Optimizing dewatering polymer usage and resulting percent cake solids at the Hornsby Bend Biosolids Management Plant
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The City of Austin (CoA) employs anaeraboic digestion at Hornsby Bend Biosolids Management Plant (HBBMP) to treat the sludge generated from upstream wastewater treatment. HBBMP applies a polymer solution to the digested sludge, and the conditioned sludge is then passed through a belt filter press for final dewatering. Recent data indicate that the final solids content of the sludge cake after dewatering (cake %TS) has fallen below CoA targets. The purpose of this research is to optimize the sludge conditioning process to maximize cake %TS, as well as evaluate potential impacts of upstream magnesium hydroxide addition on sludge volumes and final cake %TS. The polymer optimization investigation involves analysis of four measures of sludge dewaterability: specific resistance to filtration (SRF), capillary suction time (CST), specific CST and calculated cake %TS. SRF, CST, specific CST, and calculated cake %TS data are presented for sludges treated with different polymer solutions. The precipitates investigation involves analysis of upstream added magnesium hydroxide data. Theoretical calculations are presented to determine the maximum and minimum daily mass of precipitates formed and conveyed to HBBMP. These theoretical precipitate mass data are compared with historical data for total daily volume of sludge dewatered and total daily mass of dewatered sludge produced by the belt filter presses. The results of these investigations show that, at the bench scale, a BASF Zetag 8819 polymer solution containing 0.035-0.0525 mL raw polymer per 100 mL of sludge at 0.25%-0.4% concentration, aged for 10-30 minutes prior to application, will improve sludge dewaterability most and most consistently. Additionally, there was no evidence of a change in the volume of sludge processed or mass of sludge cake produced as a result of increased upstream magnesium hydroxide addition.