Water Works: The Loading Rate Wars May Not Have a Winner

I expect most of us agree that innovation is good, and that sentiment holds true in the field of stormwater management. More than two decades of focus on improving the water quality and reducing the total quantity of stormwater discharges has spawned a substantial amount of innovation in the stormwater control measure (SCM) space. We’ve learned invaluable lessons through trial and error paired with laboratory and field monitoring that allow us to design SCMs to be increasingly effective, and our SCM toolbox now is larger than ever and continues to expand.

Be it green infrastructure, an enhanced sand filter or an innovative underground solution ideal for tight urban sites, we generally have options for any given stormwater job, and, for many SCMs, overall performance has improved with iterative updates to design specifications. However, our quest for cheaper means of complying with stormwater-management standards has the potential to lead us astray if we aren’t mindful of inherent risks.

Filter Feeding

When it comes to water-quality performance, filters aren’t particularly sensitive to hydraulic loading rate. In other words, take any given filter and increase the amount of flow being sent through it and you may only see a modest drop in pollutant-removal efficiency. Obviously, there are limits, but compared with sedimentation systems such as ponds, basins or hydrodynamic separators, whose performance is largely a function of loading rate and residence time, filters are still going to capture most of the target pollutants in runoff even when we push higher flows through them. This seems attractive because a filter that’s treating more water can be smaller, translating directly to a lower cost of compliance.

This trend is playing out in real time within the SCM evaluation and certification programs such as those administered by the New Jersey Department of Environmental Protection (NJDEP) and the Washington Department of Ecology (Ecology). Put simply, the design loading rates for the filters and engineered biofilters being tested and certified under NJDEP’s laboratory protocol and Ecology’s Technology Acceptance Protocol-Ecology (TAPE) field protocol are rising. Filter-media gradations are being optimized to allow filter SCMs to meet applicable removal-efficiency standards at the highest possible loading rate. The trend toward higher loading rates is clear in the laboratory as well as the field and spans the full gamut of filter offerings. However, if we aren’t mindful of how increased loading rates impact the longevity of filters, any potential savings resulting from installation of smaller filters will quickly slip away to pay for frequent maintenance.

All Filters Clog

Increasing the loading rate to any given filter will shorten the interval between required maintenance events. All filters can and will clog at some stage, so understanding their capacity to operate and accumulate pollutants before needing maintenance should be an essential part of their design to ensure they operate for a reasonable amount of time in the field before needing maintenance. Ignoring filter longevity creates a “save now but pay later” scenario.

Part of the challenge is that our current monitoring protocols don’t provide great insight into how long a particular technology is going to last on average in the field before needing maintenance. The NJDEP laboratory protocol includes testing to estimate the mass-capture capacity of filters before reaching a failure point, but that testing is done with silica sand—research shows using silica sand for this purpose likely inflates the mass-capacity numbers in comparison to real-world stormwater that includes organics, oils and other materials that accelerate filter occlusion. There’s discussion in research circles about creating a more-realistic sediment blend for this type of longevity testing, but we’re still in the early stages of that process.

Similarly, field monitoring is more realistic, but site conditions vary widely. In an ideal world, we would collect longevity data in multiple locations to derive average longevity for a particular SCM at a given loading rate. The TAPE program has given some consideration to this type of post-installation longevity monitoring but hasn’t taken formal action to date. Meanwhile, filter SCMs recently monitored and certified by the TAPE program only functioned for a very small percentage of a water-year before needing maintenance to restore functionality, suggesting we’ve likely already gone too far with loading rate in some cases.

Ignoring the risks of rising loading rates on filter longevity isn’t a sustainable path. Maintenance budgets aren’t infinite, and clogged filters left in operation are no longer providing their intended water-quality benefit. Designers and policy makers would be wise to consider removal-efficiency performance as well as longevity when vetting and deploying filter SCMs. We all have work left to do to ensure our filter SCMs are meeting water-quality goals and are affordable to operate throughout their lifecycle, but it all starts with dialing back the loading rate