Decentralized wastewater treatment design methodology




Thomas, Michael

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The EPA considers decentralized wastewater treatment a cost-effective alternative to centralized treatment in small and rural communities, and a sustainable approach because it retains reclaimed water and nutrients within the watershed and allows management flexibility by combining various processes to address specific site conditions and to meet treatment goals (USEPA 2000, 2002). Since the decentralized wastewater treatment category covers a wide range of flows and concentrations, and due to the lack of regulatory guidance for this category, a design methodology is needed to design the treatment system.

This research addresses the problem of proprietary aerobic treatment systems based on empirical relationships from centralized application; and specified, overly conservative separate land disposal guidelines based on septic treatment from on-site applications as the available choices for designing decentralized wastewater treatment systems. The development of an integrated AT-ETA design methodology based on a mass balance approach, using the principles from the most common systems in on-site and centralized treatment in combination to derive a design methodology capable of meeting the treatment objectives of the decentralized approach is the goal of this research.

An experimentally determined microbial kinetic conversion coefficient describing the reaction rate is used to develop a design approach for an aerobic treatment (AT) unit to replace the septic tank, and a modified water balance analysis technique for the disposal component combines both absorption and evapotranspiration into an evapotranspiration/absorption (ETA) process. The ETA component is then integrated with the AT component by relating the area for disposal with the volume for treatment, considering the organic and nitrogen concentration variations, to produce a complete AT-ETA design methodology for the entire system treating wastewater derived from a combination of residential and commercial sources.

The AT-ETA methodology allows for a design based on the specific hydraulic and organic loading of the source wastewater, without the use of size-up factors to account for high strength waste from non-residential sources. The AT-ETA method considers hydrologic parameters, soil parameters, and operating conditions; integrated with specific wastewater parameters for hydraulic, organic, and nutrient constituents, using a mass-balance design approach. The results of this research are a flexible design tool that allows analysis of the various components without initial set conditions and the ability to iteratively determine a solution based on waste characteristics, site conditions, and treatment goals; to produce an efficient design with maximum treatment potential.


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sanitary and municipal engineering



Civil and Environmental Engineering