When Older is Better

The Pittsburg (Kan.) Wastewater Treatment Plant uses trickling filters and ingenuity to ward off wet-weather flows and stay in permit compliance

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In the old days, coal mining sustained the economy of Pittsburg, Kan.

Today, the abandoned mines contribute to serious infiltration and inflow that can push volume at the city’s 3 mgd wastewater treatment plant to 14 mgd and beyond. Rainwater floods the old pits and mine shafts, forcing it into the sewer system, much of it clay pipe construction.

But the wet-weather flows have not overwhelmed the treatment plant, operated by superintendent Mike Brown and his staff of six. In fact, using old technology and plenty of ingenuity, they created an effective wet-weather plan that has kept the plant in compliance, with no violations in the last 10 years.

Critical to success are two rock-filled trickling filters installed decades ago. It might be a case of older being better. “We have a plant with a large footprint and some antique equipment, but Mike and his staff make it work,” says Jim Tush, director of public utilities. “They’ve made a number of operational changes that enable the plant to withstand the I&I flows.”

Normal times

Flow enters the plant from Pittsburg’s 140 miles of sanitary sewers, passing through a bar screen before being lifted by screw pumps to a “pre-air” grit removal area. Circular primary clarifiers with scum removal gear are next in the line, followed by the trickling filters, 140 feet in diameter and 10 feet deep. In one, wastewater is fed to the media through a two-ended distributor arm. The other has a retrofitted four-ended distributor mechanism supplied by WesTech Engineering Inc.

Further treatment and nitrification is provided by a Schreiber LLC extended aeration activated sludge system. After final clarification, the effluent flows to a two-train, 16-bulb Aquionics UV light disinfection system before discharge through a cascade re-aeration channel to Cow Creek.

Biosolids and floatables from the primary clarifier move to an anaerobic digester that operates in the mesothermic range. Digester gas is used to heat the digester via heat exchange, and any excess is flared off. Secondary solids are digested aerobically. Both solids streams are conditioned with polymers before dewatering on an Ashbrook Simon-Hartley belt press. The staff takes care to select a polymer that will work effectively on both primary and secondary solids.

City trucks haul the dewatered cake to area farm fields and spread it using a city-owned tractor and slinger machine (KUHN North America). In cases where additional solids accumulate, the plant has drying beds available for additional dewatering.

When it rains

That’s the design flow scheme, but when it rains, Brown and his crew have developed a number of different paths for overflows to follow so that the plant stays in compliance. It’s like rerouting traffic when the main road becomes congested.

“We can handle up to about 6 mgd through our normal system, although at that flow rate we would bring on both UV units,” says Brown. “At normal flows, we would use only one.”

At flows of 6 to 14 mgd, the excess water passes through the old trickling filter plant and still gets treatment but is routed around the activated sludge system. “We use our tricklers, but we take the excess flow from the tricklers directly to our secondary clarifiers and then to discharge,” Brown says.

At even higher flows, which occur with some regularity during especially wet seasons, excess flow passes through the screens and is lifted by the screw pumps directly to discharge.

A number of control structures help Brown and his staff manage these wet-weather events. Maintenance supervisor Ken Bell explains that the structures are actually gates that can be raised or lowered manually to help control or divert flows. During normal weather, control structure No. 2 allows the staff to bypass the trickling filter and the pre-air grit removal step and pass flow directly to the activated sludge system.

Control structure No. 3 lets operators return flow from the secondary clarifiers to the activated sludge system for additional treatment. “But when we’re running hot and heavy, we use control structure No. 2 to store overflow ahead of the activated sludge system and prevent washouts,” says Bell. “Control structure No. 3 can be adjusted to ease flow to the secondary clarifiers.”

A gate at the head end of the plant provides additional control. “We’ve set it at about seven feet so that as soon as the pumphouse starts getting overwhelmed, we can pass excess flow directly to the screw pumps and to discharge more quickly,” says Brown.

Extra control

The Pittsburg crew has made changes to these structures to provide even more control of the overflows. They’ve raised the weirs and created a space below the weir line that they can open or close to let more or less water through. “It’s a better way to manage this water,” he says.

Another wet-weather management technique involves close monitoring of various manholes along the sewer system where known I&I issues exist. Through visual monitoring and an increasing number of flowmeters, the Pittsburg team is developing a better picture of the location and scope of the problems during rain events.

All these adjustments have paid off. The plant stayed in compliance on the basis of its seven-day and monthly average permitted levels, and things are a lot less hectic at the plant and around the system. Brown recalls that before, the plant would fill up and sewer backups were common. “We’d have the pumps on and be frantically bypassing as fast as we could,” he says. “And it might last for a day or a day and a half.”

Now, he says, bypassing seldom extends beyond four or five hours before operations return to a level his staff can handle. “We’re able to control the things we can control in managing stormwater,” Brown says. “We’ve alleviated the pressure on the plant.”

Fixing the sewers

Pittsburg is also addressing the problem of aging sewers. Tush explains that the city is using State Revolving Loan funds to replace, repair and line sewers one section at a time, and to continue installing flow-monitoring systems.

“We’re spending our funds on the sections that give us the most impact, and we still have a lot to do,” he says. “Ours was not intended to be a combined sewer system, but we’re making progress. This was a pretty leaky system.”

Two projects on the immediate horizon will also help. The old Cen-tennial pump station will be shut down, and all existing equipment will be replaced and located on an adjacent site. Centennial is ancient and is starting to rust through and fall apart. The dry well is 30 feet deep and is a safety issue. Bell says new submersible pumps being installed there will make the station much easier to maintain.

At the Southeast station, equipment is old, dating to the 1970s. “The update will include new pumps and controls, including variable speed drives,” says Bell. Both stations have been maintenance headaches, and the improvements will help ease frustrations.

“When it rains, I know I’m going to get a call to go out to one or both of these old stations,” Bell says. ­ Brown adds, “With the new equipment, we’ll have much more confidence in our system.”



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