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Oxidation of Ammonia in Source Water Using Biological Filtration (slides)
Citation:
WHITE, C. P., D. A. LYTLE, D. WILLIAMS, AND C. MUHLEN. Oxidation of Ammonia in Source Water Using Biological Filtration (slides). In Proceedings, Water Quality Technology Conference and Exposition, Phoenix, AZ, November 13 - 17, 2011. American Water Works Association, Denver, CO, #1747, (2011).
Impact/Purpose:
To inform the public.
Description:
Drinking water utilities are challenged with a variety of contamination issues from both the source water and the distribution system. Source water issues include biological contaminants such as bacteria and viruses as well as inorganic contaminants such as arsenic, barium, and iron. As a result of treatment, many water utilities face distribution system issues such as disinfection by products, biological regrowth, and corrosion. The removal of these contaminants can be costly due to expensive infrastructure improvements and high chemical costs. One process, Nitrification, is of particular importance due to the inherent disinfectant demand and secondary effects on finished water quality. Nitrification is the biological process by which ammonia is oxidized to nitrate in the presence of dissolved oxygen. Nitrite and nitrate are toxic to humans and are regulated, while ammonia is not. Furthermore nitrite and nitrate are regulated at the entry point of the distribution system rather than in the distribution system where nitrification occurs. Excess free ammonia in the source water may also negatively affect the chemical oxidation of arsenic by chlorine and hinder arsenic removal. The objectives of this study were twofold: (1) to design a biologically active filter to oxidize ammonia to nitrate (2) evaluate the impact of water quality and filter operation on filter effectiveness. Six filters, 6 feet high by 2.5 inches in diameter were constructed and bedded with anthracite-sand, small gravel, or large gravel to 30 inches. The filters were feed with raw well water containing an average of 1.1 mg/L NH3-N and 31 g/L As. The initial effluent flow rate was 200 ml/min corresponding to a loading rate of 2.6 gpm/ft2 and EBCT of approximately 12 minutes. Filters were run in parallel throughout the course of the investigation and backwashed twice a week using finished water. Concurrent with operation, water quality was monitored; analytes included ammonia, nitrite, nitrate, alkalinity, phosphate, total organic carbon, and 15 metals. Results presented will include: (1) while all filters achieved greater than 90% ammonia oxidation in approximately 35 days, nitrification in gravel filters destabilized after 100 days (2) chlorinated backwash had no affect on nitrification in anthracite-sand filters (3) filter depth profiles showed complete ammonia oxidation approximately 6 inches into the filter bed (4) decreasing the loading rate in the gravel filters failed to return ammonia oxidation to >50%.
