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Treatment of stormwater runoff

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Typical pollutants in stormwater runoff

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

Updated: August 14, 2015                                                                 Restoration


Introduction and a note of caution on treatment methodologies

"Laser particle sizing has also indicated that a considerable proportion of the particulates in road runoff are less than 10 Ám. This size fraction is difficult to capture in current stormwater pollution control devices and has been shown to contain significant quantities of heavy metals, which are of concern in aquatic ecosystems." (Drapper et al)

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=10oC, sp. gravity of particles=2.65, shape of particles=spherical) (Welch, 1935)
MaterialDiameter (mm)Hydraulic subsiding value (mm/sec)Time required to settle 1 ft.
Gravel10.01000.00.3 sec
Coarse sand1.0100.03.0 sec
Fine sand0.18.038.0 sec
Silt0.010.15433.0 min
Bacteria0.0010.0015455.0 hr
Clay0.00010.0000154230.0 days
colloidal particles0.000010.00000015463 years

Constructed Wetlands

Enter [Img: animated doors] for a bibliography on wetlands


(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:

Following is a listing of the wetlands that were researched for their ability to treat stormwater runoff as well as a map of the USA showing their location. Some of the constructed wetlands are located in regions with harsher climate and/or more intensive snow cover than in Nova Scotia (cf. Strecker et al., 1992).


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

(cf. Jones, W.W. 1996)

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?

Constructed wetlands provide storage capacity for runoff water within their basins. In addition, organic soils found in mature wetland systems act like a sponge to retain water and allow infiltration of surface water into the groundwater. This decreases not only runoff volume, but also peak discharges which may otherwise cause flooding or erosion downstream. As channelized flow enters a wetland, the velocity is reduced as the water spreads out over the wetland. Velocity is further reduced by the frictional resistance of aquatic vegetation. It is this reduction in velocity which is most responsible for sediment and nutrient retention in constructed wetlands. As the velocity of flowing water slows, it loses the energy needed to keep particles in suspension, and these particles and associated nutrients then settle out.

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?


Constructed wetland planning should not overlook the need for long-term maintenance. Additional vegetation planting may be required to speed plant coverage, replace damaged plants or to try more suitable varieties. Perimeter fencing may be required. Maintenance may be needed to control the spread of undesired plant species such as purple loosestrife. Inlets and outlets can become blocked with debris which will require periodic removal. Inlet and outlet structures should be inspected weekly and especially following big storm events. Most importantly, if the wetland functions well as a sediment and nutrient trap, it may eventually require dredging to remove accumulated materials. Thus, vehicular access to the site must be provided for maintenance vehicles and possibly dredging equipment.

Case histories of constructed wetlands and ponds:

McCarrons Treatment Facility System:

(cf. Strecker et al. 1992)

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:---

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:

(cf. Berezowsky. 1997)

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.

Lake Tahoe:

(Reuter et al 1992)

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:

(Lake Sammamish Initiative)

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:

(Urbonas and Ruzzo 1985)

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).

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:

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

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

It is recommended that this water quality volume be located, wherever possible, within the 100-year flood routing facilities.

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

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

QQ = 0.00191 a DQ1.4
in which, QQ = peak discharge from water quality outlet in m3/second
a = area of one row of perforations in the riser pipe in cm2
DQ = maximum depth of water above bottom row of perforations in riser in meters

Stormwater Ponds, Wisconsin:

(cf. University of Wisconsin-Extension UWEX and Wisconsin Priority Watersheds Program GWQ 017)

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:

Shortcomings of urban detention ponds:

(cf. Bland, J.K. 1996)

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

(cf. Wong et al. 1999)

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:

A typical stormwater management system:

[Img-CDS+Constructed Wetland]

In practice, the boundaries of these stormwater management modules need not be as distinct as in Figure 1. Early planning and identifying the uses and their priorities for each module in a stormwater management system allows improved integration of the modules and optimal utilisation of the available open space.

Examples of proprietary devices


These stand-alone proprietory devices are not expected to remove all of the typical post-development post-human occupation derived pollutants, for e.g., phosphorus, nitrogen, heavy metals, hydrocarbons, pesticides. This is because varying percentages of these post-development pollutants absorb/adsorb to particles smaller than 100 microns and/or are in dissolved form. One recommended way is to develop specially constructed wetlands to polish the effluent from these stand-alone devices. The wetland plants have to be carefully selected by `qualified and experienced wetland biologists' to remove the specific pollutants that are inevitable after the whole watershed is populated. These stand-alone devices will indeed remove larger particles to a considerable degree and/or act as gross pollutant traps thus serving as pre-treatment devices.

For a pictorial representation of a gross pollutant trap together with a constructed wetland, click on Figure 1.

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

(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.

Comparison of Estimated Removal Efficiencies (cf. Herr and Harper)
StructureRemoval Efficiencies %
Vortechs System?(10-50)?(10-50)60-80
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)
ParameterEstimated Annual Mass Load Reduction (%)
Total N30
Total P25

Capital Cost Comparison for Liquid/Solids Separation Structures (cf. Herr and Harper)
StructureRecommended Flow Rate
Estimated Installed Cost
(US $)
Estimated Installed Cost per cfs Treated
(US $)
Baffle Box18 - 4920,000 - 35,0002,800 - 1,600
CDS Unit3 - 27035,000 - 667,00012,800 - 2,470
Vortechs System0.4 - 6.022,700 - 86,50059,800 - 14,400
Stormceptor0.6 - 2.516,400 - 72,60029,000 - 27,400

UV Disinfection

Perhaps the first facility in North America, the City of Nepean within the Regional Municipality of Ottawa-Carleton has augmented the conventional retention pond treatment system with ultraviolet light instal lation. The system will handle surface runoff from new subdivisions located near the Rideau River and the settled stormwater will be subjected to a minimum 20 seconds exposure to UV radiation.


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