We have a conventional activated sludge plant. Currently, our MLSS is around 1400, average daily flow is around 3.5 MGD, primary effluent BOD varies between 90-120. Our aeration basin volume is around 1.38 MGD. Four final clarifiers, 2 ancient clarifiers are 50' in diameter and 10' deep. Two newer clarifiers that are 70' in diameter and 13' deep. We keep minimal blankets in all of them.

Typically our RAS flow is around 2 MGD. We will have an Ammonia limit next summer, so we've been expirementing with nitrification, trying to get dialed in this summer.

Lately, our MLSS have been settling to under 200 within the first 5 minutes in the settleometers. Our SVIs have been in the 80-90 range, which is great, but there seems to be quite a bit of straggler floc that I am worried is affecting our effluent Phosphorus and Fecal numbers.

Our MCRT got quite high about a month ago...15-20 days. We've been slowly lowering it and currently are trying to maintain it around 7-8 days. I set up a simple wasting calculator for the other operators, it seems that trying to maintain a constant MCRT is the simplest way for all of us to run the plant "the same way."

Also, about two or three weeks ago we were having some problems with denitrification in the final tanks. We shut off the air as much as possible in the first half of the first pass in our aeration basins ( we have two basins with three folded tanks configured for plug flow ).

Any thoughts or input would be appreciated.

This plant is being upgraded over 10 years or so, so the historical data is pretty much useless. We used to waste to 4 small, shallow primaries and had basically no control over the MLSS numbers. Now that we have a gravity thickener and can run the plant like a "real" WWTP, we are pretty much learning on the job.

We've never had a problem with TSS in the final effluent. My biggest concern is exceeding our phosphorus limit, which is 1 mg/l. We feed ferrous chloride to precipitate the phosphorous, and have been very close to violations the past couple of months. We've increased the feed rate substantially, stopped supernatant, and made adjustments to the air flow in our aeration tanks but are still near the limit on our weekly composite sample. I'm wondering if the pin floc carrying over from our finals is affecting the phosphorus.

We've also seen quite  a variation in fecal coliform numbers. We have a strong chlorine residual in our chlorine contact tank, almost twice what we've traditionally needed to keep our fecal numbers down. Our lab analyst is on vacation for two weeks, and the outside lab typically gets much larger fecal readings on our effluent than we do. Fecal violations are another concern.

We just turned on another RAS pump today to hopefully decrease the DT in the aeration tanks. It's probably 10 or 11 hours overnight with the low summer flows we're seeing right now. I'm hoping that the decrease in DT might help slow down our settling. I'd probably be more comfortable with SVIs around 120-150 as opposed to the 80-90 we're seeing right now. That seems to produce the highest quality effluent for us. As I mentioned in the original post, the plant is much different now than it was 5 years ago when I first started, and trying to find the ideal way to run it is a matter of trial and error.

I'm not sure that the sludge is settling too fast. SJH has the right way to check the effluent to determine if the P in the TSSs is causing concentration violations. I would also try to increase the sludge age to 10 days, as recommended by SJH, to improve nitrification and to see if it reduces the pin floc. I think that your high RAS rate may be causing some of the problem, the aeration basin detention time is essentially independent of the RAS (think about the inflow and outflow of the system including the clarifier). My thinking is that the high RAS may carry "young sludge" at a higher rate into the clarifiers which doesn't settle as well and consequently is discharged with the overflow rather being returned with the RAS.

Anyway, keep us posted on how things are going,

grrun

You may have a low alkalinity problem.
Check the aklalinity in the treated effluent and if it is low try adding some Magnesium Hydroxide or Lime to the aeration section to adjust the pH to an acceptable level.