Nitrogen control through decentralized wastewater treatment: Process performance and alternative management strategies

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Decentralized or onsite wastewater treatment (OWT) systems have long been implicated in being a major source of N inputs to surface and ground waters and numerous regulatory bodies have promulgated strict total N (TN) effluent standards in N-sensitive areas. These standards, however, most of which have effluent limitations of <10. mg/L TN, were generally developed without data on treatment performance and attainable compliance levels of operating OWTs designed to remove N. This paper reviews OWT technologies that rely on preanoxic or postanoxic denitrification, or simultaneous nitrification-denitrification, and frequently include compact, mechanized components. TN effluent data from 20 OWTs in 3 long-term N removal demonstration projects in Florida, Oregon, and New Zealand are analyzed and compared with the performance of 15 centralized N removal treatment plants from the US and Canada. A reliability and stability analysis shows that only one of the 20 OWTs approaches the reliability and stability of centralized plants, and can comply with a <10. mg/L TN effluent standard with a 99% probability; all of the remaining 19 OWTs have a <50% probability of compliance. The lower reliability of OWTs, many of which are energy-intensive, scaled-down models of centralized plants, is due to the inherent variability of decentralized wastewater characteristics and the challenges of operationally controlling N removal processes at the level of residences. However, the small footprint (required land area) of these compact designs offers important opportunities for retrofitting OWTs on small lots, in shoreline developments where land is at a premium and where communities wish to foster and sustain compact, village developments that reflect " smart growth" strategies. Other approaches to decentralized N management emphasizing passive, robust, ecologically engineered designs are reviewed and include natural wastewater treatment systems such as single pass sand filters with denitrifying bioreactors, which performed better than any other OWT technology; shallow trenches and drip irrigation for denitrification or plant N uptake in the carbon-rich root zone; denitrification beds/layers installed down gradient from effluent plumes; and the consideration of watershed N sinks in estimating the risks of N loading to receiving waters. These alternative approaches require further research and development, but can offer alternatives or additional treatment to mechanized OWTs. More comparative studies of long-term operation of OWTs under field conditions in other parts of the world are needed to further quantify performance capabilities. © 2010 Elsevier B.V.

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Ecological Engineering