THE EFFECT OF IN-PLANT RECYCLES ON CONTROL OF AN ACTIVATED SLUDGE PROCESS

The concept of “sludge age” or solids retention time (SRT) is a useful tool for control of the activated sludge process. SRT provides a method of controlling the microbe population mix in the treatment process. The SRT history also provides important design information. However, calculating the SRT using only the amount of sludge pumped by the waste sludge pumps, often ignores the quantities of activated sludge recycled from downstream separation processes. As shown by operating data from a case study, these quantities can be substantial. Correcting the SRT calculation, accounting for in-plant recycle streams, improves the reliability of this important guideline for operating the activated sludge process.