Rainwater Harvesting for Indoor Non-potable Use
By Sherry Bryan, Ecology Action
Rainwater harvesting is a Low Impact Development (LID) practice that provides multiple watershed and community benefits. When rain or runoff water is retained in a cistern or tank for on-site use, the volume and intensity of runoff from a property is reduced, as is the potential for downstream flooding and erosion. When implemented on a regional scale, rainwater harvesting systems can improve the hydrologic health of the watershed by reducing demand from overdrafted groundwater wells and surface waters that supply potable-quality water for non-potable uses such as toilet flushing, clothes washing, and irrigation.
Rainwater harvesting systems in California have the greatest potential for reducing potable water demand when rainwater is used to offset indoor water consumption during the wet winter months of November-March. This allows a cistern may be refilled multiple times by recurring storms because the water is being used between storms. When the California Building Standards Commission and Department of Housing and Community Development adopted Chapter 17 - Non-Potable Rainwater Catchment Systems - into the 2013 California Plumbing Code, the permitting of indoor rainwater harvesting systems was made possible in our state.
Ecology Action, a Santa Cruz County non-profit was awarded funding from the California State Water Resources Board Proposition 84 Storm Water Planning and Monitoring Grant Program to provide financial incentives to permit, construct, and monitor the first indoor, non-potable rainwater harvesting systems for toilet flushing and clothes washing in the Monterey Bay area. The water quality of eight indoor, non-potable rainwater harvesting systems was monitored from October 2014 through March 2016 to determine the most cost-effective treatment methods to meet Environmental Protection Agency (EPA) and 2013 California Plumbing Code (CPC), Chapter 17 water quality standards for E. Coli (<100 cfu/ml) and turbidity (<10 NTU). The primary objective of the monitoring study was to provide consumers, environmental health, water utility, and building and planning department staff with local data to evaluate the relative risk that stored rainwater poses to public health under different treatment scenarios.
All monitoring sites in the study incorporated the minimum CPC Chapter 17 code requirements including debris excluders less than or equal to 1/16” on gutters and downspouts conveying rainwater to cisterns, and sediment filtration less than or equal to 100 microns. Although disinfection of rainwater for indoor, non-potable uses is not required by 2013 CPC Chapter 17 three sites in the study voluntarily incorporated ultraviolet sterilization and/or chlorination to treat rainwater for indoor, non-potable and potable uses. Point-of-use water quality data from these sites were compared to post-100 micron filtration results from the same monitoring location.
Both the mean, median, and geometric means of samples for both untreated rainwater and 100 micron filtration treatment at all sites were well below the EPA and CPC allowable water quality standard of 100cfu/mL E.coli for indoor, non-potable uses of rainwater. The aggregate geometric mean of rainwater that had passed through a 100 micron filter was 1.1 cfu E.coli/100mL. Only one of 45 samples (2%) taken at post-100 micron sample taps exceeded the 100 cfu/100mL standard. Monitoring sites with trees overhanging rooftop surfaces had higher means and greater standard deviations for E.coli and total coliforms than sites with cleaner roof catchment surfaces. There was no significant difference in E.Coli levels between 100 micron filter and ultraviolet treatment in aggregate study data (p=0.18). However, there was a significant difference in total coliform levels between the 100 micron filter and ultraviolet treatment (p=0.0001).
Results of the study demonstrated that the minimum code-requirement of 100 micron filtration was the most cost-effective treatment option for meeting basic California Plumbing Code and EPA water quality standards for E.coli and turbidity. However, results also indicate that adding 1 or 5 micron sediment filtration, chlorine injection or ultraviolet disinfection can reduce or eliminate total coliforms and improve overall water quality, especially for locations where trees overhang roof catchment surfaces. To reduce system maintenance and improve water quality in the system, selecting a catchment surface that is not impacted by leaf debris is important.
Another objective of the study was to provide consumers with reliable information about indoor, non-potable system costs and the potential for rainwater harvesting systems to reduce utility bills. Rainwater harvesting system costs per gallon are typically based on the storage capacity of a tank, vs. the potential to capture and reuse rainwater throughout California’s rainy season. The monitoring study documented all design, permitting, and construction costs and metered rain water use to evaluate the return on investment and payback period of residential and commercial indoor, non-potable rainwater harvesting systems.
The residential average water savings for a home using rainwater to supply two high efficiency toilets and one clothes washer was 8,371 gallons/year in comparison with 2,065 gallons/year for a home where only two toilets were connected to the system. The indoor, non-potable residential rainwater harvesting systems in the study offset between 5-26% of a household’s average indoor, potable water use, and between 11-36% of a commercial building’s average indoor potable water use. What monitoring sites with higher offsets percentages had in common were high-efficiency fixtures that reduced system demand, large roof catchment surfaces, and a cistern capacity that maximized rainwater storage to meet monthly demand during dry winter and spring months. For example, one 4,995 gallon capacity residential RWH system supplied rainwater to toilets and a clothes washer through the entire 2014-15 drought year, when ½ of annual precipitation fell in the month of December. This case study and several others in this study demonstrate that when rainwater systems are designed to match demand with available precipitation in a drought scenario, rainwater harvesting for indoor, non-potable uses can be a reliable potable water conservation strategy, even in years with below-average precipitation. When rainwater is used for indoor, non-potable purposes, water utilities might consider basing water demand offset credits on the average amount of rainwater from the design catchment surface that is available to meet demand during lowest 25th percentile rainfall years, rather than the total rainwater harvesting capacity of the cistern.
This study concluded that at current water rates, the gains from investing in rainwater harvesting systems for toilet flushing and clothes washing did not exceed the cost of investment for the systems installed. High installation costs relative to the payback from water savings at current Monterey Bay area water utility rates ranging from $2.11-$6.80 per ccf (748 gallons) contribute greatly to negative ROI and lengthy payback period for the rainwater harvesting systems in the study. Until water utility rates have increased sufficiently to justify a positive return on investment and net present value, financial incentives such as tax credits, rebates, grants, water demand offset credits, and discounted permit and development fees are critical for encouraging home and business owners to install rainwater harvesting systems for indoor, non-potable uses. Integrating dual plumbing into the initial building design or remodel is much more efficient and less expensive than retrofitting an existing structure. Even if a client is not quite ready to invest in a rainwater harvesting system, architects and engineers should encourage clients to install dual plumbing in new construction to prepare for a future when alternate water sources are comparable in cost to paying for local and imported sources of potable water.
The Counties of Santa Cruz and Monterey and the Cities of Monterey, Pacific Grove, and Scotts Valley have all adopted, or in the process of adopting permit and inspection guidelines for rooftop rainwater harvesting systems for indoor, non-potable uses. You can see photos of the case studies and learn more at www.centralcoastgreywater.org.