Calculating wastewater costs

[Brian Tome]

I am looking at a 7 unit short sale with only one occupant and no financials (because managing partner is not cooperating with the bank).

In preparing my analysis for investors/financing, I am trying to get a rough estimate for wastewater costs.  The property is on a well (with public water available that I may hook up to in the future) but the septic is a public utility.

Is there a formula for average gallons per day use based on bedrooms/bathrooms that I could use to apply the unit costs to and estimate my operating costs?

Thanks in advance for your help.