Dual strategy for wastewater treatment

June 2009 » Feature Articles
The city of Peru, Ind., is a typical, small Midwestern city, where manufacturing and industry drive the local economy. At the end of the 20th century, this industrial city 75 miles north of Indianapolis experienced a period of economic growth. However, this welcome growth did not come without consequences. The addition of a bacon processing plant and large recreational vehicle manufacturer brought jobs to the area, but it also strained the resources of the city’s existing wastewater treatment plant.
Marcia Sherony

Project
Wastewater treatment plant upgrade, Peru, Ind.
Project engineer
Advanced Technology Services, Westchester, Ohio
Product application
Additional capacity and a Headcell grit-removal system solves suspended solids problem in wastewater treatment plant effluent.
Peru, Ind., removes grit and sludge using a mix of biological and mechanical processes.

By Marcia Sherony

The city of Peru, Ind., is a typical, small Midwestern city, where manufacturing and industry drive the local economy. At the end of the 20th century, this industrial city 75 miles north of Indianapolis experienced a period of economic growth. However, this welcome growth did not come without consequences. The addition of a bacon processing plant and large recreational vehicle manufacturer brought jobs to the area, but it also strained the resources of the city’s existing wastewater treatment plant.

The plant struggled to keep up with the increased flows. During heavy rains, the level of suspended solids in the plant effluent exceeded state safety limits. To solve this and other water treatment problems, the city opted to revamp the entire treatment plant, adding significant capacity and improving its processes.

Another serious problem facing the city of Peru was grit. There was an excessive amount of grit being transported to the plant from the aging collection system, especially during rainstorms. The facility, state of the art in the 1930s, had a Dorr Oliver grit rake and Cyclone degritter to remove grit. However, despite numerous rebuilds, the original equipment was no longer up to the task. The biosolids, which were land applied, were loaded with grit. At one point, a 3- to 4-foot buildup of grit was cleaned from the primary clarifier.

An effective grit-removal system was an important criterion to the new design. City officials were strongly considering a solids-reduction process to eliminate the need for land application of the biosolids. Advanced processes such as solids reduction must be protected from even fine grit. Another important consideration was the ability of the system to perform effectively at all flow levels, especially considering the wide swings in flows, with an average flow of only 3.5 million gallons per day (mgd) and a peak design flow of 26 mgd.

The city needed to find a highly efficient grit-removal system to alleviate these concerns. Failure to effectively remove grit and other nuisance abrasives results in downstream plugging, loss of process space, and excessive wear on mechanical equipment.

Settled solids are continuously pumped to a SlurryCup grit washer and classifier and Grit Snail dewatering escalator.
The engineering firm Advanced Technology Services of Westchester, Ohio, proposed installation of a Headcell grit-removal system, a technology capable of removing grit particles as small as 100 microns. "This was very attractive to us because grit had been such a problem in the plant," said Mike Dalquist, superintendent of Water and Sewer Management for Peru Utilities. "There wasn’t another system out there that offered that kind of efficiency in the same small footprint."

The Headcell, supplied by Hydro International (at the time, operating as Eutek Systems, an Oregon-based maker of grit-removal, dewatering, and transport systems), is a modular, multi-tray settleable solids concentrator that removes grit with minimal headloss. The Headcell Its stacked tray design provides a large surface area and short settling distances capture fine grit particles. The Headcell evenly splits flows and eliminates the short circuiting that often degrades the performance of conventional grit basins.

The small footprint of the Headcell made it easy to incorporate into the new wastewater treatment plant headworks. The high-efficiency flow distribution header evenly distributes influent over multiple conical trays. Tangential feed establishes a vortex flow pattern where solids settle into a boundary layer on each tray, and are swept down to the center underflow collection chamber. These settled solids are continuously pumped to a SlurryCup grit washer and classifier and Grit Snail dewatering escalator.

The SlurryCup uses a combination of an open free-vortex-type flow regime and boundary layer effects to capture, classify, and remove fine grit and other high-density fixed solids from grit slurries, and both primary and secondary sludge.

The grit underflow from the SlurryCup passes through a hydraulic valve, which provides secondary grit washing before being discharged into the large clarifier pool of the Grit Snail which is designed to retain fine particles. A cleated belt moving 1 to 2 feet per minute gently lifts captured solids out of the clarifier, allowing the grit to be dewatered without re-suspending it and reintroducing grit to the plant. Dewatered grit is carried to the top of the Grit Snail, where it is discharged into a disposal container for landfilling. The cleaned grit typically contains 60 percent total solids with less than 20 percent volatile or organic solids. This approach minimizes the volume and weight of material hauled to solid waste disposal sites, and reduces odors and associated complaints.

A cleated belt moving 1 to 2 feet per minute gently lifts captured solids out of the clarifier, allowing the grit to be dewatered without re-suspending it and reintroducing grit
"The volume of grit being removed has increased substantially," Dalquist said. "The original plant typically was able to remove just half a cubic yard of grit, and allowed particles the size of coffee grounds to get past the headworks building. With the new grit-removal technologies in place, the upgraded plant is removing 1.5 to 2 cubic yards of grit during average flows, and turning this into clean, dry, odorless material to be landfilled. We conducted inspections in the lab and found the grit-removal system performing exactly as advertised.

"Initially we had planned on installing a third component to remove inert material, but the new biosolids-reduction process and Headcell systems are working so well we decided against the further technology installation," Dalquist said. "This alone saved us about $1 million."

By February 2006, the new plant was online. At that time, Peru was able to retire its original treatment facility, which could only handle 4 to 6 mgd. The city is now able to treat 8 mgd, up to a peak of 26 mgd. The increased capacity ensures that the system runs optimally, even during heavy rains. The quality of the effluent that flows into the Wabash River has improved significantly—now measuring less than 10 mg per liter in total suspended solids, down from an average in "the high teens" and periodic spikes over 45, Dalquist said. Before, plant sensors "almost always" detected ammonia in the effluent; now they hardly ever do, he said.

Moreover, the wastewater plant has a sustainable means to remove grit and keep the treatment facility running smoothly long into the future. It was so successful that this model treatment plant has now been emulated by other facilities across the country.


Marcia Sherony is a national solutions manager for Hydro International’s wastewater division. She has worked with market leaders in liquid/solid separation for more than 20 years.


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