Methods of Sizing Water Quality Facilities
A comparison of different design approaches
With the increasing number of agencies throughout the United States establishing regulations for the treatment of stormwater runoff, there is now an array of approaches to selecting and sizing BMPs to address different water quality parameters. Agencies are providing guidelines on how much water needs to be treated as well as on the extent of treatment, typically expressed in percent removal.
The parameters agencies selected are typically tied to water quality issues of receiving waters. For example, in North Carolina, total nitrogen (TN) is frequently the pollutant of concern because of the effects of nitrogen on river systems. In contrast, Clean Water Services, the wastewater and stormwater utility in Hillsboro, OR, focuses on the control of total phosphorus (TP) because it is a limiting nutrient in the Tualatin River.
Most agencies, however, use total suspended solids (TSS) as a surrogate pollutant. It is presumed that effective control of TSS (typically ranging from 70 to 80%) will provide for the control of other pollutants such as TP, total metals, and total Kjeldahl nitrogen (TKN). Other agencies, such as the Washington State Department of Ecology, regulate multiple parameters. The Washington State Department of Ecology, for example, requires an 80% removal of TSS; for development where runoff is tributary to fish-bearing streams, additional treatment for soluble zinc is required as well.
The agencies not only regulate what pollutants to remove and the target pollutant removal rate but also need to establish a methodology for sizing of treatment facilities to treat a given amount of water.
A Summary of Design Approaches
There are two basic design approaches for facility sizing: volume-based design and flow-based design. Ponds, for example, are sized on the volume of water they need to hold, which is usually a multiplier of the regulated treatment volume. Peak-flow-based BMPs, such as flow-through swales, are sized based on calculation of a peak flow resulting from a design storm, unit hydrograph, and rainfall/runoff model such as the Santa Barbara Urban Hydrograph.
There are many variants and hybrids of these two approaches. A sand filter might be sized on a volume basis but can be given a sizing credit for the volume of water treated while it is filling. The average annual load method correlates the efficiency of a BMP with a flow distribution calculated from a rainfall intensity distribution from hydrologic data. Other methods include mass-based design and effluent-limit-based design. This article provides brief descriptions of each method, with examples and pros and cons.
Peak Water Quality Flow Design Storms
Many agencies set a goal to provide for the treatment of 80% of the total annual runoff. Other agencies seek to treat the first half-inch of runoff. One approach is to construct a probability distribution function (PDF) of, for example, 20 years of daily rainfall data. First, the number of storms within a given range is tabulated. The rainfall volume is then calculated by multiplying the number of storms in a given range by the mean depth of the range. These volumes are then put into a cumulative probability distribution. From that distribution range one can quickly determine which depth of rainfall corresponds to 80% of the annual rainfall. The result is a 24-hour depth of rainfall, which can be used as a design storm - which, at least probabilistically, ensures that 80% of the total annual rainfall is being treated. Of course, this design storm can then be related to a return period.
Because BMPs treat the runoff and not the rainfall, some sort of model needs to be selected to represent the rainfall-runoff relationship. Many agencies use a form of the Natural Resources Conservation Service (NRCS) TR-55 model with an SCS type of rainfall distribution or a distribution specific to the region, such as the Delmarva distribution used by the State of Delaware or the Western Washington Hydrology Model (WHM) adopted by Washington Ecology. Another consideration is what depth of rainfall is needed to cause incipient runoff. Usually, depths of 0.1 in. or less are excluded from the annual volume calculations.
One issue associated with this design approach is that, in many areas of the United States, the standard rainfall distribution has a very strong peak associated with short duration of high-intensity rainfall. This can lead to oversizing of the facility and excessive installation costs. Table 1 shows examples of the water quality (WQ) treatment flows for selected jurisdictions across the US. This table assumes a 1 ac., 100% impervious site.
disadvantage of the flow-based design is that it does not account for the volume of the runoff hydrograph. For example, consider a small, highly impervious site with a short time of concentration (Tc) that has a peak design flow of 3.0 cfs. Not far away is a large pervious site with a long Tc, which has the same peak flow of 3.0 cfs. Thus the designer would design the same size facility for each site to treat the peak flow. However, when comparing the runoff hydrographs, we see that the total runoff volume of the larger site is greater than that of the smaller site; hence the same size BMP is receiving more water and likely higher pollutant loads. The result can be a requirement for more frequent maintenance of one facility over the other.
Volume-Based Design
Volume-based designs tend to predominate in areas with intense rainfall patterns. (See Table 2 for northeastern US examples.) Volume-based design approaches are used for BMPs such as ponds, sand filters, bioinfiltration, biofiltration, extended detention basins, and variants of all of the above.
Sizing of these facilities is typically based on some multiple of a water quality volume (WQV), the regulated volume that requires treatment. A good example of this approach is the Federal Highway Administration design guidelines (FHWA-RD-96-096). This study produced a series of graphs for different regions across the United States. The volume of the basin (Vb) to the volume of the runoff (Vr) ratios (Vb:Vr) are provided as a function of the desired removal rate of TSS. To design a pond that achieves 80% removal of TSS in Zone 1 (upper northeastern United States), a 6-foot-deep pond would require a Vb:Vr ratio of about 3:1. Therefore, for 0.5 in. of runoff, the pond would require 1.5 in. of storage or about 5,400 ft.3 of storage for a 1 ac. site.
One main disadvantage of volume-based technology is the volume. Storage of large volumes of water can sometimes be problematic because of limited land availability or high land costs in highly urbanized areas.
Average Annual Load
Many jurisdictions have treatment guidelines that require a percent removal of the average annual mass load of a pollutant such as TP or TSS. Based on this concept, models have been developed to use either accumulated daily rainfall data or accumulated 15-minute intensity data to build a statistical model to design a BMP to remove 80% of the total mass in an average rainfall year.
These models are compelling because they can evaluate the performance of a BMP at all different levels of flow, rather than just at the peak flow itself. Table 3 provides a sample of this method. Percentages of a total rainfall fraction are assigned to ranges of rainfall intensity data from a specific area. A cumulative percentage of rainfall is then calculated. Using the rational method, a flow rate is calculated (this illustration assumes a 5 ac. site (Total Area A = 5 ac.) with a 90% imperviousness. Based on a performance curve for the BMP being designed, a removal efficiency is calculated for the flow rate and then a weighted TSS removal is calculated. From there, the weighted removals are accumulated. If the result shows an 80% removal, then the process is complete; otherwise, an iteration with a different size BMP is required.
Though simple, this approach makes some assumptions that require consideration. This model assumes a uniform TSS concentration and, more importantly, a uniform particle size distribution (PSD).
Because of the high frequency of low-intensity rainfall, Table 3 shows that the majority of the calculated mass removal occurs with very-low-intensity storms. This is problematic because low-intensity rainfall does not have the energy to detach larger particles such as grit, and hence the removal efficiency of the BMP will be significantly reduced. In addition, with the reduced energy the TSS concentration (mass load) is significantly reduced as well.
Therefore, this simple model approach should be evaluated with respect to the watershed size, TSS concentrations, and PSD before blanket acceptance. More sophisticated models do exist that include PSD and sediment transport mechanism as a function of intensity.
Mass-Based Design
Some agencies used mass-based design to size BMPs. In North Carolina, some regulations require that the designer calculate the annual mass load of phosphorus from an undeveloped site and then design the BMP such that the mass load of phosphorus coming from the site does not exceed predeveloped conditions.
Mass-based designs also provide for a - sanity check - on other design approaches. Any BMP designed to remove solids and other pollutants requires periodic maintenance. Therefore the designer not only needs an understanding of the load capacity of the BMP, but also should be able to at least estimate the net annual mass load. This can be simply estimated by (Event Mean Concentration) x (BMP % Removal) x (Annual Runoff Volume). This calculation might reveal that upsizing the facility might be required to reduce maintenance frequency and long-term maintenance costs.
Mass-based design frequently governs when a facility is downstream of a detention tank. The tank is designed to severely attenuate outflows with an extended hydrograph. For a flow-based design the flow is tiny, leading one to assume a tiny BMP is required. However, the mass load remains the same (unless the detention facility provides some form of pretreatment) and will drive the design, rather than the flow.
Effluent-Limited Design
Effluent-limited designs are used when an agency simply places a maximum allowable concentration of the target pollutant. Although this approach is common for industrial permitting, it is rarely used for parking lot, rooftop, and roadway BMPs.
Some agencies, however, are setting benchmark or target concentrations for effluent ranging from 20 to 50 mg/lit. TSS. Hence, the percent removal requirements diminish as the effluent approaches the target. The benchmark is also referred to as the irreducible concentration, meaning that removal is not practical below that concentration.
The effluent-limited design approach is very attractive because it focuses directly on the issue of water quality. However, setting limits implies that monitoring must be done - leading to the question of what to do about violations. In addition, the notion of a BMP implies that there is a reasonable expectation - rather than a guarantee - that a facility will perform as required.
Setting a target concentration also provides a basis from which to establish reasonable removal claims. For example, if a target TP effluent is 10 mg/lit. and studies of a BMP show a 90% removal at 5,000 mg/lit. with an effluent concentration of 500 mg/lit., it helps to put a solid perspective on the meaning of the data and the appropriateness of applying the BMP.
Methods of Establishing Equivalency
Given all these design approaches, challenges arise when one type of BMP, which is designed according to one approach, is proposed for use in areas where the regulations have a different design basis. Frequently, agencies that have volume-based criteria do not provide guidelines for flow-based technologies. Therefore, some method for design equivalency needs to be established, but this can be difficult, and there are some fundamental issues that need to be addressed.
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The "Rosetta Stone" between volume and flow is the hydrograph. To translate from one to the other, a unit hydrograph such as the SCS type must be established. Once that is complete, a peak flow design storm needs to be identified such that it demonstrates that the BMP always treats at least the first 0.5 in. (or whatever the requirement is) of runoff. My research has shown that the desirable design storm is one for which the integral of the hydrograph (volume) reaches the desired runoff volume right at the peak flow of the hydrograph.
Another criterion that can be used is to show that the design storm will treat equal to or in excess of the total annual runoff volume. This is accomplished by constructing a cumulative probability distribution of the mean annual runoff volume as shown in Figure 1, executing a series of models using the design hydrograph, and comparing the weighted volume of the water treated for all storms to the weighted volume that bypasses. Of course, as with other methods, considerations for nonuniform pollutant loads should be given.
Author's Bio: James H. Lenhart, P.E., is with Stormwater Management Inc.
July -August 2004
Methods of Sizing Water Quality Facilities
A comparison of different design approaches
With the increasing number of agencies throughout the United States establishing regulations for the treatment of stormwater runoff, there is now an array of approaches to selecting and sizing BMPs to address different water quality parameters. Agencies are providing guidelines on how much water needs to be treated as well as on the extent of treatment, typically expressed in percent removal.The parameters agencies selected are typically tied to water quality issues of receiving waters. For example, in North Carolina, total nitrogen (TN) is frequently the pollutant of concern because of the effects of nitrogen on river systems. In contrast, Clean Water Services, the wastewater and stormwater utility in Hillsboro, OR, focuses on the control of total phosphorus (TP) because it is a limiting nutrient in the Tualatin River.
Most agencies, however, use total suspended solids (TSS) as a surrogate pollutant. It is presumed that effective control of TSS (typically ranging from 70 to 80%) will provide for the control of other pollutants such as TP, total metals, and total Kjeldahl nitrogen (TKN). Other agencies, such as the Washington State Department of Ecology, regulate multiple parameters. The Washington State Department of Ecology, for example, requires an 80% removal of TSS; for development where runoff is tributary to fish-bearing streams, additional treatment for soluble zinc is required as well.
The agencies not only regulate what pollutants to remove and the target pollutant removal rate but also need to establish a methodology for sizing of treatment facilities to treat a given amount of water.
A Summary of Design Approaches
There are two basic design approaches for facility sizing: volume-based design and flow-based design. Ponds, for example, are sized on the volume of water they need to hold, which is usually a multiplier of the regulated treatment volume. Peak-flow-based BMPs, such as flow-through swales, are sized based on calculation of a peak flow resulting from a design storm, unit hydrograph, and rainfall/runoff model such as the Santa Barbara Urban Hydrograph.
There are many variants and hybrids of these two approaches. A sand filter might be sized on a volume basis but can be given a sizing credit for the volume of water treated while it is filling. The average annual load method correlates the efficiency of a BMP with a flow distribution calculated from a rainfall intensity distribution from hydrologic data. Other methods include mass-based design and effluent-limit-based design. This article provides brief descriptions of each method, with examples and pros and cons.
Peak Water Quality Flow Design Storms
Many agencies set a goal to provide for the treatment of 80% of the total annual runoff. Other agencies seek to treat the first half-inch of runoff. One approach is to construct a probability distribution function (PDF) of, for example, 20 years of daily rainfall data. First, the number of storms within a given range is tabulated. The rainfall volume is then calculated by multiplying the number of storms in a given range by the mean depth of the range. These volumes are then put into a cumulative probability distribution. From that distribution range one can quickly determine which depth of rainfall corresponds to 80% of the annual rainfall. The result is a 24-hour depth of rainfall, which can be used as a design storm - which, at least probabilistically, ensures that 80% of the total annual rainfall is being treated. Of course, this design storm can then be related to a return period.
Because BMPs treat the runoff and not the rainfall, some sort of model needs to be selected to represent the rainfall-runoff relationship. Many agencies use a form of the Natural Resources Conservation Service (NRCS) TR-55 model with an SCS type of rainfall distribution or a distribution specific to the region, such as the Delmarva distribution used by the State of Delaware or the Western Washington Hydrology Model (WHM) adopted by Washington Ecology. Another consideration is what depth of rainfall is needed to cause incipient runoff. Usually, depths of 0.1 in. or less are excluded from the annual volume calculations.
One issue associated with this design approach is that, in many areas of the United States, the standard rainfall distribution has a very strong peak associated with short duration of high-intensity rainfall. This can lead to oversizing of the facility and excessive installation costs. Table 1 shows examples of the water quality (WQ) treatment flows for selected jurisdictions across the US. This table assumes a 1 ac., 100% impervious site.
disadvantage of the flow-based design is that it does not account for the volume of the runoff hydrograph. For example, consider a small, highly impervious site with a short time of concentration (Tc) that has a peak design flow of 3.0 cfs. Not far away is a large pervious site with a long Tc, which has the same peak flow of 3.0 cfs. Thus the designer would design the same size facility for each site to treat the peak flow. However, when comparing the runoff hydrographs, we see that the total runoff volume of the larger site is greater than that of the smaller site; hence the same size BMP is receiving more water and likely higher pollutant loads. The result can be a requirement for more frequent maintenance of one facility over the other.
Volume-Based Design
Volume-based designs tend to predominate in areas with intense rainfall patterns. (See Table 2 for northeastern US examples.) Volume-based design approaches are used for BMPs such as ponds, sand filters, bioinfiltration, biofiltration, extended detention basins, and variants of all of the above.
Sizing of these facilities is typically based on some multiple of a water quality volume (WQV), the regulated volume that requires treatment. A good example of this approach is the Federal Highway Administration design guidelines (FHWA-RD-96-096). This study produced a series of graphs for different regions across the United States. The volume of the basin (Vb) to the volume of the runoff (Vr) ratios (Vb:Vr) are provided as a function of the desired removal rate of TSS. To design a pond that achieves 80% removal of TSS in Zone 1 (upper northeastern United States), a 6-foot-deep pond would require a Vb:Vr ratio of about 3:1. Therefore, for 0.5 in. of runoff, the pond would require 1.5 in. of storage or about 5,400 ft.3 of storage for a 1 ac. site.
One main disadvantage of volume-based technology is the volume. Storage of large volumes of water can sometimes be problematic because of limited land availability or high land costs in highly urbanized areas.
Average Annual Load
Many jurisdictions have treatment guidelines that require a percent removal of the average annual mass load of a pollutant such as TP or TSS. Based on this concept, models have been developed to use either accumulated daily rainfall data or accumulated 15-minute intensity data to build a statistical model to design a BMP to remove 80% of the total mass in an average rainfall year.
These models are compelling because they can evaluate the performance of a BMP at all different levels of flow, rather than just at the peak flow itself. Table 3 provides a sample of this method. Percentages of a total rainfall fraction are assigned to ranges of rainfall intensity data from a specific area. A cumulative percentage of rainfall is then calculated. Using the rational method, a flow rate is calculated (this illustration assumes a 5 ac. site (Total Area A = 5 ac.) with a 90% imperviousness. Based on a performance curve for the BMP being designed, a removal efficiency is calculated for the flow rate and then a weighted TSS removal is calculated. From there, the weighted removals are accumulated. If the result shows an 80% removal, then the process is complete; otherwise, an iteration with a different size BMP is required.
Though simple, this approach makes some assumptions that require consideration. This model assumes a uniform TSS concentration and, more importantly, a uniform particle size distribution (PSD).
Because of the high frequency of low-intensity rainfall, Table 3 shows that the majority of the calculated mass removal occurs with very-low-intensity storms. This is problematic because low-intensity rainfall does not have the energy to detach larger particles such as grit, and hence the removal efficiency of the BMP will be significantly reduced. In addition, with the reduced energy the TSS concentration (mass load) is significantly reduced as well.
Therefore, this simple model approach should be evaluated with respect to the watershed size, TSS concentrations, and PSD before blanket acceptance. More sophisticated models do exist that include PSD and sediment transport mechanism as a function of intensity.
Mass-Based Design
Some agencies used mass-based design to size BMPs. In North Carolina, some regulations require that the designer calculate the annual mass load of phosphorus from an undeveloped site and then design the BMP such that the mass load of phosphorus coming from the site does not exceed predeveloped conditions.
Mass-based designs also provide for a - sanity check - on other design approaches. Any BMP designed to remove solids and other pollutants requires periodic maintenance. Therefore the designer not only needs an understanding of the load capacity of the BMP, but also should be able to at least estimate the net annual mass load. This can be simply estimated by (Event Mean Concentration) x (BMP % Removal) x (Annual Runoff Volume). This calculation might reveal that upsizing the facility might be required to reduce maintenance frequency and long-term maintenance costs.
Mass-based design frequently governs when a facility is downstream of a detention tank. The tank is designed to severely attenuate outflows with an extended hydrograph. For a flow-based design the flow is tiny, leading one to assume a tiny BMP is required. However, the mass load remains the same (unless the detention facility provides some form of pretreatment) and will drive the design, rather than the flow.
Effluent-Limited Design
Effluent-limited designs are used when an agency simply places a maximum allowable concentration of the target pollutant. Although this approach is common for industrial permitting, it is rarely used for parking lot, rooftop, and roadway BMPs.
Some agencies, however, are setting benchmark or target concentrations for effluent ranging from 20 to 50 mg/lit. TSS. Hence, the percent removal requirements diminish as the effluent approaches the target. The benchmark is also referred to as the irreducible concentration, meaning that removal is not practical below that concentration.
The effluent-limited design approach is very attractive because it focuses directly on the issue of water quality. However, setting limits implies that monitoring must be done - leading to the question of what to do about violations. In addition, the notion of a BMP implies that there is a reasonable expectation - rather than a guarantee - that a facility will perform as required.
Setting a target concentration also provides a basis from which to establish reasonable removal claims. For example, if a target TP effluent is 10 mg/lit. and studies of a BMP show a 90% removal at 5,000 mg/lit. with an effluent concentration of 500 mg/lit., it helps to put a solid perspective on the meaning of the data and the appropriateness of applying the BMP.
Methods of Establishing Equivalency
Given all these design approaches, challenges arise when one type of BMP, which is designed according to one approach, is proposed for use in areas where the regulations have a different design basis. Frequently, agencies that have volume-based criteria do not provide guidelines for flow-based technologies. Therefore, some method for design equivalency needs to be established, but this can be difficult, and there are some fundamental issues that need to be addressed.
The "Rosetta Stone" between volume and flow is the hydrograph. To translate from one to the other, a unit hydrograph such as the SCS type must be established. Once that is complete, a peak flow design storm needs to be identified such that it demonstrates that the BMP always treats at least the first 0.5 in. (or whatever the requirement is) of runoff. My research has shown that the desirable design storm is one for which the integral of the hydrograph (volume) reaches the desired runoff volume right at the peak flow of the hydrograph.
Another criterion that can be used is to show that the design storm will treat equal to or in excess of the total annual runoff volume. This is accomplished by constructing a cumulative probability distribution of the mean annual runoff volume as shown in Figure 1, executing a series of models using the design hydrograph, and comparing the weighted volume of the water treated for all storms to the weighted volume that bypasses. Of course, as with other methods, considerations for nonuniform pollutant loads should be given.