Experience with biological nutrient removal at low temperatures

Operating experience for the first enhanced biological phosphorus (EBPR) an d biological nutrient removal (BNR) plant in Nor-way are summarized. The pr imary objective during the first years of operation was to remove phosphoru s at low temperatures both in an EBPR mode and a BNR mode. Enhanced biologi cal phosphorus removal was accomplished at 5 degreesC with 0.6 mg/L total p hosphorus 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 effluen t. Solids were separated by gravity settling only. Sufficient solids retent ion time resulted in biological phosphor-us and nitrogen removal at low tem peratures with two-thirds of the sludge production compared with chemical p hosphorus removal. In the approximate plug-flow biological reactor, a compl etely stirred tank reactor with seven compartments, a mixed liquor suspende d solids concentration of 6000 to 8000 mg/L was maintained because of the e xcellent settling characteristics of the sludge (diluted sludge volume inde x = 60 to 80 mL/g). Plug-flow mixing characteristics result in a substrate gradient that promotes growth of rapid-settling bacteria and depresses grow th of filamentous bacteria. Organic reduction in the anaerobic and anoxic z ones results in a low organic loading rate to the aerobic zones with subseq uent favorable conditions for nitrification.