What to Know About Stormwater Manufactured Treatment Devices

Stormwater quality treatment devices, or manufactured treatment devices (MTDs), are engineered structures designed to protect water resources. These innovative systems capture, filter and treat stormwater runoff before it reaches natural waterways, playing a critical role in reducing pollution and improving water quality. Stormwater control devices serve multiple functions, including sediment removal, nutrient reduction, contaminant filtration and stormwater detention. From sediment basins and wet ponds to swales and rain gardens, various MTDs exist to address current environmental needs. By incorporating these devices into stormwater control management plans, communities can create more sustainable environments, reduce flooding risks and safeguard aquatic ecosystems. In urban areas where runoff can be a significant pollution source, MTDs are essential for maintaining healthy water systems.

What Are the Different Types of MTDs (Manufactured Treatment Devices)?

What is a stormwater treatment device? MTDs, as they are commonly referred to, are integral components of stormwater control measures. They aim to remove pollutants from stormwater runoff before it reaches waterways. Each type of MTD plays a specific role.

Separation Devices

Separation devices are also called hydrodynamic separators (HDS). These are designed with a sump for sediment disposition and chambers, weirs and baffles to trap pollutants. HDS uses centrifugal force, separation and swirling techniques to remove pollutants from runoff. These stormwater quality improvement devices can remove the following from stormwater:

• Trash
• Debris
• Sediment
• Oil
• Grease

HDS can remove up to 80 percent of coarse particles and be used as pretreatment in other stormwater management technologies. The most common hydrodynamic separators include vault systems, swirl concentrators and inclined plate separators. Contech manufactures the Cascade Hydrodynamic Separator, an example of this type of stormwater quality device.

Filtration Devices

What are filtration devices for stormwater? Filtration devices provide a higher level of treatment for stormwater management technologies. Unlike HDS, which can act as a stormwater trash trap, filtration devices use sediment and filtering chambers to remove smaller particles and pollutants from stormwater.

Filtration devices can trap pollutants using vegetation or engineered media, so several different filtering methods are available for these stormwater treatment devices. These devices can remove pollutants, such as:

• Nutrients
• Metals
• Bacteria
• Total suspended solids (TSS)

Filtration devices work best in areas where high pollutant efficiencies are needed or in areas with impaired receiving waters. The StormFilter, Jellyfish Filter, and Kraken Filter are all highly rated for this type of manufactured treatment device.

Catch Basin Inserts

Catch basin inserts for treating stormwater are often used as part of a larger proprietary stormwater treatment device or system, or catch basins are used in smaller areas where other methods and devices are not so easily implemented. What is a catch basin insert? It is a storm drain inlet protection device directly installed into a storm drain or catch basin.

Catch basins work well to remove pollutants like oil, grease, heavy metals and hydrocarbons and can be made from various filter media, such as:

• Porous polymers
• Polypropylene
• Activated carbon
• Treated cellulose

Catch basins require regular stormwater maintenance to prevent clogging and ensure optimal performance. Dura Plastic Products, Inc. is one of the top catch basin brands.

What Are the Design Criteria for Stormwater Manufactured Treatment Devices?

The design of stormwater management devices is imperative. Not only must they be built to specification when it comes to building permits and codes, but they must also be part of an overall stormwater management strategy that works—so the devices must effectively control stormwater pollution. MTDs must be designed with the following objectives in mind.

Water Quality Event Treatment

A water quality event (WQE) represents a calculation concerning a certain amount of rainfall. All manufactured treatment devices should be able to handle the peak flow of stormwater runoff from any WQE, or they must bypass that flow without scouring trapped pollutants. Ensuring that MTDs handle peak flow ensures that smaller WQEs are treated and managed correctly.

Hydraulic Loading Rate

The hydraulic loading rate, or HLR, is crucial when designing manufactured treatment devices. Typically, HLR is expressed in gallons per minute per square foot (gpm/sf) or cubic feet per second per square foot (cfs/sf) of the device’s treatment area. Depending on the application, MTDs can be designed with a high or low HLR. For example, filtration systems often operate at a low HLR because of the media used within the filtration system.

Pollutant Removal Efficiency

Most regulations require that 80 percent of TSS be removed for level 5 WQEs. Pollutant removal efficiency is measured with each event. Particle size distribution, removal mechanisms, influent concentrations and additional pollutants are all factors to consider when measuring the pollutant removal efficiency of a manufactured treatment device.

What Are the Maintenance and Inspection Requirements for Stormwater Treatment Devices?

MTDs must be maintained to meet regulatory standards and protect the environment. Some key factors to consider are below.

Frequency & Record-Keeping

On average, stormwater-manufactured treatment devices should be inspected at least twice a year, and additional inspections should be performed after each major WQE or storm event. However, each MTD is different. HDS systems are often inspected biannually, while media-based filter systems are checked at least once a year.

Keeping records when inspecting or maintaining manufactured treatment devices is also imperative. This includes a maintenance log, long-term tracking, work orders, accessibility and complete inspection reports. Not only is this vital for safety, but it’s also paramount should you be audited.

Procedural Efforts

What is manufactured treatment device maintenance? Some action items to consider are listed below:

• Inspection of components: Check for damage on the entire MTD structure, including inlet protection devices and outlet structures.
• Sediment and debris removal: When the manufactured treatment device becomes full, remove sediment, trash, and debris using a vacuum truck.
• Cleaning: Remove and clean oil, grease, trapped sediments, and other pollutants.
• Vegetation management: If using bioretention systems, check for and remove invasive species.
• Filtration media replacement: Replace or clean the filter for media-based filter systems.
• Structural repairs: If any damage is noted, repair it immediately.

Generally, manufactured treatment devices are a best practice and work well to handle stormwater pollution.