Biological nutrient removal (BNR) is being selected as the cost effective option for enhancing nutrient removal at plants located in temperate climates in the Chesapeake Bay Watershed and the Long Island Sound in the U.S.A. However, there is considerable discrepancy in the reported values for temperature sensitivity for nitrification and denitrification rates in winter, as observed at different plants and those reported in higher strength wastewaters in Europe and South Africa. Full scale data show that the temperature sensitivity for nitrification is considerably lower. Denitrification rates in the primary anoxic zone are also lower, possibly because of lower influent strengths (250 to 450 mg/L COD).

Secondary clarifier limitations have been a key cost factor in attempting year round nitrogen removal as against seasonal (summer) nitrogen removal. The anoxic MCRT and anoxic volume requirements have been a critical cost factor in designs. Split flow designs, where a fraction of the influent flow is sent directly to the anoxic zone, have allowed process designs with lower anaerobic volumes. The readily available COD in the fraction of the influent which enters the primary anoxic zone enhances denitrification rates. Though the anaerobic cell receives less than 100 percent of the influent flow, excellent biological phosphorus removal can still be maintained with phosphate detergent ban wastewaters. The fraction of biomass maintained under anaerobic conditions (and the anaerobic volume) can be reduced because the phosphate detergent ban has decreased the amount of phosphorus which has to be removed for the same influent COD. Biofilm and suspended growth systems have been combined in a single reactor to reduce volume requirements for nitrogen removal.

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