Experience with biological nutrient removal at low temperatures

Operating experience for the first enhanced biological phosphorus (EBPR) and biological nutrient removal (BNR) plant in Nor-way are summarized. The primary objective during the first years of operation was to remove phosphorus at low temperatures both in an EBPR mode and a BNR mode. Enhanced biological phosphorus removal was accomplished at 5 degreesC with 0.6 mg/L total phosphorus in the effluent, and BNR was achieved at 6 to 8 degreesC with an average of 0.25 mg/L phosphorus and 5.3 to 9.6 mg/L nitrogen in the effluent. Solids were separated by gravity settling only. Sufficient solids retention time resulted in biological phosphor-us and nitrogen removal at low temperatures with two-thirds of the sludge production compared with chemical phosphorus removal. In the approximate plug-flow biological reactor, a completely stirred tank reactor with seven compartments, a mixed liquor suspended solids concentration of 6000 to 8000 mg/L was maintained because of the excellent settling characteristics of the sludge (diluted sludge volume index = 60 to 80 mL/g). Plug-flow mixing characteristics result in a substrate gradient that promotes growth of rapid-settling bacteria and depresses growth of filamentous bacteria. Organic reduction in the anaerobic and anoxic zones results in a low organic loading rate to the aerobic zones with subsequent favorable conditions for nitrification.