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Treatment of stormwater runoff

Typical pollutants in stormwater runoff

Updated: August 14, 2015                                                                 

Contents:

• Introduction and a note of caution on treatment methodologies
• Rate of settling in pure, still water
• Constructed Wetlands, and adequate sizing is imperative
  • Integrating constructed wetlands with stormwater management
    • Typical stormwater management system--- Figure 1
• Examples of proprietary devices e.g., the Stormceptor, the CDS, and the Vortechnics
• Other case histories
  • UV Disinfection, Nepean, Ontario
• References

Introduction and a note of caution on treatment methodologies

A note of caution: There have been conflicting results in the long term removal of typical stressors in storm drainage using constructed (or engineered) wetlands. Hence care has to be taken to size them adequately.

In the extreme case, treatment of urban and suburban stormwater by traditional wastewater treatment plants (WWTP) based on the tertiary removal process may be required. This may imply considerable capital costs and operation & maintenance.

Rate of settling in pure, still water

Rate of settling in pure, still water (temp=10^oC, sp. gravity of particles=2.65, shape of particles=spherical) (Welch, 1935)

Material	Diameter (mm)	Hydraulic subsiding value (mm/sec)	Time required to settle 1 ft.
Gravel	10.0	1000.0	0.3 sec
Coarse sand	1.0	100.0	3.0 sec
Fine sand	0.1	8.0	38.0 sec
Silt	0.01	0.154	33.0 min
Bacteria	0.001	0.00154	55.0 hr
Clay	0.0001	0.0000154	230.0 days
colloidal particles	0.00001	0.000000154	63 years

Constructed Wetlands

Enter for a bibliography on wetlands

• Introduction, and adequate sizing is imperative

• Design Features of Constructed Wetlands for Nonpoint Source Treatment
  • How do constructed wetlands work to reduce NPS pollution?
  • How can you enhance the functioning of constructed wetlands?
    • Loading rate
    • Hydraulic retention time
    • Water velocity
    • Soils
    • Water depth
    • Maximize edge
    • Minimize edge slope
    • Persistent emergent vegetation
  • Maintenance

• Case Histories of Wetlands:
  • McCarrons Treatment Facility System
  • Recommendations of Berezowsky, Boojum Technologies Ltd., Toronto
    • The marsh-pond-meadow system
    • The Max-Planck-Institute Process
  • Lake Tahoe, California
  • Lake Sammamish Basin, Washington
  • Shop Creek Project, Colorado
  • Stormwater Ponds, Wisconsin
    • Shortcomings of urban detention ponds

• Integrating constructed wetlands with stormwater management
  • Typical stormwater management system--- Figure 1

Introduction

(cf. Strecker et al., 1992)

Wetlands are receiving attention as attractive systems for removing pollutants from stormwater surface runoff before the runoff enters downstream lakes, streams, and other open water bodies. Wetlands have long been employed for the treatment of wastewaters from municipal, industrial (particularly acid mine drainage), and agricultural sources. The U.S. Environmental Protection Agency (EPA) encourages the use of constructed wetlands for water pollution control.

"The term `wetlands' means those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas".

A significant issue is whether natural wetland systems should be used as stormwater control measures. In general, natural wetlands have been found to be somewhat less predictable than constructed wetlands in terms of pollutant removal efficiency. This difference may be due to the fact that constructed wetlands have generally been engineered to provide favorable flow capacity and routing patterns.

People often question whether it is appropriate to use a natural, healthy wetland for such purposes. The concern is whether the modified flow regime and the accumulation of pollutants will result in undesirable environmental effects. A general consensus from the literature is that the use of a healthy natural wetland for stormwater pollution control should be discouraged.

One pre-treatment technique would be to use pond areas to provide an opportunity for suspended materials to settle out before the flows enter the wetland. Other possible options include routing inflows to the wetlands through upstream grass swales, oil/water separators, heavily vegetated areas (e.g. thick, shallow cattail area), and overland flow areas.

"....... several trends were noted. First, constructed systems were generally found to have a higher average removal performance than natural systems, with less variability; and second, larger wetlands as compared to watershed size also showed the same trend, a higher average removal performance, with less variability".

Adequate sizing is imperative:

Design Features of Constructed Wetlands for Nonpoint Source (NPS) Treatment

Constructed wetlands are employed to treat nonpoint sources such as excess runoff, eroded soil and nutrients. Natural wetlands should not be used to treat NPS pollution without first conducting a thorough environmental assessment to insure that the wetland can support the necessary treatment without becoming degraded. Likewise, using wetlands for treating toxic wastes is not recommended without extensive evaluation.

How do constructed wetlands work to reduce NPS pollution?

Nutrients such as phosphorus and nitrogen are trapped and retained in constructed wetlands by several mechanisms: burial in sediments, chemical breakdown (e.g., denitrification and ammonia volatilization), and through assimilation by aquatic plants and bacteria. The primary mechanism for phosphorus removal is adsorption to wetland soils and precipitation reactions with calcium, aluminum and iron. In most cases, phosphorus retention by vegetation is only seasonal, as it is taken up by growing plants and released with vegetation dieback in the Fall.

How can you enhance the functioning of constructed wetlands?

• Loading rate:
  • Ideally, for optimal performance, the size of the constructed wetland should be from 1 to 5 % of the size of its drainage area. Designing hydraulic loading by analyzing existing channel discharge or watershed runoff coefficients is more precise than the 1 to 5 % rule above and this more intensive hydraulic loading determination is recommended.
  • Phosphorus removal efficiency declines with increasing phosphorus loading. With high loading rates, adsorption sites become saturated and there is insufficient capacity for biotic assimilation. There are cases where overloaded wetlands have become phosphorus sources to downstream lakes.
• Hydraulic retention time:
  • The longer water remains in the wetland the greater chance of sedimentation, adsorption, biotic processing and retention of nutrients. Proper sizing of the wetland is important, but restricting the size of the wetland outlet is also effective. For wetlands with channel flow, the outlet cross sectional area should be less than 1/3 that of the inlet.
• Water velocity:
  • Peak water velocities through the wetland should not exceed 1.5 fps. High velocities can wash out rooted vegetation and scour deposited sediments. Ideally, flow velocities should be less than 0.6 fps.
• Soils:
  • fine-textured clay soils and soils with high organic matter content have more adsorption sites for retaining nutrients. Available calcium, aluminum and iron in the soil enhance precipitation reactions with phosphorus.
• Water depth:
  • Water depths less than 40 inches result in greater resistance to flow and favour aquatic vegetation.
  • The preferred depth ranges are for,
    • emergent plants= 0-1 ft of water
    • rooted surface plants= 1-2 ft of water
    • rooted submersed plants= 1.5-6.5 ft of water
  • Pools deeper than 40 inches should be included in the wetland design to maximize sediment deposition and provide winter fish habitat.
• Maximize edge:
  • Sinuous edges between the terrestrial and aquatic zones provide more resistance to flow and more edge habitat for plants and animals. Islands create additional edge, plus they provide refuge from predation for nesting birds.
• Minimize edge slope:
  • The terrestrial-aquatic boundary should have a very gradual slope. This allows for the establishment of a continuum of emergent species and reduces the erosive effects of waves hitting a sharp shoreline boundary.
• Persistent emergent vegetation:
  • Persistent emergent vegetation has stems which persist even after the growing season. This provides year-round resistance to water flow.
    • These plants include: cattail (Typha sp.), iris (Iris pseudacorus or I. Versicolor), rush (Juncus sp.), cordgrass (Spartina sp.), reedgrass (Calamagrostis sp.), sawgrass (cladium jamaicense) and switchgrass (Panicum virgatum).
    • Woody plants, such as alder (Alnus sp.), buttonbush (Cephalanthus occidentalis), black willow (Salix nigra) and others are useful edge species with persistent stems.
  • Aquatic bed or submergent vegetation removes nutrients seasonally, but does not offer significant frictional resistance to suspended sediments.

Maintenance:

Case histories of constructed wetlands and ponds:

• In contrast to the treatment of municipal or mining waste water, whose flow rates and compositions tend to be predictable and relatively constant, storm-water flows and compositions vary considerably, depending on the land uses in the catchment basin and the frequency and intensity of rain/snow events. In addition, constructed wetlands may not be effective in treating certain storm-water contaminants, such as road salt and some organic compounds.
• Pretreatment of sediment using sedimentation/siltation ponds may be necessary to prevent the sediment from choking the wetland soils and rendering them ineffective.
• Several trends were noted. First, constructed systems were generally found to have a higher average removal performance than natural systems, with less variability; and second, larger wetlands as compared to watershed size also showed the same trend, a higher average performance, with less variability. Assessment was made based on <2% and >2% size wetlands in relationship to the catchment areas, and 2% seems to be sufficient, although note the following quote, "........... at this time we are not suggesting that minimum of a 2 per cent DAR is the proper design criteria for constructed wetlands." (Strecker et al. 1992).
• "People often question whether it is appropriate to use a natural, healthy wetland for such purposes. The concern is whether the modified flow regime and the accumulation of pollutants will result in undesirable environmental effects. .......................................... A general consensus from the literature is that the use of a healthy natural wetland for stormwater pollution control should be discouraged."

McCarrons Treatment Facility System:

This facility consisted of a 30-acre detention basin with an average depth of 1.2 feet and a 6.2-acre constructed wetland with an average depth of 2.5 feet. The detention basin received stormwater and then discharged to the wetland. The contributing watershed consists of 600 acres of primarily urban land use. The predominant vegetation in the wetland consists of cattails with other emergent plant species.

Overall, they found very good results for the system. The following removal efficiencies were given:---

• % removal for Detention basin:
  • TSS-91%
  • TP-78%
  • TN-85%
  • TPb-85%
• % removal for Wetland:
  • TSS-87%
  • TP-36%
  • TN-24%
  • TPb-68%

The detention basin removed the fraction of pollutants that are more readily settled and treated, leaving the wetland with the finer, more difficult to treat pollutants.

Recommendations of Berezowsky, Boojum Technologies Ltd., Toronto:

Considering the diverse nature of the contaminants found in urban stormwater, the most effective solution appears to be a multi-stage system, such as the "marsh-pond-meadow" or the "Max-Planck-Institute" process.

• The marsh-pond-meadow system: This consists of (1) a bar screen and aeration cell using a floating surface aeratior, (2) a lateral-flow marsh planted with cattails in a sand medium, (3) a pond with aquatic macrophytes and herbivorous fish, (4) a meadow planted with red canary grass, and (5) a chlorination chamber. The removal efficiency is reported to be 77% for ammonia nitrogen and 82% for total phosphorus.
  
  
• The Max-Planck-Institute Process: This design is used in France and was a model for a system implemented in Oaklands Park, United Kingdom. The system consists of four or five stages in cascade, each with several basins laid out in parallel and planted with emergent macrophytes in gravel. The flow pattern in the first two stages is vertical, while the final ones have horizontal flow. In the French system, removal of suspended solids and BOD is good, but with poor results for nitrogen and phosphorus, perhaps because of high loading rates.

Lake Tahoe:

Wetlands treatment of stormwater has been reported (Reuter et al 1992, Verry et al 1982, Brown 1985). Results from a newly constructed wetland to treat stormwater in a cold climate region of California at Lake Tahoe were encouraging. Gravel-filled constructed wetlands (Lake Tahoe) provide a much greater surface area for bacterial attachment than is possible in natural wetlands, thereby enhancing the substratum to water volume contact ratio, and hence need less land area than natural wetlands. Constructed Wetlands are most suitable as mitigation for small development projects where land is limited. These projects include golf courses that receive fertilizers, small commercial facilities, small housing developments, etc.

They are generally limited in efficiency by the volume of water they can retain (4-8 day retention). It may be unrealistic to rely on small constructed wetlands to treat large urban areas. Other solutions like partial diversion, a combination of treatment systems, etc will have to be found. The age-old excuse of questioning the utility of constructed wetlands in cold climate regions was irrefutably proven to be false in not only the Lake Tahoe case but also in other cases elsewhere.

Lake Sammamish Basin, Washington:

State-of-the-art stormwater treatment system for removing phosphorous in runoff from a 460-acre residential development incorporates a dry pond for detention and a wet vault for pretreatment, followed by a filter with underdrains.

Shop Creek Project, Colorado:

Based on peer reviewed literature, a concept was developed in the State of Colorado, where experts advised mandatory removal of 50% of the expected post-development export of total phosphorus in urban stormwater before approving new subdivisions . They felt anything above 50% would be unduly costly. For that, their consultants recommended utilizing wet detention ponds (not dry ponds) to intercept the first half-inch of runoff (the first flush carries most of the urban pollutants, not just TP), a certain detention time (based on chemistry, etc.) followed by sand filtration prior to discharge into natural watercourses, directly or indirectly. It appears, there was a successful demonstration project, the Shop Creek Project which implemented sound engineering methodology of reducing the TP load in the storm water in a new subdivision prior to discharge into the Cherry Creek Reservoir (consult the Cherry Creek Basin Water Quality Authority, Englewood, Colorado).

• Summary of Design Criteria Recommendations for "Settleability Design Storms":
  • Fairfax County, Virginia:
    • 22.9 mm (Runoff amount)
  • Delaware Raritan Canal Comm.:
    • 31.8 mm in 2 hrs. (Rainfall amount)
  • South Florida Flood Management District:
    • 25.4 mm (Runoff amount)
    • 3 yr. storm (Rainfall amount)
  • Austin, Texas:
    • 12.7 mm (Runoff amount)
  • Central Florida:
    • 12.7 mm (Runoff amount)

After evaluation of the literature, the rainfall characteristics in Denver and the various concerns, the authors chose the following basic design parameters for the standard design:

• Provide a detention volume equal to 12.7 mm of runoff from the impervious surfaces in the watershed.
• Provide an outlet to drain the detention volume in approximately 40 hours and use a perforated riser pipe for an outlet.
• Provide a follow-up sand filter (masonry sand gradation) with underdrains. The surface area of the filter bed is to be based on a maximum hydraulic loading of 0.61 m³/sec/ha.
• Allow both wet and dry ponds, but encourage wet ponds.
• Require the pond length to be at least twice the pond width to reduce short circuiting.
• Provide a baffle structure near the inflow point to diffuse the inflow currents.
• Permit the water quality enhancement facilities to be a part of on-site or regional stormwater quantity management ponds.

The 12.7 mm of runoff from impervious surfaces can be calculated using,

V_Q = 1.27 A I
in which, V_Q = required pond volume in cubic meters
A = area of the tributary watershed in hectares
I = impervious portion of tributary watershed in percent

The water quality detention pond outlet releases its flow onto a sand filter consisting of a 30 centimetre thick layer of mortar sand over a gravel/pipe underdrain. Sizing of the filter bed depends on the allowable unit loading rate for the filter material which is built into the following equation. Thus, the surface area for the filter can be calculated using,

AF = 1.65 Q_Q
in which, AF = surface area of sand filter in hectares
Q_Q = peak discharge rate from the water quality pond in m³/sec

The peak discharge rate from the outlet can also be easily calculated using,

Q_Q = 0.00191 a D_Q^1.4
in which, Q_Q = peak discharge from water quality outlet in m³/second
a = area of one row of perforations in the riser pipe in cm²
D_Q = maximum depth of water above bottom row of perforations in riser in meters

• Examination of the equation for AF reveals that the allowable unit hydraulic loading rate on the sand filter (i.e. 0.61 m³/sec/ha) is one-fifth to one-tenth of what can be expected for clean mortar sand. The difference represents a safety factor which accounts for the clogging of the filler with time. Estimates indicate that under the permitted loading rates the filter is expected to last 2 to 6 years before the top 15 cm layer of sand has to be replaced.
• An alternative pond configuration provides for a permanent pool of water having a volume equal to 13 mm of runoff from all tributary impervious areas in the watershed.
  • This is a preferred configuration since the existing field data indicate "wet" ponds outperform "dry" ponds.
  • It is also expected that the "wet" ponds will provide greater aesthetic appeal.

Stormwater Ponds, Wisconsin:

Stormwater ponds are not a new, untested idea. They are widely used due to their effectiveness in removing pollutants, their long-term reliability and their versatility in serving other needs such as reducing the risk of flooding, and providing open space. Ponds designed for pollutant removal have enough storage to hold all the runoff from a 1.5 inch storm. In an average year, ponds this size will remove 80% of total suspended solids in the runoff. They are most effective when they have permanent pools of water 3 to 8 feet deep. A depth of 3 feet or more enhances settling rates, and prevents sediments from being stirred up and washed out during the next storm. However, depths greater than 8 feet may create problems due to thermal stratification. During summer, the water in deeper ponds stratifies into two layers that seldom mix- cooler bottom water and warmer surface water. The cooler bottom water is likely to have no oxygen. Without oxygen, water chemistry changes and pollutants such as phosphorus are likely to be released into the water rather than staying in the sediment.

Key features:

• Permanent pool: 3 to 8 feet deep
• Basinshape: 3 times as long as it is wide
• Inlet and outlet locations: at opposite ends of the basin
• Capacity: large enough to hold the runoff from a 1.5 inch storm
• Detention time: long enough for most sediment to settle out (6-24 hours)
• Emergency overflow: for storms that exceed basin capacity
• Safety features: gentle side slopes, underwater safety shelf, an outlet in the embankment, a trash rack over the outlet pipe, and signs
• Maintenance features: an access road, a forebay where much of the sediment settles, and an emergency drain to completely dewater the pond
• Landscaping: to provide attractive scenery and open space for the neighborhood and to prevent nuisances such as geese (an unmowed landscape with shrubs and trees also discourages geese)

Shortcomings of urban detention ponds:

Urban detention ponds, typically with well-cropped lawns leading directly up to water's edge have been partly responsible for the phenomenal increase in Canada geese across many states. Literature cites a typical Canada goose dropping frequency at 28-92 times/day, dry wt. of droppings ranges 1.17-1.9 g, and dry phosphorus content at 1.34-1.0%. In addition, the geese are known to be carriers of Salmonella, Chlamydia, and the vectors for swimmers itch. A study on Green Lake, Seattle estimated that 52% of the annual phosphorus budget was attributable to waterfowl.

Hence it is necessary to design ponds and wetlands in a manner that they would not be too conducive to the geese.

Integrating Constructed Wetlands With Stormwater Management

Stormwater management is a subset of land use planning and urban design. Both exercises must be coordinated and consider the downstream impact of urban development with respect to water use and management and aquatic ecosystem conservation.

Stormwater management involves the use of many devices and techniques with a range of purposes and benefits, including:

• flood protection and flow control
• water quality improvement
• landscape and recreational amenity
• provision of wildlife habitat

A typical stormwater management system:

• Gross pollutant trap (GPT) – to trap artificial and natural litter and coarse particles like gravel and sand.
• Pollution control pond/constructed wetland inlet zone – to trap sand- to silt-sized particles and improve water quality. This module can have some secondary benefits, including landscape aesthetics and flow attenuation.
• Macrophyte zone i.e., an area of plants such as rushes, reeds and sedges – to improve water quality through the trapping of fine particles and soluble pollutants. This module can have some secondary benefits, including wildlife habitat and flow attenuation.
• Lake/island – to provide passive recreation, landscape enhancement and wildlife habitat. Depending on the outlet structure, lakes can significantly attenuate flow. Lakes can also provide water quality benefits, but this function can be compromised if the lake attracts large populations of wildlife, which can degrade water quality.
• Flood retarding basin – to protect downstream areas from flooding and to control stream hydrology. This module can provide more open space within the urban landscape. Stormwater treatment modules located in flood retarding basins can benefit from the extra hydrologic control provided by the basin.

Examples of proprietary devices

• Caution
• Removal of Gross Pollutants From Stormwater Runoff Using Liquid/Solid Separation Structures for four in-situ technologies, the Vortechnics, Stormceptor, CDS Technologies, and traditional baffle boxes
  • Comparison of Estimated Removal Efficiencies
  • Estimated Net Mass Reduction in Stormwater Constituents Achieved Based on 70% TSS Removal
  • Capital Cost Comparison for Liquid/Solids Separation Structures
  • The Stormceptor
    • Recommendation on possible applications/constraints

  • CDS (Continuous Deflective Separation)
    • Recommendation on possible applications/constraints

  • Vortechnics Engineered Products, now available in Nova Scotia
    • Hydro-Brakes
    • Customized Flow Controls

Caution:

Removal of Gross Pollutants From Stormwater Runoff Using Liquid/Solid Separation Structures for four in-situ technologies

(cf. Herr and Harper)

During 1998-99, evaluations were conducted for the City of Orlando, the City of Winter Haven, and the City of Atlantic Beach related to the removal of gross pollutants. Based on information found in the literature and information obtained from technology manufacturers. removal efficiencies were estimated and compared for the four separate technologies.

The evaluation considered removal efficiencies for litter, debris, and coarse sediments; estimated inmitial cost; and operation and maintenance requirements.

Based on removal efficiencies for coarse sediments, removal efficiencies were, estimated for common stormwater constituents, including total nitrogen, total phosphorus, total suspended solids, BOD, and heavy metals. Based on typical fractions of particulate matter in runoff, liquid/solid separators are capable of removing approximately 20-50% of nutrients and heavy metals under ideal conditions.

Limitations of liquid/solid separators must be understood when considering these systems for retrofit applications. While performing the evaluations, it became apparent that there is insufficient field data to accurately predict the removal efficiencies for various gross pollutants contained in stormwater runoff in the United States.

Gross pollutants in stormwater runoff generally consist of litter, debris, and coarse sediments. Most gross pollutants cannot be sampled by traditional automatic samplers, and gross pollutants are often overlooked when evaluating the impact of stormwater runoff on receiving waters.

• Litter is typically defined as human-derived material, including paper, plastic, metal, glass, cloth, or any other man-made material.
• Debris is typically defined as any natural organic matter transported by stormwater runoff, such as leaves, twigs, and grass clippings.
• Coarse sediments are defined as inorganic particulates. Particle diameters of inorganic particulates considered as gross pollutants vary from 5 mm (or 5,000 µm) to much smaller diameter suspended solids.

Comparison of Estimated Removal Efficiencies (cf. Herr and Harper)

Structure	Removal Efficiencies %
	Litter	Debris	Sediments
Vortechs System	?(10-50)	?(10-50)	60-80
Stormceptor	?(10-50)	?(10-50)	60-80
CDS	98	98	?(10-50)
Baffle Box	?(10-50)	?(10-50)	60-80

The removal of sediments from stromwater runoff using liquid/solids separation structures will remove a portion of the particulate fraction of various pollutants contained in runoff which attach to sediment particles. A typical distribution of dissolved and particulate pollutant runoff fractions for residential runoff is provided in Table.

However, particulate matter contributing to loadings of nutrients and havey metals in stormwater runoff is typically 500-100 &microm or smaller. The removal efficiencies for particles of this size range from 20-70%, with lower removals at smaller particle sizes. For purposes of this evaluation, a removal efficiency of 50% is assumed for particles in the 0.1-0.5 nun range.

Estimated Net Mass Reduction in Stormwater Constituents Achieved Based on 70% TSS Removal (cf. Herr and Harper)

Parameter	Estimated Annual Mass Load Reduction (%)
Total N	30
Total P	25
TSS	70
BOD	20
Cadmium	15
Chromium	18
Copper	15
Lead	38
Nickel	15
Zinc	33

Capital Cost Comparison for Liquid/Solids Separation Structures (cf. Herr and Harper)

Structure	Recommended Flow Rate (cfs)	Estimated Installed Cost (US $)	Estimated Installed Cost per cfs Treated (US $)
Baffle Box	18 - 49	20,000 - 35,000	2,800 - 1,600
CDS Unit	3 - 270	35,000 - 667,000	12,800 - 2,470
Vortechs System	0.4 - 6.0	22,700 - 86,500	59,800 - 14,400
Stormceptor	0.6 - 2.5	16,400 - 72,600	29,000 - 27,400

UV Disinfection

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