Whereas industrial bioprocesses turn out a product for sale, waste treatment processes are most desirable when the products are carbon dioxide and water that are released to the environment. The microbial cells must be treated for disposal, so the less cell mass the better.
Except for some industrial wastes, most feed streams to a waste treatment process are quite dilute and would not sustain a dense microbial population. With low numbers of cells, treatment rates would be slow resulting in large vessels and long detention times to reach acceptably low concentrations for the treated waste. Instead of discharging cells, a high percentage is collected (usually by sedimentation) and recycled to the process. The result is many more cells, higher treatment rates, and a large fraction of cells that are starving or dead because of insufficient nutrients.
In the R.P.I. archive are several hypertext presentations related to biological treatment of wastes.