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The National Pollutant Discharge Elimination System permit program has played an important role in helping to keep nonpoint source pollution from entering our waterways. Developers are required by law to obtain water quality permits for new development, including retrofit projects. There have been a number of policy changes over the years under this program as the science of storm water has advanced. Now the time has come once again to advance storm water public policy, as new research is exposing a key consideration in storm water design.
It is essential that storm water designs prevent removed pollutants from being flushed out from manufactured treatment devices and resuspended downstream during larger storm events. A handful of third-party research initiatives have begun to shine light on this issue. The research highlights distinct differences in storm water treatment devices’ ability to prevent the resuspension of pollutants.
When one compares the results of these studies with what is happening today in many parts of the country, it is evident that there are several storm water treatment devices being approved and installed in the field well beyond their performance capabilities. The big loser in this debate is North America’s precious waterways, which unnecessarily are receiving additional tons of polluted sediment every time it rains.
Resuspension Fact-Finding
As one example, research studies at the University of Florida Department of Environmental Engineering Sciences tested and compared two popular full-scale manufactured storm water treatment devices. The results clearly demonstrate significant performance differences between the two technologies when comparing resuspension capabilities.
A common hydrodynamic separator with a screen as part of its design released shockingly high levels of coarse sediment as its hydraulic loading rate was raised. This phenomenon often is referred to as “resuspension” or “scour.” Significant resuspension was measured even when sand-sized particles were preloaded into the sump at 50% of the recommended maintenance level and at hydraulic loadings as low as 100% of the typical operating rate.
The study went on to state that frequent maintenance and cleaning of the device would be required for successful performance. This means that the treatment system would be an unrealistic storm water solution for the majority of existing land owners and developers—and their budgets.
A different type of unit—the Stormceptor—experienced virtually no sediment release, even with fine particles (fine silts and sand) preloaded into the unit at 100% of the recommended maintenance level and at hydraulic loading rates representing 500% of the typical operating rate.
Rethinking Testing Protocols
Some systems appear to perform properly in “clean-unit” laboratory testing where no sediment is preloaded into the sump. The same units will fail, however, under the real-world conditions experienced in the field.
Until recently, few regulators wanted to address this issue through policy changes. Their laissez-faire attitude approaches a general thought process of “if it has a bypass, then the device must prevent resuspension.” As a result, manufacturers’ marketing claims and outdated storm water policies override the need to update regulatory requirements based on sound science, third-party research and unbiased data.
The vast majority of pollutant removal testing is conducted on “clean” units. While this makes it easier to measure what was removed initially from runoff water, in real applications these units contain captured solids removed during previous rain events.
Testing standards and design requirements are beginning to change to better reflect performance capabilities and the realities of the field. For example, the New Jersey Department of Environmental Protection (NJDEP), which oversees the Technology Acceptance and Reciprocity Partnership program, recently introduced a new resus-pension testing protocol. Without conducting this test, devices must be installed off line to minimize resuspension. NJDEP has gone a step further and required third-party testing, as the majority of testing results to date have been conducted by the manufacturers themselves
Critical Design & Performance Considerations
Factoring resuspension and pollutant release into storm water designs is critical in order to prevent pollutants from being released into downstream water bodies. In the case of pretreatment in a treatment system or treatment train, resuspension can lead to premature maintenance requirements and/or the failure of downstream best management practices (BMPs)—commonly filtering and infiltration practices.
The storm water design and regulatory community historically has focused on conveyance and moving storm water as quickly and efficiently as possible from point A to point B. Water quality treatment requires a different mind-set because treatment flow rates need to be well defined. What particle size distribution (PSD) are you trying to remove? Have you properly designed your system to prevent pollutant resuspension?
Accepting a treatment flow rate without having all the critical design and performance parameters defined and accounted for can be detrimental and result in states falling far short of water quality goals. It should not be surprising if field testing results demonstrate poor performance when total suspended solids and resuspension have not been factored properly into storm water treatment designs.
Examples of tools developed to assist the design community can be found in Dupage County, Ill., where officials have developed a checklist to ensure important design parameters are being factored into the county’s storm water designs (visit www.dupageco.org/dec/).
The Center for Watershed Protection also has recognized the significant need for coupling performance and design of proprietary BMPs with treatment flow rates and a clearly defined PSD (Tool No. 8: “BMP Performance Verification and Checklist”).
The Challenges Ahead
We may not be able to rely on maintenance, at least for the short term. “It’s hard to attain rigorous enforcement and keep systems routinely monitored,” said one storm water industry insider who wished to remain anonymous. “Our group’s budget and resources have been cash-starved for several years. We hope that’s changing, but with state budgets in flux, it will take time.”
Although research demonstrates that all storm water treatment systems are not created equal, engineering and storm water design professionals often do not have the time to dig into the various treatment options. There are some simple things the design community can request, including third-party testing, to back up manufacturers’ marketing and performance claims.
Where focus and action is needed most is on the regulatory side. The majority of regulations need to be tightened if these issues are going to be dealt with properly and storm water quality objectives met. Without holding storm water treatment devices to a rigorous standard that confirms their claims with full-scale lab test data, environmental damage to our water bodies will continue.
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