Jerusalem has approximately 896,112 sq.m. of roof space on public buildings, which could yield a water saving of 384,253 cu.m. annually.
By MIMI KAPLANYears until the Savings Equals the Investment(photo credit: JERUSALEM INSTITUTE FOR POLICY RESEARCH)
Atop a 1,000-sq.m. roof of a school building, rainwater runs into gutters at the perimeter. Through the gutters, the water flows into a large reservoir, connected to the school’s toilets by a separate line to avoid cross connections with city water.A pressure switch activates only upon a toilet flush in the school bathroom, causing the water 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 a rainwater catchment system that Amir Yechieli’s company, Yevul Mayim, has installed at 150 schools in Israel. The systems are used to teach students concepts of conservation and research methods like data collection and arithmetic.Though there have been many sponsors of the systems, such as Keren Kayemeth LeIsrael-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 has the collection potential of approximately 429 cubic meters annually, or 2,681 bathtubs full of water, given Jerusalem’s average rainfall. This is 429 cu.m. 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), which comes out to NIS 4,269 per cu.m. 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 cu.m.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 repay the other two over time.Third, a third of the capital cost could be raised by the school’s local community, and two-thirds paid for by a five-year loan that the school takes from a commercial bank (at the market interest rate).Fourth, the municipality could take a five-year loan (at the market interest rate) 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-sq.m. rainwater catchment system to equal the NIS 30,000 capital cost is depicted in the graph.Jerusalem has approximately 896,112 sq.m. of roof space on public buildings, which could yield a water saving of 384,253 cu.m. annually.Expanding the number of rainwater catchment systems on Jerusalem’s roofs would enable the city to continuously save more water, energy and money.Jerusalem Institute for Policy Research, en.jerusaleminstitute.org.il