Stormwater runoff from urban developments is a major source of water pollution. Runoff from rain and melting snow traverses impermeable surfaces, picking up oil, grease, industrial discharges, airborne fallout, assorted chemicals and nutrients, bacteria, dirt, trash and other contaminants. Stormwater runoff is derived from construction sites, small commercial sites, large urban mall parking lots, residential streets, and roadways and freeways.
The basis for stormwater management and treatment is the Clean Water Act (CWA), which was passed in 1972. Section 402(p) was added to the Clean Water Act in 1987 to require implementation of a comprehensive two-phase approach for addressing stormwater discharges under the NPDES program. These EPA stormwater regulations began in 1990.
Phase I established that an NPDES permit is required for stormwater discharge from municipalities with separate storm sewer systems that serve a population greater than 100,000, for large construction sites, and for certain defined industrial activities. Under Phase II regulations (promulgated in November 1999), smaller municipalities and small construction sites are also required to obtain NPDES permits for stormwater runoff. Phase II is scheduled to take effect March 10, 2003.
The U.S. Environmental Protection Agency (EPA) is the federal governing body for the CWA. However, much of the implementation and enforcement action has been delegated to individual states.
Stormwater discharges to surface water are under increased scrutiny in the Pacific Northwest. The main concern here is the effect of sediments on the life and reproduction of endangered fish, primarily salmon. This recognition of the potential adverse effects of stormwater on streams and lakes has led to the increasingly common practice of collecting and infiltrating stormwater to groundwater. As with surface water, stormwater discharge to groundwater may require a permit from the state regulatory agency.
For purposes of this course, target constituents are defined as chemicals (e.g., nitrogen) and water quality parameters (e.g., TSS) that need to be reduced or removed to protect groundwater and surface water. Most target constituents belong to the following categories:
Total Suspended Solids (TSS)
Nutrients (nitrogen and phosphorus)
Pesticides/herbicides
Petroleum hydrocarbons
Pathogens
Metals
Salts (chloride)
Section 1
DESIGN CRITERIA AND ASSUMPTIONS
Design criteria are quantitative standards upon which the design is based. Design assumptions are the decisions that must be made during the design process. Typical preliminary performance goals for a stormwater management system are:
Facility and infrastructure protection
Surface water and groundwater protection
Aesthetics/architectural compatibility
Reliability
As stated, the performance goals are relatively qualitative and therefore might be achieved in a number of different ways. Design criteria and assumptions provide the constraints to direct or guide the development of a design that meets the performance goals.
For a preliminary design report, the primary design criterion is the design storm event. Selection of design storm events will typically be based on regulatory requirements, both state and local. For example, city standards may require the use of the 10-year 6-hour storm event for the “design storm.”
The general methodology used here is to:
| 1. |
| Select the design storm |
| 2. |
| Obtain the total precipitation for this storm, from weather records |
| 3. |
| Calculate the peak flow and total water volume for the storm, using some type of model. |
For example, assume that the 10-year 6-hour storm is selected as the design storm. Total precipitation for the 10-year 6-hour storm at a given location is 1.78 inches. The peak flow and total water volume corresponding to a rainfall event of 1.78 inches over the area in question must be estimated through modeling.
The actual flow will be site-specific. For
example, flows from mall area parking lots
will depend on many factors, including
parking lot layouts, grading, and
stormwater catch basin locations.
Flow-through treatment units must be sized to handle peak flows to treat the water quality design volume for the design storm event. The system could also be “over- designed” in that it could treat a larger storm event than required. This would provide a higher degree of environmental protection at some additional cost.
The “Rational Method” is an empirical runoff formula that has gained wide acceptance because of its simple intuitive treatment of storm runoff. This method relates runoff to rainfall intensity, surface area and surface characteristics. Further refinement of stormwater peak flows and volumes should include consideration of “time of concentration” and should consider the use of alternative runoff prediction methods. As a replacement for the 6-hour storm, the time of concentration (i.e., 5, 15, 20, 30, 60 minutes) could be calculated for each specific site.
Here are examples of several design assumptions:
| 1. Stormwater will be infiltrated to groundwater to minimize effects on surface water flow conditions and to minimize stormwater conveyance requirements. Discharge to surface water would be the other option. |
| 2. Stormwater flows in excess of peak flow from the design storm event may bypass treatment units, provided the water quality design volume has been treated. High-flow bypasses are used when the actual flow exceeds the design capacity of the treatment unit. For example, piping may be designed to handle the 25-year storm, while the treatment units may only be designed for the 6-month storm, which will have a significantly lower peak flow and total volume. A significantly higher capital cost might not be justified to treat the occasionally high flow. Alternatively, the treatment units could be “over-sized” to treat a higher, less frequent, peak flow. 3. Stormwater flows in excess of water quality design volume may bypass treatment units. This is similar to the peak flow case discussed above. |
| 4. Stormwater treatment should be effective year-round. Cold weather may significantly affect treatment efficiency, especially of vegetated systems such as grassy swales. |
| Runoff from roofs will be handled separately from stormwater flows from parking lots and streets, and will be discharged to groundwater through dry wells or other appropriate infiltration devices. Runoff from roofs has been documented by research studies to be of substantially better quality than stormwater from parking lots and streets. |
| Infiltration of roof runoff is considered to be a stormwater Management Practice (BMP), in that it decreases the volume and peak flow of lower-quality water to be treated. |
6. Use of existing wetlands in the area for stormwater treatment is acceptable, if such use provides a level of stormwater treatment that meets water quality goals and is protective of surface water and groundwater.
7. Surface features should be attractive and enhance the site; otherwise, surface features should be minimized.
PRELIMINARY TREATMENT DESIGN
The goals of stormwater treatment are to reduce the concentrations of target constituents to levels that are protective of groundwater and surface water. Most target constituents belong to the following categories:
Total Suspended Solids (TSS)
Nutrient constituents (nitrogen and phosphorus)
Pesticides/herbicides
Petroleum hydrocarbons
Pathogens
Metals
Salts (chloride)
Here is the most common source of petroleum hydrocarbons in stormwater:
Identification of these target pollutants is based on the following criteria:
Identification of constituents by the EPA as having moderate or high potential of adversely affecting groundwater quality during stormwater infiltration
Comparison of estimated concentrations of constituents in stormwater runoff from commercial areas with applicable water quality criteria for the protection of groundwater and surface water
Identification of constituents as sources of water quality impairment per a nutrient management plan or Total Maximum Daily Load (TMDL)
The EPA describes the stormwater pollutants that have the greatest potential of adversely affecting groundwater quality during stormwater infiltration as consisting of six classes of pollutants:
| Nutrients Pesticides/herbicides Organic chemicals (other than pesticides/herbicides – primarily petroleum hydrocarbons) Pathogenic microorganisms Heavy metals and other inorganic compounds Salts |
| In other words, all of the pollutant categories are listed except TSS. TSS discharged in stormwater will affect surface water quality (and potentially aquatic life such as salmon) but will not significantly affect groundwater quality. |
According to EPA regulations, an urban stormwater Best Management Practice (BMP) is any “technique, measure or structural control that is used for a given set of conditions to manage quantity and improve the quality of storm water runoff in the most cost-effective manner” (Beck, D.E., “Storm Water Treatment Technologies,” CE News, November 2001).
| For this course, a BMP will be regarded as any action which will result in reduced stormwater pollution, excluding actual treatment. For a given project, BMPs will likely include: |
| Public education regarding the impacts of land use activities on stormwater quality and the effects of stormwater discharges on receiving waters Establishing spill prevention plans Encouraging public cooperation and participation in control efforts (e.g., stormdrain stenciling) Sweeping of paved surfaces |
All of these BMPs can have a large effect on stormwater quality. For instance, new higher- efficiency street sweepers are available to remove much of the potential pollutant load prior to storm events. A recent study performed on Wisconsin freeways showed a 45 percent reduction in total suspended solids using this new technology (Bannerman, R., Wisconsin Department of Natural Resources, personal communication, 2002).
Traditional stormwater treatment consists of such systems as non-vegetated ditches and non-engineered ponds, neither of which necessarily provide treatment. Traditional treatment is generally not very effective compared to advanced technologies, and has become
somewhat obsolete with the recent focus on meeting EPA Phase II rules governing stormwater discharges under the National Pollutant Discharge Elimination System (NPDES).
3. Advanced Treatment Technologies
Advanced treatment technologies are engineered systems which provide treatment for stormwater at a given capacity. Advanced technologies may provide for either surface (above-grade) or underground (below-grade) treatment and discharge.
The remainder of the course will focus on advanced treatment technologies, particularly engineered systems.
PRELIMINARY TREATMENT DESIGN
Surface (Above-Grade) Treatment Systems
Systems such as detention and retention ponds, wetlands and grassy swales are above-grade and typically discharge to surface water. These systems can enhance water quality by removing pollutants through the interaction of sedimentation, filtration, adsorption and biological processes. To be effective, ponds, wetlands and swales usually require large areas and dense vegetation.
The surface area requirements of surface treatment systems may not be viable for existing developments, and may not be cost-effective for new developments when compared to other advanced treatment technologies. If discharging to groundwater, a shallow water table may limit the depths of ponds/swales/wetlands that could be constructed on-site.
Large areas would be required to store a given volume of water, and both water and soils in/below the ponds would be subject to freezing. Freezing not only limits the degree of treatment provided by the ponds/swales/wetlands but also severely limits infiltration rates. Ponds excavated into gravels likely would have to be lined with low-permeability materials to prevent direct infiltration of water to the aquifer without treatment
Existing wetlands would be subject to the same limitations as constructed wetlands, i.e., given their shallow depth, they could likely treat only small amounts of stormwater and their treatment efficiency may be hindered during cold weather.
Subsurface (Below-Grade) Treatment Systems
Other advanced treatment technologies are engineered systems, which are typically available as fabricated, stand-alone systems. Most of these systems are proprietary in that they are only manufactured by one company. These systems may discharge to either surface water or groundwater. Most systems are available in a variety of sizes, depending on the anticipated water flow rate; for very large
areas, multiple units can be linked together through parallel piping.
Subsurface treatment systems can be divided into three types of devices:
Sedimentation
Swirl concentrator or
Filtration
| Several of these systems are shown in Table 1. A number of other systems are available; however, the systems in Table 1 were chosen because they are representative, and independent cost and performance data are available. | |||
| Table 1. Advanced Treatment Technologies for Groundwater Discharge | |||
| Type | Product | Company | |
| Sedimentation | Catch basin Stormceptor Stormvault Hydro-Kleen | Many CSR Hydro Conduit Jensen Precast Weaver Manufacturing | |
| Swirl concentrator | Vortechs CDS | Vortechnics CDS Technolofies | |
| Filtration | Sand filter Storm filter | US Filter and many others Stormwater Management, Inc. | |
Sedimentation devices rely on simple settling as their primary mechanism for pollutant removal. Heavy particles such as suspended solids will settle to the bottom, while oil/grease will rise to the water surface. Treated water is typically removed from the middle of the unit so that both solids and oil/grease remain trapped inside. A catch basin is the simplest sedimentation device and has been in use for many years. The other technologies have generally been developed and refined during the past 10 to 20 years.
In swirl concentrators, tangential (off-center) inlets create swirling motions to force settleable solids to the center of the swirl chamber, where they separate from the stormwater and fall to the bottom of this chamber. Lighter-weight pollutants (such as oil) and floatables are separated at the outer edge of the vortex.
Filtration devices have been used for many years in treatment of industrial and municipal waters, but have been used for stormwater treatment only recently. The sand filters or multimedia filters typically used for water treatment are not necessarily suited for stormwater because of its intermittent flow and varying quality. Filtration devices such as the StormFilter have been developed specifically for stormwater. The StormFilter combines both sedimentation and filtration processes.
Here is a photo of a new large StormFilter installation:
In Table 2, brief product descriptions and target constituents for each product shown in Table 1 are listed.
Table 2. Summary of Product Descriptions
| Product | Product Description | Target Constituents2 |
| Stormceptor | A weir insert is placed in a round manhole vault to improve hydraulics, thereby improving removal efficiency and sediment retention. During low flows, the insert directs the flow downward and then laterally towards the walls of the sump. Excess flow above the design capacity flows directly across the insert device towards the outlet. | Settle able and floatable solids, oil/grease and particulate pollutants |
| Storm vault | A wet vault consisting of multiple chambers in series separated by baffles. Contains standing water or dead storage, which enhances treatment. | Settle able and floatable solids, oil/grease and particulate pollutants |
| Hydro-Kleen | Two types available: box and tapering cylinder. Box: water directed to vertical chamber on one side for sediment; water overflows to second chamber where it passes through media. Tapering unit: collects sediment in perimeter trough; water overflows to center to pass downward through media. | Hydrocarbons, organically bound metals, PCBs, pesticides, VOCs, sulfides |
| Vortechs | Vortex separation with the swirl device placed in a rectangular, shallow vault. Available in nine standard sizes. | Settle able and floatable solids, oil/grease and particulate pollutants |
| CDS | Circular device; flow is directed to create circular flow like a vortex, but removal occurs as the water passes through a screen around the outer perimeter. Removal is induced by countercurrent flows on opposite sides of the screen, which also prevents clogging of the screen. | Settle able and floatable solids, oil/grease and particulate pollutants |
| Sand Filter | Many diverse designs, from standard slow sand filter (standard water treatment design) to several varieties strictly for storm water–lineal box, open basin or closed vault (similar to Storm Filter). Common features are a pretreatment basin for capturing large solids; gravity flow downward; and sand media overlaid on gravel or coarse sand above an under drain system. | Settle able and floatable solids, oil/grease and particulate pollutants |
| Storm Filter | Vertical cylinder with media of various types placed in the cylinder. Water enters laterally through the filter and enters a vertical center well which exits to an under drain system. One standard-size cylinder (15 gpm). Number of cylinders is a function of design peak flow. Pretreatment may be desirable, as defined by the manufacturer. | Varies with media. All remove settle able solids. Some remove dissolved phosphorus or metals. |
1) From “Investigation of Structural Control Measures for New Development” (Larry Walker Associates, Inc., prepared for the Sacramento Stormwater Management Program, November 1999)
2) As stated by manufacturer
A paper has been written reviewing a suggested methodology for the preliminary evaluation of stormwater filtration systems by James H. Lenhart of Stormwater Management, Inc. This information, which is available under “Published Papers” at www.stormwaterinc.com, will be paraphrased in the following section. This material deals with a specific subset of stormwater treatment (advanced treatment
systems which employ filtration), but the most of the methodology is applicable to stormwater treatment in general.
Overview
As the need for efficient stormwater treatment grows, there will be an increasing number of commercial filtration systems offered to regulators. As part of their due diligence, the regulator goes through an evaluation process to determine if the proposed system will meet some basic criteria. If the facility meets those criteria, a pilot project is identified to evaluate the facility in the field. This section addresses some of the stormwater filtration fundamentals which all filter systems should meet prior to implementation of a pilot study.
System Hydraulics
This is an evaluation of how water flows through the system. Important factors are:
1. Evaluate the hydraulic grade line at the design flow rate. Typically, calculations should be performed from the point of downstream control to ensure the system could convey the peak design flow. This analysis should include head losses due to the media, pipe entrance, exit and barrel.
2. Check scour velocities in tanks and pipes with particular reference to where sediments are deposited or where high-energy flows can dislodge or scour the filtration media. For example, at what velocity does the inlet pipe discharge into the filter bay?
Media Hydraulics
According to experts in the field, media hydraulics are an important part of filtration which are often not well understood by the user. The factors are
Media Type
Structural Considerations
1.Structural integrity is critical. Make sure the treatment units are reviewed by structural engineers to ensure that anticipated traffic loads can be handled.
2.Water tightness is required by many agencies. Evaluate vertical and horizontal joints for design integrity. Vertical joints are more difficult to control due to differential settlement. Some agencies require a water tightness test prior to acceptance. All joints below the permanent pool elevation need to be watertight.
3.Buoyance measures need to be considered. In areas of high groundwater, measures should be taken to prevent system flotation.
4.Constructibility considerations are important. Many times, what is simple on the plans is difficult during constructions. Does the company have a track record of successfully constructing and installing treatment facilities?
- 1. Considerations
The fact that stormwater flow is intermittent is an important consideration for most advanced treatment systems. Many systems include “bypass” features to avoid trying to ineffectively treat too much water during infrequent high flows; consequently, some water is discharged without treatment.
Some generalities can be applied to most potential stormwater applications (Beck 2001):
Even the best system will not remove all pollutants
Some systems target different sets of primary pollutants
Most systems incorporate some type of detention to provide sedimentation
The relative compactness of advanced treatment systems may not provide sufficient volume for optimum settling of fine-grained particles, and may allow re-suspension of accumulated sediments
Maintenance is essential to ensure long-term performance
Advanced treatment systems can be used in conjunction with each other and/or in combination with traditional systems
Pollutant concentrations can vary substantially during storm events, and from event to event
Based on the design assumptions and comparison of the relative strengths and weaknesses of surface treatment methods versus subsurface treatment methods, it is the author’s view that subsurface treatment methods are preferable most of the time. The remainder of this course will focus on subsurface advanced treatment technologies, although some performance data for surface treatment methods is also presented for comparison.
- 2. Previous Data for Advanced Stormwater Treatment Technologies
A comprehensive report has been compiled by the Center for Watershed Protection (CWP) to compare removal performance of various parameters for several technologies. The CWP is funded through a grant from the U.S. EPA, Office of Water, and maintains a Stormwater Manager’s Resource Center web site at www.stormwatercenter.net.
In the CWP report, both average percent removal and concentration data are shown for the approximately 135 studies compiled in the report. The advanced treatment technologies for surface water discharge (ponds, wetlands and grassy swales) are shown separately in the report; unfortunately, all other advanced technologies are grouped together under the heading of “Filtration.” However, this provides a comparison between these three options for surface treatment and the group of options for subsurface treatment.
Another thorough comparison report was developed for the City of Sacramento (Larry Walker Associates, Inc., “Investigation of Structural Control Measures for New Development,” prepared for the Sacramento Stormwater Management Program, November 1999). In this report, performance and cost data for 14 proprietary technologies were compared to those for grassy swales and sand filtration.
Six of these 14 technologies are discussed in this course:
Stormceptor
Stormvault
Hydro-Kleen
Vortechs
CDS
StormFilter
- 3. Comparison Criteria
Based on previous experience with storm water systems, and on the two comparison summaries discussed, several criteria were selected to compare treatment systems. Assume that TSS, oil & grease (O&G) and nutrients are anticipated to be the primary pollutants in stormwater. Therefore, removal of these pollutants is very important.
| The other criteria chosen are: |
| Ease of maintenance Capital cost (based on a design capacity of 1 cubic feet per second [cfs]) Annual operation and maintenance (O&M) cost, also based on a design capacity of 1 cfs Flexibility. Treatment modifications must be possible without significant changes to conveyance systems or land use |
TSS Removal
In comparable studies, as may be expected, filtration is more effective at TSS removal than ponds, wetlands or grassy swales. However, the three surface treatment options are still quite effective (see Table 3). The median concentration after filtration, in the numerous studies compiled by the CWP, was 11 mg/L. Grassy swales were also shown to be nearly as effective as sand filtration in the
Sacramento study (Larry Walker Associates, 1999).
| Technology | Percent Removal | Median Effluent Concentration (mg/L) |
| Wet Ponds Wetlands Grassy Swales Filtration | 80 76 81 86 | 17 22 14 11 |
Table 4. Ranking of Treatment Technologies For Groundwater Discharge
| Technology | Percent Removal TSS O&G | Ease of Maintenance | Capital Costs1 | Annual O&M Costs1 | |
| Stormceptor | 26 -93 | 43 | Simple | $16,700 | $1,000 |
| Stormvault | 24 – 50 | 38 | Simple | $12,400 | N/A |
| Hydro-Kleen | N/A | N/A | Moderate | $3,900 | N/A |
| Vortechs | 84 | N/A | Simple | $17,500 | $400 |
| CDS | 70 | N/A | Simple | $11,800 | $400 |
| Sand filter | 79 – 97 | 69 – 84 | Moderate to difficult | $10,000 – $80,000 (3) | N/A |
| StormFilter | 43 – 92 | 69 – 81 | Moderate | $39,600 | $4,500 |
1) Based on 1-cfs capacity 2) Larry Walker Associates, 1999 3) CWP, 2000 |
Oil and Grease Removal
Only four of the seven technologies had removals for O&G, which met the Sacramento study guidelines. Sand filtration and the StormFilter had the highest removals and were essentially equivalent for this parameter.
Nutrient Removal
Average nutrient removal efficiencies for various stormwater treatment technologies are presented in Table 5. According to the data, filtration is most effective at phosphorus removal while grassy swales are most effective at nitrogen removal. However, median effluent concentrations for all technologies are similar.
Table 5. Nutrient Removal Efficiency Comparisons
| Total Nitrogen | Total Phosphorus | |||
| Technology | Percent Removal | Median Effluent Concentration (mg/L) | Percent Removal | Median Effluent Concentration (mg/L) |
| Wet Ponds | 33 | 1.3 | 51 | 0.11 |
| Wetlands | 30 | 1.7 | 49 | 0.20 |
| Grassy Swales | 84 | 1.1 | 34 | 0.19 |
| Filtration | 38 | 1,1 | 59 | 0.10 |
| 1) From “Stormwater Treatment Practice Pollutant Removal Performance Database,” 2000 |
Maintenance Issues
Listed below are some important issues relating to maintenane and operation of all water quality facilities. The complete reference (Lenhart and Harbaugh, “Maintenance of Stormwater Quality Treatment Facilities,” 2000) may be found under “Published Papers” at www.stormwaterinc.com.
Maintenance is not occurring on many systems. After design and installation, many facilities are not being maintained and cease to function as intended. In many cases, continuous build-up of pollutants could trigger much higher maintenance costs in the future
Failed facilities are frequently encountered. Stormwater treatment design is still relatively new and in many cases, what appears good on paper does not function in the field. Facilities fail due to poor design, extreme flows, lack of maintenance or poor construction methods. Many times, the facility function does not match the land use or pollutant load
Facilities transformed into landscape amenities are a common problem. Swales can be quickly transformed into a grassy area with short mowing heights, fertilizer and herbicide application, etc. These practices can easily defeat the water quality function of the system
Here is an example of a grassy swale which has become a landscaping amenity rather than a treatment system:
Improper disposal of residuals is frequently encountered on both the public and private sides.
Poor maintenance access results from inadequate design review. Maintenance access needs to be part of the engineering plan review process.
Recovery periods following maintenance need to be part of the scheduling process. Vegetated systems typically require time for the vegetation to recover or earthwork to stabilize. In many cases, the system may need to be placed off-line until the reestablished vegetation can handle design velocities.
Inspections of private facilities by public inspectors can be problematic for many types of BMPs. Depending on the BMP, the end result of the inspection can be subjective and lead to questioning whether a facility needs maintenance, or if adequate maintenance has been performed.
Source control programs should be part of the maintenance and inspection guidelines. If pollutant loads exceed expectations or unexpected pollutants are discovered, the pollutant should be identified and source control implemented if possible.
Maintenance of upstream facilities needs to be done at the same time the water quality facility maintenance is completed.
Start-up and construction activity can greatly impact a facility during the first and second years of operation. Tracking of soils and immature landscaping can lead to excessive TSS loading of a system. Often, maintenance frequency is increased to address this problem. Over a period of time, the TSS load decreases, with a corresponding increase in other pollutants such as oil and grease.
Based on the author’s water treatment experience, and product descriptions provided by each vendor (Table 2), the ease of maintenance for each of the six engineered stormwater treatment systems being considered was subjectively graded. The Stormceptor, Stormvault, Vortechs and CDS systems were all thought to be “simple” to maintain, because they are sedimentation or vortexing devices, which have no parts to replace (Table 4). Sediment and floatables must be periodically removed.
The Hydro-Kleen and StormFilter devices were deemed “moderate” to maintain because they contain media, which must be replaced, in addition to sediment and floatables to periodically remove. Sand filters are ranked as “moderate to difficult” to maintain. In general, sand filter systems are moderately difficult to maintain because sand must be periodically changed at the top of the unit, probably on a more frequent basis than the StormFilter. For a site with a shallow water table, horizontal sand filter layouts would be difficult to maintain, as shallow filter units and low overhead working space would be required.
Capital Cost
Capital costs for this example are based on 80 percent TSS removal at a design capacity of 1.0 cfs (Larry Walker Associates, 1999). The HydroKleen system is the least expensive at approximately $4,000, while the Stormceptor, Stormvault, Vortechs and CDS systems are in the $10,000 to $20,000 range (Table 4). The StormFilter would cost approximately $40,000, while a sand filter is potentially the most expensive option at a capital cost of approximately $10,000 to $80,000.
Annual Operation and Maintenance (O&M) Cost
| Annual O&M costs are also shown in Table 4. Costs for a 1-cfs system are anticipated to be moderate for all systems, with the StormFilter the highest at $4,500/yr. Direct O&M cost estimates for the sand filter, Stormvault and Hydro- Kleen systems were unavailable, although the Stormvault and Hydro-Kleen costs should be less than $1,000/yr. The sand filter costs should be similar to the StormFilter. |
| 4. Overall Ranking |
| From Table 4, it is apparent that filtration technologies such as a sand filter or StormFilter will provide the best storm water treatment, followed by the other systems. In Table 3, it was shown that filtration is anticipated to provide slightly better treatment than ponds, wetlands and grassy swales. This statement is probably true for both sand filters and the StormFilter. The StormFilter has higher capital and O&M costs than the other proprietary systems; however, its capital cost is much lower than the sand filter. The StormFilter system is more flexible than other proprietary technologies because a variety of different filter media may be used, and switching from one media to another is simple. |
In summary, many types of stormwater treatment technologies are available. Among these, engineered proprietary technologies probably provide the most effective removal of target constituents and have the best probability of being approved by regulatory authorities. Removal efficiency, capital cost, maintenance cost and maintenance issues will vary by technology, and must be among the
factors considered by the designer or regulator.
| Section 1: Conclusions |
| Conclusion |
| Design criteria are quantitative standards upon which the design is based. Design assumptions are the decisions that must be made during the design process. Design criteria and assumptions provide the constraints to direct or guide the development of a design that meets the performance goals. The primary design criterion for a preliminary design report is the design storm event. |
| The general methodology using the storm event is to: |
| 1. Select the design storm | |
| 2. Obtain the total precipitation for this storm, from weather records | |
| 3. Calculate the peak flow and total water volume for the storm, using some type of model | |
| The actual flow will be site-specific. For example, flows from mall area parking lots will depend on many factors, including parking lot layouts, grading, and stormwater catch basin locations. Models such as the “Rational Method” are used to predict runoff volumes. The designer will need to make numerous design assumptions. These include how water greater than the design flow or volume will be handled, and the fate of each stormwater source such as roof runoff. |
| The general methodology using the storm event is to: |
| The goals of stormwater treatment are to reduce the concentrations of target constituents to levels that are protective of groundwater and surface water. Most target constituents belong to the following categories: |
| Total Suspended Solids (TSS) Nutrients (nitrogen and phosphorus) Pesticides/herbicides Petroleum hydrocarbons Pathogens Metals Salts (chloride) |
| All of these categories may affect surface water, and all except TSS may affect groundwater. Potential control (treatment) technologies for stormwater are: (PAGE 5 of Conclusion missing picture) 1.Best Management Practices (BMPs)— “any technique, measure or structural control that is used for a given set of conditions to manage quantity and improve the quality of storm water runoff in the most cost-effective manner” 2.Traditional technologies— primarily non-vegetated ditches and non-engineered ditches; generally not very effective 3.Advanced technologies— engineered systems which provide treatment at a given capacity Advanced treatment technologies may be surface or subsurface. Most subsurface treatment systems are engineered systems, which are typically available as fabricated stand-alone systems. These may be divided into three types of devices: Sedimentation Swirl concentrator or Filtration Several variations of each type of subsurface treatment system, which have different mechanisms for pollutant removal, are commercially available To perform a successful evaluation of stormwater filtration systems, the following categories should be considered: System hydraulics Media hydraulics Media type Structural considerations The stormwater treatment designer must realize that all treatment systems have their limitations, and should keep these limitations in mind when deciding on performance goals and design assumptions. Comprehensive reports are available in the literature that compares stormwater treatment performance for many technologies. Several criteria were selected to compare treatment systems using these data: Removal efficiency for TSS, oil and grease (O&G) and nutrients Ease of maintenance Capital cost Annual operation and maintenance (O&M) cost and Flexibility |
For TSS, filtration technologies are generally more effective than surface treatment systems (ponds, wetlands or grassy swales). However, these three surface treatment options are still quite effective.
For O&G, filtration is the most effective technology. According to the data, filtration is most effective at phosphorus removal while grassy swales are most effective at nitrogen removal. However, median effluent concentrations for all technologies are similar.
| The list of maintenance issues for treatment systems includes: |
| Lack of maintenance Failed facilities Facilities transformed into landscape amenities instead of treatment systems Improper disposal of residuals Poor maintenance access Lack of recovery periods following installation or maintenance Inspections which are problematic or too subjective Lack of source control programs Lack of maintenance of upstream facilities, and Excessive TSS loading at start-up Costs vary significantly among engineered treatment systems. For systems based on 80 percent TSS removal at a design capacity of 1 cubic foot per second (cfs), capital costs ranged from $4,000 to $80,000. Annual O&M costs ranged from less than $1,000/yr to $4,500/yr. In summary, many types of stormwater treatment technologies are available. Among these, engineered proprietary technologies probably provide the most effective removal of target constituents and have the best probability of being approved by regulatory authorities. Removal efficiency, capital cost, maintenance cost and maintenance issues will vary by technology, and must be among the factors considered by the designer or regulator. Sources of more information on stormwater treatment design include published papers and reports, public websites and vendor websites. |
| OTHER SOURCES OF INFORMATION |
| Sources of information used for this course include: “Published papers” at www.stormwaterinc.com Beck, D.E., “Storm Water Treatment Technologies,” CE News, November 2001. Larry Walker Associates, Inc., “Investigation of Structural Control Measures for New Development,” prepared for the Sacramento Stormwater Management Program, November 1999. http://cfpub.epa.gov/npdes, which has information on all of the EPA stormwater regulations. www.stormwatercenter.net, which is a web site made possible through a grant from the EPA Office of Water. The Stormwater Manager’s Resource Center (SMRC) site is managed and published by the Center for Watershed Protection, Inc., located in Ellicott City, Maryland. The SMRC web site was funded to provide Phase II communities with the tools and techniques necessary to protect their watersheds, and to enhance and restore their local water resources. Numerous papers are available for downloading at this site. Information on proprietary stormwater treatment technologies may be obtained from the following vendor websites: www.stormwaterinc.com — Stormwater Management, Inc. www.vortechnics.com — Vortechnics www.rinkermaterials.com/stormceptor — Rinker Materials www.mycelx.com — MYCELX Technologies |