I personally prefer to use sludge age: Inventory in system/ inventory leaving system per day. If there is a high amount of inventory in the final clarifiers this should also be included in the system. If TSS lost to the effluent is significant this should be included in addition to the mass amount of sludge wasted from the system.
While using sludge age, also track and monitor other things such as SVI, F/M ratio, MLSS concentration, and effluent performance. Every couple sludge cycles or more often if there is a lot of variability, an advanced microscopic evaluation evaluating floc structure, filament abundance, protozoa and metazoan, type of filamentous etc. is useful to see which direction things are going, and identify the cause for any changes in the above parameters. Knowing what the plant looks like under the scope during “good conditions” is very useful because it provides a great baseline for troubleshooting when and if the time comes.