Pumps are the heart of any wastewater treatment plant (WWTP). Adopting secondary treatment has increased the number of pumps at many facilities and increased vulnerability to often stringy, fibrous solids in the flow. This frequently induces clogging at the impellers of aging or inappropriately engineered pumps; this can compromise—even paralyze—efficiency as raw influent is converted into acceptable effluent discharge.
Because of several uncommon features of the 15-acre municipal plant's infrastructure, operators of the Fort Atkin-son, Wis., WWTP consider the reliability of pumps throughout the 2.7-mgd facility a top priority. They restored predictable performance at a critical link in the process chain with a retrofit in June 2001, replacing the original impellers of the Model 3127 pumps (manufactured by Trumbull, Conn.-based ITT Flygt) with Model 488 “N” impellers. The replacement impellers, derived from the new generation of N pump technology, maintain high efficiency even during long run times. As a result, the pumps draw less electricity because the motors are not bogged down by clogs. The retrofit resolved recurring clogging of the two 10-hp pumps that move waste-water drawn from the primary clarifiers and thickened waste activated sludge to three aerobic digesters.
The plant, opened in 1974, underwent a major upgrade in 1992 and 1993 to address flow, loading, and effluent quality considerations necessary to meet levels set for state permits. Improvements included installation of the extended-air activated sludge process to handle new ammonia nitrogen limits, primary and secondary clarification, aerobic digestion with sludge dewatering, and sodium hypochlorite disinfection with sodium bisulfite dechlorination.
The municipal system's 65 miles of collection lines and three lift stations deliver 60% of the plant's influent from industrial sources and the balance from residential connections. The state permit limits the biochemical oxygen demand (BOD) to 30 mg/L, 30 mg/L total suspended solids (TSS), 1.5 mg/Lphosphorous, and 3.3 mg/L ammonia. The plant's annual production, however, exceeds those levels, averaging 5 mg/LBOD, 7 mg/LTSS, 0.7 mg/Lphosphorous, and 0.2 mg/L ammonia.
The treated effluent discharges into a river. The dewatered sludge is stockpiled and seasonally applied to area farm fields in compliance with mandated procedures.
START AT THE HEADWORKS
The plant's staff gained a firsthand appreciation of the importance that two submersible pumps play following processing by the three aerobic digesters. These units were the Achilles' heel of this step in the process flow, according to plant foreman Tim Reel and maintenance technician Mike Paul. They attribute the problem in large part to the design of the plant's headworks, which lack bar screens. Instead, the incoming influent first passes through four 60-inch grinders. The suspended material then enters a wet well, where it often reforms into rag masses before reaching the grit removal system. Next, the effluent reaches the primary clarifiers where the fibrous material settles out before continued treatment.
“The design of the headworks only passed the clogging problem downline,” Paul said. “When the pumps at the digesters got older and the impellers experienced wear, we began having problems.”
At that point, the wastewater often carries 2.5% to 3% heavy solids. The two 450-gallons-per-minute (gpm), or 51 total dynamic head, pumps in the 18-feet deep wet well experienced recurring clogging as fibrous material enveloped and choked the impeller. Maintenance personnel had the unpleasant task of hoisting the clogged pumps out of the wet well, clearing them of debris, and restoring the units to full performance, said Paul Christensen, waste-water supervisor at the facility. Meanwhile, the plant process was fully interrupted.
“We experienced three or four failures a year created by rags and other material,” he said. “The first sign of impending trouble would be a steady drop in flow from the rated 450 gpm to perhaps 240 gpm—and eventual stoppage.”
The design at this wet well posed a roadblock to maintaining the process flow when one of the pumps malfunctioned. “Our plant is designed so that the two pumps, set in a common wet well, must run in tandem,” he said. “The lack of redundancy left us no real option but to continue running a choked pump until it tripped offline.”