Are some of your restaurants' water bills much more expensive than the others? This could be because it is using more water, but it could also be from your water utility charging more for sewer.
For example, a large southeastern city that provides water to the city and the nearby communities just outside the city, has two different charges for both water and sewer.
The water charges for inside the city is about the same as the water charges for outside the city. However, the sewer charges for inside the city are almost four times as high as those outside the city.
How do you get the sewer side of the restaurant's water bill down?
Water utilities do not meter sewer use because the assumption is that all of the water you bring into the restaurant is going to go down the sewer at some point. So for ease of calculation and billing, the amount of water used is also used to determine the amount of sewer.