Establishing Cause-Effect Relationships in Activated Sludge Plants - what Can Be Controlled

Plant wide control problems in the activated sludge process have been revisited. The ultimate criteria for controlling a complete plant have been systematically addressed only infrequently. In principle, the effluent has to meet the standard at a minimal cost. Basically, the process is at the time driven by disturbances, mostly via the influent flow, but sometimes created within the plant. An essential task for any control system is to either minimise the consequences of the disturbances or to even eliminate them, before they hit the process. In this paper the main purpose is to systematically classify cause-effect relationships between available knowledge and open questions. This qualitative reasoning is meant to better define what can be achieved in the operation of a plant. The dynamic of an activated sludge system span over several orders of magnitude, from seconds to months. These different time scales allow many control actions to be decoupled. As a result, slowly varying variables can be controlled while assuming the fast dynamics to be instantaneous. Likewise fast varying variables may be manipulated while the slow modes are considered constant