re-use is vital
We haven't done any hard measurements, but with major attention to water re-use we are pretty sure we've got our water use down to 2.5:1. Water used for wort cooling is held and re-used for all our cleaning and sanitizing needs, plus we installed a heat-on-demand system that uses only this water source for mash water. Hand washing, re-using both cleaners and sanitizers, timing tank cleaning so that you can do this... and NOT bottling. Single biggest waste of water in a brewery, as far as I can tell.
The other issue is how well any wastewater system will cope with your chemicals. Digesters of any sort will be very unhappy with both caustic and yeast. Both need to be treated before they hit the drain. Yeast can easily be killed by blending with sanitizer (which is easy, since you've just used it to clean the finishing tank into which you're transferring), and caustic will have to be pH balanced. A pre-digester/pre-septic holding tank will allow you to get your wastewater pH balanced before it goes into the actual digester. You can also use alternative chemicals like PWB from Five-Star which is actually helpful to digesters, and use Peroxyacetic Acid as a sanitizer.
Unfortunately, there are very few city engineers out there who will understand this stuff, you need to get it all drawn up by an engineer who does get it in order to impress them.
Canada's Certified Organic, on-farm microbrewery