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Stormwater Treatment

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

Updated: May 08, 2004

September 11, 2004            Our related web pages

Contents:

Introduction

Sewage Management

Constructed Wetlands

Stormwater Management

Typical stormwater management system--- Figure 1

Multi-Treatment Tanks

Examples of proprietary devices ................ a note of caution

Other case histories

Aluminum application

UV Disinfection, Nepean, Ontario

Prologue

References

Introduction

Unit Loadings (Nonpoint and Point sources), and Bacteriological Contamination

Street surface pollutants associated with various particle sizes

Typical distribution of dissolved and particulate runoff fractions for residential runoff

Common road runoff pollutants and sources

Mean pollutant concentrations in runoff from urban and rural highways

Approximate relationship between Unit Areal Loadings from Nonpoint Sources in North America

Unit Urban Phosphorus Export Coefficients in Dartmouth and Cole Harbour, HRM, NS

Aerial deposition of total phosphorus in Nova Scotia and Ontario

Rate of settling in pure, still water

Cover descriptions for various land uses, summary of curve numbers and impervious percentages by land use

Rationale for urban stormwater treatment

An alternate and preferred methodology would be to establish "Lake Carrying Capacities" (e.g., Regional District of Muskoka, Ontario) subsequent to public hearings and then develop planning/zoning regulations to achieve the desired Lake Carrying Capacities of lakes not already adversely impacted (to various degrees). Click here for an overview of relatively undisturbed lakes within Nova Scotia.

For example, typical handbooks on E&S controls do not address post-development phosphorus export since literature cites small particle sizes (0-70 microns) for phosphorus (TP adsorbs/absorbs to minute particles). It is not defensible to compare our lakes with lakes elsewhere which are either naturally more eutrophic or lakes in parts of the world which experienced heavy development pressures as a result of the industrial revolution a century or more back. The only credible ones that are valid are those that compare with natural background value together with direct aerial deposition (of TP, the values are in the range of 1-3-4 ug/l for clearwater lakes) as well as consulting long term local residents if any on any deterioration in visible water quality (cf. Hutchinson, Neary and Dillon, 1991).

As far as phosphorus contribution via stormwater in urban areas is concerned, in reality, it probably would be watershed specific and has to be investigated on a lake by lake basis. Literature cites various nonpoint sources of phosphorus in urban stormwater such as decaying vegetation, automobile traffic, pet wastes, abnormal bird population, tree canopy over impermeable surfaces, the efficiency of street cleaning practices as well as the amount of dislodged vegetation, any local increase in aerial deposition, overuse of fertilizers, etc.

With a few exceptions, stormwater contribution to the phosphorus loading in a suburban/rural area (with onsite systems) in newer subdivisions is not expected to be a major component because of the lower density of development (large lot sizes, fewer people, hence fewer sources of TP), lower impervious area (only a small fraction of a typical urban area), and a significant amount of forest litter.

In typical urban areas where almost all of the land gets developed over a period of time, the increase in TP contribution is dramatic. Approximate values are: a background 0.05-0.075 kg/ha for forest land in Halifax and Wolfville soils (Hart et al, 1978) as opposed to 1.1 kg/ha (Ontario urban mean, Waller & Novak, 1981) cf. TP Predictive Modelling.

• A more detailed study in an urban residential area of the Halifax Regional Municipality by Vokey (1998) reported TP export coefficients of 0.53 kg/ha.yr for a storm outfall catchment area of 7.3 ha in the Settle Lake subwatershed, and an export coefficient of 0.57 kg/ha.yr for a storm outfall catchment of 57.6 ha in the Bissett Lake subwatershed.

And excess phosphorus is just one of the inevitable post development pollutant contributions. There are several others as follows:

Unit Loadings (Nonpoint and Point sources), and Bacteriological Contamination

For an extensive summarized synopses of pollutants in developed areas, download Synopsis-3 (in MS WORD 6.0)

Some examples of the post development pollutants are as follows:

Street Surface Pollutants associated with various particle sizes

Typical distribution of dissolved and particulate runoff fractions for residential runoff

Common road runoff pollutants and sources

Mean pollutant concentrations (µg/l) in runoff from urban and rural highways

Approximate relationship between Unit Areal Loadings from Nonpoint Sources in North America

Approximate relationship between Unit Areal Loadings from Nonpoint Sources (USEPA, 1976)

	Average (Kg/ha/yr)				Range (Kg/ha/yr)
	TN	TP	TSS		TN	TP	TSS
Forest	2.5	0.2	250		1-10	0.005-1	40-400
Range/Pasture	5	0.3	400		2-10	0.2-0.6	10-1,000
Cropland	10	0.6	1,600		1-40	0.03-0.7	300-4,000
Urban	5	0.8	2,000		2-20	0.25-5	200-5,000
Feedlots	1,000	250	---		700-1,500	100-400	---
Precipitation	10	0.25	---		1-100	0.05-1	---
Lake Sediments    Aerobic Conditions    Anaerobic Conditions	--- ---	20 150	--- ---		--- ---	5-40 100-200	--- ---

Unit Urban Phosphorus Export Coefficients in Dartmouth and Cole Harbour, HRM, NS

Aerial deposition of Total Phosphorus in Nova Scotia and Ontario

Rate of settling in pure, still water

Cover descriptions for various land uses

Constructed Wetlands

Enter for a bibliography on wetlands

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

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

Introduction

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".

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 includes (Figure 1):

• 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.

Multi-Treatment Tanks

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- now in common use in Nova Scotia

Recommendation on possible applications/constraints

CDS (Continuous Deflective Separation)- now in common use in Nova Scotia

Recommendation on possible applications/constraints

Vortechnics Engineered Products, now available in Nova Scotia

Hydro-Brakes

Customized Flow Controls

Introduction

In Brief:

• Situation: Urban stormwater runoff from the City of Milwaukee's Ruby Avenue Garage washes sediments and pollutants down the storm sewer leading to Lincoln Creek.
• Solution: An experimental Multi-Treatment Tank
• Benefits: Improves water quality at difficult-to-manage sites, cleans up polluted runoff close to its source, removes pollutants other treatment methods cannot, and is hidden from view
• Drawbacks: Suitable for small sites only
• Statistics: Serves 0.16 acre drainage area, handles runoff from 0.5 inches of rain, main settling chamber holds 5600 gallons, and costs US$88,000 (relatively high cost because this is a new, unique device)
• Design: The MTT consists of three components: a catch basin, settling chamber, and filter. The catch basin includes a mesh bag with polypropylene balls. A honeycomb of PVC plates with angled PVC sections between them, `inclined tube settlers' runs the length of the chamber. Finally, only the filter can remove the dissolved pollutants like trace metals and organic compounds that remain. The filter is made up of a mixture of sand, peat and activated carbon overlying layers of sand and gravel

Download the following assessment (in PDF format)

per Email d/December, 17, 1999 from Prof. Robert Pitt, P.E., PhD, DEE, Professor, Dept. of Civil and Environmental Engineering, University of Alabama at Birmingham, Alabama.

"If it is a small area (only a ha or so), the MCTT works extremely well. ............... generally are too under-sized to provide significant control for most pollutants. Other proprietary devices seem to also be undersized (to reduce cost). Hydraulics and settling theory dictate large commitments of land for important removals."

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- now in common use in Nova Scotia

Recommendation on possible applications/constraints

CDS (Continuous Deflective Separation)- now in common use in Nova Scotia

Recommendation on possible applications/constraints

Vortechnics Engineered Products, now available in Nova Scotia

Hydro-Brakes

Customized Flow Controls

Caution: 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.

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)

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.

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 µm 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.

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

Other case histories of treatment

Aluminum

Note of Caution: There has been rising concern among scientists regarding the probable long term deleterious effects to the environment from the indiscriminate use of chemicals inclusive of aluminum compounds.

Stormwater treatment by UV Disinfection

Prologue

It will not affect small land owners and the onus will be on larger developers (i.e. their consultants) to design in keeping with certain "Lake Carrying Capacities". In the case of the presence of several land owners, centralized municipal systems with usage fees similar to sanitary systems may be more applicable. An intelligent municipality will always be able to develop innovative approaches.

For example, when Lake Tahoe (which was ultra-oligotrophic i.e. very poorly fed with nutrients) turned oligotrophic (poorly fed) there was a major outcry by experts in limnology and other lake users which led to some leading research over the past two decades. We understand that the bi-state authority does not as a rule (with rare exceptions) allow direct discharge of stormwater even in unserviced areas (i.e. rural). They place the onus on the consultant to design innovative systems.

References

• Berezowsky, M. 1997. Constructed Wetlands for Remediation of Urban Waste Waters. In Eyles, N. (Ed.). Environmental Geology of Urban Areas. Geological Association of Canada. 203-213.

• Bland, J.K. 1996. A gaggle of geese ... or maybe a glut. In LakeLine, North Am. Lake Manage. Soc. 16(1):10-11,45-47.

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