Two-Stage Activated Sludge Systems in Municipal Used Water Purification

The two-stage, activated sludge process is utilized in plants possessing two separate, activated sludge circuits. It consists of high-loaded first and low-loaded second stages and was developed in the 1970s in Europe in order to treat high-strength used water influents. Subsequently, the tightening of the effluent discharge requirements with respect to nitrogen and phosphorus in the 1990s meant that the process ceased to make further progress. However, in the twenty-first century, it is attracting significant attention, as it offers great potential for the improvement of the net energy balance of used water treatment plants. On the one hand, as compared to the conventional activated sludge system, the high-loaded stage facilitates increased COD capture in the activated sludge, and on the other, alternative methods for backload treatment as a replacement for the conventional nitrification/denitrification enhance process efficiency. Therefore, numerous researchers are investigating the increased carbon capture mechanisms of the high-loaded sludge and the implementation of the nitritation and anammox process as a backload treatment.