Expert: David L. Russell, PE - 8/22/2008

Question
Dear Mr. David,
 We have SBR system treatment plant. The plant had been operating for 2 year yt we fail to bring down the COD to 100 ppm. Our influent and effluent are 8000 and 200 ppm. We had attempted to mitigate this by
1) extend reaction time (10 hours aeration, 1 hour settling and 1 hour decanting), and
2) Dose urea to maintain activated sludge (MLSS- 5000 mg/L.

 There are a few options and recommendations from the consultant;
1) Our system can't break down dissolved COD,
2) Our system may not contain the right bugs and require to dose synthetic bacteria such as actizyme,
3) Install pretreatment system such as Dissolved Air Floatation, and
4) Extend aeration duration.

 For your info, we encountered severe brown foaming problem before extending the aeration time, yet this fail to assist us in cutting down COD concentration.

 Greatly appreciate in advance for your precious respond.

Answer
First, your influent COD is quite high. Have you looked at the balance or C:N:P ratio of your influent and for your MLSS?  The CNP ratio for ideal treatment should be around 150:5:1. Have you run some respirometer tests? Your MLSS is high, but considering the wastewater, it's probably OK.
Here's what I suggest as a trial.  Find some 0.2 -0.4 micron filter paper and get a sample of your incoming waste and settle the solids. Take an aliquiot of the  filtered influent supernatant. Then run a COD on that to determine the soluble COD. Repeat the test on the effluent supernatant after filtration.  
If you have a severe brown foaming problem that is probably an indicator of dissolved detergent surfactants (I am guessing that it is alkyl benzene sulfonates but I may be wrong.) If the product foams significantly after you remove the solids, you probably have ABS. The compound is biodegradable, but it takes an aeration time of about 24 hours or so, as I recall. Depending upon the level of ABS in the influent, it may be passing through your waste stream and giving you a high result.  If so, you are going to have to perform a sewer survey of industrial users and go to the supermarkets and other areas to find out if anyone is using conventional ABS.
Hope that helps.  If not, contact me again.

Dave