I like how the question is ...

I like how the question is framed; as some have commented, we normally just look at all the streams and figure out which should be combined and which shouldn't be. The question points us to what should the principles be that would guide this activity.

Let me give it a shot:

Should combine when:

1. It leads to better equalization and balancing e.g. one variable stream with another more constant stream or multiple batch streams with some continuous streams.

2. It leads to reduced concentration where this may be required.

3. There are much smaller streams that do not have much load and would be impractical to segregate and provide separate treatment.

4. It is cheaper and more practical to build one large plant vs several smaller plants if segregated.

Should segregate when:

1. It is a highly polluted and high load stream requiring a pretreatment step before it can be safely combined with other streams

2. The stream is relatively clean but having a large flowrate, which if combined will require a much larger, combined plant. I.e. it is cheaper or more practical to build smaller multiple plants.

3. The stream has significant flow but different pollutants compared to others requiring different treatment technology.