A point to remember
Many sewage departments think that water in = water out. While this is basicaly true for residences and the majority of businesses its not for us. Make sure you get credit for beer produced as this will reduce your bills!!
As for amount you can use up to 10X beer produced. Though 5X is a more efficent target.
You also have to think about BOD. (Biological Oxygen Demand) This can be reduced by sending solids- trub/yeast/hops to solid waste though filtering/seperation operations.
Good luck. I think all of us will be hearing more form our sewer departments in the future!