"Keeping the microorganisms in balance is an ongoing challenge here," said Donnie Higgins, plant supervisor who has worked at the facility for 18 years. "We have to keep our mixed liquor balanced at 1100 mils/per liter to maintain an efficient aerobic process. Any higher than that and we experience a high die off rate. Most other plants typically run in the range of 2,000 to 4,000 mil/per liter, even though we waste out about the same."
Upgrades and modifications
Vernon built the 2.0 mgd WWTP in the 1950s, which is now a 60-acre facility that discharges into the adjacent Pease River. Several major upgrades and process modifications altered the plant in the mid-1980s and again in the mid-1990s in response to regulatory mandates. The upgrades aimed at improving the operating parameters.
Sludge is drawn from the bottom of the primary clarifier into the digester while the liquid enters one racetrack equipped with four rotors and then into another wet well. At that stage, four 15-hp Flygt pumps with VFDs pump it to an aeration basin, and from there is channeled through a splitter box that sends flow through two secondary clarifiers. Sludge from the bottom of these secondaries is part return activated sludge (RAS) used to reseed the process. Waste activated sludge (WAS) advances into the thickener before being pumped to the digester. The clean, clear water advances through sand filters ahead of the UV system and then through a flow-measuring device and into the discharge line.
The sludge, containing approximately 5% solids, from the primary digester and thickener is advanced to the sludge digester where it recirculates through a boiler that keeps the digester at a constant 95° F. After passing through a belt press, the compressed cake is collected and the byproduct water captured and sent back to the influent. The cake has been sent to a landfill since the utility abandoned land application in 1996.
Lacking any redundancy, equipment reliability is essential to the operation. Prior to a recent upgrade, the digester recirculating pump proved particularly problematic. The belt-driven, 8-inch unit had been a chronic—and costly—problem.
“The pump failed up to eight times a year, which would paralyze the process flow for several days pending repairs. It could cost us up to $2,000 to repair the pump even if we did the work ourselves," Higgins said. “Repairing this pump was always a major project. It would cost us $6,000 just for the parts to completely rebuild it.”
Upgrading the pump
Higgins decided to upgrade the sludge recirculating pump to an ITT Flygt Model NZ3153 after learning that the company’s well-known submersible pumps could be configured to operate in a dry well mode.
Flygt’s N-pump technology was ideally suited for handling the thicker sludge recycled into the digester, he added. N-pumps feature a patented, semi-open, self-cleaning impeller that represent an award-winning innovation in wastewater-handling technology. The leading edges of the impeller vanes are kept unobstructed, where fouling often sets the stage for clogging other pumps. The impeller passes across a stationary relief groove that clears any snared fibrous solids, grease or sludge, creating a self-cleaning flow path through the pump.
"Reliability was the major reason for selecting the digester recirculating pump from ITT Flygt," Higgins emphasized. "Flygt had supplied three, two-speed pumps in our main influent pit, four at the racetrack and others in lift stations. They never had given us any problems and had been easy to work on and maintain."
The digester pump has been operating flawlessly since being installed at the plant's dry pit. "We've never had to pull it for a clogged impeller," Higgins added, "and have never been off line since."