Solving a Headworks Headache
Growing Poteau, Okla., community benefits financially from switch to grit-resistant slurry pump at municipal wastewater plant headworks
- By Eugene L. Smith
- Jan 01, 2004
Cost-effective engineering has enabled the City of Poteau, Okla., to achieve major savings while undertaking recent upgrades to municipal wastewater treatment facilities. The approach could provide a model for other communities that must stretch their resources in this troubled economy.
The 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."
The New Approach
A common sense approach characterizes the multiphase projects guided by the city's consulting engineers, WDB Engineering, based in Tulsa, Okla. 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 employee hours are needed now for maintenance caused by shortcomings in the previous design and equipment of the plant.
As presently configured, the 3-million-gallon-per-day (mgd) plant supports a 58-mile long collection system comprised of 6 inches to 15 inches of concrete, segmental clay and more recently adopted fused polyethylene lines. The plant uses chlorine disinfection, following an aborted use of ultraviolet (UV), with the activated sludge process. Sludge is disposed of on hayfields at the municipal airport and the treated effluent is discharged through a 24-inch line that extends from the plant to the Poteau River.
The use of UV lasted approximately one year and with 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."
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.
"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 2 feet of sand and gravel in the bottom. As a consequence, we had experienced problems with the aeration process."
An Equipment Alternative
Working with Automatic Engineering Inc., a manufacturer's representative for ITT Flygt Corp., 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 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 an ITT 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 almost two years ago to empty the flooded mine shaft during the heroic rescue of entrapped Pennsylvania coal miners.
The slurry 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 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 pump conversion ran only 10 percent of the originally considered structural modifications at the headworks, the Public Works official emphasized.
The pump changeout at the headworks followed significantly more ambitious upgrades reflecting the same cost-effective philosophy. A 2-inch rain back then prior to the pump change-out would have spawned a bypass with the attendant regulatory violation. Stricter environmental regulations also mandated secondary treatment in the process chain.
The plant underwent a physical expansion from 1.4 mgd to the present 3 mgd (5-mgd hydraulic rating) and the upgrade to the activated sludge process. Two digesters were built and an existing 90-foot diameter clarifier converted into an activated sludge basin for extended aeration. The armature and center apparatus were salvaged and used in one of two 75-foot diameter clarifiers also added during the project for secondary treatment. Because the adjacent creek used for discharge until then had an irregular flow, the state ordered that the 24-inch effluent line lead directly to the river. In addition, the holding capacity was increased by 300 percent to safeguard against former bypass during strong rains.
An Engineering Solution with Economic Benefits
The process conversion project lasted a year and further demonstrates the economies of sound engineering solutions. Whereas a new 3-mgd plant built in that area would have cost approximately $5 million at the time, according to David B. Wyatt, PE, a principal at WDB Engineering, the Poteau Public Works Department achieved the same results for a $2-million retrofit. "They enlarged the plant capacity from 1.4 mgd to 3 mgd and it now complies fully with stricter discharge regulations that once had them facing significant fines," Wyatt said.
Meanwhile, the city has continued a cost-effective line replacement and relining program in stages to correct infiltration and inflow through aging runs of the collection system, mostly 8-inch lines. The Public Works Department performs many of the replacement projects, using its own employees, construction equipment and a recently acquired TV-inspection system to plan the ongoing line upgrades.
"I can replace line using our workforce for about $22 per foot, compared to more than $40 per foot using contractors," Collins said. "We normally reline at creek crossings or where a conflict exists with underground utility runs or cuts that would intrude into high-traffic roadways or rail lines."
The upgrades to the wastewater facilities serving this small community demonstrate that economy and efficiency are not mutually exclusive.
Inflow and Infiltration Links
Wastewater Treatment Links
This article originally appeared in the 01/01/2004 issue of Environmental Protection.