The use of the return activated sludge (RAS) process in the wake of the Clean Water Act improved treated effluent quality at scores of wastewater treatment plants across the nation.
However, at the wastewater treatment plant in Cocoa Beach, Fla., the RAS process upgrade in the mid-90s caused a recurring problem for the pumps installed for additional flow and control.
When the utility switched to advanced secondary treatment for nutrient removal and filtration compliance, the absence of a primary clarifier at the headworks allowed rags, hair and other fibrous solids to reach the settling basin following initial oxidation. The fibrous material that had passed through the step screen, grit separator and first-stage treatment reached the bottom of the clarifier and continually clogged the RAS pumps at the start of secondary treatment.
“The material began to clog the RAS pumps with increasing frequency,” recalled Darby Blanchard, director of water reclamation for the city. “The failures sapped the maintenance budget and decreased the 6 mgd plant’s efficiency.”
“When the pumps clogged and tripped, we first tried resetting them,” he said. “If that didn’t work, we would reverse the leads on the pump motors and spin them backward. But that’s not an acceptable practice because backflushing a pump can cause an impeller to loosen and spin itself off the driveshaft. So more often than not, we had to pull and clean the RAS pumps every week,” Blanchard recalled. “It was dirty and dangerous work because sharp objects hidden in the debris could slice through a work glove.”
According to Don Hoover, plant operator, “pulling and cleaning the clogged pumps was an awful task that should not have to be done. However, the continual fouling of the pumps necessitated frequent cleaning in order to avoid disruptions in the treatment process.”
“Time spent fighting those failures exceeded more than 800 man-hours a year,” Blanchard said. “At $26 per hour, something obviously needed to be done.”
Immediate improvements
In search of a solution, short of building a primary clarifier at the headworks, Blanchard called in Ellis K. Phelps Co. The Orlando-based distributor for ITT Flygt first ruled out any problems with specified flow rates, air binding and other factors as the cause.
Attention was then focused on ways to prevent the unavoidable debris from choking the RAS pump impellers. Blanchard agreed to upgrade one of the four RAS pumps to a unit with the newly patented “N” impeller that ITT Flygt had recently introduced.
Blanchard recalled, “the test pump featured a newly designed self-cleaning impeller that inherently resisted clogging.”
He continued, “E.K. Phelps told me that most of the rags, stringy materials and solids would be cleared from the impeller and pumped away. If the clogging problem was eliminated, the operating efficiency of the pump would not deteriorate.”
Finally, Blanchard said, “they also told me that the new pump had been operating at several other sites and had proven to consume significantly less power because it wasn’t bogging down.”
The pump upgrade delivered immediate operational improvements, Blanchard noted. Enough so that he soon authorized upgrades of the remaining three units. The pumps ran problem free and the flows through the RAS pumps, all equipped with variable frequency drives, remained more constant, an indication of energy savings.
Describing the new pumps, Hoover stated, “they’ve operated perfectly since we installed them. There haven’t been any flow problems or clogging. I’ve never seen another type of pump that would perform well under these circumstances.”
“Since the full changeout was completed within months after the test pump success, the upgrade to ITT Flygt Model N-3127 pumps has clearly stood the test of time,” Blanchard said. “The N impeller has proven its ability to remain clog-free. It seems we now pull the pumps only every six months for routine maintenance. We no longer have a clogging problem.”
Innovative facility
The utility serves a modest population of 12,000 residents that jumps fourfold when vacationers descend on the white sand beaches of the six-mile long barrier island between the Atlantic Ocean and the Banana River Lagoon.
The utility is known throughout the state as an innovator. For example, Cocoa Beach contracts to treat 2 mgd of wastewater from Patrick AFB and another 275,000 gal. from the Canaveral Port Authority.
Those revenues help underwrite future capital improvements. The contract with Patrick AFB underwent renegotiating, to include connection fees, with the recent privatized redevelopment of housing now underway on the base.
Recent capital improvements include 20 mg of onsite storage for treated effluent and a 5-mg tank offsite that holds another 650,000 gal. of treated effluent piped in from the city of Cape Canaveral.
The storage supports a successful subscriber-based irrigation utility for the treated effluent. The irrigation utility has become the primary disposal mechanism for the plant’s treated effluent.
“By using recycled water for lawn irrigation, our citizens and our municipal golf course save a considerable amount of money,” emphasized Blanchard. “This is particularly true of our large condominium developments that can use 50,000 to 80,000 gpd to irrigate their grounds. There are currently approximately 3,000 subscribers to the non-metered system.”
Even more recent construction projects have produced administration, maintenance, lab and operations facilities.
An underdrain filter system, PC-touchscreen controls, security technologies and safer disinfection process also went into place. Blanchard now is studying the feasibility of a likely multi-jurisdictional plant that would convert dry sludge into a marketable soil enhancer. Once applied to orchards and other farmland, the dry sludge presently generated off the plant is now disposed of at a landfill.
Cocoa Beach corrected the onetime weak link in integrating RAS into the municipal wastewater treatment plant’s process chain. Blanchard still considers adding a primary clarifier someday but for now the pump upgrade has proven a cost-effective means of restoring reliability and durability to the RAS process.
