Water quality considerations for stormwater and rainwater harvest and use/reuse
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Water quality considerations for stormwater and rainwater harvest and use/reuse

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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
Link to this table

Pollutant	Sources	Potential Concerns
Nutrients 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
Chlorides	De-icing chemicals water softening chemicals	Corrosive to metal pipes/plumbing Plant toxicity (irrigation)
Pathogens	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
Metals	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
Link to this table

Constituent (concentrations reported in mg/L)	Annual			Twin Cities Snowmelt⁴
Constituent (concentrations reported in mg/L)	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
Link to this table

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
Link to this table

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
Shingles 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
Link to this table

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
Link to this table

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
Link to this table

Constituent (concentrations reported in mg/L)	Wisconsin Data¹					Twin Cities Highways²
Constituent (concentrations reported in mg/L)	Arterial Street	Feeder Street	Collector Street	Collector Street	Residential Driveway	Twin Cities Highways²
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. Link here for a summary of 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.
In spring, organic litter including pollen may increase Biochemical Oxygen Demand (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 for up-to-date treatment requirements during the design of each stormwater harvest and use system project at NWRI's website. Post-storage treatment processes available to reduce risk of exposure are described below.

Risk-based treatment requirements for stormwater harvest and use systems

Link to this table

Risk Level	Description	Site 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.

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

Link to this table

Measure	Limit
Turbidity (NTU)	<1
E. coli (MPN/100 mL)	2.2
Odor	Non-offensive
Temperature (degrees Celsius)	MR*
Color	MR
pH	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 pretreatment, 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
Link to this table

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
		Micro-filtration	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
		Nano-filtration	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
		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	X	X
		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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