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Rolling topography can present an uphill battle for many wastewater utilities, particularly when certain types of development—hospitals, nursing homes, daycares, restaurants and schools—add high-solids wastewater along the lines.
The Summit County, Ohio, Department of Environmental Services needs more than two hundred pump stations just to keep the service area’s wastewater flowing on even the best days. The county’s relatively high elevation amid its rollercoaster geography requires 108 pump stations and more than another 100 grinder-type units to serve the municipalities within the service area. Rated from 25 gal per minute (gpm) up to 75 gpm, several of these pumps presented daily problems before an advanced pump technology instilled reliability.
The utility’s collection system carries 15 million gal per day (mgd) to the 8-mgd Fishcreek or the 4-mgd Springfield wastewater treatment plants. Akron's wastewater treatment facilities treat the remaining 25% of the daily flow.
Hilly geography requires more than 10 pump stations for the utility.
It is Ben Duke’s responsibility to keep the lines flowing. He began his career at the utility decades ago as a vacuum truck operator, went on to maintain the pump stations and now heads the group responsible for the utility’s many lift stations. There was a time when he routinely planned on sending crews daily to unclog one or more pump stations because they incurred chronic blockages due either to their design or to byproduct grease from restaurants and a variety of medical waste flushed into the collection system from healthcare facilities, assisted living and nursing homes.
The problem tied up a portion of his already limited workforce. The situation finally led him to explore pump technologies that might mechanically resolve the problem.
ITT Water and Wastewater studied the problem and recommended using a Flygt “N-pump" on a trial basis. The new-generation N-pump has proprietary features specifically engineered to combat clogging problems. Much of the clogging at this and other problem-prone stations was aggravated by the constrictive 3-in.-diameter throat of the existing pumps adopted from the Ten States Standards, a manual is referenced by ten Midwestern states, including Ohio, Illinois, Iowa, Michigan, Minnesota, Missouri, New York, Pennsylvania, Wisconsin and the Province of Ontario.
Duke was led to try the patented N-pump technology developed by ITT Water and Wastewater. These pumps have an inherent improvement due to a 4-in.-diameter discharge that, together with the unique impeller design, streamlines the passage of material. In field tests where recurring blockages were an issue elsewhere, the innovation not only eliminated clogging but was delivering significant power savings that would accrue throughout the expected service life period of over 50 years.
As Duke explained, "The semi-open, screw-shaped impeller would prevent clogging and complement the clear flow of fibrous material through the 4-in.-diameter discharge versus the common 3-in.-diameter throat adopted in the Ten States Standard."
In addition, the leading edge of the N-pump’s rotating impeller passes across a stationary relief groove located in the pump suction. The dynamic action cleans and pumps away any rags, stringy materials and solids from the impeller without losing hydraulic efficiency. The engineering is more reliable than traditional chopper pumps that can grind rags and other debris but are less effective against hard foreign materials. By eliminating debris-induced drag and allowing the N-pump to maintain RPMs, excess energy consumption is eliminated, often by more than 40%.
“We retrofitted the Station Number 10, our worst problem, with one N-pump and it performed admirably thereafter,” Duke said. He has since added the N-pump technology to two other pump stations and committed to a fourth whenever the District has available budget.
The field trial that replaced an existing 88-hp pump with a smaller, 66-hp N-type pump proved so successful that the county utility modified two other pump stations with an N-pump kits that delivered comparable reliability improvements. The District’s forthcoming SCADA system will enable the utility to closely monitor the stations and to remotely activate their more effective N-pump at the triplex station whenever the two conventional pump units transmit indications of clogging. Experience has shown the single N-pump can handle the flow until a crew reaches the station.
New technologies such as the N-pump, can level the playing field for wastewater utilities confronting challenging topography and other contributory factors. Some believe that as the N-pump gains acceptance, widely adopted design specifications may undergo revision to reflect the upgrade.