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Primary Drinking Water from Wastewater Effluent: Pilot Test Experience at the Miami Dade South District WWTP

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The Miami-Dade Water and Sewer Department (MDWASD) owns and operates the South District Wastewater Treatment Plant in Goulds, Florida. This is a 225 mgd peak flowrate secondary oxygen activated sludge treatment facility that discharges effluent via deep injection wells. Based upon the detection of nitrogen species in the monitoring wells for the deep injection well system, MDWASD entered into a Consent Order with the Florida Department of Environmental Protection (FDEP) to address these concerns. The Consent Order requires a significant increase in the level of treatment for the facility. The resulting effluent standards and requirements for this facility include high level disinfection (HLD) as defined by the FDEP, Primary Drinking Water Standards, addressing peak flowrates, and future participation in the Comprehensive Everglades Restoration Program (CERP).

Primary Drinking Water Standards were addressed by a series of analytical efforts plus the implementation of an anoxic zone in the fifth of six stages of the oxygenation tanks at this facility. Peak flowrates were analyzed and are being addressed in the near term via secondary treatment plus HLD treatment for 285 mgd of peak hourly flowrate, and in the future, via the use of high rate clarifiers plus HLD treatment for peak flowrates above 285 mgd and up to a potential future peak hourly flowrate of 450 mgd. HLD standards per the regulations are based around filtration to less than 5 mg/l of TSS on a continuous basis plus disinfection. With the need for continuous compliance to a standard that has typically only been applied to reuse systems, which typically have alternate means of effluent disposal, it was apparent that these conditions were unique.

To address the unique circumstances, a literature search and review of similar facilities was performed. These investigations resulted in the need to pilot test deep bed sand and disk filtration processes in order to confirm potential for continuous compliance and define appropriate design criteria.

Initial pilot test screening of these two alternative filtration approaches resulted in elimination of the disk filters from further consideration and testing since the data did not support compliance at “acceptable” loading rates (3 gpm/sf and higher). Deep bed mono-media sand filter tests resulted in supporting their application for this HLD upgrade and the use of a 6 gpm/sf hydraulic loading rate as a full-scale demonstration design value. The Consent Order allows this one year demonstration with a recommended loading rate with possible, subsequent process supplementation. Criteria from the pilot testing also supported the need to ensure about 15 % redundancy of filters, plus capability to backwash and air scour up to 15% of the filters to address solids and minimize the potential for filter plugging. On-line TSS metering devices were also tested. Results of this testing indicated benefit for control of pilot filter target influent TSS levels, but inconclusive data for filter effluent TSS monitoring. The data indicated that the on-line TSS meters should be further tested in the full-scale system with less variability of loading conditions and generally better and more focused control of the meters.

Document Type: Research Article


Publication date: January 1, 2006

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