Grit in biosolids was destroying the dewatering centrifuge at the Council Bluffs (Iowa) Water Pollution Control Plant. Damage to the scroll bowl and other wear cost $30,000 to $40,000 per rebuild, sometimes done twice a year. Grit also accu-mulated in the anaerobic digesters, reduced their capacity, and was difficult and costly to remove.

When the plant began a preliminary treatment process upgrade, division superintendent Hank Pangelina worked with the consultant to find a grit removal solution to install using existing structures. The consultant liked vortex separators because of their long history. That equipment, however, relies on velocity to remove grit. Efficiency diminishes as flows return to normal after rainstorms. Pangelina wanted a system with the same peak-flow effectiveness that worked at average flows.

“I knew about the Eutek (Hydro International Wastewater) TeaCup headworks grit removal and washing system, and that led us to the newer HeadCell, SlurryCup, and Grit Snail system,” says Pangelina. “The HeadCell units are built to trap specific-sized grit particles.” Since Hydro International installed the system in 2004, the dewatering centrifuge is back on a standard maintenance schedule. The system is meeting everyone’s expectations.

Loess hills

The Water Pollution Control Plant serves 19,000 residential and 1,600 commercial, industrial, and institutional accounts. It operates a two-stage trickling filter system followed by integrated fixed-film activated sludge treatment. Effluent is discharged to the Missouri River.

Grit in the biosolids has been a problem since the plant opened in 1974. “We’re a river town with a shallow water table, so the plant receives fine sands from the Missouri River,” says Pangelina. “Our second source of grit is the Loess Hills, a rare geologic feature. Loess dirt is a light, gritty, porous material that erodes easily and winds up here.”

The original plant had two coarse bar screens with 1-inch openings. They discharged to two aerated rectangular grit chambers with a chain and rake collection system. The settled grit was raked into a bucket elevator and deposited in a 20-cubic-yard roll-off container. During dry weather, waste haulers emptied the container twice a week.

Some grit still accumulated in the anaerobic digesters and caused wear on the centrifuge. “We did major repairs on the machine once a year, and that’s abnormal,” says Pangelina. When repairs were imminent, the staff pulled the biosolids inventory as low as possible to provide a four-week work window. Occasionally, repairs lasted five weeks. “Timing repairs with our dewatering needs was a constant challenge,” says Pangelina.

Built to size

Engineers from Hydro International worked with city consultants to design the system as part of a retrofit of the old plant’s 15-foot-deep north and south aeration basins. The design called for 12-foot-diameter trays with seven per stack, two stacks per basin to achieve the required 75-micron removal. Building 12-footers was a first for the company.

The engineers wanted to do a particle-size analysis of the grit in the influent, but the heavy rain necessary to move the material through the system never arrived. “Grit particles coated with FOG, soap, and organics settle more slowly than equal-sized clean sand particles,” says Pangelina. “Grit removal systems must be sized to capture those smaller particles. Without the analysis, we decided on 75-micron removal.”

During installation, the plant headworks was bypassed for almost a year. The staff built screens to trap debris in the sewage before it damaged pumps and filled digesters, then cleaned them two or three times per shift. Handling the frozen material during winter made the work cold and miserable. “It was a challenge, and I give my guys a lot of credit,” says Pangelina. “They worked hard to keep things moving.”

Settling particles

After the retrofit, each aeration basin had two stacks of seven trays in a stainless steel frame. The trays, resembling plastic funnels, are stacked one inside the other with a small gap between them.

Wastewater flows through the bar screen set, down a stainless steel channel, and to a proprietary manifold that evenly distributes the flow to each HeadCell unit and its seven trays. Instead of falling 15 feet to the bottom of the old aerated grit tank, particles now settle 12 inches before hitting the bottom of the tray. Once captured, grit works its way down the center of the stack until it reaches a collector box in the bottom of the tank.

A pump sends the grit in the collector box to the SlurryCup, which uses an open free vortex and boundary layer effect to capture, classify, and remove fine grit, sugar sand, snail shells, and high-density solids. It separates organic content from the grit and returns it to the plant for treatment.

Grit and fine abrasives go to the Grit Snail dewatering device and settle onto an escalator belt of slow-moving stepped cleats. Dewatering begins as particles are gently lifted out of the unit’s clarifier pool at 1 to 5 fpm. Grit-free water flows over a weir and out the clarifier. The belt carries dewatered abrasives to the top of the machine and discharges them into the dump container.

No pump station

“Installing other grit removal systems would have required building a pump station or upgrading our force main pump stations to operate at a higher head,” says Pangelina. “Since we didn’t have as much headloss across the HeadCell units, we avoided those expenses.”

The HeadCell units require only quarterly maintenance. “We take one side down, dewater it, and clean it with a hose,” says Pangelina. “We then do a quick inspection to make sure the fasteners are tight before putting it back online. They are trouble-free.”

Pangelina received numerous phone inquiries about the system and some visitors. As a result, two other Iowa treatment plants are installing HeadCell units as part of upgrades.