Single-stage nitrification-denitrification

(Nitrosoma and Nitrobacter) achieves a maximum rate x in the activated sludge process at dissolved oxygen (do) concen trations of approximately 2 mg/1 or above. The rate decreases to zero as the do con centration decreases to zero. In contrast, while denitrification by facultative bacteria occurs in both anoxic and aerobic systems, the most rapid denitrification occurs with do concentrations of zero and with ample sources of readily available organic carbon to serve as electron donors for the bacterial reduction of the nitrate or nitrite. In biological treatment of wastewaters, oxidation of carbonaceous material, nitrifi cation, and denitrification all occur within a single process if sufficient bacterial solids retention time (srt) is provided for devel opment of the nitrifying organisms. Un fortunately, optimum process operating conditions for oxidation and for subsequent denitrification are thermodynamically an tagonistic; that is, the presence of the more powerful oxidant oxygen (electron accep tor) suppresses the use of N03~ (electron acceptor) in the biological oxidation of the carbonaceous material in the wastewater. In conventionally aerated biological sys tems, efficient oxidation of the carbonace ous and nitrogenous materials, in either a single- or two-stage process, is achieved under aerobic reactor conditions. The aerobic conditions produce the nitrate prod uct but also remove the organic materials that act as readily available electron donors for rapid denitrification. In addition, the do in the water significantly suppresses or minimizes the biological reduction of the nitrate or nitrite as it is produced. Clearly, while in municipal wastewaters the biological potential exists for both ni trification and denitrification without sup