Ubivis: Don't forget, that there might be a monsoon and all the sewers are flooded, setting the town in danger.
Indeed. It might have to be able to cope with several different test scenarios. You could also have a Super Bowl Sunday scenario, where everybody flushes the toilet simultaneously during half-time.
You could have levels comprising both residential and business districts, which would have different throughput levels at different times of the day.
You could include some sort of sewage treatment facilities, and make requirements for how much pollution is allowed in the sewer system's final output (into nature).
The possibilities are, if not endless, then at least varied.
It's funny, once you sit down and think about it, the most unlikely things could form the basis of some pretty interesting games.