Atop a 1,000 square meter roof of a school building, rainwater falls and runs down into gutters at the perimeter. Through the gutters, the rainwater flows into a large reservoir either by the force of gravity, or with the help of sub-pumps. The reservoir is connected to the school’s toilets by a separate line in order to avoid cross connections with city water.  A pressure switch activates only upon a toilet flush in the school bathroom, causing the rainwater to flow once again and refill the toilet. Through these pipes, reservoirs and pumps, this system captures the valuable natural resource of rainwater and puts it to use.
This is the model of rainwater catchment system that Amir Yechieli’s company, Yevul Mayim, has installed at 150 schools in Israel so far. Once built, the systems are used as educational tools by the schools, to teach students concepts of conservation and research methods like data collection and arithmetic. Though there have been many national and international sponsors of the systems like the Jewish National Fund, Rotary International and the Jerusalem Foundation, the systems are viable as a financially and environmentally sustainable investment in their own right.
The system described above has the collection potential of approximately 429 cubic meters annually, or 2,681 bathtubs full of water, given Jerusalem’s average amount of rainfall. The actual volume of rainwater moving from roof to toilet depends on the total amount of rainfall , its distribution, the storage capacity, and number of toilets hooked up to the system. This is 429 cubic meters of rainwater that would not incur the energy costs of treatment and pumping, or the direct financial cost of NIS 9.95 (including value added tax) per cubic meter, which comes out to NIS 4,269 annually at the water rate for public buildings.
The average cost of a modest rainwater catchment system for a school rooftop in Jerusalem is NIS 30,000 with negligible operating and maintenance costs. This capital cost could be paid for in a number of ways, all of which assume use of all of the 429 cubic meters, and that money would be set aside for repairs. First, as mentioned above, a philanthropic organization could pay the full capital cost. Second, a public-private partnership between the school, the municipality and a private installation company could be created to finance and build the project. The initial capital cost could be split between the three partners, and the school could pay the other two back over time through a negotiated percent of the avoided cost. Third, a third of the capital cost could be raised by the school’s local community, and two thirds paid for by a 5-year loan that the school takes from a commercial bank (at market borrowing terms). And fourth, the municipality could take a 5-year loan (at market borrowing terms) from the national government to pay for the creation of rainwater catchment systems. The number of years it takes for the money saved by installing a 1,000 square meter rainwater catchment system to equal the NIS 30,000 capital cost is depicted in the graph. Jerusalem has approximately 896,112 square meters of roof space on public buildings, which yields a potential water saving of 384,253 cubic meters annually. Expanding the number of rainwater catchment systems on Jerusalem’s roofs through any of these four financial mechanisms would lead the city to continuously save more water, energy, and money.