Grit Removal: A Gritty Project

Jan. 11, 2016
Iowa water pollution control plant replaces aerated grit basin to improve fine loess sand removal

About the author: Marcia Sherony is national sales manager for Hydro Intl. Sherony can be reached at [email protected].

A successful project at the Council Bluffs (Iowa) Water Pollution Control Plant replaced an outdated aerated grit basin to improve fine loess sand removal and reduce digester cleaning in the existing tanks and sludge dewatering centrifuge repair. 

A stacked tray grit separation system (Eutek HeadCell) was retrofitted into the existing aerated grit basin, significantly reducing overall installed project costs. Eutek SlurryCup and Grit Snail systems provided clean grit suitable for landfill disposal.

Improving Fine Loess Sand Removal at Council Bluffs

Council Bluffs Water Pollution Control Plant has handled all of the community’s wastewater treatment needs since 1974. Most of the original equipment is still in operation, including the existing pretreatment facilities, primary clarifiers and two-stage trickling filters that culture bacteria on rock media to decompose organic solids. The plant added an activated sludge basin in 1997 to increase treatment capacity. It can handle 18 million gal per day for maximum daily flows and 30 million for peak flows.

The plant used its original headworks through 2003. The prior systems consisted of two bar screens with 1-in. openings and two aerated grit basins. In the grit basin, centrifugal blowers and diffused aerators suspended the biological particles; grit fell to the bottom of the basin and a chain-and-flight mechanism moved the grit onto bucket elevators, then to a belt conveyor, and finally into a collection container for disposal.

Plant supervisor Hank Pangelina said the old grit removal system allowed too much of the fine loess sand from local hills that trickled into the waste stream to pass through to the aeration basins.

Too Much Grit Getting Through

“We weren’t satisfied with the grit removal we were getting,” Pangelina said. “Too much was getting through, and it was creating a lot of wear and tear on the centrifuges downstream. We were spending thousands of dollars a year maintaining the centrifuges, and still they were wearing out faster than they should. We decided it was time for an upgrade.”

Plant operators went through an evaluation process and settled on a new gravity-forced vortex grit removal system. The system consists of four HeadCell grit-removal units, two SlurryCup grit washing units and one Grit Snail dewatering escalator. The system was provided by Hydro Intl.’s U.S. Wastewater Div., based in Hillsboro, Ore. The Council Bluffs wastewater staff looked at three alternatives before choosing the HeadCell option.

Gravity-Forced Vortex Grit Removal System

The gravity-forced vortex grit removal system offered a sizeable upgrade in grit removal efficiency, as well as a significant saving in concrete costs, and the promise of lower operating costs going forward. “The cost savings and the innovative nature of the HeadCell units really tipped the scales in favor of this option,” said Keith Hobson, vice president of Fox Engineering Associates of Ames, Iowa, which consulted on the project for the city.

The HeadCell is a modular, multiple-tray grit concentrator that removes grit as small as 75 μ with minimal headloss. 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 which moves it down to the center underflow collection chamber. These settled solids are continuously pumped to a SlurryCup washing and classification unit, which delivers the washed and concentrated slurry to a Grit Snail dewatering system.

The SlurryCup uses a combination of an open free vortex and the boundary layer effect to capture, classify and remove fine grit, sugar sand, snail shells, and high density fixed solids from grit slurries, and both primary and secondary sludge.

The Grit Snail captures fine grit and abrasives by providing sufficient clarifier area to retain 50-μ particles. A slow-moving, cleated belt gently lifts grit from the clarifier pool without re-suspending captured fine grit particles, which would allow them to escape with the clarifier overflow.

The four units—each 12 ft in diameter with seven trays—fit into the existing 20-ft-wide grit basins with some modifications for flow distribution. A portion of the old grit basin no longer  was needed and provides pre-aeration of the incoming flow prior to the biological process. During average flow conditions only one half of the system is operated.


“The fact that it was more compact was very important,” Pangelina said. “The HeadCell stacks fit right into our two tanks. Our engineers are pretty creative. They did a nice job of retrofitting our existing facility. If we had to tear that up, it would have cost multi-millions.”

Because the HeadCell has no moving parts, it operates without any control requirements and minimal maintenance. The SlurryCup, Grit Snail, grit pumps and grit dewatering belt require normal maintenance and operation checks. Pangelina said the system has worked well and saved the city a significant amount of money.

“It’s performing really well,” he said. “One of the indicators we had was, in the anaerobic digesters we haven’t seen grit. When you take a digester down and clean it out, you can see it. We used to have 3 ft of grit. Now we have very little.”

The wear and tear on the equipment no longer is a problem anymore either, Pangelina said. Before Council Bluffs installed the system, he said, the staff had to take the centrifuges down several times a year to repair abrasive damage or erosion on the scroll. Since then, they have only had to repair the scroll once.

“We’ve seen a number of paybacks,” Pangelina said. “There’s less wear on the centrifuge, I don’t have to take the digester off line and clean it. We didn’t have to build another pump station and maintain it. All in all, this system has done what we wanted at a very reasonable cost.” 

Download: Here

About the Author

Marcia Sherony

Sponsored Recommendations

Blower Package Integration

March 20, 2024
See how an integrated blower package can save you time, money, and energy, in a wastewater treatment system. With package integration, you have a completely integrated blower ...

Strut Comparison Chart

March 12, 2024
Conduit support systems are an integral part of construction infrastructure. Compare steel, aluminum and fiberglass strut support systems.

Energy Efficient System Design for WWTPs

Feb. 7, 2024
System splitting with adaptive control reduces electrical, maintenance, and initial investment costs.

Blower Isentropic Efficiency Explained

Feb. 7, 2024
Learn more about isentropic efficiency and specific performance as they relate to blowers.