One of my favorite green building technologies is rainwater harvesting: the capture, treatment and use of rainwater for uses inside the building such as toilet flushing and cooling-tower makeup water (to replace water lost by evaporation and back-flushing). This is such a simple and obvious thing to do in much of the country that one wonders why it has taken so long to be considered. In addition to water conservation, rainwater harvesting can help reduce stormwater runoff from building sites.
Imagine even a modest half-inch rainfall on a 24,000-square-foot roof. That event will generate 1,000 cubic feet, or about 7,500 gallons, of clean free water. In a climate like the Pacific Northwest, or anywhere that receives light rainfall a good part of the year, this system could be quite productive. Assuming one could collect 80% of annual rainfall of 35 inches, one would harvest about 420,000 gallons for reuse. Basic treatment with a sand filter and ultraviolet light would make it suitable for toilet flushing and similar non-potable uses. What could be simpler? Nothing, except that you can expect to pay $20,000 to $50,000 for such a system, and it's not in most building budgets.
But wait! That may not be the end of the story. Many urban areas have quite expensive charges for storm-drain hookups. I have found many cases where the impact fees or system development charges that are avoided by a 100% rainwater reclamation system were greater than the total cost of the rainwater collection and treatment system. In that case, a building owner is "money ahead" to install it. In one California university project, just the cost of installing the storm sewers to take water off the site and to connect to the town's storm drains was greater than installing two 20,000-gallon tanks to hold runoff from the 100-year rainfall event and provide toilet flushing for a good part of the year.
One caution: don't expect rainwater to provide all of your needs, unless you are prepared to treat it to potable water standards and make that case to local code officials. In addition, the taller the building, the lower a percentage of annual needs the system will supply, because you've only got one roof for collection purposes, but more toilet and sink fixtures for each added story. Another caution: runoff from parking lots is often too polluted to collect and treat with simple systems, so you can't count on using it.
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