Cities Need To Test New Stormwater Systems

By James H. Lenhart

As the need for efficient stormwater treatment grows, there will be an increasing number of commercial filtration systems on the market. As part of their due diligence, utilities will need to evaluate the systems to ensure they meet basic criteria. If the facility meets these criteria, a pilot project should be conducted to evaluate the facility in the field.

To perform a successful evaluation of stormwater filtration systems, the following categories have been developed:

• System Hydraulics
• Media Hydraulics
• Media Type
• Structural Considerations
• Maintenance Considerations
• Ongoing Support
• Additional Considerations

System Hydraulics

To evaluate the simple hydraulics of water flow through the system, consider these factors:

1. Evaluate the hydraulic grade line at the design flow rate. Typically a backwater calculation from the point of downstream control should be performed to ensure the system can convey the peak design flow. This analysis should include media losses, pipe entrance, exit, and barrel losses.
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, what velocity does the inlet pipe discharge onto the filter bay?

Media Hydraulics

This is a key factor of filtration that is least understood. The assessment includes:

1. Evaluate the specific flow rate (q) through the media. The specific flow rate is in units of gallons per minute per square foot (gpm/ft²). Given the specific flow rate times the surface area (A) of the filter the total flow rate (Q) can be calculated (Q=qA). A good reference point is rapid sand filtration with rates of about 4 gpm/ft². In general, the higher the rate, the higher the head loss. Finer media is typically more efficient for Total Suspended Solids (TSS) (and other pollutants) removal but has high head loss characteristics. Coarser media can handle higher flow rates but are less efficient in TSS removal.
2. Compare the design-specific flow rate to specific flow rates in lab and field studies. Does the specific flow rate of the proposed design match the specific flow rate associated with performance data. For example does performance data have a rate of 2.0 gpm/ft² while the design rate is 20.0 gpm/ft²? For proper filter design this an absolute factor. Also evaluate different model sizes of the proposed BMP. Given the same driving head, the specific flow rate should be the same. If the specific flow rates vary with model size, this should raise questions.
3. Consider the thickness/head loss of the media. The thickness of the media coupled with the specific flow rate determines the amount of contact time the water has to be treated. The longer the contact time the more effective pollutant removal will be, particularly when soluble pollutants are being removed though reactive processes. Thicker media will have higher removal rates but will increase head loss. Thicker media will also increase media costs and maintenance costs. For porous media such as perlite, TSS removal efficiency increases with thickness since there is more opportunity for particles to be captured as the water follows a tortuous path though interstitial pores. For fine media such as sand, the majority of TSS capture is at the surface and media thickness has less influence on TSS removal.

4. Contact time should be calculated. For example if the flow rate is doubled and the thickness is reduced by half, the contact time is one fourth. This will have a direct impact on pollutant removal effectiveness. It is important to check the contact time of the design versus the test data presented.
5. Fouling and occlusion of media will ultimately control the specific flow rate through the media. One critical aspect is the surface of the filter. Filtration media that is effective at trapping fine solids will accumulate a thin layer on its surface. Thicknesses as little as 1 mm can control the filtration rate and can reduce the specific flow rate to very small amounts, i.e. < 0.1 gpm/ft². Without cleaning mechanisms to prevent surface clogging, this problem becomes endemic to all filtration systems.
6. Horizontal Bed vs. Vertical Filters. Horizontal bed filters operate by ponding water on their surface. The driving head causes the water to percolate through the media. 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. However, horizontal bed filters have the benefit of a constant pressure head (equivalent to the depth of water) over the entire filter surface while vertical filters have a reduced driving head (equivalent to _ the depth of water) due to a triangular pressure distribution.

Media Type

1. What are the physical properties of the media for sediment removal? Most 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 create higher head loss and have higher clogging factors. This tradeoff is a fundamental consideration.
2. What chemical properties and mechanisms remove stormwater pollutants? Many types of pollutants such as nutrients, metals, oils and greases are soluble and can be removed through chemical and/or biological processes including cation exchange, precipitation, chelation, adsorption, etc.

When claims are made for soluble pollutants, there needs to be a documented process by which these reactions take place. 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 compared to what is generated from the site. Reaction kinetics also cause a slowing of pollutant removal rates as media saturation increases and pollutant concentration decreases.

The reviewer also should consider if the media adds constituents to runoff. For example, organic media such as leaf compost can elevate ortho-phosphorus. In some watersheds this is not a desirable media and an alternate should be selected.

1. Do the properties of the media change over time? Stormwater is a complex mixture of sediments, nutrients, organic matter, bacteria, etc. Many times a media may perform well short term, but long term may be compromised by biological decomposition, bacterial slimes, or simple decomposition by continuous saturation in water. Cellulose based media such as corn cobs, rice hulls, etc. can decompose when exposed to these elements.
2. Proprietary media and multiple media need to be evaluated. There are concerns that systems using proprietary sole source media may complicate long-term maintenance. Systems that can facilitate multiple media options, i.e. both proprietary and non proprietary, offer clear advantages in that the owner is not "locked up" with a single media. Additionally multiple media systems have the versatility to tune different media to site specific pollutants.

Structural Considerations

1. Structural integrity is critical. Make sure the structures are reviewed by structural engineers to ensure expected 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. Buoyancy measures need to be considered. In areas of high groundwater measures should be taken to prevent system floatation.
4. Constructability considerations are important. Many times what is simple on the plans is difficult during construction. Consider construction loads, back filling, etc. Is there a track record for constructing and installing facilities?

Maintenance Considerations

1. Maintenance contracting is critical. It is the nature of filters to occlude with captured TSS, hence maintenance is required. Acceptance of all systems should be with a maintenance contract by a bona fide maintenance provider.
2. Maintenance frequency varies from site to site. Typically, conducting maintenance more frequently than yearly costs more over the project life cycle than upsizing the facility. Conversely, maintenance scheduled for periods greater than one-year result in high first costs that increase life cycle costs.
3. Maintenance costs are critical. If a contractor states a cost, ask if they will sign a contract. Frequently, costs are underestimated as they do not include mobilization, heavy equipment rental, and mileage costs. Consult with a local maintenance provider when in doubt.
4. Facility access will always be needed. Even manholes are equipped for access. Facilities will need to be accessed for cleaning of media, washing of sidewalls, repairs, media installation, and facility inspection. Review plans for height restrictions, ventilation and extraction ports. Make sure the facility also is accessible by the required equipment.
5. Check the weight of the media. How is the media being extracted? How much would a media "vessel" that is full of sediment and has a high water content weigh. Is it practical that it be removed or lifted. As a rule of thumb use a minimum of 70 lbs/ft³ for a lightweight media, such as perlite. (Perlite weighs about 5 lbs/ft³ when fresh.

Product Support

1. Does the manufacturer warrant the product? Typically there is a one year warranty on the structure and components.
2. Does the manufacturer provide support to the owner? Filtration systems are more effective than simple settling or vortexing devices and do require media replacement. It is important that the manufacturer supply on-going, long-term support to ensure proper operation.

Other Considerations

1. Check references, and speak with other agencies where the facilities have been used. Use the scope of the information to establish credibility. Remember, because of the variable nature of stormwater runoff, all types of facilities will have examples of poor performance. This includes ponds, swales, filters, settling devices, etc. but overall the assessment should be positive.
2. Integrate all of the considerations above 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, upsizing of the facility to the test values is warranted.
3. Are data presented consistent with pollutant concentrations and characteristics typically associated with stormwater runoff? Common claims include:

• 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 & 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. Does the data presented reflect the reality of what typically is found in the field?
• Performing studies conducted at a fraction of the design flow rate. (i.e. if a filter is designed to a high specific flow rate it should be tested at that rate.

Conclusion

These factors are meant to serve as a guideline for the preliminary review of new products. If this review meets the satisfaction of the reviewer(s) the next consideration should be pilot facilities which lead to system acceptance. If a submittal does not appear to met the criteria listed above, the design engineer or reviewer needs to seek clarification or redesign prior to setting up a pilot program.

About the Author

James H. Lenhart, P.E., is the Vice President of Engineering and Research of Stormwater Management Inc. He is a nationally recognized expert in stormwater management technologies and has over 12 years experience in consulting engineering and research and development.