Growing, intertwined water challenges confront us: Storm water is polluting watersheds, and there is a global shortage of clean water and growing demand for freshwater.
A three-year study of a Key Largo, Fla., single-family, dual-source residential rainwater harvesting system (served by both treated rainwater and municipal water separated by a backflow prevention valve) concluded that a well-designed and maintained residential rainwater harvesting system can reduce or eliminate storm water runoff, diminish municipal water demand and deliver high-quality, competitively priced potable water. The 2014 study was written by Tamim Younos, Ph.D., and me, and was published jointly by our respective organizations, the Cabell Brand Center for Global Poverty and the American Rainwater Catchment Systems Assn. (ARCSA).
Relying on Rainwater
From August 2011 to August 2014, all 150,500 gal of rain that fell on the 1,956-sq-ft roof were retained on the property. Of the 150 in. of rain that fell during the period, 72%—or 108,850 gal—was captured and treated for potable and non-potable consumption by the home’s two full-time residents and periodic guests. Overflow from the 7,500-gal poured-concrete cistern was diverted to an onsite spreader swale. As a result of the rainwater use, municipal water consumption was 33.9 gal per capita per day (gpcd), or less than one-third of the utility-wide average of 108 gpcd.
The system was designed and maintained in accordance with ARCSA guidelines and ARCSA/ASPE/ANSI Standard 63-2013, with the possible exception that the home has copper gutters. Standard 63, released 11 years after the home was built, prohibits copper roofing for potable systems due to leaching risks. Regardless, no annual test results indicated greater than 10% of the 1.3-ppm U.S. Environmental Protection Agency (EPA) maximum contaminant level for copper.
National Testing Laboratories Ltd. performed annual primary and secondary water quality tests of treated rainwater from the kitchen cold spigot. Water samples were shipped overnight to the laboratory in lab-provided, ice-packed plastic foam containers and processed less than 24 hours later.
Samples were tested for all EPA primary and secondary drinking water standards, including microbiological contaminants, physical factors, and organic and inorganic contaminants, as well as many potential contaminants not addressed by current EPA standards. With the exception of one pH result (8.8 compared with the 8.5 standard), all tests of treated rainwater easily surpassed EPA standards, with no trihalomethanes or other organic contaminants.
Subjectively, the water was soft, clear and tasted good, with annual hardness results from zero to 17 mg/L and turbidity from zero to 0.4 ntu. Treated rainwater is the home’s primary source for drinking, cooking and bathing, because its quality is superior to the published utility water quality.
The rainwater harvesting system includes a prefiltration and first-divert system consisting of two Leaf Eater Advanced self-cleaning rain heads and two SafeRain adjustable first-flush units. After the first flush, rainwater enters the first half of the divided tank through calming inlets. When the first half-tank is full, additional water decants to the second. Each half has a foot valve located 6 in. above the bottom. Water can be drawn from either or both valves by a 0.75-hp jet pump. Water pressure is maintained by an 80-gal pressure tank.
For treatment, the rainwater flows through three canisters of a Pura Big Boy UVBB3 filtration and ultraviolet (UV) system, including a dual-density polypropylene sediment filter (nominal 25-µ prefiltration and 1-µ post-filtration), a 5-µ nominal carbon block, and a UV chamber for disinfection.
The treated rainwater flows through a manifold of selection valves used to manually shift the source from rainwater to utility water or back, for any or all of three subsystems: toilets and hose bibs, showers and lavatories, and a dedicated kitchen spigot, according to rainwater availability. When rainwater is plentiful, no utility water is consumed. As cistern levels decline during the relatively dry season, rainwater is reserved for human consumption. When cistern levels drop to approximately 1,000 gal, utility water is used for all purposes except for the dedicated kitchen spigot.
Costs & Projections
The system’s initial fixed material and labor costs totaled $13,186. The annual costs of filters, a new UV bulb, electricity consumed and an estimate for unscheduled maintenance totaled $531, while the cost of water was zero. This was compared with the upfront utility water meter cost of $3,750, fixed costs in the monthly water bill and water cost of $5.75 per 1,000 gal. The end-of-study (12th year since commissioning) cost of treated rainwater was $0.0349 per gallon compared with utility water’s $0.023. The cost for both is projected to be $0.0186 per gallon in the system’s 25th year.
Beyond the 25th year, in this estimation, the cost of treated rainwater will be less than utility water. Because the upfront costs for a rainwater system are likely to be greater than the upfront costs for utility water, but even inexpensive utility water costs more than free rainwater, a durable rainwater system will deliver less expensive water after the upfront cost differential has been offset by harvesting greater quantities of free rainwater. A less expensive rainwater harvesting system or more expensive utility water thus would deliver a quicker return on investment (ROI).
Beyond simple ROI calculations, other factors that could affect broader value comparisons are unpredicted maintenance costs, central sewer savings, net present value calculations, rebates, utility water rate increases, a rainwater system lifespan of more (or less) than 30 years, and storm water runoff avoided costs. For some, the quality and security of water delivered by an onsite rainwater harvesting system provides compelling, albeit unquantifiable, benefits.