Principles in the design of single-sludge activated-sludge systems for biological removal of carbon, nitrogen, and phosphorus

The function of the single-sludge activated-sludge system has expanded from chemical oxygen demand (COD) removal to include, progressively, nitrification, denitrification, and phosphorus removal, all biological. With such complexity, design procedures based on fundamental behavioral patterns are required. The fundamental principles of a steady-state model for biological excess phosphorus removal (BEPR) that is based on the current understanding of the mechanisms of BEPR are described. The model can be used for design and to predict the expected system response under constant flow and load conditions. With the aid of the model, the influence of various parameters on the magnitude of phosphorus removal and the interactions between BEPR and nitrification and denitrification are examined, Principle variables that influence phosphorus removal are an influent wastewater characteristic-readily biodegradable chemical oxygen demand concentration-and process design parameters-anaerobic mass fraction and its subdivision and sludge age. Also, the model indicates that, in agreement with observations, preventing recycling of nitrate to the anaerobic reactor is one of the main considerations in design.