GP is partly right. Most of the cost of sewers is fixed cost: employee salaries, building and maintaining X kilometers of sewers, etc. Some is variable: chemicals, but a small part.

If you, a single person, cut your water usage in half, you pay half as much. But if everybody uses half as much, the system still needs about the same amount of funding. So now you double the per-unit price, and everybody pays the same they were before spending money on water saving features. In this case, even if each person used half as much water, the total water needed isn't cut in half because the sewers need more water to function.

(Also, water isn't "used"; most of it's transported, cleaned, transported, dirtied, cleaned again, transported)