Application of the Streaming Current Detector in Sludge Conditioner Selection and Control

Abstract

The streaming current detector (SCD) can be utilized to measure charge characteristics of a suspension or sludge which can be related to zeta potential. In this study, the SCD was used as a tool for polymer selection and dose control toward improved sludge conditioning prior to centrifugation. In laboratory studies, polymer addition to sludges was followed by controlled mixing then measurement of streaming current (SC) and capillary suction time (CST). Plant studies employed facilities at an 18.8 MGD wastewater treatment facility in Warminster, PA, USA. Anaerobically digested sludge was conditioned with varying doses of cationic polymer and dewatered using two parallel solid-bowl centrifuges. The polymer caused a transition from negative to positive charge in both conditioned sludge and centrate. The polymer dose range for optimum dewaterability, as indicated by CST and solids recovery, corresponded to near-zero SC in all cases. Even when greater mixing intensities or times increased the polymer demand, the charge transition was similarly related. This correspondence was used to successfully confirm a calculation method establishing velocity gradient and mixing time similitude between jar tests and in-line mixing prior to the centrifuge.