jeffryy:

Five mg/L nitrate is enough to float a sludge due to denitrification.

Hope that this helps..

Thanks for the reply.

Does anyone know if the rising sludge can be prevented by adding some carbon source to the SBR? I measured the COD after the anoxic period  and it is quite low  (30-40mg/L)and I am guessing that it is not denitrifying properly despite the DO being less than 0.1 mg/L already. but then again, I had added some sucrose a few times before during anoxic period and did not see any improvements.

Temperature is at room temperature approx 20 deg C.
Thank you

It appears a short aeration phase prior to the settling may be needed to keep things aerobic during the settling and decant cycle. The best way to prevent denitrification is to have oxygen present.

Also sludges with lots of filaments are more prone to denitrification, this is due to the large internal void spaces in the floc (easier for the nitrogen gas to float them). A microscopic eval is a good idea to see if any measures can be taken to improve the sludge quality.

Hi Ryan, (Sludge Judge),
Seeing that it was I who recommended the increase in MLSS I will explain briefly by I made this comment:

This is a summary of the pertinent information provided for the operation of the SBR:
8 h each cycle [feed 30min; mixing 6hrs (incld 4h aerobic + 2 hr anoxic); settle 1h; decant 30min].
Sludge 11 days.

The sludge age of 11 days is relatively short and is an indication of "higher" loading &/or "lower" MLSS.

Activated sludge is typically expected to denitrify about 0.8 to 1.1 grams Nitrate per kg MLSS per hour, (depending on MLVSS content, temperature, pH, etc).

Perhaps the make up of this sludge or some of the other operating parameters are reducing the denitrification rate to the lower end of this scale, (or even below the lower end of that scale).

If that is the case then an increase in the MLSS could have the effect of increasing the denitrification achieved over a specified time, (i.e. more sludge acting on a set mass of Nitrate in a set time = improved denitrification; provided all other parameters are optimised for denitrification).

Hence the suggestion that "An increase in MLSS might also help."

Hope that this clarifies the thinking behind that suggestion.

You're welcome Ryan.

You're getting fairly good denitrification already. I think the problem lies in that there's two hours of unaerated time before the settling and decant cycles. You need some DO present to prevent denitrification during those stages.  

The only way to prevent the nitrate formation is to prevent nitrification. If you have an ammonia limit this won't be possible.

best regards,

Ryan

hi, I have seen a sample of the SBR in microscopes and it does have more filamentous.. so I think that is what s causing the rising sludge.

The feeding is already mixing and I think it is considered anoxic already. I am thinking of extending the initial anoxic period too, but worried that the carbon source may not be enough for nitrification during aeration as well..

note that the filaments aren't responsible for the rising sludge/ denitrification is....having a filamentous sludge can trap more nitrogen gas (produced during denitrification) and float flocs more easily

Ok, so hopefully this newer SBR was innoculated with sludge wasted from your other SBRs (all operating under the same conditions as I understand it). If it wasn't and you want a SBR like the other ones, restart one with their WAS...you'll get where you want to be much quicker.
If you want another 4L SBR at 3100 mg/L. It would take you a minimum of 5 days assuming you had 3 other SBRs running identically (I can send you a simple spreadsheet)

As I have understood your posts:
There is no denitrification in old SBRs
There is some denitrification in the new one but some or all of it is occuring in the settling phase (enough to float your sludge)

Both cases can be remedied with suggestions in older responses
Good luck.