Stormwater re-use and rainwater harvesting

Rain water harvesting is the practice of collecting rain water from impermeable surfaces, such as rooftops, and storing for future use. There are a number of systems used for the collection, storage and distribution of rain water including rain barrels, cisterns, evaporative control systems, and irrigation.

Example of a residential rain barrel - Stillwater, MN

Design Criteria

• The system should be watertight, have a smooth interior surface, be located on level and stable ground, have a tight-fitting lid, good screens on the inlet and outlet and have an emergency overflow device.
• To prevent the breeding of mosquitoes, empty the water in less than 5 days or place a fine screen over all openings.
• Material can withstand the pressure of water over long periods of time.
• Disconnect and drain rain barrels and cisterns in the winter to prevent freezing and deformation of the rain water harvesting system.

Management suitability

• Water Quality (V_wq) - High¹
• Channel Protection (_Vcp) - Med.
• Overbank Flood Protection (_Vp10) - Low
• Extreme Flood Protection (Vp₁₀₀) - Low
• (Recharge Volume (V_re) - High¹

¹Assumes water is drained to a vegetated pervious area. does not apply to volume of runoff that bypasses the system.

Mechanisms

• Infiltration¹
• Screening/ Filtration¹
• Temperature Control
• Settling
• Evaporation
• Transpiration¹
• Soil Adsorption¹
• Biological/ Micro. Uptake¹

¹Assumes water is drained to a vegetated pervious area. does not apply to volume of runoff that bypasses the system.

Pollution Removal

• Total Suspended Solids - 100%¹
• Nutrients - Total Phosphorus/Total Nitrogen- 100%¹
• Metals - Cadmium, Copper, Lead, and Zinc- 100%¹
• Pathogens - Coliform, Streptococci, E. Coli- 100%¹
• Toxins - Hydrocarbons, Pesticides- 100%¹

¹Assumes water is drained to a vegetated pervious area. does not apply to volume of runoff that bypasses the system.

Site factors

• Drainage Area - Rooftop
• Max. Slope - NA
• Min. Depth to Bedrock - NA
• Min. Depth to Water Table - NA
• SCS Soil Type - can be used in C&D soil types with modifications (e.g. underdrains) - NA
• Poor Freeze/ Thaw Suitability - NA
• Potential Hotspot Runoff - Suitable

¹Assuming water is drained to a vegetated pervious area. Does not apply to volume of runoff that bypasses the system

Benefits

• Protects water supplies by reducing use during peak summer months.
• Mimics the natural hydrology of the area by infiltrating a portion of the rain water falling on the site.
• Reduces volume of storm water being delivered to downstream waterbodies.
• Results in cost savings by reducing municipal water bill.

Limitations

• Not suitable for the following roof types: tar and gravel, asbestos shingle and treated cedar shakes.
• Depending on the design, requires a certain amount of operation and maintenance.
• Proprietary systems can be expensive.

Description

Rain water harvesting can be accomplished using rain barrels and/or cisterns. Rain barrels are typically located at the downspout of a gutter system and are used to collect and store rainwater for watering landscapes and gardens.

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.

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 are typically used to irrigate landscapes and gardens on a regular basis 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.

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 water losses. 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.

Executive Summary

Recently, stormwater reuse is being looked to as more than a water conservation practice but also as a viable alternative to help meet stormwater management requirements. This Minimal Impact Design Standards (MIDS) workplan was focused specifically on stormwater reuse for irrigation of non-food crops, such as turf and ornamental landscaping, based on a request from the MIDS technical team.

The goal of the work completed as part of this MIDS workplan was to:

• Perform a literature review of select documents related to water reuse and summarizing the available information in the context of stormwater reuse for irrigation including:
  • general discussion about stormwater reuse,
  • concerns related to stormwater reuse systems,
  • existing guidelines, standards, treatment requirements, and (draft) code related to stormwater reuse,
  • typical contaminants and concentrations in stormwater, and
  • components of a typical reuse system for irrigation.
• Conduct preliminary modeling considering growing season/irrigation season use rates to estimate the relationship between stormwater volume reduction credits versus storage capacity including and estimated range of annual volume, phosphorus, and suspended solids removals for these systems.
• Conduct a total of three meetings with state agency staff and the MIDS technical team to begin discussing the guidelines and standards currently available, regulatory jurisdiction and concerns related to reuse systems and how these concerns may impact future guidelines, and the potential impact of these reuse system on stormwater management.

Based on the work completed, the following is a summary of the major conclusions related to stormwater harvesting and reuse for irrigation and the suggestions for the next steps:

Summary of Conclusions:

• One of the major concerns related to stormwater reuse for irrigation (and other uses) is the public health risk due to exposure to pathogens, such as E. coli, and potential for crosscontamination of the potable water supply.
• The lack of national guidance for stormwater reuse has resulted in differing use and treatment guidelines/standards among state and local governments, and in many areas, rainwater and stormwater harvesting is largely unaddressed by regulations and codes. Additionally, many of the existing standards were originally developed for the reuse of reclaimed water (treated wastewater) rather than stormwater.
• Often, the treatment requirements ultimately come down to the risk of exposure to pathogens determining the most stringent levels of treatment. Many jurisdictions evaluate stormwater reuse projects based on whether the application area has restricted or unrestricted public access. However, the level of treatment required by each municipality can influence the number of harvesting and reuse systems that are actually implemented.
• Currently, the State of Minnesota does not have a state-specific code or guidance applicable to stormwater harvesting and reuse and generally relies on the State of California Water Recycling Criteria (2000) as guidance for water reuse projects. At present, there is limited regulatory jurisdiction by the various state agencies over stormwater reuse systems for irrigation.
• From the stormwater management perspective, there is a large range in the expected impact of a stormwater reuse system for irrigation on average annual volume and pollutant load reductions, ranging anywhere from 1 percent to upwards of 90 percent average annual removal. There are several variables that can impact the expected removal efficiency of the reuse system, including the reuse storage volume, the expected area for application, the irrigation rate and season, and any potential pretreatment prior to reuse (e.g., reuse from a wet pond meeting National Urban Runoff Program, NURP, criteria). Assuming that the reuse storage volume is optimized to the contributing watershed and there is sufficient application area to utilize the stormwater, typical volume reduction and pollutant removals from reuse alone would be expected to range from approximately 20 to 50 percent on an average annual basis.

Summary of Suggestions for Future Work:

• Development of a workgroup with representatives of each state agency, including the Department of Labor and Industry (DLI), the Minnesota Department of Health (MDH), the Minnesota Pollution Control Agency (MPCA), the Minnesota Department of Natural Resources (MDNR), and Minnesota Department of Agriculture (MDA), focusing on stormwater reuse (for irrigation) to begin clarifying the roles and jurisdiction for each agency and any associated guidance. The intent of the work group would be to facilitate discussions surrounding stormwater reuse systems, jurisdiction of the various agencies, and potential guidance (appropriate water quality and treatment standards) for these systems.
• Completion of health risk assessments of non-potable water sources (including stormwater) and the potential uses for these sources, including investigation into cases of human illness related to stormwater reuse systems. These assessments will help begin quantifying the actual health risks associated with stormwater reuse (for irrigation) and to begin understanding if the current water quality guidelines (from the various programs in other states) are too stringent, appropriate, or not stringent enough and to help better define levels of required treatment. These assessments would eventually lead to the development of statewide water quality guidelines (or standards) and treatment requirements that would help guide the design of stormwater reuse systems (for irrigation and potentially other uses).
• Because one of the major demands for stormwater reuse systems in Minnesota is irrigation of golf courses and athletic fields from existing stormwater ponds, it is important to understand the actual water quality in stormwater ponds (after settling), in the context of the public health concerns, irrigation equipment function, and impact of stormwater pollutants on plant health. A comparison of the levels observed in actual stormwater ponds to current stormwater reuse water quality standards/guidelines would help regulators begin understanding if additional treatment, such as filtration or disinfection, is needed for reuse systems utilizing water from stormwater ponds.
• The purpose of the preliminary modeling analysis performed as part of this MIDS work was to begin understanding the potential impact of stormwater reuse for irrigation with regards to stormwater management standards. Performance curves were developed based on specific assumptions related to the various parameters. Therefore, these curves only apply to sites that would meet the specific assumptions that were included in the modeling analysis. However, it is expected that there would be significant variability in the model parameters related to the stormwater reuse. Therefore, additional modeling would be necessary to develop a full range of performance curves related to cover the potential site conditions and variability in watershed area, stormwater storage volume, application area for irrigation, irrigation rates, and irrigation periods to be incorporated into the MIDS Calculator.

Overview of Stormwater Harvesting and Reuse for Irrigation

Stormwater harvesting and reuse is a practice of collecting and reusing stormwater for a potable (for consumption) or non-potable applications. Outdoor irrigation is considered a non-potable water use. For this work plan, we have assumed irrigation of non-food crops, such as turf and landscaping. For the purposes of this document, stormwater is defined as runoff collected from roof and ground surfaces, including roadways, driveways, parking lots and other impervious areas. Rainwater is defined as runoff from roof surfaces only. Some of the literature sources reviewed as part of the development of this memorandum place emphasis on rainwater only, while others focus on stormwater for harvesting and reuse. Additionally, some of the documents and standards reviewed were originally developed for the reuse of reclaimed (treated wastewater).

The following documents were reviewed as part of the development of this memorandum:

• Stormwater Harvesting and Reuse: Literature Review (EOR, 2011 (draft))
• Guidelines for Water Reuse (EPA, 2012)
• Managing Wet Weather with Green Infrastructure Municipal Handbook Rainwater Harvesting Policies (EPA, 2008)
• Metropolitan Council Stormwater Reuse Guide (Metropolitan Council, 2011)
• Metropolitan Area Master Water Supply Plan (Metropolitan Council, 2010)
• Minnesota Stormwater Manual, Version 2 (MPCA, 2008)
• Municipal Wastewater Reuse. (MPCA, 2010)
• Water Use in Minnesota. (University of Minnesota Water Resources Center, 2010 (draft)).
• Chapter 16 Alternative Water Sources for Nonpotable Applications (Uniform Plumbing Code, 2012 (draft))
• Chapter 17 Nonpotable Rainwater Catchment Systems (Uniform Plumbing Code, 2012 (draft))
• Contech Webinar – Rainwater Harvesting as a Runoff Reduction Tool for Areas with Moderate to High Intensity of Rainfall (Contech, 2012)
• Managing Mosquitoes in Stormwater Treatment Devices (California Department of Health Services, 2004)
• Australian Guidelines for Water Recycling: Managing Health and Environmental Risks (Phase 2) Stormwater Harvesting and Reuse (Natural Resource Management Ministerial Council, the Environment Protection and Heritage Council, and the National Health and Medical Research Council (Australia), 2009)
• Watering Lawns and Other Turf (University of Minnesota Extension, 2009)

There are several overarching goals for the implementation of stormwater harvesting and reuse systems. These goals include (EOR, 2011 (draft)):

• reduction of stormwater pollutant loads and flows to surface waters, helping achieve local stormwater management requirements,
• reduction in the size of other stormwater Best Management Practices (BMPs) needed to achieve local stormwater management requirements,
• reduction of the demand on potable water sources, and
• reduction of stress on the existing water supply infrastructure.

Additionally, stormwater harvesting and reuse systems can be used to help achieve Leadership in Energy and Environmental Design (LEED) and other sustainable design credits related to stormwater quantity and quality as well as water efficiency.

The scale of stormwater harvesting and reuse systems can range from small residential systems to very large commercial systems. According to the EPA, when harvested rainwater is re-used, it generally is best for irrigation and non-potable uses of water closets, urinals, and HVAC, as these uses require a lesser amount of on-site treatment than potable uses (EPA, 2008). Because of this, one of the most common reuse applications of stormwater and rainwater is urban irrigation (EOR, 2011 (draft)), which can include irrigation of athletic fields, golf courses, parks, landscaping, community gardens, and creation of water features (Metropolitan Council, 2011).

Nationally, outdoor water uses represent 58% of the domestic daily water uses while for hotels and office buildings, outdoor uses represent 10 to 38% of the daily water uses, respectively (EPA, 2008). In Minnesota during the summer, as much as 50% of potable water supply is used for outdoor, non-potable uses. During hot weather and extended periods of drought, Twin Cities’ property owners will use 45 to 120 gallons of treated drinking water per person per day for outdoor uses with peak usage on large lots and new turf reaching as much as 200 gallons per person per day (Metropolitan Council, 2011).