Stormwater water quality regulations are getting more restrictive, and best management practices (BMP) are evolving to meet the challenges of lower benchmarks and permit limits. BMPs are considerably more complex and costly than they’ve ever been. At the high end, systems such as electrocoagulation require a dedicated footprint, unrelenting maintenance and tens to hundreds of thousands of dollars. At the low end, a grassy strip or small retention basin is low cost, low maintenance and can be very effective at reducing contaminants in stormwater runoff.
Treating water for heavy metals is a two-fold problem. First, metals are commonly attached to particulate matter which may or may not be susceptible to settling in a basin. Second, those particulates are in equilibrium with the stormwater that surrounds them, so the metal ions are continually shuttling between attaching to the particulate matter and dissolving in the stormwater solution. In short, if you have one, you almost certainly have the other – and you’ve got to capture both.
Boom protecting a small creek from industrial runoff activity.
Using booms and socks for heavy metal treatment is not usually part of early treatment discussions, but it should be.
The use of booms and socks, those long snaky tubes that are often seen on construction sites, is a proven method for controlling erosion. The concept is simple. As sheet flow encounters the boom, the water slows down and water-borne soil and solids drop out of solution in front of the boom. When properly used, erosion control booms keep bare soil where it’s supposed to be and prevent trash from entering waterways. Those same concepts can be applied to heavy metal remediation.
Capturing as many solids as possible substantially reduces particulate-bound heavy metals. When the solids are finely divided and tend toward suspension, treatment plans call for a physical filtration that can trap fine particles, which is why the media inside treatment booms is a critical factor in success. Also, because particulate metals are usually accompanied by dissolved metals, the media inside the boom needs to act concurrently as a sorption media. Using a flocculant in the boom can increase the particle size to promote entrapment, but flocculants need time to work and often the velocities inside a boom are too great to get the full benefit.
Booms are versatile and can be placed anywhere the water flows. When used at the periphery of a site, around catch basins, drains or downspouts, booms capture diffuse sheet flow. But just because they can be placed there, that doesn’t mean that they should be. Considerations such as water volume, traffic patterns, foot traffic and needed treatment all influence the appropriateness of using booms in stormwater treatment.
- A boom is only as successful as it’s functional height. In other words, if the boom is continually overtopped because the collection area is too large or the average rainfall is too heavy, much of the water will pass over the top of the boom without actually traveling through the boom, which is what slows down the water velocity and promotes sorption via contact with the interior media surface. Most booms for metals treatment are 8 inches in diameter or less. Booms can be stacked one on top of each other to minimize overtopping, but they need to be secured against toppling during heavy rains.
- Good ground contact is critical for maximum treatment. Again, it’s the traveling through the boom that maximizes treatment. If water can circumvent the boom by traveling underneath it, or if there is a gap between boom ends, the water will bypass the treatment area. Fortunately, heavy metal treatment booms are usually filled with granular media that is friendly to hugging the ground surface. When placing booms, smooth out the wrinkles and spend some time getting good ground contact, generously overlapping the ends, and making sure that stacked booms don’t have any gaps.
- If the boom is placed in an area where upstream ponding is a concern, consider choosing a boom with a larger media particle size. The larger media will allow more water to pass through, but the tradeoff is that there will be less contact time with the media. Metals concentrations on the downstream side may suffer as a result. Booms are ideal for sites with sheet flow where a large linear area needs to be treated, but there is not a lot of volume in any one area.
- Media choice matters. Choose carefully. Manufacturers of booms for heavy metals use a wide variety of materials, and they are not all created equal. Compost, biochar, peat, proprietary media, zeolite and carbon are all available in booms, but they do not have equal activity or capacity for heavy metals. Even sand and gravel will remove metals at the outset, but the capacity is very limited. It’s important to choose booms with the same care as used in choosing media for a tank or vessel.
Even though they are low-tech, inexpensive, and effective, booms are not without maintenance. Some of the solids that settle out during treatment will plug the media over time, which leads to more incidences of overtopping. Further, if the boom is doing its job, solids will accumulate on the upstream side. Those solids have to be removed periodically to minimize sedimentation inside the boom. The geotextile fabric that encases the treatment media degrades over time, particularly if it is exposed to UV light, and the media inside the boom itself can get loaded with metals. The boom life can be extended by rolling the boom over about halfway through its life cycle. That will expose new fabric to the sunlight and will transfer the freshest media to the bottom of the boom where the bulk of the water travels.
If your stormwater treatment plan calls for heavy metal remediation, factor a peat-based boom into your strategy. It can be a low-cost BMP that pays off in big rewards. Reach out to APT for more information.