WEF Discussion Forums
Operation and Maintenance
aerobic digester-very high solids-odor problem
The title sums it up. My 9mgd activated sludge plant has three aerobic digesters. They have been receiving 90k gal. of thickened WAS for the past 15 months. Now it is extremely thick, and emmitting a very offensive odor. I'd welcome any comment that might point me toward the right approach to alleviate this mess. Thanks
When we had aerobic digesters we could not transfer enough oxygen to meet the demand once the %TS reached 3.5 to 4.0 %. The larger plants would use oxygen instead of air to meet their demand. Check organic acids to determine if they are the source of the odor. Organic sulfur compounds can be another source once the sludge is septic.
I would dewater and lime stabilize or landfill enough sludge so as not to overload your digesters. Once to digesters are acclimated and well digested then you can add more raw sludge just so that you keep some DO.
You can find studies on dairy waste that show O2 transfer drops way off as you go from 2% to 3% solids. Even if you are using fine bubble difusers, the air has to come together to form a big enough bubble to push through the sludge. Big bubble = bad transfer.
Hi Jim! With the thick sludge in the aerobic digester, you are getting affected by the Alpha Factor. If you do a search for "Understanding the Alpha Factor" you will find an article that might explain it to you.
The thicker the sludge, the harder it is to transfer enough air into it to stabilize it. If you thin out your sludge (down to 2.5%ts or lower) and watch the D.O., you will start to notice that the D.O. will transfer, and the sludge will begin to digest. To run the thicker solids in the digester, you could add more horsepower in blowers, but that's big bucks.
A quick way to determine if you are starting to aerate the biosolids would be to measure the off gas from the digester. If you get hydrogen sulfide readings, and not ammonia, then you are not adding enough air. If the hydrogen sulfide drops, and the ammonia picks up, you are headed into the right direction.
What is the TSS, VSS, pH, DO, SOUR and temp of the digesters? Also, how would you describe the smells? If there is lots of H2S smell, can control that by raising the pH to 7 1/2 to 8. What kind of aeration system is installed? I've used some evergreen and noscent essential oils from odor control co. in a 200,000 gal digestor with some success. Good luck.