A healthy stormwater system should be able to handle the most torrential of downpours with confidence. These heavy downpours can dramatically swell the affected watershed; how do properties without the broad real estate for detention ponds keep from contributing to floods? Underground vaults.
Proprietary systems are prefabricated or customizable stormwater management assets that use designs and technology that businesses like yours may be responsible for. They’re a type of Best Management Practice (BMP) that are ingeniously designed to reduce the space necessary to process runoff and stay out-of-sight. Like traditional surface-level BMPs, these manufactured devices treat stormwater before discharging to another BMP or a body of water. Because proprietary systems come in a range of designs and sizes, they are highly variable in performance, design, flow rates, unit processes, and clearly storage capacity.
There are two major design types for proprietary systems: separation and filtration. Separation units can be further broken down into to chambered and hydrodynamic. In chambered BMPs, the runoff is channeled through several compartments where settling of sediment particles and flotation of hydrocarbons (seen as an oil sheen on top) takes place. On the other hand, hydrodynamic devices work through inducing a swirling motion to the incoming stormwater flow to aid in the settling of sediment particles. Filtration BMPs pass runoff through filter cartridges or filter media, physically or chemically removing some of the solid pollutants from the stormwater.
Underground stormwater BMPs can be both proprietary and non-proprietary, and are devices installed below ground for stormwater treatment. These systems include retention tanks, tunnels, vaults, pipes and storage chambers, and treat runoff through processing. Methods include sedimentation, screening, filtration, and hydrodynamic separation. They can be integrated into parking lots, sidewalks, vegetated landscaping, and this versatility makes them a worthwhile consideration for many development types, including distribution centers, strip malls, and even housing developments.
There are pros and cons with placing stormwater treatment belowground. With a dedicated maintenance team, while they may be out of sight these systems are never out of mind. Without regular checkups, it can be hard to see problems developing or to know when maintenance is needed (especially for low-flow systems with a higher susceptibility to clogging). Performance isn’t as easily measured or documented as with traditional, surface BMPs so it is even more important to stay on top of upkeep. Surface BMPs are still the baseline, and often local jurisdiction requires demonstrating that the underground BMP performance will be comparable in the event of a deluge. Surface BMPs have an established track record and provide additional environmental benefits such as infiltration, groundwater recharge, and mitigation of the heat island effect.
Despite these drawbacks, underground proprietary BMPs are growing in popularity and are a good option for many sites. One of the primary reasons for this is that they require less land surface. This is especially useful in urban areas where land is at a premium and dual use of the surface can occur. Proprietary systems can be cheaper than traditional BMPs depending on the needs of the site, even when you take maintenance into consideration. The minimized footprint significantly cuts back on landscaping, providing savings at no cost to the watershed. Another benefit is that the systems can be chosen to target specific pollutants of concern. For this reason, they are sometimes used in conjunction with other BMPs to provide additional, targeted, water quality improvement. Where possible, underground systems can be incorporated into vegetated landscapes and set up to allow infiltration comparable to detention ponds and infiltration basins.
The frequency of maintenance required by an underground BMP depends on the specific model, the conditions of the site, and the amount of stormwater flow.1 Site selection plays a large role in determining ease of maintenance. Underground BMPs should be constructed in areas where visual inspections can be conducted, and the necessary maintenance equipment can gain access as needed. Maintenance tasks depend on the type of system, and for most proprietary devices this information is included in an owner’s manual and maintenance guide. It is important to understand what the ongoing maintenance costs will be so that you can properly budget for a properly planned maintenance program moving forward (and note that confined space entries are costly by product of inspecting). Because these systems aren’t entirely visible, it is especially important to stay ahead of upkeep, and that’s where we come in. Preventative maintenance reduces cost in the long run and makes it easier to prepare for visits from county and federal inspectors. To give a better picture of potential maintenance plans, let’s take a look at some common systems that are helping businesses like retailers control their runoff:
StormFilters are comprised of connectable structures that contain rechargeable cartridges, filled with a special blend of media that traps particulates and absorbs pollutants. The cartridges have a large surface area and are siphon-activated to bring in stormwater evenly across the device. This design helps reduce surface blinding and maximizes media contact to extend the lifespan of the filters. The media can be customized to target site-specific pollutants, which is an advantage for sites that have particular pollutants of concern. In addition to the media customization, the cartridge housing structures are also modifiable to accommodate a wide range of flows, footprints, and hydraulic conditions.
AQUALIS’s interactions with StormFilters fall in line with manufacturer-recommended actions and follow regulatory mandates to ensure the longevity of the systems. This lets us address the current needs and anticipate future concerns on a case-by-case basis. Some of our services include the cartridges and removing sediment, jet vacuuming the vault during cartridges changes, maintaining the surrounding area, and assessing all physical components of the device.
Image: Contech vault in a state of failure, avoidable through proper and preventative maintenance.
Filterras are ingeniously designed to seamlessly integrate with landscaping. A tree sits on top of a media-filled area resembling soil, which inconspicuously absorbs stormwater to begin the treatment process. Runoff enters the Filterra system through a curb-inlet opening or pipe and flows through a specially designed filter media mixture contained in a landscaped concrete container. This media traps pollutants, which decompose onsite and are incorporated into the tree itself as it grows. The treated water is discharged through an underdrain system at the bottom of the container. Maintenance for the Filterra is mostly what would be necessary for landscaping upkeep of a regular planter (e.g. debris and trash removal). Since the functioning of the system is dependent on this media, it is critical to stay on top the upkeep. For example, the mulch needs to be removed and replaced based on flow through and site-specific factors.
Similar to Contech’s Filterra, the BioMod uses plant root uptake to integrate stormwater management into landscaption. This bioretention system uses filtration and biological uptake to remove total suspended solids, metals, nutrients, gross solids, trash and debris as well as petroleum hydrocarbons from stormwater runoff. The BioMod works in series, with the number of modular concrete sections dependent on the needs of the site. These modules are filled with layers of aggregate, bioretention media, plantings, and mulch. BioMod is non-proprietary, the bioretention media can be any low-flow rate media that meets the requirements of the local regulatory agency. Maintenance for the BioMod is both aesthetic and functional: trash and debris need to be removed regularly, and the media of choice should be replaced as needed before the filtering efficiency drops off.
The DVS pulls stormwater in through two vortex tubes that operate in parallel. The configuration of the vortex tubes and the extended flow path increases residual time (the total time any given droplet spends working its way through the DVS), allowing solids to settle out. Sediment is collected in a storage area and internal baffles separate trash, debris, and petroleum hydrocarbons from the water. The system comes in several manhole and vault sizes, allowing for easy integration into existing site plans. Routine maintenance includes removal of accumulated pollutants through the vortex tubes with a vacuum truck. Site conditions like parking lot size and annual rainfall determine the performance of the systems and how frequently maintenance visits should occur. Having a dedicated maintenance team to keep track of the system is highly recommended to keep the vortex tubes working at full throttle.
While these systems are in the dark, it doesn’t mean you need to be. Underground stormwater management systems are a worthwhile BMP option that can save you space, time, and money. The examples provided above are just a few of the many vault designs tracked and vetted by AQUALIS. We follow new releases and updates to monitor performance and ensure best fit for our clients. Knowing when to use a surface BMP and when an underground system would be more appropriate depends on a variety of factors: site conditions and local regulations come to mind. If you’d like to learn more, think your sites would benefit from using this technology, or want to get to know how the vaults you already have been doing, don’t hesitate to reach out to us!