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What Process are using to monitor your sludge inventory and why?


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.

best regards,
Ryan Hennessy


Author: wastewatermicrobiologyconsulting

I provide wastewater microbiology testing and support for third party clients such as engineering companies, chemical companies, contract operation companies, and bioaugmentation companies

3 thoughts on “What Process are using to monitor your sludge inventory and why?

  1. I have worked with sludge age and sludge inventory, as well as sludge retention time. Both of these, in conjunction with the parameters Ryan mentioned, allow operators to manage the solids in their system as well as provide maximum performance. It must be kept in mind that when altering the sludge age or SRT that one never, ever, ever, attempt to change more than one variable at a time. Change two variables together and you will not know which one had the desired effect.


  2. Thanks Ryan for starting a very good discussion. I believe sludge inventory management is one of the most crucial issue related to performance of biological wastewater treatment. The irony is the parameters used for controlling sludge inventory are more often derived parameters based on certain measurements and like BOD/COD measurement , they are sort of indirect indicators of biological status of the reactor. Each method F/M ratio, MLSS/MLVSS, SRT control , OUR control has its advantages and limitations. Thus sticking to any one method might not be helpful to maintain healthy performance of the reactor. Moreover, the dynamic behavior of WWT is one of the major challenge We face all the time to maintain an ideal or so called optimistic condition in the reactor. I believe there exist an operating window for each plant based on its design, loading rates, climatic conditions and treatment which can be defined using those parameters mentioned above.

    Liked by 1 person

    • Sludge age is a very basic and fundamental parameter in any biological WWT. It has a definite influence on the bio-system, but it is not the only determining factor. (it should be regarded as the foundations of a house, it determines a lot, but there are many different houses possible with the same foundation, and sometimes, things change…). It is the job/art to find the right/best (range) of SRT that gives best results under the given set of side/boundary conditions. I believe that SRT also should be seen in the time frame/time window of its magnitude; it is the “time constant” (mathematical approach) of the slow dynamics of a WWTP. The latter sentence does NOT mean that there are no short (or very short) term dynamics.


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