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The City of Waukegan's 18-mgd conventional water treatment plant (WTP) currently collects sedimentation basin residuals and filter backwash waste in an on-site Reclaim Basin. Decanted water from the basin is recycled to the head of the plant, while the solids are discharged to the sanitary sewer. However, these practices have led to several operational restrictions and water quality concerns.

Because the WTP's agreement with the sanitary sewer district restricts discharges to once per week and further prohibits discharges during holiday weeks or weeks of heavy rain, solids accumulate in the Reclaim Basin. The accumulation of solids leads to intensive manual cleaning of the basin and may also delay the start of a filter backwash if sufficient volume is not available in the basin for the backwash waste. Additionally, when solids accumulate in the basin the decant water that is recycled may introduce taste and odor components into the raw water supply.

As a result of these concerns, the City of Waukegan initiated a study to identify a residuals handling plan for the water treatment plant residuals that accomplished the following goals in the most cost-effective manner: (1) provided the greatest operational flexibility, (2) improved water quality, and (3) reduced maintenance requirements.

The recommended plan involved both operational improvements and capital improvements. Operational improvements included: (1) more evenly distributing backwashes throughout the week, (2) separating filter backwash residuals from the more concentrated sedimentation basin residuals, (3) discharging the sedimentation basin residuals directly into the Sludge Well of the Reclaim Basin, and (4) operating the Reclaim Basin as a batch thickening unit for filter backwash residuals. Capital improvements included modifications to the Reclaim Basin and construction of a gravity thickening unit. The gravity thickening unit is intended to reduce sanitary sewer discharges and provide pretreatment for a future dewatering process.

The operational limitations and water quality concerns that led the City to initiate the study will be presented. In addition, both the near-term and long-term proposed system improvements will be presented.
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Document Type: Research Article

Publication date: 2007-01-01

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