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Related Articles


Activated Sludge

Activated Sludge

Activated Sludge

I find that the preferred method for process control in conventional activated sludge, extended aeration, oxidation ditch systems in the MLSS control as the analysis is simple and straightforward. I am interested in hearing about the forums members experience on this topic. Are there plants that use F/M, SRT as the primary process control parameter and if so what is the main reason?

Thanks in advance.

RE: Activated Sludge

The extended aeration system typically uses a retention time of 24 hours.

On large system, the large volume of the retention time is not cost effective. With a larger system, additional control input is necessary.

RE: Activated Sludge


Whilst the MLSS control is often used, i believe SRT control is the most appropriate way to operate plants that are being used for nutrient removal.
SRT control maintains a certain mix of bug population but the MLSS will vary with the load although the change in MLSS will always trail behind the load changes. If this mix of bugs gives the best performance then you would want to maintain it.

MLSS control sees that population mix vary as the load changes because you have to waste more as the load increases. Likewise it means a constant assessment of the waste rates. If its going to be done precisely than the MLSS concentration of the waste stream should be analysed as well and the amount required worked out. However most operators work it out by trial and error or take a guess and see what happens. Constant F/M is somewhat similar but requires even more analysis.

SRT control wastes a certain volume regularly(say every day. The volume to be wasted is a certain fraction of the total activated sludge volume, typically the aeration. Wasting is normally undertaken from the fully mixed aeration. Waste MLSS then equals the aeration MLSS .
For example if your aeration was 1,000,000 gallons if you wasted 5% or 50,000 gallons per day then you would have a 20 day SRT.
If you wanted to move to a 25 day SRT you would waste 40000 gallons per day instead. SRT control usually takes about 2 x SRT to settle down and the changes in bug population to come into effect. MLSS will vary with load but unless it gets way out of hand you need to resist the urge to waste more as the MLSS starts to increase.

"Any water can be made potable if you filter it through enough money"

RE: Activated Sludge

ashtree your reply is very interesting it sounds like you have a really good understanding of the activate sludge operations. Are you working in the operations or a wastewater process design engineer? So what you are saying is that in case the SRT is changed the system takes 2xSRT to stabilize, I believe this may be the steady state conditions you are referring to.


RE: Activated Sludge


I am predominantly in operations but have been involved in many process designs as well. If you understand the design you can often make a process work better or you at least know where the strengths and weaknesses are which allows you to focus your attention more on the weaker areas. Many process designers are divorced from real world operational problems and experience.

With regards to SRT changes some trends can often be seen before this but it will take at least 2 SRT for the system to stabilise at the new SRT.

Think of this example to help the understanding of why this occurs. If you had a tub that contained 100 red balls which is constantly being mixed up and you wanted to change them to blue. But you can only change the balls to blue by taking 5 balls a day selected totally randomly from the tub and replacing them with 5 blue balls.
The first day you will take out 5 red balls and you will now have 95 red balls and 5 blue balls.
The next day there is a high likelihood that you will take out another 5 red balls but you could theoretically remove 5 blue balls.
And so it goes on.
In this case we have a SRT of 20 days but there is still a high probability of many red balls in the system. After 2 x SRT there may still be the odd red ball but it will predominantly be blue. Given that SRT is only an average of the time that a bug will spend in the system it is every chance that even after 3 SRT there could still be a red ball in the system but the odds are diminishing quickly.

SRT control is a powerful tool if understood and can often be easily implemented depending upon your plant configuration.

"Any water can be made potable if you filter it through enough money"

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