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

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ABSTRACT
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.

INTRODUCTION
Miami-Dade Water and Sewer Department (MDWASD) owns and operates three major wastewater treatment facilities that manage about 370 mgd on an annual average basis. Their South District Wastewater Treatment Plant (SDWWTP) located in Goulds, Florida provides secondary treatment to about 90 mgd of domestic wastewater and discharges the resulting effluent via a deep injection well system. This facility is currently rated and permitted capacity is 112.5 mgd annual average flowrate, 225 mgd on a peak hourly basis. Future wastewater discharges and lift station improvements result in an anticipated increase in treatment needs to 131.25 mgd annual average flowrate, with 450 mgd on a peak hourly basis.

Based upon the detection of nitrite in the injection well monitoring system, MDWASD entered into a consent order with the Florida Department of Environmental Protection (FDEP) in April of 2004. Concerns for potential leakage of treated effluent into a possible underground source of public drinking water led to the Consent Order requirements for implementing a High Level Disinfection (HLD) system and for producing effluent that meets primary drinking water (PDW) standards prior to disposal. The C.O. indicates that HLD must be placed in service by April 29, 2009. Further, recent EPA regulations require that all Deep Injection Well implement HLD treatment by the end of 2010. The final rule is entitled “Revision of Federal Underground Injection Control Requirements for Class I Municipal Disposal Wells in Florida”. These regulations make HLD requirements widespread in their impacts, not just an impact to the MDWASD SDWWTP.

Compliance with the majority of the PDW Standards will not be an issue for the existing SDWWTP. Also, MDWASD was able to expeditiously implement an anoxic zone in their high purity oxygenation system to address the nitrogen concerns. However, compliance with the Consent Order requires expeditious design and construction of secondary treatment process improvements for reliability and of HLD upgrades. HLD Upgrades are defined by the Regulations which indicate the need for filtration followed by disinfection. Of key concern for ensuring compliance with the HLD standards imposed by the Consent Order is the requirement for effluent Total Suspended Solids (TSS) less than 5 mg/l on a continuous basis under various filter influent loading conditions. Typically, WWTP’s with reuse capability that are required to meet these HLD standards also have alternate methods of effluent disposal which provides relief from the “continuous” compliance concern. In addition to filter influent loadings based upon typical secondary clarifier effluent TSS levels of 20 mg/l or less (about 10 – 20 mg/l), the filters must also be capable of handling Influent TSS of 45 mg/l, which is the weekly average TSS permit limit for the secondary WWTP, as well as higher TSS levels which are expected, although infrequently.

In addition to primary drinking water standards and HLD requirements, the C.O. required MDWASD to address peak flows at the SDWWTP and develop a peak flow proposal. The peak flow proposal that resulted defined the use of high rate clarifiers (HRC’s) for addressing peak flows (flows above the capacity of the existing biological treatment system). Pilot testing conducted at the SDWWTP for implementation of HLD upgrades also addressed testing of HRC’s for raw wastewater treatment. Testing conditions included HRC effluent blended with the effluent from the secondary process plus filtration as well as effluent from the HRC’s plus secondary clarifiers blended and then filtered. As a result, pilot system drawings and aerial photos show the HRC’s. However, these HRC tests are associated with potential future conditions and were considered as preliminary and not as critical as filtration tests of secondary effluent in order to address HLD requirements. Based upon the need to expedite the HLD upgrades, inconsistent preliminary results of testing with HRC’s and other considerations, HRC pilot tests were not fully completed as part of this program and therefore are not addressed further in this paper (other than inconsequential references / illustrations on some figures).

A literature search and check on WWTP’s with treatment standards similar to HLD standards determined that the C.O. required continuous TSS compliance limit is unique, and that specific design data for compliance with these standards are not available. This preliminary investigation, however, did support that deep bed filters (a more traditional technology) and disk filters (new, potentially economical technology) were most suitable for further investigation.

As a result, a pilot test program was planned and conducted with these two selected technologies, utilizing trailer-mounted pilot filter units, sized for roughly at 0.3 mgd.

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