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underground presence of several unmarked and abandoned vaults and structures. These items were not shown on the as-built draw- ings. The contractor worked with RETTEW to modify the construction approach, com- pleting the base for the oxidation ditch in three concrete foundation pours while the structures were simultaneously removed. Because of the close proximity to a nearby creek, groundwater also became an issue during excavations and construction. The contractor brought in large pumps to dewater the excavations for the oxidation ditch and kept the project on schedule and within the original cost estimates. The original estimate of construction costs for the work was $10.4 million. However, approximate total cost after bids were returned was $8.6 million. With the change orders and change in construction approach due to the unforeseen circumstances, the final costs were approximately $9.6 million. The Process The newly installed BRN processes enabled the facility to meet PADEP nutrient discharge requirements. BNR typically refers to a system treating wastewater to an effluent level of total nitrogen 8 to 10 mg/L and total phosphorus 1 to 3 mg/L. In a BNR wastewater treatment facility, the microorganisms used to treat the wastewater are intentionally stressed, resulting in several nutrient-important results: the conversion of ammonia to nitrates and nitrites, the release of nitrogen gas, and the uptake of phosphorus. To ensure these biological conversions occur, the microorganisms' environment must be controlled, keeping the pH balance, alkalinity, carbon, and sludge age at correct levels. By controlling the microorganisms, their environment, and the phases of the installed oxidation ditches, MAWSA completes biological nitrogen removal through nitrification/ denitrification, as well as some biological phosphorus removal. The Next Step ENR is a step further than BNR, resulting in effluent levels as low as total nitrogen of 3 mg/L and total phosphorus of 0.3 mg/L or less. Traditionally, wastewater treatment facilities obtain ENR with the addition of filters or chemicals. At MAWSA, however, the facility's staff has been able to reach ENR-level discharges through optimizing its oxidation ditches. The authority's staff wanted to optimize the facility for its primary discharge requirements, so they tested and fine- tuned processes to reach lower effluent discharge levels. The goals were both to improve effluent discharge levels and find operating and cost efficiencies for the facility, something that was possible because of the facility's unusual quality influent. MAWSA's facility does not feature typical quality influent. Rather, its influent has low biochemical oxygen demand (BOD) concentrations, featuring monthly averages of 160 mg/L, with lows as little as 50 mg/L. These concentrations result in a low food-to-microorganism ratio. After careful testing of the process, MAWSA determined that by operating at an older sludge age, it was able to compensate for the low food-to-microorganism ratio, as older microorganisms require less food than younger ones. MAWSA now operates with a sludge age of 40 to 50 days, while a typical oxidation ditch will operate with a sludge age of 20 to 30 days. This environment, which supports an older sludge age, allows MAWSA to meet ENR conditions within its existing oxidation ditch process, during which nitrogen removal is obtained biologically. Phosphorus is also partially obtained biologically but also via chemical participation in the clarifiers. The Pros And Cons Operating the oxidation ditches at an old sludge age has several advantages. The older microorganisms require less dissolved oxygen than younger microorganisms. These conditions result wateronline.com n Water Innovations NUTRIENTREMOVAL 23 Month And Year Average Monthly Total Nitrogen (mg/l) Average Monthly Total Phosphorus (mg/l) August 2014 2.12 0.11 September 2014 2.36 0.19 October 2014 1.99 0.16 November 2014 2.05 0.11 December 2014 1.70 0.10 January 2015 1.92 0.10 February 2015 2.13 0.10 March 2015 2.21 0.09 April 2015 2.17 1.38 May 2015 1.75 0.40 June 2015 2.41 0.20 July 2015 1.67 0.08 Average 2.04 0.25 MAWSA's WWTP Effluent Quality Data A summary of MAWSA's treatment plant total nitrogen and total phosphorus concentrations from August 2014 through July 2015 (the most recent data available) Plant improvements included new pumps for return sludge and waste-activated sludge.