Rainwater harvesting is a practice that has been used by environmentalists for many years, but has only recently become more popular. By connecting underground cisterns or above-ground containers, people can collect up to 0.6 gallons of water for every inch of rain that falls on one square-foot of roof. In other words, if it rains one inch, and you have a 1,000 square-foot roof, you can collect about 600 gallons of water. With two inches of rain, you can collect up to 1,200 gallons.
The problem, however, arises with mosquitoes and other harmful vectors. Since mosquitoes with harmful parasites and diseases have recently been spreading, it is crucial to ensure a biologically sealed rain capturing system. This is often done by having a series of filters or screens that seal the water from the open-air. By doing so, the mosquitoes cannot access the water to lay their eggs, and must find elsewhere to breed.
This method of “recycling” rainwater is seen as much safer than reusing greywater because rainwater is often much cleaner than greywater. There are, however, systems to recycle greywater. These systems are often very expensive because they sometimes must filter out soaps or other biological contamination.
To read about rainwater collection systems, go to: http://www.mercurynews.com/breaking-news/ci_13987135?nclick_check=1