February 19, 2025
Passing the Test
The stormwater sector in the US is highly regulated, and these regulations are what help to ensure that our surface water is appropriately treated to remove damaging pollutants before they are able to reach the environment.
In implementing these regulations, some states have developed or adopted testing protocols in order to ensure that manufactured treatment devices (MTDs) and other stormwater best management practices (BMPs) meet performance standards. Systems and technologies that verifiably meet these standards are certified or approved and thereby made available to engineers.
Prominent testing frameworks include New Jersey’s NJCAT and Washington’s TAPE programs, with many jurisdictions around the country also accepting certifications from these programs.
The great strength of these testing protocols is that they level the playing field, create consistency and provide engineers with a list of options that meet performance standards. However, in addition to that strength there is also an opportunity.
In order to create consistency and a level playing field, the tests typically require systems to demonstrate that they remove a certain percentage of a particular pollutant; all systems that pass the test achieve that percentage removal rate by definition, so the only differentiation between them that engineers can make is in the maximum treatment flow rate (MTFR) of the system. As an example, one system might remove 50% of total suspended solids (TSS) at an MTFR of 5 cfs, while another system might remove 50% at 6 cfs.
This can be helpful, but what is not always well understood is that the MTFR and its associated removal rate is not a simple number; rather it is a weighted aggregate of different removal rates at a range of different flows, designed to provide an overall average removal rate over the course of a typical year of annual precipitation.
For example, a system might remove 70% of TSS at 10% of MTFR and remove 30% of TSS at 150% of MTFR, with a range of other removal rates for the other different flow rates in between. Overall, with different weightings given to the different flow rates, that might equate to an aggregate removal rate of 52% at the given MTFR.
In this way, the test accounts for seasonal variations in rainfall, ensuring that the system provides greater protection from the more frequent smaller storms that generate the bulk of the pollutant load, while also providing protection from the less frequent but more intense storms. In other words, the test attempts to simulate the flows that a system might encounter in the real world.
While testing protocols such as these are a good approximation of real-world conditions, they are necessarily an abstraction, however, and a range of confounding environmental factors can affect performance, including temperature, influent particle size and the hydraulic characteristics of the surrounding network.
Under these conditions stormwater treatment systems don’t always operate predictably. For example, stormwater samples tested at our Portland, ME hydraulics lab has indicated that for the Up-Flo® Filter, higher pollutant loading rates correlate with higher percent removals: as the concentration of TSS increases, so does the percent removal.
Our new Hydro-Shield™ Advance hydrodynamic separator has been recently certified by the New Jersey Corporation for Advanced Technology and achieved the protocol’s 50% TSS removal rate with the highest flow rate of any system under its newest test protocol, and while I’m delighted that we’ve developed the leading stormwater separator on the market, I know that the system’s truest test will be under the conditions that it will face in the field.
And of course we should keep in mind the non-performance aspects of systems such as these. For example, smaller footprints can mean that installation is easier, cheaper and—most importantly—safer for contractors, while improved maintenance access can help to ensure that systems continue to perform as designed long after they’ve been installed. After all, test results doesn’t mean much if a system has been left to clog up.
Therein lies the opportunity. The test results indicate which companies have the technical expertise to design a system that performs, but this should be the start of the conversation rather than the end. All stormwater separators remove TSS, but which are optimized for maintenance? Which are quick and easy to install? Which are made sustainably? Looking beyond the system itself, which companies provide reliable technical information, design support and tools? Finally, which companies provide reasonable lead time delivery and installation support after the sale is made? All these things must be considered for the design engineer to make the right choice for the project owner.
Testing protocols such as NJCAT and TAPE are necessary and valuable, and they provide the consistency and minimum performance levels that we need to protect our environment from stormwater pollution. However, if engineers focus overly on the results in the lab then they run the risk of underemphasizing the way a system will perform where it really matters: in the ground.
I would urge specifying engineers to use approvals, verifications and certifications as an initial menu of options, but then to partner with providers that go beyond the test to deliver the real-world performance and practical considerations that ultimately improve design, installation and long-term stormwater management.
