POLYMER CONTROL IN A CO-THICKENING DAFT USING AN ON-LINE SUSPENDED SOLIDS PROBE

King County's South Treatment Plant has operated dissolved air flotation thickeners (DAFTs) since 1987. These circular DAFTs are designed to co-thicken primary sludge and waste activated sludge. Considerable effort has been directed at optimizing the performance of these DAFTs. Polymer use has received considerable attention due to its sizable cost. The historic polymer dosing strategy is based on maintaining a turbidity setpoint in the DAFT underflow. Though effective, this strategy is reactive in nature and suffers from significant time delays. Many other factors can influence underflow turbidity as well. In June 2004, a new polymer control strategy based on solids loading rate was evaluated. DAFT feed solids were measured using a HACH Solitax sc-100 Suspended Solids probe with wiper. The probe and control strategy proved to be reliable and effective at reducing polymer use and producing more consistent thickened sludge concentrations. The new polymer control strategy has allowed South Plant to realize a 20-30% reduction in polymer use while maintaining thickened sludge concentrations.