Grit-resistant slurry pump for Oklahoma wastewater treatment plant

The Poteau, Oklahoma community of 8,000 residents serves as a commerce center for a large swath of the scenic Poteau River Valley. Local government has placed a higher emphasis on reducing the cost of some services in recent years. The Public Works Department has experienced a reduction in force from 20 to 16 field personnel, with three operators assigned to the wastewater treatment plant. Many line improvements are done by department crews rather than contractors, usually at half the cost.

"Our system is growing now at about 10 percent a year," said Mark Collins, Public Works Director. "For the past decade, we’ve been involved with a series of projects that led to better methods and improved material specifications intended to meet both immediate and long-term objectives."

Making Existing Facilities Work More Efficiently

A common sense approach characterizes the multi-phase projects guided by the city's consulting engineers, WDB Engineering, based in Tulsa. By preserving existing infrastructure instead of resorting solely to new construction, the utility has saved millions of dollars while achieving operational improvements that overcame capacity constraints and brought the treatment plant into consistent environmental compliance. In addition, fewer man-hours are needed now for maintenance caused by shortcomings in the previous design and equipment of the plant.

As presently configured, the 3-MGD plant supports a 58-mile long collection system comprised of 6" to 15" concrete, segmental clay, and more recently adopted fused polyethylene lines. The plant uses chlorine disinfection, following an aborted use of UV, with the activated sludge process. Sludge is disposed of on hayfields at the municipal airport and the treated effluent discharged through a 24" line that extends from the plant to the Poteau River.

The use of UV lasted approximately one year, and provided poor results. “The state came down because our fecal coliform was consistently out of compliance and told us that the UV simply wasn't working,” Collins said. “The notice of violations kept mounting up and an administrative order finally came down. Then the UV manufacturer went out of business. We pulled the system and went with chlorine.”

An Effiective Alternative Headworks Modification

A recent project resolved the chronic failures of a diaphragm pump in the grit chamber at the plant headworks. The sloped pit holding the pump was cast into the headworks at the time of construction in 1978 and interfaces with a cyclone-type separator. The arriving influent passes through an automatic bar screen and then reaches the grit chamber whose previous diaphragm pump failed weekly due to blockages or cuts in the diaphragm. The problem, attributed to incoming sand, sharp objects and other material brought in by inflow and infiltration (I&I) of leaking lines, became so pronounced that the Public Works Department initially anticipated a costly reconstruction of the grit chamber.

The Department started smoke testing the lines to identify sources. An initial project was then funded to repair only the leaking joints but that proved ineffective. “The I & I became evident in the early 1990s,” Collins recalls. “It presented an even greater problem following a project in 1999.”

“The pump was off line more than it operated and we kept getting written up by the Department of Environmental Quality for inadequate screening,” he added. "The magnitude of the problem became especially obvious when we remodeled the plant in 1999. We were then limited to only one discharge point at the plant and lost storage capacity when one of our two former lagoons was eliminated and converted into a landfill. The state said one lagoon would be insufficient, which is when we began our various line replacements.

Working with Automatic Engineering, Inc., a manufacturer's representative for ITT Flygt unit, and Flygt's Southwest regional office, the city's consulting engineer explored an equipment alternative to building a $250,000 gravity type grit separator at the headworks. Jim Smith, District Manager for Flygt, and David Hickman, with Automatic Engineering, studied the headworks for a Flygt submersible pump that would reliably perform the mission.

The grit chamber presented a large, concrete cone cast into the bottom of the channel from where the influent was pumped through a grit separator at grade before flowing into four large pumps at the start of the treatment process. Smith and Hickman recommended a Flygt HS5100 submersible pump whose casehardened impeller would provide the durability absent with the diaphragm pump. In addition, the Model HS5100 pump has an extended shaft with a small agitator to stir up solids and maintain efficient suspension. The Model HS5100 slurry pump is normally used in abrasive industrial, power plant and mining wastewater environments and was among those used approximately two years ago to empty the flooded mine shaft during the heroic rescue of entrapped Pennsylvania coal miners.

The HS5100 has higher sustained efficiency than a diaphragm pump. A diaphragm pump looses efficiency as its suction and pressure side check valves wear in such abrasive applications. Because of the method of installation of the HS5100, both the installing and, if service is eventually required, removal is much simpler. There is no suction side piping used for the HS5100, as is needed for the diaphragm pump. Given the huge difference in cost and operational intrusion between reconstruction and a pump replacement, WDB Engineering recommended that the city first try the hybrid pump.

The submersible pump neatly centered into the bottom of the grit chamber from where it pumps influent straight up and into the inlet of the hydrocyclone. The solids are spun to the sides of the hydrocyclone and drop to the bottom where they are discharged through a line to a trough equipped with an auger-like screw that lifts and transfers the solids into a plastic disposal bucket.

The cost of the Flygt pump conversion ran only ten percent of the originally considered structural modifications at the headworks, the Public Works official emphasized.

The installation of the new Flygt pump, as well as other continuing upgrades to the wastewater facilities serving this small community clearly demonstrate that economy and efficiency are not mutually exclusive.