An Innovative Design for Residuals Collection Improves Settled Water Quality
- Z. Michael Wang, P.E., Ph.D, BCEE, David Laliberte, - Hazen and Sawyer
- Brent O’Neill, P.E., Illinois American Water
An innovative fixed grid residuals collection system was designed and has been in operation successfully since the 2010 upgrade of a 6 mgd water treatment plant (WTP). We verified and optimized the design using pilot studies and computational fluid dynamics (CFD) modeling tools. Continued full-scale operation has demonstrated that the fixed grid system has lowered capital and maintenance costs, improved residuals removal efficiency, saved water required for cleaning the basins, and reduced the down time and frequency of basin cleaning.
Before the upgrade the plant sedimentation basins had no sludge removal mechanism. The settled water turbidity was high. Several alternative sludge collection systems were considered and evaluated. However, the stacked double path sedimentation basin configuration presented unique challenges. The irregularly spaced supporting columns in the lower path made mechanical sludge collectors not applicable. Additionally, the sedimentation basins needed to be manually washed monthly, which was labor intensive, water demanding, and requiring taking one unit out of normal operation.
The final design adopted an innovative fixed grid sludge collection system. The collection nozzle orientations vary according to the local flow direction, so that the nozzles have the maximum projected opening area facing the approaching flow and in the direct path of particle settling. To optimize the nozzle orientation, CFD modeling was used to simulate a detailed flow pattern and particle settling paths in the sedimentation basins.
Initially implemented on pilot scale, the system was installed in a portion of the basin. Continued onsite side-by-side operation comparison demonstrated that the new system reduced effluent turbidity and residuals accumulation. The design was then expanded to full-scale based on the success of the pilot study. The improvements indicated by CFD modeling were validated by the real operation. Operators at the WTP love it.
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