4.9.2 Design

The following general design considerations should be considered when designing rain water harvest systems:
  • Rain water harvesting systems should be sized according to rainfall data (daily or sub-daily data preferred where available) and proposed indoor and outdoor water needs. The sizing of the collection system should only include non-pollution-generating tributary impervious surfaces.
  • Cisterns should be covered to prevent mosquito breeding. The cover will protect the water from sunlight and minimize algae growth.
  • Screens on the gutter and intake of the outlet pipe should be included to minimize clogging by leaves and other debris.
  • Below-grade cisterns should have tie downs per manufacturer’s specifications to avoid the floating of the cistern resulting from elevated groundwater levels.
  • Flow control structures, overflows, and clean-outs should be readily accessible and alerts for system problems should be easily visible and audible (WSUPSP, 2012).

In 2009, the State Building Code Council adopted the 2009 edition of the Uniform Plumbing Code. Significant changes were made to Chapter 16, which governs the use of reclaimed water. The previous plumbing code did not distinguish reclaimed water from rain water. The new adopted code has a separate set of regulations that govern some aspects of rain water harvesting (for indoor use only). This code went into effect July 1, 2010 and is codified under WAC 51-56-1600.

Rain water harvesting systems should only collect water from roof surfaces and not from vehicular or pedestrian areas, surface water runoff, or bodies of standing water (WAC 51-56-1400).

Depending on its size, an above-ground cistern will be treated as a structure under locally-adopted building and zoning codes. As a structure, a cistern would need to be set back from property lines and meet local height, bulk, and dimensional standards. Access to the structure may need to meet confined space requirements (check local requirements). The components of a rain water harvesting system will depend on the rainfall pattern, physical setting, water needs, and stormwater management goals and are described below. Catchment or Roof Area
The roof material should not contribute contaminants (such as zinc, copper or lead) to the collection system (WAC 51-56-1600). The National Sanitation Foundation (NSF) certifies products for rain water collection systems. Products meeting NSF protocol P151 are certified for drinking water system use and do not contribute contaminants at levels greater than specified in the USEPA Drinking Water Regulations and Health Advisories (Stuart, 2001). Guidelines for roofing materials include:
  • Enameled standing seam metal, ceramic tile or slate are durable and smooth, presumed to not contribute significant contaminants, and are the preferred materials for potable supply.
  • Composition or 3-tab roofing should only be used for irrigation catchment systems. Composition roofing is not recommended for irrigation supply if zinc has been applied for moss treatment.
  • Lead solder should not be used for roof or gutter construction and existing roofs should be examined for lead content.
  • Galvanized surfaces may deliver elevated particulate zinc during initial flushing and elevated dissolved zinc throughout a storm event (Stuart, 2001).
  • Copper should never be considered for roofing or gutters. When used for roofing material, copper can act as an herbicide if rooftop runoff is used for irrigation. Copper can also be present in toxic amounts if used for a potable source.
  • Treated and untreated wood shingles and shakes should not be considered for rain water collection systems (WSU-PSP, 2012). Gutters and downspouts
Gutters are commonly made from aluminum, galvanized steel, and plastic. Rain water is slightly acidic; accordingly, collected water entering the cistern should be evaluated for metals or other contaminants associated with the roof and gutters. See below for appropriate filters and disinfection techniques. Do not use lead solder for gutter seams. WAC 51-56-1600 states that copper or zinc gutters and downspouts shall not be used; however, if existing gutters and downspouts are already in place, the interior shall be coated with NSF-quality epoxy paint.

Screens should be installed in the top of each downspout. Screens installed on gutters prevent coarse (e.g., leaves and needles), but not fine debris (pollen and dust) from entering the gutter. Gutters will still require cleaning and access should be considered when selecting gutter screens. First flush diverters
First flush diverters collect and route the first flush away from the collection system. The initial flow from a storm can contain higher levels of contaminants from particulates settling on the roof (e.g., bird droppings). A simple diverter consists of a downspout (located upstream of the downspout to the cistern) and a pipe that is fitted and sealed so that water does not back flow into the gutter. Once the pipe is filled, water flows to the cistern downspout. The pipe often extends to the ground and has a clean out and valve (WSU-PSP, 2012).

The Texas Rainwater Harvesting Manual recommends that the first 10 gallons of water be diverted for every 1,000 square feet of roof (applicable for areas with higher storm intensities) (Texas Water Development Board, 2005). However, local factors such as rainfall frequency, intensity, and pollutants will influence the amount of water diverted. In areas with low precipitation and lower storm intensities, roof washing may divert flows necessary to support system demands. Roof washers
Roof washers are placed just before the storage cistern to filter coarse and fine debris. Washers consist of a tank (typically 30-50 gallons), a course filter/strainer for leaves and other organic material, and a finer filter (typically 30-microns or less). Roof washers should be cleaned regularly to prevent clogging as well as prevent the development of pathogens (Texas Water Development Board, 2005).

WAC 51-56-1600 governs roof washers. The following provisions apply:
  • All rain water harvesting systems using impervious roof surfaces shall have at least one roof washer per downspout or pre-filtration system. A roof washer or pre-filtration system is not required for pervious roof surfaces such as green roofs. Roof washers and pre-filtration systems shall meet the following design requirements:
  • All collected rain water shall pass through a roof washer or pre-filtration system before the water enters the cistern(s).
    • If more than one cistern is used, a roof washer or pre-filtration system shall be provided for each cistern. EXCEPTION: Where a series of cisterns are interconnected to supply water to a single system.
    • The inlet to the roof washer shall be provided with a debris screen that protects the roof washer from the intrusion of waste and vermin.
    • The roof washer shall rely on manually operated valves or other devices to do the diversion. Storage tank or cistern
The cistern is the most expensive component of the collection system. Cisterns are commonly constructed of fiberglass, polyethylene, concrete, metal, or wood. Larger tanks for potable use are available in either fiberglass for burial or corrugated, galvanized steel with PVC or polyethylene liners for above ground installations. Tanks can be installed above ground (either adjacent to or remote from a structure), under a deck, or in the basement or crawl space. Above-ground installations are less expensive than below-ground applications. Aesthetic preferences or space limitations may require that the tank be located below ground, or away from the structure. Additional labor expenditures for excavation and structural requirements for the tank will increase costs of subsurface installations compared to above-ground storage (Stuart, 2001). Multiple tank systems are generally less expensive than single tanks and the multireservoir configurations can continue to operate if one of the tanks needs to be shut down for maintenance.

WAC 51-56-1600 governs cisterns. The following provisions apply:
  • All cisterns shall be listed for use with potable water and shall be capable of being filled from both the rain water harvesting system and the public or private water system (WAC 51-56-1600).
  • The municipal or on-site well water system shall be protected from cross-contamination in accordance with Section 603.4.5 of the Uniform Plumbing Code.
  • Backflow assemblies shall be maintained and tested in accordance with Section 603.3.3 of the Uniform Plumbing Code.
  • Cisterns shall have access to allow inspection and cleaning.
  • For above grade cisterns, the ratio of the cistern size shall not be greater than 1:1 height to width. An engineered tank with an engineered foundation may have a height that exceeds the width (subject to approval of the authority having jurisdiction). The ratio for below grade cisterns is not limited.
  • Below grade cisterns shall be provided with manhole risers a minimum of 8 inches above surrounding grade. Underground cisterns shall have tie downs per manufacturer’s specifications, or the excavated site must have a daylight drain or some other drainage mechanism to prevent floating of the cistern resulting from elevated groundwater levels.
  • Cisterns shall be protected from sunlight to inhibit algae growth and ensure life expectancy of tank.
  • All cistern openings shall be protected from unintentional entry by humans or vermin. Manhole covers shall be provided and shall be secured to prevent tampering. Where an opening is provided that could allow the entry of personnel, the opening shall be marked, “DANGER - CONFINED SPACE.”
  • Cistern outlets shall be located at least 4 inches above the bottom of the cistern.
  • The cistern shall be equipped with an overflow device. The overflow device shall consist of a pipe equal to or greater than the cistern inlet and a minimum of 4 inches below any makeup device from other sources. The overflow outlet shall be protected with a screen having openings no greater than ¼-inch or a self-sealing cover. Pumps and pressure tanks
Adequate elevation to deliver water from the storage tank to the filtration and disinfection system and the house at adequate pressure is often not available. Standard residential water pressure is 40-60 pounds per square inch. Two methods are used to attain proper pressure: 1) a pump with a pressure tank, pressure switch, and check valve; or 2) an on-demand pump. The first system uses the pressure tank to keep the system pressurized and the pressure switch initiates the pump when pressure falls below a predetermined level. The check valve prevents pressurized water from returning to the tank. The on-demand pump is self-priming and incorporates the pressure switch, pressure tank, and check valve functions in one unit (Texas Water Development Board, 2005).

Where a pump is provided in conjunction with the rain water harvesting system, the pump shall meet the following provisions per WAC 51-56-1600:
  • The pump and all other pump components shall be listed and approved for use with potable water systems.
  • The pump shall be capable of delivering a minimum of 15 psi residual pressure at the highest outlet served. Minimum pump pressure shall allow for friction and other pressure losses. Maximum pressures shall not exceed 80 psi. Back flow prevention
Rain water is most commonly used to augment an existing potable supply for uses that don’t require treatment to potable. Typically, such systems augment an existing supply because the cistern will likely run dry or near dry in the summer. Chapter 16 The Uniform Plumbing Code as adopted by Washington State has provisions that govern how to dual plumb such systems to prevent backflow and subsequent contamination of the potable water supply. Water Treatment
Water treatment falls into three broad categories: filtration, disinfection, and buffering.

Filters remove leaves, sediment, and other suspended particles and are placed between the catchment and the tank or in the tank. Filtering begins with screening gutters to exclude leaves and other debris, routing the first flush through first flush diverters, roof washers, and cistern float filters. Cistern float filters are placed in the storage tank and provide filtration as water is pumped from the tank to the disinfection system and the house. The filter is positioned to float 10-16 inches below the water surface where the water is cleaner than the bottom or surface of the water column (Texas Water Development Board, 2005).

Types of filters for removing the smaller remaining particles include single cartridges (similar to swimming pool filters) and multi-cartridge filters. These are typically 5-micron filters and provide final mechanism for removing fine particles before disinfection. Reverse osmosis and nanofiltration are filtration methods that require forcing water through a semi-permeable membrane. Membranes provide disinfection by removing/filtering very small particles (molecules) and harmful pathogens. Some water is lost in reverse osmosis and nanofiltration with concentrated contaminants. The amount of water lost is proportional to the purity of the feed water (Texas Water Development Board, 2005).

  • Ultra-violet (UV) radiation uses short wave UV light to destroy bacteria, viruses, and other microorganisms. UV disinfection requires pre-filtering of fine particles where bacteria and viruses can lodge and elude the UV light. This disinfection strategy should be equipped with a light sensor and a readily visible alert to detect adequate levels of UV light (Texas Water Development Board, 1997).
  • Ozone is a form of oxygen produced by passing air through a strong electrical field. Ozone kills microorganisms and oxidizes organic material to CO2 and water. The remaining ozone reverts back to dissolved O2 (Texas Water Development Board, 1997). Care must be exercised in the choice of materials used in the system using this disinfection technique due to ozone’s aggressive properties.
  • Activated carbon removes chlorine and heavy metals, objectionable tastes, and most odors.
  • Chlorine (commonly in the form of sodium hypochlorite) is a readily available and dependable disinfection technique. Household bleach can be applied in the cistern or feed pumps that release small amounts of solution while the water is pumped (Texas Water Development Board, 1997). There are two significant limitations of this technique: chlorine leaves an objectionable taste (this can be removed with activated charcoal); and prolonged presence of chlorine with organic matter can produce chlorinated organic compounds (e.g., trihalomethanes) that can present health risks (Texas Water Development Board, 1997).

For potable systems, water must be filtered and disinfected after the water exits the storage reservoir and immediately before point of use (Texas Water Development Board, 2005).

As stated previously, rain water is usually slightly acidic (a pH of approximately 5.6 is typical). Total dissolved salts and minerals are low in precipitation, and buffering with small amounts of a common buffer, such as baking soda, can adjust collected rain water to near neutral (Texas Water Development Board, 1997). Buffering should be done each fall after tanks have first filled.