As others have noted, the problem is metering, or rather, lack thereof. This can actually come up in two ways:
- Water usage that does not count as sewer usage
This is the initial question. It affects different users to varying degrees. I have heard of some places billing a little differently if you have a swimming pool, as a single fill of a pool can be as much, or more than, a month of regular water but not sewer usage. The flip side of pools is that in many areas pool maintenance companies, landscapers, etc. can pay a fee to make use of hydrants for such purposes, which particularly for pools is a big benefit as a hydrant can typically fill up a pool much faster than a regular residential water connection.
If everyone gets credit for "some water doesn't go in the sewer" then there really is nothing to do - if everyone gets 10% credit, then everyone's bill has to be dropped by 10% volume but raised 10% per gallon to compensate because the sewage plant still needs to be paid for. So end result is "nothing", unless there is separate metering for large water-not-sewer users.
The one exception I could see, but which should be incorporated into the billing, is if you have municipal water but no sewer connection - i.e., septic tank. That should be easy enough - pay for 100% of the water, but nothing for sewer.
- Sewer usage that does not come from water usage
When I first encountered this it didn't make any sense! It turns out that there are some places using well water but municipal sewer. In that case, the sewage has to be metered in order for the utility to know how much to bill for sewage.
So really any combination is possible. But metering doesn't come for free. A classic example is lighting in front of houses. Originally this was gas and later electric. But in both cases often not metered, because a meter - and meter reading (in the days before remote reading) is not free. A calculated price is determined based on the type of light and billed accordingly. Sometimes, in the case of electric, the bulbs burn out and years later the next owner of the house gets their first electric bill and asks what the "lighting charge" is for - and finds out it was for a light that doesn't work...and that the previous owner paid for even though it wasn't working. Really.