Dinkar,

I suggest increasing sludge age to 20 days and maintaining dissolved oxygen concentration between 1.5 and 2.5 mg/L.

Dear Dinkar, I have seen your Agra prjct docs. having details of bio- remediation..etc..  A few suggestions can be studied/ attempted on a pilot scale, In order to get BOD less than 5 ppm, HRT would have to be minimum 8 to 10 days.. with appropriate plants/ root zones in 3 phases.. At the start, deep ponding around 5m depth.. and followed by phyto-r in next 2 phases. If cascading flows are possible from a higher to lower levels then the water gets oxygenated helping bio- r and bio-fixation in roots. Well wishes, Ajit Seshadri, The Vigyan Vijay Foundation, Delhi.      

MBBR should be able to achieve soluble BOD less than 5 ppm in some hours. Certainly HRT of days isn't normal (@Ajit Seshadri) and MBBR doesn't have sludge age so it cannot be increased (@Michael Lever).

You might have shedding of excess biofilm which is degradable in the BOD test. This can be solved by filtration (e.g. a sandfilter). 

You could also have too low pH or oxygen . Othervise, it seems the MBBR is overloaded. Adding more carriers or a new tank in a second stage could help.

Dinkar,

Professor Andersen is correct, I thought you had a Membrane Bio-reactor (MBR).

With your results you need to determine whether the BOD is soluble or particulate. If the BOD is mainly particulate, coagulation and sand filtration should be effective.

If the BOD is soluble, assuming the DO is adequate, you will need more capacity as outlined by Henrik Andersen. I don’t see ozone as a solution.

Thanks all for valuable suggestions.

special thanks to Henrik for his deeper thought.

thanks & regards

dinkar