Dynamics of nitrification in the activated sludge process. (1975)

Renewed interest in the problems cre ated by the d scharge of nitrogenous compounds to receiving waters has focused attention on the form and concentration of nitrogen in wastewater treatment plant effluents. Most wastewater treatment plants are designed, operated, and evalu ated based on the reduction of carbona ceous 5-day biochemical oxygen demand (bod5), rather than on the total bod of the effluent (carbonaceous and nitrogenous). As a result, ammonium is the primary form of nitrogen in domestic wastewater treat ment plant effluents and is commonly found in concentrations up to 20 to 30 mg/1. If a theoretical oxygen requirement for ni trification of 4.5 mg oxygen/mg ammonium nitrogen oxidized to nitrate is assumed, an effluent with 20 mg NH4+-N/1 can exert a nitrogenous oxygen demand of up to 90 mg/1, which is considerably greater than that contributed by the ultimate car bonaceous demand of secondary biologi cal treatment plant effluents (?30 mg/1). The conversion of ammonium to nitrate within a biological treatment plant would, therefore, represent a significant improve ment in effluent quality. The activated sludge process may be used to obtain nitrification if conditions suitable for the retention and accumulation of nitrifying bacteria are maintaned. The conditions necessary for nitrification may be expressed in terms of sludge age, pH, temperature, and dissolved oxygen (do) concentration. The concentration of nitrifying bacteria will depend on their specific growth rate and on the rate at which they are discharged from the system through the effluent and waste sludge streams. Several laboratory, pilot, and full scale investigations have been conducted with nitrifying activated sludge systems and have demonstrated that high efficiencies of nitrification are obtained at sludge ages of approximately 4 days or more.1-6 It is important to note, however, that, at tem peratures less than 20 ?C, the sludge age required to maintain high nitrification ef ficiencies increases markedly. Until the recent application of mathe matical models based on kinetic principles, some of the basic concepts regarding ni trification in the activated sludge process were not clearly understood. Downing et al.1 were among the first to develop a kinetic model for nitrification in the acti vated sludge process. They concluded that consistent nitrification would be expected in activated sludge plants only if the growth rate of the nitrifying bacteria were greater than the rate of loss through the effluent and sludge wasting streams. Using this concept and experimentally determined values for growth rate coefficients, they were able to obtain reasonable predictions of the degree of nitrification in activated sludge plants.7 Although several mathematical models have been developed for the nitrifying ac tivated sludge process,1'3> 8_11 these usually have been steady-state models. Because domestic wastewater treatment plants op erate under time-varying loads, the use of steady-state models to describe these processes is inconsistent with their opera tional nature. In this regard, a full under standing of the kinetics of nitrification and the response of nitrifying systems to dy namic inputs may be necessary before more effective control of nitrification is possible. The primary objective of this research was to develop a dynamic model for nitri fication in the activated sludge process that