Evaluation of Media-Based Stormwater Filtration Systems

6. Consider fouling and occlusion of the media. Fouling is a general failure at the surface of the filter caused by the growth of biological films in association with accumulations fines, organics and nutrients. Occlusion is more associated with the progressive accumulation of solids which reduces and ultimately stops flow through the filter. Occlusion can be at the surfaces or a result of filling the pores of a depth filter. This will ultimately control the specific flow rate through the media. One critical aspect is the texture of the surface of the filter. In general, finer media can remove finer particles but have a much higher clogging factor (see Photo 1). Sand can retain about 1.1 pounds of dry sediment per cubic foot before it rapidly clogs and fails (Lenhart and Calvert, 2007), whereas a coarser perlite media can retain about 6 pounds of sediment per cubic foot.

Filtration media that is effective in trapping fine solids will accumulate a thin layer of solids on its surface (schmutzdecke) that begins to occlude the filter and reduce the flow rate. An accumulated layer as thin as 1 millimeter will control the filtration rate and reduce the specific flow rate to very small amounts (< 0.1 gpm/ft2). For example, a horizontal sand filter that builds up a schmutzdecke demonstrates this. Without cleaning mechanisms to prevent surface clogging, this problem becomes endemic to all filtration systems. The following are two important aspects to consider regarding the filter surface:

• Know and examine the percent open area of the outer wall of the filter vessel, which includes the filter body and any internal mesh (see Photo 2). The lower the percent open area, the more susceptible the housing is to surface clogging and failure. If the surface of the filter is exposed to light, algal growth can rapidly clog the filter surface.

• Understand the capability of the technology to prevent surface fouling by sediments. It is a fact that effective filters will eventually clog with sediments; the only question is how long that will take. Clearly, if a filter has no active mechanism to remove accumulated sediment from a filter surface, its life will be less than a filter that does.

Pretreatment by settling will help, but research indicates that most fouling is by fine sediments, organic matter and bacterial growth, all of which are difficult to remove by pretreatment employing rapid settling. In fact, research at Monash University in Melbourne, Australia, (Siriwardene et al, 2005) indicates that particles of 20 µm or smaller cause clogging of sand filters.

7. Compare a horizontal bed with vertical filters. A horizontal bed filter operates by ponding water on its surface such that the driving head causes the water to percolate through the media (see Figure 1). Depending on how it’s designed, the flow through the media may be saturated or unsaturated flow.

One characteristic of horizontal bed filters is that all collected sediments will impact the surface of the filter, thus reducing longevity, whereas vertical filters allow much of the sediment to be deposited on the floor, away from the filter media. Horizontal bed filters have the benefit of a constant pressure head (equivalent to the depth of water) over the entire filter surface. Vertical filters have a reduced driving head (equivalent to half the depth of water; see Figure 2).

Media Type

A variety of different filter media have a long track record of successful use for stormwater filtration, including sand, peat and compost. Recently, perlite, zeolite, carbon and other “exotic” media have expanded the choices for targeting specific pollutants.

Consider the physical properties of the media used for sediment removal. Most filter media remove solids by mechanical processes. The gradation of the media, irregularity of shape, porosity and surface roughness characteristics all influence TSS removal characteristics. Finer media are more effective at removing TSS than coarse media, but they create higher head loss and have higher clogging factors. This tradeoff is a fundamental consideration. Finer media with a less uniform gradation also typically results in a lower flow rate at the same amount of driving head.

Understand the chemical properties and mechanisms used to remove stormwater pollutants. Many types of pollutants such as nutrients, metals, and oil and grease are insoluble or free-form, and they can be removed through chemical and/or biological processes. Common processes are cation exchange, precipitation, chelation and adsorption. When claims are made for soluble pollutants, there needs to be a documented process by which these reactions take place. In addition, these reactions have limits in terms of sorption capacity and reaction kinetics. For example, media may have a sorption capacity of “X” mg/kg of media. Given the mass of the media, the total mass of pollutant that can be removed can be calculated and then compared with what’s generated from the site. Reaction kinetics also cause a slowing of pollutant removal rates as media saturation increases and/or pollutant concentration decreases.

The reviewer also should consider if the media can add constituents to runoff. For example, organic media can elevate ortho-phosphorus in filter effluent as it leaches from the media. In nutrient-limited watersheds, this is undesirable, and an alternate should be selected. Other media can raise or lower pH. In all cases, media should be tested for aquatic toxicity and rejected if shown to leach chemicals or degrade in forms or quantities that impair downstream biota.

Evaluate whether the properties of the media will change over time. Stormwater is a complex mixture of sediments, nutrients, organic matter, bacteria and other pollutants. Many times, media may perform well in the short-term, but in the long-term may be compromised by biological decomposition, bacterial slimes or simple decomposition by continuous saturation in water (see Photo 3).

For example, cellulose-based media such as treated pulp, corn cobs or rice hulls will decompose when exposed to these elements. Media that swells or shrinks on wetting and drying cycles also can be problematic because it changes the bed properties. Swelling media can prevent design hydraulic loading rates from being reached and can damage media containment equipment. Shrinking media can open up preferential pathways in the media bed and lead to short-circuiting. Filter media should be free draining. Permanently submerged media can lead to anoxic conditions, causing anaerobic decomposition and release of many of the trapped pollutants.

Check media availability, cost and quality. Media that is only available in small production quantities or does not meet all specifications should be avoided. Some systems can accommodate multiple-media options either mixed or layered within the same bed or as alternative bed materials. Variable-media systems have the versatility to fine-tune media to site-specific pollutants as well as adapt to future improvements in media effectiveness. The media provider should be able to demonstrate media QA/QC history to know the product is meeting specification (Hills, 2017).

Structural Considerations

Structural integrity is critical. Most subsurface stormwater filtration systems are designed to handle traffic loads, so it’s important to evaluate the structure for integrity and design life. Make sure the structures are reviewed by structural engineers to ensure expected traffic loads can be handled.

Water tightness is required by many agencies. Evaluate vertical and horizontal joints for design integrity. Vertical joints are more difficult to control because of differential settlement. Some agencies require a water-tightness test prior to acceptance. All joints below the permanent pool elevation need to be watertight.

Buoyancy measures need to be considered. In areas of high groundwater, take measures to prevent system floatation.

Constructability considerations are important. Many times what appears simple on the plans can be difficult during construction. Consider construction loads, backfilling, etc. Does the contractor have a track record of constructing and installing similar facilities?

Consider materials of construction. Stormwater runoff can be very corrosive. The presence of numerous nuts and bolts, differential metals contact, pivot points and hinges, and galvanized parts are all potential candidates for corrosion and ultimate failure.

Maintenance Considerations

Proper operation and maintenance is critical to success and should be considered when evaluating stormwater filtration systems.

Product Support

Does the manufacturer warranty the product? Typically there’s a minimum one-year constructability warranty on the structure and components.

Does the manufacturer provide support to the installing contractor or owner? Filtration systems are more effective and more complicated than simple gravity settling or vortex separation devices. It’s important that, if requested, the manufacturer be able to supply the contractor with installation support for complex structures. Furthermore, the manufacturer should be prepared to offer ongoing, long-term owner support to ensure proper operation and maintenance.

Other Considerations

To establish credibility of performance, installation, operation or maintenance claims, it’s wise to check references and speak with other agencies where the facilities have been installed. Remember, because of the variable nature of stormwater runoff, all types of facilities—ponds, swales, filters, settling devices and others—will have examples of poor performance, but overall the assessment should be positive. Checking manufacturers’ claims is critical. A large number of reports do not necessarily imply a system is well tested or verified.

Integrate all the considerations into an overall assessment of how a proposed system matches performance data from prototypes. This is a critical review. If the design flow rates, media thickness, etc., do not match the studies, increasing the size of the facility to match the test values is warranted.

Make sure the data presented are consistent with pollutant concentrations and characteristics typically associated with stormwater runoff. Common claims that illustrate inconsistencies include the following:

• Performing a study with sand or grit and equating the percent removal to the removal of silts and clays in the field.

• Performing a study with very high concentrations of oil in water to assess oil and grease (O&G) removal and then using these data to claim high percentage removals on stormwater runoff, which has much lower concentrations of O&G.

• Performing studies conducted at a fraction of the design flow rate. (If a filter is designed with a high specific flow rate, it should be tested at that rate.)

Evaluate lab data vs. field data. Lab data are collected under controlled conditions and can provide a lot of insight on filter behavior. Studies successfully executed at the laboratory level include analysis of TSS removal at different flow rates and hydraulic behavior. Properties that can’t be evaluated in the laboratory include fouling characteristics, maintainability, and pollutant removal characteristics with complex hydrology and water chemistry. Laboratory data provides rapid data collection and good insight to filter performance but should not be used as a sole method to judge performance.

Programs for Filtration System Verification

New Jersey and Washington have implemented statewide programs to evaluate filtration technologies (and other technologies) to ensure that the technology can meet the water-quality objectives. The programs, known as the NJCAT and TAPE processes, are well documented and available on the internet.

In short, these programs outline monitoring protocols, treatment goals, reporting standards and other criteria that all new BMPs must be subjected to for final verification and approvals.

Conclusion

These factors are meant to serve as guidelines for the preliminary review of new products. If this review meets the satisfaction of the reviewer(s), the next consideration should be pilot facilities that lead to system acceptance. If a submittal does not appear to meet the aforementioned criteria, the design engineer or reviewer needs to seek clarification or redesign.

REFERENCES

• Siriwardene, N.R., A. Deletic, and T.D. Fletcher, 2006. “Preliminary studies of the development of a clogging prediction method for stormwater infiltration systems,” Proceedings, 4th International Conference on Water Sensitive Urban Design, Melbourne, Australia, April, V1.211-V1.218.

• Lenhart, James H., P.E., and Paula P. Calvert, 2007. “Mass loading and mass load design of stormwater filtration systems,” Environmental Water Resources Institute, Proceedings, April 2007.

• M. Hills, A. Macleod, J. Lenhart, 2016. “Ensuring bioretention system performance success: Guidance for the verification of bioretention media via quality assurance and control testing,” World Environmental and Water Resources Congress 2016, pp. 364-370.

Authors

James H. Lenhart, P.E., D.WRE

Jim Lenhart is the Chief Technology Officer for Contech Engineered Solutions. He was founder of Stormwater Management Inc. and was also owner of Stormwater Northwest, where he consulted with companies that provide products and services in stormwater-related markets. Lenhart has authored more than 50 papers on the subject of water quality and stormwater treatment. He is a professional Agricultural and Environmental Engineer with more than 25 years of experience in consulting engineering, and research and development. He served as an adjunct instructor of Civil Engineering at Portland State University, where he taught hydraulics and water resources engineering. Lenhart currently is an active member of the Water Environment Federation, the Water Environment Research Foundation, ASCE EWRI, and serves as Chair of the Urban Water Resources Research Council. Lenhart holds a BS in Plant Sciences, a BS in Agricultural Engineering, an MS in Water Resources Engineering and is a Diplomate of the American Academy of Water Resources Engineers.

Vaikko Allen

Vaikko Allen currently serves as Director of Stormwater Regulatory Management for Contech Engineered Solutions, where he assists regulators, engineers and environmental organizations in the development and implementation of stormwater regulations. Throughout his 20 years of stormwater management experience, he has managed stormwater system monitoring programs and development initiatives, and participated in numerous workgroups, providing technical guidance on TMDL implementation, hydromodification, low-impact development strategies and industrial stormwater compliance. Allen holds a bachelor’s degree in Environmental Science and Policy from the University of Southern Maine with a concentration in Water Resources.

Jeremy Gray

Jeremy Gray is the Filtration Product Manager for Contech, directing the strategy, leadership, and execution of the Jellyfish and StormFilter product lines. Since starting with Contech in 2004, he has held a series of roles with increasing responsibility within Contech’s Stormwater segment. These experiences managing projects across several stormwater markets with varied treatment strategies have given him the vision to assist in the development and design of Contech’s filtration products overall and on hundreds of individual projects. Gray has a B.S. degree in Mechanical Engineering from Tufts University.