The city of Vernon, Texas, is among scores of municipalities across the nation whose wastewater treatment plants have undergone numerous modifications to comply with the U.S. Clean Water Act.
For some, the addition of secondary treatment and related upgrades presented few problems, but for others, including the facility in Vernon, the changes have proven more challenging. The Vernon plant and process management have adjusted over the years to increasingly stringent regulations, industrial load and even brine with nitrates off the city’s recent ion exchange water treatment plant. To continue operating in compliance, the plant’s performance relies equally on the resourceful staff and new technologies.
“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 1,100 mil/L to maintain an efficient aerobic process. Any higher than that and we experience a high die-off rate. Most other plants would typically run in the range of 2,000 to 4,000 mil/L, even though we waste out about the same. Other operators and instructors have no explanation when we describe this phenomenon while attending operator training seminars.”
Plant upgrades
Ranching and other agriculture-related enterprises are the predominant livelihoods in Vernon. Those who live and work in the area are willing to take on jobs that others elsewhere would outsource—including the staff that operates the Vernon Wastewater Treatment Plant.
The city of Vernon built the plant in the 1950s on what is now a 60-acre facility that discharges into the adjacent Pease River. The utility has 4,250 service connections and 11 lift stations along the collection system for the 12,500 residents. Although permitted at 2 mgd, the activated sludge plant normally operates at only half that daily capacity, Higgins said.
Several major upgrades and process modifications altered the plant in the mid-1980s and again in the mid-1990s in response to regulatory mandates. Both projects significantly transposed the plant’s infrastructure and aimed at improving the operating parameters.
The original anaerobic process that utilized an Imhoff tank, primary clarifier, trickling filters and chlorination has been replaced with an aerobic process. The Imhoff tank was converted into a dual-chamber aeration basin. Other work added a secondary clarifier and restored a once-abandoned oxidation ditch. Ultraviolet disinfection replaced chlorine disinfection in the more recent upgrade.
The plant’s infrastructure was developed at two elevations, with an approximate 12-ft difference. The influent enters the plant through a 24-in. main at the lower portion of the facility, where it passes through a grit chamber and a bar screen before advancing to the primary clarifier.
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 enters another wetwell. At that stage, four, 15-hp Flygt pumps with VFDs pump it to the portion of the plant at the higher elevation, where it reaches another aeration basin. From there, it 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 used to reseed the process. Waste activated sludge 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. A separate mixing pump keeps the digester stirred. 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.
Reliable operation
Higgins is especially proud of his staff, who have learned to successfully manage the plant’s reliability. Their performance routinely exceeds mandatory permit requirements. TSS in the treated effluent averages 1 mg/L daily versus the 15-mg/L limit set by the state. CBOD runs less than 2 mg/L versus 10 allowed by the permit, and ammonia has been kept at 0.04 mg/L avg. versus 3 mandated by the permit, since adding larger and more efficient diffusers and blowers.
Lacking any redundancy, equipment reliability is essential to the operation. Prior to a recent upgrade, the digester recirculating pump proved particularly troubling. This previous belt-driven, 8-in. 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. Higgins attributes the recurring failure to the pump’s water-cooled packing that would dry out and eventually melt, causing the sleeve to warp and eventually ruin the seal that damaged the bearings as the unit grew progressively hotter.
“It got expensive because the packing alone cost $80, and it could cost us up to $2,000 to repair the pump, even if we did the work ourselves,” Higgins said. “It would cost us $6,000 just for the parts to completely rebuild it. Repairing this pump was always a major project.”
Higgins decided to upgrade the sludge recirculating pump to an ITT Flygt Model NZ3153 after learning that the company, best known for submersible pumps, also manufactured the required dry pit unit. The N-pump technology was ideally suited for handling the thicker sludge recycled into the digester. The pump features a patented, semi-open, self-cleaning impeller. This design keeps the leading edges of the impeller vanes unobstructed where fouling often sets the stage for clogging in other pumps. The leading edges of the impeller vanes on an N pump pass 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 this manufacturer,” Higgins explained. “Flygt had supplied three, two-speed pumps in our main influent pit, four at the racetrack and others in lift stations. These pumps had never given us any problems and had been easy to maintain and work on.”
The upgrade has performed flawlessly since being installed at the plant’s dry pit. “We’ve never had to pull it for a clogged impeller,” Higgins added, “and we’ve never been off line since.”
RSS: WWD Articles
.gif)
