Eliminating the Ripening Spike and Saving Water - Optimizing Filter Backwash at a 30 MGD WTP

Authors:

  • Jay Jackson, PE - Hazen and Sawyer
  • James Amburgey, PhD, PE - UNC - Charlotte
  • Barry Parsons - City of Greensboro

Extended Terminal Sub-fluidization Wash (ETSW) was evaluated to provide improvement in filter performance and efficiency at the City of Greensboro’s 30 MGD Townsend WTP. ETSW can be used to reduce filter ripening time, eliminate the ripening spike, and to provide for enhanced filter performance for compliance with LT2 Bin 2.

Filter backwash procedures should be evaluated to ensure that adequate wash rates and bed expansion are achieved. At the same time, the backwash sequence should be optimized to ensure that excessively long backwash periods and/or excessively high wash rates are not practiced. It is important to remember that backwash rates should change (up to 50%) proportional to the water temperature.

Backwash rates should be sufficient to expand the filter media. These rates can be calculated and then confirmed by field tests on the water plant filters. Calculations and field instruments were used to confirm wash rates and bed expansion at the Townsend WTP. Turbidity measurements of the backwash waste water were taken every minute for an extended backwash period, and the time to remove 95% of the total turbidity was determined. The results showed that a significant reduction in wash water use could be realized while still effectively cleaning the filter.

Ceramic media as a cap to existing media was evaluated as a means to extend filter run times.

For more information, please contact the author at jjackson@hazenandsawyer.com.

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