Small Footprint Alternative to Secondary Clarification and Tertiary Filtration
Challenge
For Pennsylvania’s Western Butler County Authority (WBCA), peak flow events due to inflow and infiltration (I&I) have gone hand-in-hand with aging sewage system and treatment plant infrastructure. Resulting overflow events at the Water Pollution Control Plant (WPCP) raise concerns for the quality of effluent being discharged to Connoquenessing Creek, a surface water source for the region.
Built in 1978 and expanded in 1999 – but with some components dating back as far as 1939 — the WPCP uses an activated sludge treatment process with three aeration tanks, each followed by a clarification tank. Clarified water then passes through sand filtration and a chlorination system before discharge. The plant is designed for an average daily flow (ADF) of 2.2 million gallons per day (MGD) and a peak hour factor (PHF) of 4.5 MGD. These days, however, the plant sometimes sees peaks of 18 MGD – flooding the plant’s excess flow tanks and sending solids over the clarifier weirs and into downstream processes.
The Taron™ System is a flexible alternative to increase treatment capacity and achieve discharge limits.
Solution
Recognizing the need to address such overflows and to plan for continued growth in the region, WBCA undertook a flow monitoring study and an evaluation of potential solutions. Among the options considered for modernizing the plant’s treatment technology was the Taron™ activated sludge filter from Xylem’s Sanitaire® brand.
The Taron activated sludge filter is designed to replace secondary clarification and tertiary filtration in an activated sludge treatment process. Able to operate at higher mixed liquor suspended solids (MLSS) concentrations than a traditional secondary clarifier, the Taron filter allows plants to increase the biological capacity of existing bioreactor tanks while also requiring a much smaller footprint for solids removal.
To evaluate the technology for WBCA, Xylem installed a four-disc Taron activated sludge filter in a standalone process. Raw wastewater influent was fed from the plant’s intake to a dedicated bioreactor tank for the Taron filter, where it underwent anaerobic, anoxic, and aerobic activated sludge processes before flowing downstream to the Taron filter.
Applied directly in the activated sludge, the Taron filter develops a dynamic sludge cake layer on micro mesh panels, providing two filter media to separate out the solids. The filtration rate is controlled by the rotational speed of the discs and a continuous filtrate backwash system that removes the sludge cake layer after each rotation. Depending on the application, the Taron filter can produce effluent with total suspended solids (TSS) of 10 mg/L to less than 5 mg/L.
Result
The Xylem team monitored WBCA’s Taron pilot daily and measured effluent TSS from 24-hour composite samples. The effluent was then circulated back to the main aeration basins.
The effluent TSS was less than 5 mg/L, on par with typical effluent quality after tertiary filtration. This low TSS was achieved with MLSS concentrations of up to 14 g/L, compared to the main treatment trains’ maximum 5.9 g/L and average around 4 g/L.
For the last four months of the pilot, the Taron filter was evaluated under diurnal flow patterns that better reflect actual treatment plant operation. To provide optimal filtration of the biological floc as the flow changes throughout the day, the rotational speed of the filter discs must be adjusted. The higher the inflow, the faster the disc rotation. These adjustments are made automatically through a programmable logic controller (PLC) connected to the Taron filter — based on flow meter and level sensor readings in the pilot, and based on plant inflow in a full-scale installation. During the evaluation of diurnal flow for WBCA, the Taron filter continued to provide lower, more stable effluent TSS than the plant’s secondary clarifiers.
Throughout the nearly year-long pilot, the Taron activated sludge filter successfully demonstrated its ability to reduce TSS, at MLSS concentrations ranging from 8 to 14 g/L. For utilities like WBCA that have limited land availability, as well as construction challenges for the existing land, the implementation of Taron filters as part of the activated sludge process can provide a flexible alternative to increase treatment capacity and achieve discharge limit.