Challenge

The Racine, Wis., Water Utility draws its raw water from Lake Michigan, and needed to increase the capacity of its clarification facilities in order to improve turbidity control, especially during the spring runoff from melting snow and ice.

The expansion was complicated by the fact that the Utility’s existing concrete sedimentation basins were situated underground, and were hemmed in by city streets on three sides, with the water treatment plant on the fourth.

Solution

The Utility’s engineering consultant—Camp, Dresser & McKee, Inc., Chicago—recommended a plan to increase clarification capacity by retrofitting the existing sedimentation basins with Zimpro® inclined plate separators. The two existing basins were each about 4,850 square feet in area.

In the CDM design, each basin was retrofitted with inclined plate separator “packs", increasing the effective settling area of each basin by seven fold. The retrofit was accomplished in two phases in order to provide an uninterrupted flow of drinking water to the community.

In plate separators, flocculated water flows upward between the inclined plates, and solids settle out on the plate surfaces and slide down the plates into thickening and collection zones below. Clarified water flows through v-notch weirs and into a top-mounted effluent trough. Sand filtration and disinfection follow.

Results

Performance tests were conducted using Lake Michigan water. The inclined plate separators exceeded the required turbidity removal, using substantially less chemicals than allowed by the specifications. Flow-weighted turbidity during testing averaged 1.57 NTUs in the effluent, with influent turbidity of up to 36 NTU. Since then, influent turbidities of more than 100 NTU have not impacted effluent turbidity.

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