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Use of Soil Filtration and UV Disinfection for Advanced Wastewater Treatment

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While guidelines exist, currently there are no federal regulations governing treatment requirements for reclaimed water. In their absence, many states have adopted California's reclaimed water regulations, found under Title 22 of the California Code of Regulations (CCR) as well as the UV Disinfection Guidelines for Water and Water Reuse, published by the National Water Research Institute and the American Water Works Research Foundation in 2003 (NWRI/AwwaRF 2003 Guidelines). These documents provide treatment requirements based on the type of reclaimed water application being proposed (e.g. irrigation, aquifer recharge, streamflow augmentation).

The City of Dinuba, California has been using percolation as a disposal method for their unfiltered secondary wastewater effluent for some time. Now viewed as a resource, the City plans to extract up to 4 mgd of percolated municipal wastewater from recovery wells for use in landscape and golf course irrigation. The extracted effluent will be disinfected through an invessel UV rector to meet Title 22 recycled water criteria for unrestricted landscape irrigation. Title 22 includes limits on turbidity (2 NTU average 24 hour), coliforms (2.2 MPN/100mL 7-day median), and a minimum 5-log reduction of poliovirus during operation.

While not directly written into Title 22, the California Department of Health Services (CaDHS) closely abides by the UV design guidelines in the 2003 NWRI/AwwaRF Guidelines. These guidelines are meant to ensure that UV systems are adequately sized to meet treatment objectives. The guidelines vary depending on the type of treatment prior to UV disinfection. For post media filtration UV systems it is recommended that a UV design dose of at least 100 mJ/cm2 and a design UV transmittance of 55 percent be required for this application. Post membrane filtration, the recommended UV design dose is 80 mJ/cm2 and design UV transmittance is 65 percent.

Although filtration prior to UV is recommended, water quality investigations indicated that it is not appropriate for this specific application. Samples of the extracted percolated effluent (taken from recovery wells) consistently verified its high quality, with consistent UV transmittance ranging from 90 to 91.8 percent. In addition the UV transmittance data showed little variability, with a lower 10th percentile UV transmittance of 91 percent. The turbidity of the extracted effluent was less than 0.1 NTU. Samples were also collected to track male specific bacteriophage removal through the plant, the percolation ponds, and to the extraction wells. A comparison of the bacteriophage levels in the secondary effluent compared to the extracted effluent (none detected) from the recovery wells (Figure 1) showed that there was at least a 2-log reduction of male specific bacteriophage due to percolation. Total and fecal coliforms were also non detect in the extracted effluent. Therefore the high quality of the reclaimed water (i.e. low turbidity, high UV transmittance, and no measurable microorganisms) verified that filtration after extraction from the recovery well was not necessary for this application.

The design of this system provided regulatory and design challenges due to the reclaimed water having a UV transmittance typically found in drinking water treatment applications. There are provisions supporting this approach in the 2003 NWRI/AwwaRF Guidelines to design around a less conservative UV transmittance if substantial data is acquired. Additionally CaDHS has allowed less conservative design UV transmittances with backup data. During this project it was found that avoiding filtration and designing the UV system for a higher UVT would result in appreciable equipment cost savings for the City of Dinuba. In order to design the UV system at a higher UVT and a lower UV dose (60 mJ/cm2) the City will need to receive 2-log virus removal credit for percolation from CaDHS and collect enough data to support design at a higher UVT. The final design will initially treat a flow of 2.9 mgd and meet all NWRI/AwwaRF and California Title 22 recycled water criteria.
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Document Type: Research Article

Publication date: 01 January 2007

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