Rainwater catchment system for flushing toilets
I'm building a modern multi-family complex in a very urban environment and have several large (1,250) gallon storage tanks planned to be installed under driveways. The buildings are 3 stories tall and incorporate green roofs (approx 20% coverage of entire roof). There are four roofs consisting of approx 500-650 sq ft each. Each rooftop is coupled to its own water storage tank approx 40 feet below the roof. I plan to install a pump in the tank to pump water to the toilets. The tank will have an overflow (to city storm water drains) and there will be an inlet w/ball valve for city water when the tank runs low (e.g. during dry spells).
We get roughly 25" of rain annually here so I calculate that the systems will save approx 7,500 gallons on the smaller roofs and 9,750 gallons of water on the larger roofs annually. The average person flushes their home toilets 5 times/day. I'm using dual-flush toilets which consume 1.6 gallons for #2s and .9 gallons for #1s. Assuming the average person will do two #2s and 3 #1s this comes to 6 gallons per day per person. The smaller units will have approx 3 people living in each one and the larger ones 4 people. This comes out to 6,570 gallons for the smaller units and 8,760 gallons for the larger ones... which matches the annual rainfall totals quite well. I realize that the summer months will be dryer but still, the tanks can hold 1,250 gallons so this should help buffer dry spells (a full tank will provide approx 2 months of flushing before going dry with no rain).
Has anyone here ever installed a system like this before? Some people tell me it's a glorious waste of time and money, not to mention the additional items that need to be maintained (pump, float valves, tank, etc). But it just seems like the right (and responsible) technology to integrate into a sustainable project. Am I nuts?