Grit Washer - Thorn Creek Sanitary District, IL (2003)

The Thorn Creek Basin Sanitary District’s Wastewater Treatment Plant in Chicago Heights, Illinois, was experiencing periodic rejection of waste solids at the landfill disposal site due to excess water.

A substantial portion of the excess water was contained in organic laden grit removed from the plant’s two grit removal tanks. Settled grit is removed from the grit tanks by a bucket elevator system and discharged to a horizontal screw conveyor that empties into a dumpster. The District decided to add a grit washer at the screw conveyor discharge point to remove water and organic material from the grit before discharging to the dumpster.

NIES Engineering, Inc. reviewed 5 installation alternatives, selecting an arrangement with orientation of the grit washer at a 60 degree angle to the axis of the screw conveyor. This minimized the cost of the building expansion required to house the new grit washer while maintaining maintenance access to the equipment and allowing for a bypass grit discharge point from the screw conveyor in case the grit washer was out-of-service.

Purchase of the grit washer equipment was bid separately and the Parkson Tru-Grit 400 grit washer, rated at 15 cuft/hour, was selected. In this unit, grit enters the top of the barrel section while washwater is injected at the bottom so that descending grit flows countercurrently to the rising water flow. The grit is also stirred to help separate organic matter, organic laden water is discharged from the top of the unit and clean grit settles to the bottom where it is removed by a shaftless spiral screw conveyor. The project construction cost of $373,300 also included rehabilitation of existing bucket elevator equipment and replacement of existing screw conveyor equipment, construction of a building expansion, HVAC system upgrades, enhanced equipment controls suitable for SCADA integration, site paving and piping improvements, and electrical and lighting system enhancements.

The District solved the excess water problem, removed 98% of organic material from the grit and reduced the total quantity of solids to be disposed at the landfill.

Key Points

  • Multi-faceted
  • Cost Effective Design

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