King County South Treatment Plant in Renton, Wash., has found monitoring the solids loading to the dissolved air flotation thickener (DAFT) tanks yields the timely signal needed for effective control of polymer dosing during sludge thickening.
The trick, according to Curtis Steinke, process analyst at the King County South Treatment Plant, was to find a suspended solids sensor that stands up to the task in such a harsh environment. He found his answer at work in another process at the same plant.
Since the late 1980s, operators at the King County South Treatment Plant have been utilizing DAFT tank effluent turbidity measurements to control polymer dosing via an automated single-loop-cascade control system. While automated, this reactive strategy suffered a four-to-six hour delay between a dosing adjustment and follow-on underflow turbidity.
“The effect was delayed adjustment for a condition that existed hours earlier,” said Steinke.
He explained how this reactive dosing strategy—based on whether the underflow turbidity was above or below the set-point—at times underdosed or overdosed polymer with respectively increasing or decreasing solids loading.
In June 2004, the staff decided to evaluate a more proactive approach to DAFT polymer dosing.
Steinke learned that an associate had been using a SOLITAX sc Turbidity and Suspended Solids Sensor with sc Controller manufactured by the Hach Co., to monitor mixed liquor suspended solids measurements at the plant’s aeration basin, for purposes of automating sludge volume index monitoring. The sensor had proven during its yearlong deployment in the mixed liquor channel that its dual-beam infrared/scattered light photometric detection technology is immune to color interference and that its readings correlate well to laboratory analysis.
Steinke installed a unit of the SOLITAX sc Sensor and its sc100 Controller to monitor the mixed sludge (primary and activated sludge solids) in DAFT feed for about two months. He compared readings, ranging from 3,000 to 12,000 mg/L to grab sample measurements and observed the sensor’s resistance to fouling. This trial-by-fire convinced him this sensor was the answer for online, real-time monitoring of sludge mass loading to the DAFTs.
Controller output automates control
Steinke then linked the signal from the suspended solids sensor to the plant SCADA system using a full-scale 4-20 mA output from the sensor’s sc100 Controller. With real-time mixed sludge flow rate, the sensor value, and polymer flow and concentration, the SCADA system calculated and adjusted polymer flow rate to consistently meet the set point of pounds of active polymer to dry ton mixed sludge.
He noted that the controller allows for adjusting the sensor measurement with an operator-entered factor that correlates sensor measurement to traditional gravimetric suspended solids determination. Operators have revised this factor, due to changes in overall water quality conditions, three times over the six months the sensor has been monitoring DAFT solids loading.
After just three weeks of successful automated dosing control, the King County South Treatment Plant converted additional DAFT tanks to the same dosing-control approach.
The bottom line
The bottom line reflects the value of reliable monitoring instrumentation: the South Treatment Plant now sees doses of 2.3 to 2.7 lb per dry ton, compared to the turbidity-guided approach that yielded doses ranging from 3.0 to 4.0 lb per dry ton.
In just a few months, the new DAFT polymer dosing control strategy realized a 20% to 30% reduction in polymer consumption—the equivalent of an estimated $30,000 to $40,000 savings in chemical costs over a year.
“We’re saving operating costs while we’re maintaining the thickened sludge concentration of 6% to 6.5% that keeps the overall solids treatment optimized—the thickening, digestion and dewatering processes,” Steinke concluded.
