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Pollutants in harvested water

The composition of stormwater is highly variable in space and time due to differences in land use and rainfall events. This variability is an extremely important consideration when evaluating the feasibility of a stormwater harvesting and use system and determining what level of treatment is necessary to achieve the water quality criteria of the end use.

Common pollutants in stormwater runoff include nutrients, sediments, heavy metals, salinity, pathogens, and hydrocarbons (Typical stormwater pollutants, summary of sources and potential concerns for harvest and use table below). Water quality of stormwater varies depending on the type of land uses in the drainage area, such as commercial, industrial, residential and parks/open spaces. Typical urban stormwater quality characteristics for the Twin Cities and two other cities are summarized in the following tables.

Typical stormwater pollutants, summary of sources and potential concerns for harvest and use
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Pollutant Sources Potential Concerns
  • Nitrogen
  • Phosphorus
  • atmospheric deposition, sediment (adsorbed nutrients)
  • organic debris
  • fertilizer runoff
  • animal feces
  • combined sewer overflows
  • Support growth of algae or unwanted microbial growth on the water surface in the storage unit.
Organic Matter
  • Organic debris (leaves, flowers, pollen, twigs, insect carcasses, etc.)
  • Decomposition in tank can result in low dissolved oxygen levels, nuisance odors, and release of pollutants from sediments
Suspended Sediment
  • paved surfaces
  • areas of bare soil/poor vegetative cover
  • construction activity
  • stockpiles
  • May clog pump intake or distribution
  • Increases maintenance of storage
  • De-icing chemicals
  • water softening chemicals
  • Corrosive to metal pipes/plumbing
  • Plant toxicity (irrigation)
  • animal feces (including bird feces on rooftops)
  • insects/vector organisms
  • drainage area activities such as waste management
  • sewage overflows or leaking sewers
  • Human health risk
  • Vehicle exhaust
  • Roofing materials
  • Drainage area activities that are potential sources of metals (e.g. vehicle fueling or repair)
  • Plant toxicity
Organic Chemicals
  • Pesticides/herbicides
  • Industrial chemicals and solvents
  • Petroleum-derived chemicals
  • Drainage area activities that are potential sources of organics (e.g. herbicide/ pesticide use or waste management)
  • Plant toxicity
  • Human health risk
  • Animal health risk

Typical concentrations of pollutants in stormwater runoff and snowmelt runoff for select cities
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Constituent (concentrations reported in mg/L) Annual Twin Cities Snowmelt4
Twin Cities1 Marquette MI2 Madison WI Storm Sewers Open Channels Creeks NURP5
Cadmium 0.0006 0.0004
Copper 0.022 0.016
Lead 0.060 0.049 0.032 0.16 0.2 0.08 0.18
Zinc 0.111 0.203
Biological Oxygen Demand 15
Chemical Oxygen Demand 169 66 169 82 84 91
Total Kjeldahl Nitrogen 2.62 1.50 3.52 2.36 3.99 2.35
Nitrate + Nitrite 0.53 0.37 1.04 0.89 0.65 0.96
Ammonia 0.2
Total Phosphorus 0.58 0.29 0.66 0.7 0.56 0.54 0.46
Dissolved Phosphorus 0.20 0.04 0.27 0.25 0.18 0.16
Chloride3 230 49 116
Total Suspended Solids 184 159 262 148 88 64
Volatile Suspended Solids 66 46 15

1 Event mean concentrations; Reference: Brezonik and Stadelmann 2002
2 Geometric mean concentrations; Reference: Steuer et al. 1997
3 Geometric mean concentrations; Reference: Waschbusch et al. 1999
4 Reference: Oberts, G. (Met Council). 2000. Influence of Snowmelt Dynamics on Stormwater Runoff Quality.
5 Reference: Median concentrations from more than 2,300 rainfall events monitored across the nation; EPA, 1983

Water quality considerations of runoff from specific source areas

The source area from which stormwater is collected largely determines the water quality characteristics of harvested stormwater (Summary of pollutants typically found in stormwater by source area below). Most stormwater is collected from a mix of source areas, however stormwater harvested for use can often be collected from one dominant source area since the catchment area of the systems tend to be smaller than other larger scale stormwater BMPs. This section discusses the unique water quality considerations for stormwater harvest and use systems for the following source areas

  • Hard roofs
  • Green and brown roofs
  • Paved surfaces
  • Green spaces

Summary of pollutants typically found in stormwater by source area
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Source Area Solids Total Suspended Solids Particulate Nutrients Dissolved Nutrients Bacteria Metals Chlorides Grease, Oil Pesticides Other Chemicals
Hard Roofs
Green and Brown Roofs
Paved Surfaces
Green Spaces
Sedimentation Basins and Detention Ponds

● = relatively high concentrations
○ = relatively low concentrations

Hard roofs

Hard rooftops may be composed of a variety of materials (ex. clay/concrete tile, asphalt/composite/wood shingles, metal, slate, or rubberized roofs). See table below.

Common roofing materials and water quality considerations
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Roofing Material Water Quality Considerations
Metal Roofs
  • Runoff may contain high levels of zinc, copper, and lead (see here)
Sheet Roofing (PVC)
  • Recommended for non-potable use only
Tile roofs (clay, ceramic, cement, fiberglass)
  • Need periodic cleaning - debris may accumulate between tiles
  • Recommended for non-potable use only
  • asphalt
  • composite
  • three tab asphalt
  • Recommended for non-potable use only
  • Some shingles manufactured prior to 1980 may contain asbestos in trace amounts
  • May not be suitable for irrigation if shingles have been treated for mold eradication or with herbicides
Shingles – cedar shakes/wood shingles
  • Recommended for irrigation only
  • Shingles retain moisture, support mold, algae, and insects, may be treated with fire retardant or other chemicals

The table below provides a summary of typical roof runoff quality in Minneapolis and Wisconsin.

Typical Roof Runoff Quality in Minneapolis and Wisconsin
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Constituent Minneapolis1 Wisconsin2
E. coli (#/100 mL) 764
Total Solids (mg/L) 126
Total Solids (mg/L) 10 19
Total Hardness (mg/L) 44
Total Nitrogen (mg/L) 0.421
Ammonia-N (mg/L) 0.268
Nitrate-N (mg/L) 0.586
Total Phosphorus (mg/L) 0.104 0.24
Total Dissolved Phosphorus (mg/L) 0.076 0.11
Soluble Reactive Phosphorus (mg/L) 0.065
Cadmium (mg/L) 0.0004
Copper (mg/L) 0.0075 0.01
Lead (mg/L) 0.0032 0.01
Zinc (mg/L) 0.101 0.363

1 Arithmetic mean concentrations; Reference: Minneapolis Public Works, City of Minneapolis Neighborhood Rain Barrel Partnership Project, 2008 2 Highest geometric mean concentration reported; Reference: Roger T. Bannerman and Richard Dodds, Sources of Pollutants in Wisconsin Stormwater, 1992

High metal concentrations in rooftop runoff are a major water quality consideration for harvest and use systems. See table below. Although runoff collected from rooftops is generally high quality compared to other sources of stormwater (NAS 2016), certain roof materials may adversely affect the quality of harvested rainwater (Common roofing materials and water quality considerations Table). Other water quality concerns for rooftops include pathogens which may be found in bird or animal feces and organic litter from tree canopy which may contribute to biological oxygen demand.

Concentrations of Zinc, Copper, and Lead in Roof Runoff Based on Roof Material Type
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Metal Roof Materials Runoff Concentration (mg/L)
Zinc New uncoated galvanized steel 0.5-10
Old uncoated galvanized steel 1-38
Coated galvanized steel 0.2-1
Uncoated galvanized aluminum 0.2-15
Coated galvanized aluminum 0.1-0.2
Other (aluminum, stainless steel, titanium, polyester, gravel) <0.002
Copper Uncoated copper 0.002-0.175
Uncoated galvanized steel <0.003
Clay tiles 0.003-4
New asphalt shingles 0.01-0.2
New cedar shakes 1.5-27
Aged/patinated copper 0.9-9.7
Lead Uncoated galvanized steel 0.001-2
Coated and uncoated galvanized steel <0.0001-0.006
Painted materials <0.002-0.6

Zinc data: Clark et al. (2008a,b); Faller and Reiss (2005); Förster (1999); Gromaire-Mertz et al. (1999); Heijerick et al. (2002); Mendez et al. (2011); Schriewer et al. (2008); Tobiason (2004); Tobiason and Logan (2000); Zobrist et al. (2000)
Copper data: Clark et al. (2008a); Gromaire-Mertz et al. (1999); Karlen et al. (2002); Wallinder et al. (2009); Zobrist et al. (2000)
Lead data: Clark et al. (2007); Davis and Burns (1999);Förster (1999); Gromaire-Mertz et al. (1999); Good (1993); Gumbs and Dierberg (1985); Mendez et al. (2011); Shriewer et al. (2008)

Green and brown roofs

Filtrate from green and brown roofs may require little or no treatment since green and brown roofs are effective at removing sediment, although soluble nutrient concentrations (nitrogen and phosphorus) may be elevated and water may be colored.

Paved surfaces

Paved surface source areas include parking lots, sidewalks, driveways, and roadways. The table below provides a summary of water quality characteristics for several types of paved surfaces. Runoff from paved surfaces can contain higher levels of chlorides, solids, and hydrocarbons. Harvest and use systems collecting runoff from paved surfaces will likely require some sort of first flush diverter to bypass very high concentrations of pollutants in spring snowmelt, and potential toxic spills in the drainage area, Treatment may also require filtration units capable of removing fine solids and hydrocarbons.

Urban Stormwater Quality Characteristics from Paved Surfaces
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Constituent (concentrations reported in mg/L) Wisconsin Data1 Twin Cities Highways2
Arterial Street Feeder Street Collector Street Collector Street Residential Driveway
Cadmium 0.0028 0.0008 0.0017 0.0012 0.0005 0.0025
Chromium 0.026 0.007 0.013 0.016 0.002
Copper 0.085 0.025 0.061 0.047 0.02 0.023
Lead 0.085 0.038 0.062 0.062 0.02 0.242
Zinc 0.629 0.245 0.357 0.361 0.113 0.123
Nitrate-Nitrite 0.77
Total Phosphorus 1.01 1.77 1.22 0.48 1.5 0.43
Total Dissolved Phosphorus 0.62 0.55 0.36 0.07 0.87
Chloride3 11.5
Total Suspended Solids 993 1152 544 603 328
Suspended Solids 875 1085 386 474 193

1 Arithmetic mean concentration; Reference: Roger T. Bannerman and Richard Dodds, Sources of Pollutants in Wisconsin Stormwater, 1992
2 Reference: University of Minnesota Water Resources Center, Assessment of Stormwater Best Management Practices Manual, 2008
3 Data represents chloride concentrations during monitoring season, typically April through October. Chloride concentrations in winter snowmelt grab samples have been found to be as great as 3,600 mg/L.

Green spaces

Green space source areas include lawns and park areas (see ‘Open Space’ land use in the Concentrations of contaminants found in stormwater table). Green spaces typically have lower concentrations of pollutants compared to stormwater source areas. Due to the presence of pets and/or wildlife (particularly Canadian geese), these areas may have very high concentrations of pathogens and require disinfection treatment for certain end uses.

Seasonal considerations

In addition to variability in stormwater quality from different source areas, stormwater quality also varies with season. The Typical Annual and Snowmelt Urban Stormwater Quality Characteristics table above illustrates water quality characteristics of snowmelt in the Twin Cities Metropolitan Area. Seasonal considerations include the following.

  • Snowmelt can have very high concentrations of chlorides and sediment from winter road de-icing practices (see Typical Annual and Snowmelt Urban Stormwater Quality Characteristics table above).
  • In spring, organic litter including pollen may increase BOD and residual solids on pavement may increase TSS.
  • In fall, leaf litter may contribute to BOD, nutrients and solids.

Treatment requirements

Water quality criteria have been developed for stormwater harvest and use systems in many states and are summarized in Toolbox R.3b of the 2011 Met Council Stormwater Reuse Guide. The only existing criteria in the State of Minnesota are for stormwater harvest and use systems regulated by Chapter 17 of the Plumbing Code (see below). The National Water Research Institute (NWRI) is currently sponsoring an Independent Advisory Panel to develop national risk-based treatment requirements for stormwater harvest and use systems (see below). Treatment requirements will be based on risk of exposure to harvested stormwater instead of based on achieving end-use water quality criteria. For each level of risk, certain levels of treatment and risk barriers will be required. A risk-based approach for stormwater harvest and use treatment is more appropriate than end use-based water quality criteria due to the wide range of harvested stormwater quality, pathways for exposure, and project specific circumstances.. Please check this page for the most up-to-date treatment requirements during the design of each stormwater harvest and use system project. Post-storage treatment processes available to reduce risk of exposure are described below.

Risk-based treatment requirements for stormwater harvest and use systems

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Risk Level Description Site Barriers Filtration Disinfection Other treatment barriers
Low exposure No direct physical contact None
Medium exposure Direct physical contact Signage
High exposure Ingestion or inhalation Restricted access

National Water Research Institute (NWRI) Independent Advisory Panel findings

The primary public health concern with stormwater reuse systems is pathogenic microorganisms. Traditionally, water and wastewater systems have been monitored using fecal indicator organisms (FIO) such as E. coli. The presence or concentration of FIO in a water or wastewater samples was assumed to be indicative of other waterborne pathogens. The FIO were useful because they were expected to be present in water contaminated with fecal waste. However, there are a number of limitations with the use of FIO including (a) FIO may not always be present in stormwater, (b) FIO are not necessarily representative of other pathogen groups, (c) grab samples analyzed for FIO cannot be used for continuous monitoring, and (d) FIO are more difficult to measure consistently than other surrogate parameters. Therefore, state agencies are currently working on procedures for designing and monitoring stormwater reuse systems in more effective ways.

Water quality monitoring and control systems are used commonly to assess the operation, performance, and status of a given component or process. The fundamental purpose of performance target monitoring of a stormwater reuse system is to ensure that the treatment barriers that have been put in place to meet the specified water quality targets are operating as intended.

Most non-potable water systems utilize a number of unit processes in series to accomplish treatment, known commonly as the multiple barrier approach. Multiple barriers are used to improve the reliability of a treatment approach through process redundancy, robustness, and resiliency.

When multiple treatment barriers are used to achieve pathogen removal, the contribution from each barrier is cumulative. In addition to these treatment barriers, operational and management barriers are used to ensure that the systems are in place to respond to non-routine operation. The technical barriers can be monitored using operational and critical control points.

This image shows Risk characterization stormwater reuse
Risk characterization stormwater reuse

Minnesota Plumbing Code

The new 2015 Minnesota Plumbing Code, Minnesota Rules, Chapter 4714, took effect Jan. 23, 2016. The code now includes the design and installation of harvesting rainwater from building roof tops in Chapter 17, Nonpotable Rainwater Catchment Systems. Nonpotable rainwater catchment systems are acceptable for use to supply water to water closets, urinals, trap primers for floor drains, industrial processes, water features, vehicle washing facilities, and cooling tower makeup water provided the design, treatment, minimum water quality standards, and operational requirements are in accordance with Chapter 17 of the code. Designs must be approved by a Minnesota registered professional engineer.

Rainwater catchment systems use for plumbing applications listed above in combination with lawn irrigation must meet the requirements of Chapter 17. System components used solely for lawn irrigation, such as irrigation pumps and piping mounted outside of buildings are not subject to the requirements of Chapter 17. The conveyance of the rainwater catchment system is still governed by the plumbing code. Minimum water quality standards are now described in Chapter 17.

Minimum water quality for rainwater catchment systems

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Measure Limit
Turbidity (NTU) <1
E. coli (MPN/100 mL) 2.2
Odor Non-offensive
Temperature (degrees Celsius) MR*
Color MR

* MR = measure and record only; Treatment: 5 micron or smaller absolute filter; Minimum .5-log inactivation of viruses

Treatment systems

Stormwater harvest and use systems require some level of pre-treatment, similar to other stormwater BMPs, such as:

  • source control
  • debris/coarse solids removal, and
  • suspended solids removal.

These pre-treatment processes are not discussed here. Additional post-storage treatment requirements for stormwater harvest and use systems defined in the Risk-based treatment requirements for stormwater harvest and use systems table above, including:

  • dissolved solids removal,
  • disinfection, and
  • other additional treatments (such as chlorine residual removal and pH adjustments),

are summarized in the Post-Storage Treatment Process Considerations below.

Post-Storage Treatment Process Considerations
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Post-Storage Treatment Process Description and Considerations Treatment Alternatives Target Pollutants Capital Cost O&M Level Energy Needs Advantages over Alternatives Disadvantages over Alternatives
Dissolved Solids Removal Filtration generally is used to remove residual solids that will not settle spontaneously from harvested water through sedimentation or which may become re-suspended in storage. Filters come in a variety of different types and sizes. The type of filter depends on the class of pollutants targeted for removal. Coarse & fine filters
  • Suspended solids
Med Med Med
  • Lower overall O&M costs than other filtration
  • Does not remove micro-organisms
  • Suspended solids
  • Micro-organisms
Med Med Med
  • Smaller footprint required
  • May reduce disinfection requirements
  • Captures microorganisms
  • Higher capital costs
  • Higher O&M costs including membrane replacement, energy, performance monitoring, and residuals disposal
  • Dissolved salts
  • Bacteria/ viruses
  • Proteins
Med Med High
  • Requires less energy than reverse-osmosis and ion-exchange filters
  • Requires large amount of pretreatment to remove metals that cause scaling and particulates that cause biofouling
  • Produces a larger waste stream than reverse-osmosis
  • Dissolved salts
  • Dissolved solids
  • Ions
  • Bacteria/ viruses
High High High
  • Highest removal efficiency
  • Produces the smallest waste stream
  • Commercially available
  • Requires more energy than nanofiltratoin
Ion-exchange filter
  • Charged ions
  • Dissolved salts
High High High
  • Requires the least amount of pretreatment
  • Produces the largest waste stream
  • Requires more energy than nanofiltration
  • Make-up water is required to continuously wash membranes
Disinfection Disinfection processes kill, remove, or deactivate pathogenic microorganisms in harvested water. Chlorination – injects chlorine into stormwater
  • bacteria
  • viruses
  • other pathogenic organisms
Low Low Low
  • Most common disinfection technology
  • Least cost
  • Requires calibration of dosage control devices
  • Does not kill cysts
Ultra-violet light (UV) radiation – stormwater is passed over an ultraviolet lamp
  • bacteria
  • viruses
  • other pathogenic organisms
Med High High
  • No byproducts
  • Minimal energy requirements compared to chlorination/ozonation
  • Requires cartridge filters ahead of the UV light, with routine cleaning of filters – UV is ineffective on unfiltered stormwater
  • UV lamps must be replaced periodically
Ozonation – diffused ozone released through a fine bubble diffuser at the bottom of the storage tank (possible with stormwater but rarely used) Med Med Med
  • Also removes dissolved organics
  • More effective disinfectant than chlorination
  • Treatment of off-gases required
  • High energy requirements
  • Corrosion protection required
  • Requires monitoring of influent to adjust doses
  • Requires routine check for leaks
Other treatments (e.g., pH adjustment) Treatment for pH adjustment may be needed if the end use of harvested water requires a neutral pH or if harvested water will come in contact with metal pipes or surfaces. Rainwater tends to be slightly acidic and harvested stormwater may retain this characteristic. Acidity can cause metal pipes to corrode leading to contamination of harvested water. Chemical additive
  • acidic or alkali substances
Low Low Low
  • N/A
  • N/A

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