the rate of dewatering. A series of tests were conducted to assess the impact of biosolids and polymer make down water temperatures on dewatering since cooling of ADB in storage tanks was possible at the WWTPs. The CST of the conditioned biosolids samples decreased as biosolids temperature increased, indicating that biosolids temperature impacts on the rate of dewatering. The impact of temperature on the CST (or rate of dewatering) was more pronounced at Galt and Kitchener than at Waterloo. Additional tests conducted using a 3 × 3 factorial design with the modified centrifugal technique indicated that there was no discernible difference in cake solids content as a function of either polymer make-up water or biosolids temperature. The contradictory results from the differing methodologies prevented a definitive conclusion about whether dewatering of ADB is significantly impacted over a temperature range between 15 and 30 °C when emulsion polymer is used for conditioning.