NITRITE LEVELS IN AN INCOMPLETE NITRIFYING PLANT, AND HOW INCREASING DENITRIFICATION COULD DECREASE NITRITE AND DISINFECTION ISSUES

摘要

Nitrite levels above 1 ppm in the secondary clarifier effluent require increased doses of chlorine for disinfection. An incomplete nitrifying plant was studied for 1 year and the causes of increased nitrite levels in the secondary clarifier effluent coincided with high concentrations of ammonia oxidizing bacteria (AOB) and total bacteria (TB). When Nitrospira was present in concentrations near 10~(13) cells per liter and when Nitrospira and Nitrobacter were both greater than 10~(12) cells per liter effluent concentrations were below 1 ppm nitrite. When increased to >10~9 copies per liter, nitrite increased in both the aeration tank and the secondary clarifier effluent. During chlorination of the foaming event, the concentration in the effluent increased. When AOB growth rate started to increase in March and heterotrophic growth rate in April concentration of nitrite increased in the aeration tank, which was followed by increased nitrite in the secondary clarifier effluent until mid-June. Experiments were conducted to determine if recycle of a percentage the aeration tank mixed liquor would help to decrease nitrite levels. However, the experiments with aeration tank volumes (10, 20 and 50%) combined with anoxic zone volumes (90, 80 and 50%) produced mixed results, which corresponded with the finding that denitrification although greatest in the summer only periodically decreased nitrite levels in the aeration tank.