Five Case Histories of Automatic Sludge Age Control

Control of the sludge-wasting rate from an activated sludge process is considered one of the most important control parameters. Manipulation of the wasting rate has the overall objective of influencing the biology of the activated sludge process. Although the biology can be affected by influent conditions and other operational variables such as dissolved oxygen concentration, the wasting rate is generally considered to be the most influential variable. This conclusion is supported both by mathematical analysis of microbial growth and by practical experience in the field over many years.Although the sludge age concept is used at many facilities, automatic control of sludge wasting based on sludge age is not yet commonly used. Clearly there is a potential for improved benefits from controlling the plant more closely. Benefits attributed to automatic sludge age control in the WEF manual Automated Process Control Strategies (1997) include reductions in permit violations, increases in design capacity, control of nitrification, and control of filamentous organisms in the aeration basins.Automatic sludge age control at five different facilities was evaluated as part of a Water Environment Research Foundation project on automation. The five plants utilized four different implementations of sludge age control with one plant implementing two different techniques. Each implementation had different levels of instrumentation and technical complexity. The paper and presentation will evaluate each automatic sludge age control strategy implementation, develop the implicit and explicit assumptions, and present advantages and limitations of each.