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Clog-resistant lift stations benefit wastewater utility
Clogged pump stations are among the most common problems confronting wastewater utility systems. The city of Haverhill, Mass., was no exception, but five recently completed duplex stations are engineered specifically to avoid such incidents, according to Paul Jessel, the city’s supervisor of wastewater collection.
Jessel based his expectation on the prefabricated fiberglass stations applied to the upgrades. The units have a unique floor configuration that directly complements the efficiency of a high-performance selfcleaning pump included in the prefabricated package. The future benefits should include better use of his five-man field force and vital wastewater collection for more than 900 acres of industrial property now better positioned to undergo jobcreating development.
“I believe the combination of the prefabricated station and self-cleaning pump offers us a proactive solution to preventing clogging at those new facilities,” Jessel said.
Chronic Clogging Solution
The sewage infrastructure of Haverhill, a community of 68,000, shares much in common with that of other long established New England municipalities. The collection system’s 35 pump stations now consist of eight fitted with electric submersibles, including the five new installations. There are 17 flooded suction stations, six vacuum primes and four air-ejector types along more than 987,000 ln ft of 8- to 72-in. gravity pipe and 4- to 42-in. force mains. More than 143,000 ln ft of the collection system originated in the late 1800s and the system included more than 295,000 ln ft of combined sanitary and storm water lines, Jessel said.
The utility’s success in resolving the chronic clogging of a submersible station with the Flygt N-pump from ITT Water & Wastewater led Jessel to make the selfcleaning submersible pump his utility’s preferred specification. The combined features of the pump and a pre-engineered alternative to traditional site-built lift station construction earned application as a fully integrated solution for the five recent station projects.
“That first pump station to receive an N-pump served a lot of restaurants nearby that would plug the sump almost monthly with built-up grease,” Jessel said. “It would take three men with a pump truck four hours or more to pull the pump and clear it with the original old-style pumps.”
In 2000, the clog-prone station was retrofitted with an N-impeller to control the grease with great success.
“Instead of plugging monthly, sometimes weekly, we have a blockage there maybe once a year. We wanted the same reliability in our latest five stations,” Jessel said.
The recent construction produced four new stations and one replacement. The 6-ft-diameter lift stations were assembled largely at grade level, lowered into 15-ftdeep holes and then grouted into place. The factory-manufactured TOPS (The Optimum Pump Station) package produced a facility with features that overcome the clogging experienced in many traditional site-built, cylindrical wet wells with flat circular bottoms that are prone to collect built-up sludge and odorous surface blanket scum. For the five recent projects, the factory-manufactured stations were installed in the duplex facilities equipped with 6.5-, 15-, 30- and 34-hp N-pumps sized to accommodate existing and expected future flows.
The TOPS concept also simplified procurement because a single-source supplies the fiberglass inserts, pumps, sealed wall penetrations and required fittings, mounting hardware, vent pipe, and level and pump station controls. The split, grate-type aluminum access cover that completes the package has several important safety features that top out this pump station construction alternative.
Complementary design features are at the heart of the clog resistance of the preengineered package, according to Jessel.
The convergent slope of the sump floor encourages incoming solids and other debris to settle into a trough molded into the bottom of the fiberglass insert. This improves the suction power at the pump inlet and the selfcleaning volute of the pump itself. The combined design features optimize the fluid handling of a lift station, especially those facilities with high solids and debris in their flows.
The twofold design features inherently eliminate “dead zones” where sludge can build up in nonagitated areas of traditional flat-bottom wet wells. In one of the new facilities prone to accumulating a blanket of surface scum, any minuscule amount of residual sediment is flushed down-line at the startup of a pump cycle by a 15-second blast of recirculated wastewater from a Mix-Flush valve. Even the slicker fiberglass walls prevent buildup of residue that creates offensive odor and inevitable public complaints, especially in residential areas.
The two contractors who assembled, set into place and grouted the new stations cited the easier installation, reduced completion schedules and clog-resistant design as major initial and long-term benefits of the prefabricated concept. With virtually no clogging or snarled impellers expected, the pumps will operate at full efficiency and use up to 40% less energy—an important consideration in this era of rising utility costs.
Although it is too early to quantify the cost benefits from fewer clogs, Jessel said he will continue to use the units as the city’s preferred specification for both new and retrofit station upgrades.