Water Online

September 2014

Water Online the Magazine gives Water & Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

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BNR 101: Tutorial R egulatory activity in the Midwestern United States, as in other parts of the country, has spurred increased interest in nutrient management among utilities throughout the region. Discharges into the Great Lakes or water-quality-impaired streams, as defined by each state, will soon be required to meet nutrient discharge limits. This raises questions for utilities about how to cost- effectively and sustainably achieve compliance with new phosphorus discharge limits. The Illinois Environmental Protection Agency (EPA) is considering a total phosphorus (TP) effluent limit — probably 1 mg/L TP — as the first step, and other state agencies are likely on a similar tack. Utilities that need to meet these lower limits will want to consider enhanced biological phosphorus removal (EBPR). Unfortunately, biological removal of phosphorus is frequently misunderstood; a solid understanding of the EBPR process and the needed wastewater characteristics that drive EBPR is essential for effective use of the technology. Enter EBPR The U.S. EPA's first major technical publication on phosphorus removal, which dates back to April 1976, addressed chemical precipitation of phosphorus. The discovery of biological phosphorus removal has since revolutionized the wastewater treatment industry with a lower operating cost and ability to produce a valuable end product with a high agronomic value. But understanding influent data analysis requirements, the role of fermentation, and how collection system practices affect the process is necessary for effective EBPR planning. EBPR is simply the biological uptake of phosphorus by selected microorganisms called phosphorus-accumulating organisms (PAOs). While the actual uptake of phosphorus occurs under aerobic conditions, PAOs must first be conditioned by exposure to volatile fatty acids (VFA) under anaerobic conditions. PAOs store food under anaerobic conditions and then process the stored food once under aerobic conditions. The preferred foods for PAOs are VFA: acetic, propionic, and butyric acids. PAOs expend energy to transform VFAs into a chemical form for storage, and they obtain energy for VFA storage by breaking phosphorus bonds within themselves. This results in the release of ortho-phosphate which is the conditioning step needed to trigger the aerobic "luxury phosphorus uptake." If PAOs are exposed to enough VFA, they will deplete their energy reserves and become stressed. This stress causes PAOs to overreact and accumulate more phosphorus in their chemical energy storage banks. While good aeration is all that is needed for phosphorus uptake to occur, the aerobic uptake of phosphorus is dictated by the amount of VFA stored and energy/phosphorus released in the anaerobic zone. Therefore, EBPR process success is primarily determined by influent wastewater quality and the amount of VFA that is present in proportion to the amount of phosphorus to be removed. Fermentation Can Help Fuel The Process The recommended minimum ratio of chemical oxygen demand to phosphorous (COD:P) is 40:1. This influent COD:P ratio is correct but misleading. The authors have seen plants with a >40:1 COD:P ratio work great for phosphorus removal, while other plants with a similar ratio struggle. EBPR performance varies from plant to plant with similar influent wastewater COD:P ratios because not all COD is the same when it comes to EBPR. Some wastewater treatment plants (WWTPs) have enough VFA in the influent to satisfy the EBPR needs for good phosphorus removal. If the VFA as COD:P ratio is over 8, EBPR will perform well. Good EBPR performance is defined as producing an effluent ortho-P concentration below 0.3 mg/L. Because effluent permits focus on TP, a 1 mg/L TP effluent permit can be easily met with a 0.3 mg/L ortho-P concentration and an effluent TSS less than 10 mg/L. Although having a sufficient supply of VFA in a WWTP influent is ideal, some plants have little VFA in their influent and still achieve effective EBPR 11 wateronline.com ■ Water Online The Magazine What Everyone Should Know About Enhanced Biological Phosphorus Removal As phosphorus effluent restrictions become more stringent, many utilities will need to step up their treatment. Black & Veatch advises on the potential of EBPR. By Dave Bunch, Ed Kobylinski — lead author, Dave Koch, Tom Ratzki, and Mark Steichen

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