Combining Aerobic & Anoxic Conditions Within One Basin

With increased effluent pollution restrictions on municipal wastewater treatment plants, cities are pressed to find ways to economically increase their effluent quality with little to no funding. Many new NPDES permits require the reduction of nitrate and/or phosphorous, as nutrient removal becomes more prevalent due to the effects of nitrogen and phosphorus on discharge receiving streams. Biological Nutrient Removal (BNR) is a term describing the removal of nitrogen and phosphorus by utilizing the naturally occurring biology in activated sludge. Typically a BNR plant upgrade requires additional capacity in the form of an anoxic or anaerobic basin, which requires significant funding to complete. According to a fact sheet published by the US EPA “Biological Nutrient Removal Process and Costs”. A plant between the flows of 0.1 and 1.0 MGD required $6,972,000 per MGD for a BNR system upgrade, on average. One option to avoid the construction of an additional anoxic basin is to convert the aeration system to an intermittent aerobic/anoxic cycle structure. However, typical aeration equipment designed to operate 24/7 lacks the turndown capability required to promote anoxic conditions. Retrofitting these oxidation ditch systems with the proper aeration equipment and am aerobic/anoxic cycling strategy can eliminate the need for an additional anoxic basin.