Hi,  I am operating a Waste Water Treatement plant in Ireland that has very poor settling sludge. which is not allowing the MLSS in the aeration basins to get to high (currently 3500mg/l). The sludge coming of the aeration basins has an SVI of about 950 unless its splits, and the sludge blankets are very high only about 300mm from the top of the tanks, hence we are getting high s/s in the final effluent. Currently we do not use any chemical addition so am wondering if we should be adding poly etc to the plant to lower the SVI. The RAS rate back to the aeration has a ratio of above 0.8 of the plant, should this be more? Everything I read seems to indicate that it should be around this figure yet the sludge blanket remains very high. What would be the most likely effect on the plant if I was to increase the RAS, I have seen filamentous develop because of this more in the summer time. Our filamentous count at present is around 3.0. Can anyone give me any ideas what other tests can be carried out to remedy the situation?

Hi LMDBlader,

I agree 100% with Flocdocs' post!

best regards,

DS

What is the sludge age? A young sludge will settle slowly and increasing the RAS reduces the time under aeration. While you are looking for filaments with the microscope, I suggest that you try to reduce the RAS to half the current rate and see if that increases the freeboard in the clarifier.

Check out: http://water.me.vccs.edu/courses/ENV149/operationb.htm
for operation and control strategies.

grrun

Hi LMDBlader,
What type of wastewater are you treating? What is the F/M? What is the DO? Why do you want to increase the MLSS? What is the design MLSS? What filaments are you seeing in the biomass? What is the final effluent quality? Was the plant working better before you reduced the WAS? What was the MLSS at that stage?
I would agree with flocdoc and DS that you need to identify the filament as that will suggest what remedial action to take. Your SVI is 271ml/g which indicates bulking sludge.

Best regards,
Cgillen

Hi,

In Activated sludge systems, MCRT and SRT are the same and should be calculated as: total sludge present in system (so: AT + Decanter) devided by the total mass flow of sludge out of the system, so the sludge amount wasted daily from the decanter (Q,w * MLSS,w) + sludge lost with the effluent (Q,e * SS,e).
Not only the sludge under aeration is active, sludge is also active (lots of things happen intracellularly) in anoxia (denitrification) and anaerobic zones (Bio-P process).


In systems with attached growth (retained in reactor), MCRT and SRT are different, SRT being: Sludge amount in suspension/Sludge out. MCRT = Total sludge amount (attached + suspended)/sludge out. The website to which was referred to is confusing.

Long SRT is not per se a cause for bulking, reducing the WAS does not necessarily promotes the growth of Filaments.

best regards,

DS

In reply to C.Gillen
Point 1.The wastewater we are treating is mainly domestic sewage, with some industrial effluent but nothing with high BOD or CODs', storm water and river infiltration also enters our plant via sewers.
Point 2.The F/M ratio currently is 0.02 but previosly has been higher 0.08.
P.3. DO is controlled to 2mg/l
P.4. The design MLSS is 3500mg/l, I was stopping the wasting as the MLSS across the 3 tanks was down to 2700mg/l.
p.5 I havent had the filaments identified as yet, and agree that this will help diagnose the problem.
P.6 The final effluent quality is good, and can be better, COD, BOD, TP & TN & NH4 would be pretty well controlled, SS however may exceed 5-10times per month. High flows coming into the plant from storm water tend to lift the blanket (freeboard) which remains quite high as you can see with the SVI.
P.7. We always have been restricted by the amount we could waste to our dryer, but after changes to the dryer have been made we now find ourselves in a position that we can control the plant to 3500mg/l, but since this has been the case, our sludge blankets in the clarifiers have been very high.
We have previosly after dry weather in the summer had to dose chlorine to try and reduce bulking sludge, but this is more like a pin floc that seems to jsut wash out in large amounts, previously the bulk floc would go over the weir in  a big lump.

IN reply to grrun
Thanks for you honesty, between the few posts I was scared to change anything. The sludge age would normally be a lot lower 17 days or so, as the calculation basis itself on the wasting rate it may not be a true indication of the sludge age.

I am going to find out what filaments are causing the problem and take corrective action.

Thanks very much for your help and comments