Upgrading a wastewater treatment plant

Cost-effective engineering has enabled the City of Poteau, Oklahoma to achieve major savings while undertaking recent upgrades to municipal wastewater treatment facilities. The approach, using a grit-resistant slurry pump from ITT Industries' Flygt unit in the treatment plant headworks could provide a model for other communities.

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.

As presently configured, the 3-MGD plant supports a 93 km long collection system comprised of 15 to 38 cm 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 61 cm line that extends from the plant to the Poteau River.

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," Mark Collins, Public Works director recalls. "It presented an even greater problem following a project in 1999.

"A former clarifier being converted at that time to an activated sludge basin revealed how bad the I&I problem had grown because it had accumulated 60 cm of sand and gravel in the bottom. As a consequence, we had experienced problems with the aeration process," said Collins. Working with Automatic Engineering, Inc, the engineers explored an equipment alternative to building a gravity type grit separator at the headworks. 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. A Flygt HS5100 submersible pump was recommended with a casehardened impeller that would provide the durability absent with the diaphragm pump. In addition, the pump has an extended shaft with a small agitator to stir up solids and maintain efficient suspension. The slurry pump is normally used in abrasive industrial, power plant and mining wastewater environments.

The pump has higher sustained efficiency than a diaphragm pump. A diaphragm pump loses efficiency as its suction and pressure side check valves wear in such abrasive applications. Because of the method of installation of the pump, both the installing and, if service is eventually required, removal is much simpler. There is no suction side piping used for the pump, 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 centred 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 10% of the originally considered structural modifications at the headworks, the Public Works official emphasised.