Maximum Nitrogen Removal in the Step-Feed Activated Sludge Process

Abstract

This paper presents a mathematical framework that can be used to determine the flow distributions in a step-feed activated sludge process for achieving the greatest nitrogen removal efficiency. The model indicates that the nitrogen removal efficiency in a step-feed activated sludge process is highly dependent on the BOD to TKN ratio of the wastewater. For typical domestic wastewater, which has a relatively high BOD to TKN ratio, the step-feed process will outperform the Modified Ludzack-Ettinger (MLE) process for nitrogen removal, if the flow to each step is optimally distributed. Nitrogen removal performance of four step-feed activated sludge plants operated by the Los Angeles County Sanitation Districts is calculated with the developed model using plant specific water quality data and operating conditions, and is compared with actual observations over a one-year period. The calculated nitrogen removal efficiencies match closely with the actual data at these four plants indicating the model is a useful tool for predicting nitrogen removal in a step-feed activated sludge process. To achieve the greatest nitrogen removal efficiency in a step-feed activated sludge process, the operator may consider the use of two or three daily adjustments of primary effluent flow to each step based on diurnal fluctuations of BOD and TKN. An example is provided to illustrate the application of such an operating strategy and the potential enhancement of nitrogen removal.