I’m thankful to all fellows ...

I’m thankful to all fellows for responding to my question.

The project is in India. The drain is passing through residential areas hence receiving only domestic sewage. We have fixed about 11-12 sampling points in complete stretch of 14 km & 5 lateral lines. Our main focus is on estimation of BOD in these samples. Based on BOD we adjust doing of bacterial consortium containing strict & facultative bacteria. We are doing at the point of origin of each lateral and in main drain we have fixed 6 dosing points.
Normally initial BOD is ranging between 150 to 300 ppm. At the outlet of drain before it falls in the river we successfully achieve BOD 20-30 ppm. The process is completely electricity free. 
We are receiving marvelous results in terms of reduction of organic matter, control of emission of H2S etc but authorities are more interested to know about mass balance.
According to my experience calculating mass balance accurately in this type of water body is not possible because of following reasons:

1. The width and depth are not constant somewhere width is 50 m but after 500m it remains 5-6 m only because it’s not a channelized structure with proper boundaries. So it can’t be considered as bioreactor.

2. Based on our analysis the anaerobic strains are found growing in the sediments thus controlling tge emission and generation of odors completely. So there is no proper distribution of bacterial colonies in the complete volume of water.

3. initially we noticed the high loss of bacterial colonies but after 15 days they stabilize in the system and we started receiving substantial reduction in pollutants as mentioned above

I would like to know whether I’m right in my concepts or it’s possible to calculate the mass balance

Appreciate your comments on above mentioned details.