Do You Know Your Sludge Age?

Solids Residence Time (SRT) is calculated at treatment plants based on steady-state assumptions and using empirical moving averaging, and as such is an approximation of the real age of the sludge. A simple method is presented that plant operators can use to determine the true Dynamic Solids Residence Time (DSRT) at any given day at their plant. It is shown that the DSRT is particularly useful if influent fractionation has been performed at the plant. The DSRT is more accurate in predicting process performance than the conventional SRT, even under rapidly changing process conditions, and it provides information about the age of individual fractions of the activated sludge. Three plants where DSRT in the winter is not sufficient to maintain nitrifiers or denitrifying methylotrophs are used to demonstrate the DSRT calculation method. The DSRT can be compared against the critical (washout) Mean Cell Residence Time (MCRT) and is able to predict the impending loss or establishment of an active microorganism group such as nitrifiers.