Biological process is the secondary stage in most of wastewater treatment plant. Is there any wastewater treatment plant without undergoing biological process?
It all depends on what is in the waste water, and how much is allowed to be left after treatment.
Treatment can mean anything from a settling tank, to a filter or screen, to oxygenating, to centrifuging, to active biological break down, and more. In some cases, treatment means it flows thru a "magic pipe" and is simply discharged when no inspectors are looking.