Grit To Green

A combined grit separation, washing and dewatering process leaves a Wisconsin treatment plant with lower operating costs and two desirable byproducts.
Grit To Green
One of two RoSF 4 COANDA grit washer units (Huber Technology). One unit operates at all times. By rotating the units, the plant can keep one on hand for standby and perform maintenance on one unit while the other is online.

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The team at the Isle La Plume Wastewater Treatment Plant in La Crosse, Wis., wanted to optimize treatment process efficiency and effectiveness, but its grit screw system was not performing to expectation.

Because of the odor, hazardous nature, and sheer weight of the organics-laden grit, the plant’s disposal fees were significant. Cleaning the grit would immediately reduce disposal costs, so the plant team set out on a process improvement initiative. They solved the problem by installing two RoSF 4 COANDA grit washer units from Huber Technology.

“We didn’t realize that what we were doing was unique until we listened to comments made during tours of our facility regarding plant automation and the green contributions we’re making in our community,” recalls Jerod Greeno, plant superintendent.

Cutting the weight

The Isle La Plume plant is one of two serving the city of La Crosse and several neighboring entities. The system includes 26 sanitary lift stations, and 188 miles of sanitary sewers that handle 10.5 mgd from 85,000 residents.

Isle La Plume’s grit challenges revolved around weight reduction. Annual cost to aggregate and haul-off grit consumed a significant portion of the plant’s budget. Because the existing process could not properly clean the grit and separate the organics at pretreatment, the plant team was left with a useless and rather odiferous and hazardous byproduct.

Annual grit haul-off alone cost $7,530. The odor and harmful fumes from the organics in the grit made it undesirable and costly to landfill. The plant team knew the organics were the problem and that a grit-washing process would reduce the weight and the hazardous material content. If the organics removed could be further treated, both the grit and organic material could be turned into useful byproducts and more easily and economically removed from the plant.

Excellent fit

The RoSF 4 COANDA grit washer met those objectives and easily integrated with the plant’s existing automation. The system combines grit separation, washing and dewatering in one process and a single, compact unit. Combined classifying and sorting enables high separation efficiency and effective washing performance.

The solids in the flow (grit particles and organic material) are separated by way of flow diversion and flow velocity reduction. Grit particles sink to the bottom of the tank. The flow pattern in the system leads to more than 95 percent separation of 0.20 to 0.25 mm grit particles.

The separated grit is then washed, detaching organic matter from the mineral grit particles. After removal of the organics, the clean grit is removed through a classifying screw, statically dewatered and discharged into a container. The organic material left in the grit washing plant is removed automatically for further treatment.

The Isle La Plume plant installed two units, one of which is operational at all times. By rotating the units, the plant never runs the system without the grit washers in place and never suffers system downtime from unit maintenance.

Startup required only priming of the system with enough grit for processing and then adjusting the minimum levels to fit the plant team’s preferences. Greeno describes the system as working flawlessly with little operator attention. Periodic gearbox maintenance and oil changes are the only interruptions to the units’ operation.

Positive outcomes

Since the washing system went online, grit haul-off fees have been reduced significantly — through reduction in both volume and hazardous content. Whereas previously 125 tons of grit was hauled at a cost of $65 per cubic yard ($7,340 a year), now 48 tons are hauled (a 79 percent reduction) at $12.50 per yard, totaling $600 a year.

“The reduction in grit quantity and weight is so significant that we no longer need a dedicated hauling person, since our haul-off schedule has been reduced to one 7-cubic-yard dump container every five weeks,” says Greeno.

The plant eliminated risk from the previous haul-off process and made the process much less equipment- and labor-intensive. Previously, the material was hauled to a dumping yard via a vat-dumping and load car system. The grit washing system removes organic material so effectively that it can be delivered directly to the treatment process without human intervention.

The plant now produces sandbox-grade grit. “The RoSF 4 COANDA system leaves us with no waste,” Greeno observes. “Both the grit and organics byproducts are quite useful. In fact, the city landfill actually wants our treated organics to use as cover, and the grit is suitable for use in all sorts of landscaping applications.”

The dumping yard where the offensive grit had been stored is no longer needed and has been turned into part of a biking trail that runs from one end of La Crosse to the other.

About the author

T.R. Gregg is national marketing manager with Huber Technology, a supplier of liquid-solid separation solutions for municipal/industrial wastewater treatment systems. He can be reached at trgregg@hhusa.net.   



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