Cost and Effectiveness of Stormwater Treatment Technologies

When faced with the decision to select effective and appropriate stormwater treatment technologies, also denominated Best Management Practices (BMP), the vast range of options can be daunting. The need for effectiveness data is especially acute, since the investments made at a watershed scale can be quite significant, and needs to be justified. In an attempt to understand the success or shortcomings of various projects, users and manufactures of stormwater mitigation technologies were surveyed for information about the citing, effectiveness, maintenance and cost of stormwater treatment systems they have installed or designed.

Prior to conducting the survey, a number of resources were identified that provide useful information. In 1996 the US Environmental Protection Agency and the American Society of Civil Engineers began the International Stormwater BMP Database (www.bmpdatabase.org) in an effort to consolidate BMP performance studies. The site has since grown to include other partners and services that provide performance summaries, BMP evaluation protocols, and other resources. The California Stormwater Quality Association also provides valuable information in four handbooks available at their website (http://www.cabmphandbooks.com/). Each handbook is sector-specific and focuses on new development/redevelopment, construction, industrial/commercial or municipal stormwater projects. A study by Weiss et al. (2007) also is a valuable planning tool and provides an evaluation of the cost and effectiveness of contaminant removal (total suspended sediment and phosphorus) for dry extended detention basins, wet basins, sand filters, constructed wetlands, bioretention filters, and infiltration trenches. Another study by Sample et al., (2003) presents new methods for evaluating stormwater controls and BMPs within a land development context. Fifield (2004) also provides a considerable number of designs, with information for cost estimation and assessing effectiveness.

To complement these various references, a survey was conducted. The survey was conducted via the internet between the months of September and December 2007, receiving voluntary responses from stormwater BMP users and manufacturers across various sectors and geographic locations throughout the US. The study was developed to acquire information regarding expected versus actual costs, effectiveness of pollutant removal, and situational applicability of structural BMPs from the perspective of industry practitioners. Technologies including infiltration basins and trenches, vegetated swales or buffers, porous surfaces, and media filters were highlighted in the survey, while information regarding the effectiveness of other technologies was solicited as well.

Survey Respondents
A combined total of 189 complete responses were received from users and manufactures of stormwater technologies. A majority of respondents were BMP users, with 40% from local governments, 53% from consulting firms and 8% from universities and other respondents. User responses were solicited in five categories that asked if they had installed any or all of the following types of BMPs: vegetated swales, infiltration basins, porous pavements, media filters or other. A breakdown of responses for each of these categories is shown in Table 1.

The experience that each user had with stormwater BMP systems varied greatly, with respondents reporting to have installed between 1 and 500 systems. Among all technology types, the average number of systems installed was 27. Figure 1 shows the average number of installed vegetated swales, infiltration basins, porous pavement, media filters and “other” technologies

Results From Stormwater BMP Technology Users
Users of stormwater BMPs responded to questions about BMP installation locations, functional characteristics of the BMPs such as maximum flow and removal efficiency, and expected versus actual cost for capital, installation, and maintenance of vegetated swales, infiltration basins, porous pavements, media filters and other technologies. It is important to note that the technologies included in the “other” category varied widely and summary statistics for this category include information from the following technologies:

Baffle boxes
Bioretention cells
Catch basin inserts
Catch basin screens
Concrete washout systems
Oil skimmers
Downspout filters
Dry ponds
Dry wells
Flow-through boxes
Green roofs
Hydrodynamic separators
Inlet inserts
Manufactured treatment devices
Oil water and grit separators
Rock check dams
Silt fences
Sediment logs and wattles
Stormceptor
Swirl separators
Underground detention
Water treatment systems with sand filter

Site Characteristics
To understand the situational characteristics of locations where various stormwater BMPs were installed, users were asked about the climate, precipitation, level of development, and severity of erosion at the BMP installation locations. Climate information was solicited with the question, “what regime best characterizes the climate in the area of use? (arid, semi-arid, temperate, sub-tropical, or tropical.)” A majority of respondents reported installing systems in temperate or semi-arid climates, with fewer respondents reporting installing systems in arid, sub-tropical, or tropical climates (Figure 2). The large number of respondents installing systems in temperate or semi-arid climates may result from the greater level of development in these climates compared to arid, sub-tropical, and tropical climates.

To gauge the intensity of precipitation, erosion, and development at BMP sites, users were asked, “On a scale of 1 to 5 please rate the relative intensity of precipitation, erosion, and development at the site where the technology was installed. (1= minimal; 5= severe).” Responses suggest that most stormwater BMPs are installed in locations that are relatively well developed, with porous pavement and media filters installed in more developed areas, and infiltration basins and vegetated swales installed in slightly less developed areas (Figure 3). Average reported precipitation intensity ranged from 3 to 3.75 on a scale of 1 to 5, with porous pavement users reporting the highest precipitation intensity. The average reported intensity of erosion was not significantly different between technology types, with an average of 2.5 on a scale of 1 to 5. It was interesting to note that there was little variability in the various intensities, across all BMPs, as indicated by the error bars.

A series of three questions about stormwater BMP characteristics were asked to identify trends in treatment/flow capacity, perceived removal effectiveness and required maintenance. First, respondents were asked, “What is the technology’s maximum flow interception capability? Please specify units.” Although many respondents did not address the question, the responses that were received were converted to m³/s for comparison. Note that 1 m³/s is equal to 15,850 gal/min or 70 acre-feet/day. The reported minimum, average and maximum flow capacities for vegetated swales, infiltration basins, and media filters are shown in Table 2. Information about porous pavement was not considered because of the low response rate. Results indicate that among the BMP users that responded to this question, media filters generally had the lowest maximum interception capacity with an average of 0.31 m³/s and infiltration basins had the highest interception capacity with an average of 0.41 m³/s.

To understand the removal efficiency of these constituents by various BMPs, the following question was posed to survey respondents, “On a scale of 0-5 Please indicate the removal efficiency for each of the constituents below. (0= the technology does not address this constituent, 5=complete removal.)” Responses vary greatly by technology and constituent and are shown in Figure 4. According to survey respondents, porous pavement and media filters performed best for most of the constituents, except trash (for porous pavement) and bacteria, metals and nutrients (for media filters). In most cases the removal efficiency observed by these respondents was good to very good, scoring between 3 and 4. In general, vegetated swales were seen as least efficient, even for sediments and nutrients.

Results from the question, “On average, how frequently do you perform maintenance on the technology named above? (More than once a month, once a month, once every 6 months, once a year, less than once a year)” showed that the most common maintenance interval among all technologies is either once every six months or once a year (Figure 5). The vegetated swales had the largest distribution of maintenance intervals, with a skew towards more frequent maintenance. On the other hand, porous pavement had the smallest distribution of maintenance intervals, falling mostly within the 6 months or 1 year interval.

BMP Cost
Structural BMP implementation cost depends upon the size, location, and the design selected. As opposed to surveying BMP users about their total costs, users were asked about how much the BMP project costs varied from what they had anticipated using the following question: “For this technology how different were actual capital, installation, and maintenance costs from estimated capital, installation, and maintenance costs? (much less than expected, somewhat less than expected, same as expected, somewhat more than expected, much more than expected, or not sure).”

A summary of responses is shown in Figure 6 and indicate that vegetated swale projects are generally more likely to have capital and installation costs that are similar to what was anticipated, whereas infiltration basins and porous pavement, media filter and other projects have shown more variation in anticipated versus actual costs for capital and installation. In general the variation is towards higher expenses than expected, across all three cost categories. These results indicate that vegetated swale projects are generally most likely to stay on budget for capital and installation in comparison to other types of projects.

Porous pavement projects most frequently stayed on budget for maintenance, followed by vegetated swales, other technologies, infiltration basins, and media filters. It is interesting to note that media filter maintenance was “somewhat more than expected” in 41% of projects reported, indicating that maintenance budgets for this technology are frequently underestimated.

Stormwater Technology Manufacturer Survey
The manufacturers that responded to the survey represent various types of stormwater technologies including: (Table 3)

The majority of the stormwater technologies (78%) addressed in the manufacturer’s survey are intended to mitigate only stormwater as opposed to both stormwater and combined storm and municipal sewers (CSO) which represented 22% of respondents.

Site Characteristics
Manufacturers were asked to provide information regarding characteristics that need to be considered for selecting various stormwater technologies, including climate, soil type and slope. The manufacturers’ responses indicate that their technologies are suitable for most climates with a slight trend showing that more technologies are suitable for dry arid climates than for wet tropical climates. At least 80% of the respondents said that the technologies are suitable for all of the climates in the survey (Table 4).

The manufacturers indicated that most of the stormwater technologies are suitable for all of the soil types with the highest proportion of respondents indicating that the technologies are suitable for somewhat erodible and moderately erodible soils (Table 5 and Figure 7). Some of the respondents indicated that the question was not applicable, reflecting technologies that are not installed directly into soils, such as drain inserts. The respondents indicated that most of the stormwater technologies are suitable for areas with moderate or slight slopes, while areas with steep slopes were the least suitable for stormwater technology application (Table 6 and Figure 8).

Based on the responses to the three site characteristics (climate, soil stability and slope) it is interesting to note that manufacturers considered that climate was much less of a constraint for technology selection than soil stability or slope. Given the small number of responses by type of technology, the survey results cannot be expressed in terms of specific technologies, but the overall result is that it is important to consider soil stability and slope when selecting a BMP.

BMP Performance and Cost
Manufacturers were also asked to provide information with regards to removal efficiency for several common pollutants, maintenance intervals, flow rates and costs. With regards to pollutant removal efficiency, the manufacturers were asked to report on a scale of 1 to 5, where 1 was poor and 5 was excellent. They could also choose not to answer, if insufficient data was available or their technology was not targeted to a particular pollutant. There is a wide range of responses (Table 7), from technologies that deal well with many pollutants, to those that are only useful for a small range. Although an “average” removal efficiency is not very meaningful, the information in Figure 9 serves to highlight that certain pollutants can be targeted with many technologies with high removal efficiency, while metals, organics, bacteria and nutrients tend to be more challenging.

Recommended maintenance frequencies vary significantly (Table 8). The manufacturers indicated that the most common maintenance frequency for the stormwater technologies is once a year (Figure 10), which is somewhat longer than experienced by the users, but of course this is very technology dependent.

Given the wide diversity of technologies represented, it is not possible to make a meaningful comparison of average or maximum flow rates. In addition, the cost of these systems is quite dependent on the specific site conditions. Nevertheless, it is useful to note that on average the manufacturers expect their technologies to be handling on the order of 0.14 m³/s (~2,200 gal/min) per unit system, with a small variance among technologies (Figure 11). The technologies can handle up to 1.2 m³/s on average, but there is a wide variation in the maximum flow rates that can be handled. A future survey could evaluate the relationships between flow rate, cost, and performance, which are very important.

The average capital, installation, and maintenance costs vary from less than $10,000 to over $40,000. Capital costs were the highest with installation and maintenance costs second and third respectively. There is also a large range in capital and installation costs depending on the type of stormwater technology used (Figure 12).

Conclusions
The responses from the users indicated that most of the experiences are in temperate climates. However, there was no significant difference in use when considering precipitation, erosion or development intensity. Based on the users’ perspective, the vegetated swales appeared to have the lowest removal efficiency for the seven classes of pollutants considered, while they reported that the porous pavement had the highest removal efficiency. Since the number of user responses was very low for porous pavement, this may not reflect the experiences of other users. Infiltration basins were slightly better rated than media filters for the removal of most classes of pollutants.

Based on the information from the manufacturers, most of their systems are applicable in any climate, and soil type is not typically a deciding factor. Most technologies can be applied in a wide range of slopes, although they are more applicable in flat to moderate slopes than in steep ones. The removal efficiency reported by the manufacturers for the various BMPs had a much wider range than the range reported by the users, which is perhaps based on practical experience.

Most users reported maintenance intervals of between six and twelve months across all the various BMPs. This is in contrast with the manufacturers, which generally recommended a maintenance interval of around 1 year. From the users’ perspective, the cost of vegetated swales was closest to their initial expectations, while the cost of the other three classes of technologies tended to be leaning towards somewhat higher than expected. The manufacturer’s information indicated a wide range of costs, due to the large variance in flow and capacity among the systems discussed in the survey.

This information should provide useful guidance to potential users of stormwater BMPs. As seen, some design characteristics are not very critical, while other characteristics play a more important role in the decision-making process, such as removal efficiency and cost.

Jenny Phillips, Reni Keane-Dengel, and Arturo A. Keller are with the Bren School of Environmental Science and Management at the University of California, Santa Barbara.

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