High intensity land use patterns and increasing pressure on water resources require effective stormwater management solutions in tight spaces. Rainwater harvesting programs collect runoff from rooftops, parking lots and other surfaces and reuse the water for such things as irrigation of gardens and municipal ballparks, washing patio furniture and lawn watering. Additionally, harvested rainwater when approved could be used indoors for non-potable uses such as toilet and urinal flushing. Indoor use designs are subject to review by the Plumbing Plan Review Program of the Minnesota Department of Labor and Industry in accordance with the Minnesota Plumbing Code and would require pretreatment practices including filtration and disinfection. The effect is volume control, reduced flooding and erosion, and less demand for treated potable water.
This fact sheet discusses the benefits of rainwater harvesting, highlights existing programs and provides conceptual designs for a variety of effective rainwater harvesting systems.
Rainwater harvesting programs serve multiple benefits. The collected rainwater can be used for purposes that would otherwise require potable, tap water. This reduces the cost of tap water to the owner and conserves potable water resources. All of the water captured and subsequently infiltrated (e.g. used for irrigation) removes 100 percent of the solids, nutrients, metals, pathogens and toxins that would otherwise have washed off, drained to the storm sewer, and then reached downstream waterbodies.
Harvesting and re-using rainwater decreases the impact of stormwater runoff to our lakes and streams; it protects the environment and minimizes localized flooding and erosion. It has additional benefits in urban areas, including, but not limited to, an increase in soil moisture levels for urban greenery. In addition, it can be used to meet regulatory requirements for stormwater volume control and water quality.
Programs designed to promote rainwater harvesting and reuse can incorporate any combination of the elements below.
Rainwater harvesting can be accomplished using rain barrels and/or cisterns. Rain barrels are typically small scale (25 to 100 gallons) and located at the downspout of a gutter system. They can also be linked to expand the overall storage volume. They are used to collect and store rainwater for watering landscapes and gardens or washing patio furniture. The simplest method of delivering water is by the force of gravity. However, more complex systems can be designed to deliver the water from multiple barrels connected in a series with pumps and flow control devices.
The total volume of storage available from rain barrels represents roof runoff from relatively small rainfall events, typically substantially less than one inch of rain over the surface. This is a small volume for a single rain barrel, but cumulative effects of rain barrels installed across a watershed include volume reduction and water quality treatment since typically the first half to one-inch of runoff contains the dirtiest water. During wet weather, there will likely be little or no storage available because of prior filling and little demand for irrigation water. If the ground can absorb it, consider discharging collected water onto vegetated areas between rainfall events to maximize rainwater capture and infiltration even if unnecessary for irrigation.
The following general guidance provides an overview of the items to consider in rain barrel design, installation and operation.
The utilities of Austin, [ http://www.rochestermn.gov/departments/public-works/stormwater-management/actions-for-clean-water/pollution-prevention/rain-barrelsRochester] and Owatonna, MN, teamed up to offer a $10 rebate to water customers for rain barrel purchases of over 40 gallons in volume. Martin County SWCD in Fairmont, MN, coordinated with Hormel Foods Corporation to salvage their 55 gallon food-grade drums to recycle for use in its Rain Barrel Program. St. Croix County, Wisconsin, runs a program retrofitting food-grade barrels into rain barrels and selling them for $30. Many cities provide a limited supply of rain barrels to residents at a reduced cost.
In the past, Nine Mile Creek Watershed District has sponsored a Rain Barrel Decorating Event with optional art competition in Bloomington and Hopkins, MN, for residents living in the District. The District supplied rain barrels at discounted price and free painting supplies for decorating the barrels onsite.
In 2007, the City of Minneapolis supplied 2,000 rain barrels to residents at the reduced cost of $45. The barrels were available through a $100,000 grant from the Environmental Protection Agency and in partnership with Minneapolis/Metro Blooms and the Green Institute. Monitoring was conducted by the City of Minneapolis to determine the volume control and water quality benefits of rain barrels (Neighborhood Rain Barrel Partnership Project, 2008). Data indicated that the average 50-gallon rain barrel could capture a 0.26-inch precipitation event, or 64 percent of the 28 precipitation events monitored. Additional benefits could be realized by increasing available storage to limit overflow (e.g. plumbing barrels in series).
For case studies of projects utilizing cisterns, link here.
Cisterns have a greater storage capacity than rain barrels and may be located above or below ground. Due to their size and storage capacity, these systems (often large polyethylene drums) typically collect runoff from areas larger than residential rooftops such as commercial parking lots.
Collected water is typically used to irrigate landscapes, gardens, and ballparks on a regular basis (e.g. feeding an automated irrigation system) reducing the strain on municipal water supplies during peak summer months. Again, cisterns may be used in series and water is typically delivered using a pump system. Pump systems in cisterns can be designed with a floating level that shuts off the pump and converts the water source to a municipal supply when cistern levels are too low.
St. Anthony Village, MN, constructed an underground cistern that collects stormwater runoff from adjacent roadways and filters backwash from the adjacent water treatment plant for reuse. The cistern is designed to hold 500,000 gallons of water which is used to water the adjacent City Park and City Hall. A surface stormwater pond can also overflow to the cistern and be used for irrigation. The $1.5 million project was designed and
St. Anthony Village, MN, constructed an underground cistern that collects stormwater runoff from adjacent roadways and filters backwash from the adjacent water treatment plant for reuse. The cistern is designed to hold 500,000 gallons of water which is used to water the adjacent City Park and City Hall. A surface stormwater pond can also overflow to the cistern and be used for irrigation. The $1.5 million project was designed and installed in conjunction with an adjacent road project making connection to storm sewer easier; the cistern overflows to storm sewer when full.
The storage capacity of a rain barrel or cistern is a function of the catchment area, the depth of rainfall required to fill the system and the volume and timing of water use. A general rule of thumb in sizing rain barrels or cisterns is that one inch of rainfall on a 1,000 square foot roof will yield approximately 600 gallons of runoff. The website, Rainwaterharvesting.org, provides additional design guidance. More complicated rainwater harvesting designs entailing 5,000+ gallon cisterns and pumps for irrigation require a more thorough investigation of regional rainfall patterns and irrigation rates.
A rainwater harvesting ordinance may be an option for MS4s. The City of Tucson, Arizona requires that commercial developments provide at least 50 percent of their irrigation needs with harvested rainwater.
Hotspots are facilities, activities or landuses that historically or currently produce higher levels of stormwater pollutants and/or present a higher potential risk for spills, leaks or illicit discharges of stormwater pollutants. Caution must be exercised to avoid collection of stormwater from hotspots when routing the saved water to a pervious area. For more information on hotspots identification and awareness, see Potential Stormwater Hotspots (PSHs) in the Minnesota Stormwater Manual.
Rain barrels and cisterns require inspection and minor maintenance to ensure the structures are leak-proof and have not been compromised by weathering or puncture. Rain barrel hoses, spigots and seals can require upkeep. Irrigation pumps and electronic management systems associated with large stormwater reuse systems require regular inspection and maintenance. Inspection is needed as a result of sediment and debris wash-off from roof and other impervious surfaces; it can accumulate if not filtered prior to discharge to the rain barrel or cistern.
Rain barrels typically cost between $50 and $230 for a 55 gallon drum depending on the manufacturer and inclusion of accessories and/or installation (see Rain Barrels: More Than a Drop in the Bucket). Rain barrels can be easily constructed by residents using a standard food-grade plastic 55-gallon barrel which can be obtained for approximately $15 to $20. The Low Impact Design Urban Design Tools website, designed by the Low Impact Development Center, provides additional cost guidelines.
Cisterns are considerably more expensive than rain barrels ranging from $200 to $10,000 due to size, materials, and structural requirements. Very large scale stormwater reuse systems (e.g. at public buildings or commercial sites) vary in cost based on complexity of the system, the scale of the system and the existing land use prior to installation. The reuse system in the Village of St. Anthony (see Cisterns above) was particularly costly due to size and connectivity with the filter backwash from the adjacent water treatment plant, which added an additional layer of complexity.