Stormwater cistern

Jana, when you say the cistern is capable of receiving 100% of the site's runoff, I assume that the cistern is sized to hold a certain volume of water based on a certain storm event (e.g. 1-year, 10-year, 50-year, etc.). You mention a small amount of water will be reused in flushing fixtures and for irrigation, so I'm also assuming that the cistern does not get completely emptied when the pumps to the municipal system kick on. I would suggest you verify with the engineers how much water will be held and reused for flushing and irrigation. If that volume is equal to or greater than volume calculated using the Runoff Treatment Equivalents in the LEED Reference Guide, I think you can make a convincing case that 100% of the TSS from the average annual storm is treated and removed on-site. There are also some best practices that should be considered, such as pretreatment and screening (before the runoff enters the cistern) to prevent debris and sediment from building up in the cistern and entering the toilet and irrigation lines. If the pretreatment system is a hydro-dynamic treatment device, it may well be capable of removing 80% of the TSS from 90% of the average annual rainfall itself. You just need to make sure it's sized properly.