Environmental Engineer:
Applied Research and Practice

Energy-Saving Benefits of Denitrification

Authors: Diego Rosso, BCEEM, and Michael K. Stenstrom, Ph.D., P.E., BCEE

Abstract

Nitrogen removal in wastewater treatment can be achieved by introducing anoxic zones in biological reactors within an activated sludge process, operating at medium or long mean cell retention time (MCRT). Anoxic zones at the head of the process also function as biological selectors and provide benefits in addition to nitrogen removal, including improved stability by avoiding filamentous bulking, enhanced removal of many recalcitrant pollutants and reduced energy consumption due to the oxygen credit and higher oxygen transfer efficiency. Improved oxygen transfer occurs because the readily biodegradable organic compounds are used by the denitrifiers for nitrate reduction. These organic compounds, which are surfactants, would otherwise reduce oxygen transfer efficiency, increasing plant operating costs. We tested 22 treatment plants which included either conventional, nitrifying-only, or nitrifying-denitrifying (NDN) operations. Off-gas tests confirm that oxygen transfer efficiency for NDN operations is higher. Our economic analyses show that NDN operation can have the lowest aeration costs, contrary to long-standing beliefs. The net operating costs can be lower than conventional, short MCRT operation and are always lower than nitrifying-only operation. However, depending on the local plant situation, expansion of aeration volume and/or clarifier area might be necessary, and the operating savings could be offset by debt service on plant expansion.

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