Stormwater Treatment: A Look at Various Methods, Hydrodynamic Separators

June 28, 2002

Since the passing of the Clean Water Act, the industry has made great strides in improving the quality of point source discharges to the environment. As treatment technologies continue to improve, non-point source pollution becomes a more significant contributor to environmental degradation.

This contamination is introduced into the environment from a variety of sources: agricultural runoff containing excess nutrients, sediment-laden runoff from open construction sites, and parking lot runoff carrying trash and oils are each significant sources of non-point source pollution. The next step toward clean water has been focused on this non-point source pollution.

Thirty years ago, stormwater management had one simple goal: collect the water and get it off the site as quickly and efficiently as possible. Looking at rivers and streams today, one can see the erosive effects of this type of design. During the last fifteen years, many programs have been implemented at the state and local levels to prevent this type of environmental degradation. Under Phase II of the NPDES program, these state and local programs will soon be federally mandated.

These programs will require the use of best management practices (BMPs) to prevent non-point source pollution. Any program, structure, or activity that reduces the pollutant load entering the environment is a BMP. Street sweeping reduces the load of sediment entering the environment; nutrient management programs for agriculture limit the introduction of excess nutrients into streams. Engineered BMPs are also included in site designs by the civil engineer to remove contaminants from the stormwater runoff before it is discharged.

Stormwater runoff from a typical parking lot can contain sediments, oils, trash and other debris, heavy metals, nutrients, and a wide variety of other pollutants. To remove these contaminants, the engineer has a variety of BMP choices, including ponds, wetlands, infiltration systems, filters, and hydrodynamic structures.

A stormwater pond is an engineered structure, sized according to anticipated storms and with a controlled release that discharges water according to the water level in the pond. Because it relies on volume and retention times to remove pollutants from stormwater runoff, a pond takes up a large amount of space. And it is been found that using ponds can create many other environmental issues and is costly to maintain.

Wetland plants have the capability to remove pollutants from stormwater through their natural biological processes. A constructed wetland utilizes these plants in an engineered marsh to improve the quality of stormwater runoff, and also creates habitat for wildlife and waterfowl within the wetland and its buffer areas. More than any other BMP, a constructed wetland depends on the hydrology of the site to operate properly; a permanent water pool that can withstand a drought is essential to the installation. The wetland also requires pretreatment to prevent sediment from choking the vegetation.

An infiltration system is designed as a groundwater recharge device, and utilizes the soils on site as a filter medium. Such a system collects the stormwater and allows it to percolate through the soils and back into the groundwater. Because it depends on the native soils to filter and discharge the water, an infiltration system is not feasible for every site. The soil types, underlying geology, slopes, and hydrology of the site must be considered when designing an infiltration system. And here too there is a risk of clogging the installation.

Stormwater filters collect the water and pass it through a bed of sand or other media to remove contaminants from the water. The most common type of stormwater filter is a sand filter, which may be constructed in a concrete structure or designed into a small detention area. While they are capable of excellent pollutant removal, filters are also susceptible to clogging and are costly to maintain.

Hydrodynamic separators, such as those produced by BaySaver, Inc., are structures designed to remove suspended sediments, oils, and floatable debris by physical processes. Usually installed as an underground structure, a hydrodynamic separator is most often used on sites with large paved areas where space is at a premium. This makes these structures a good treatment choice as a stand alone. This type of installation relies on sedimentation and flotation to remove and retain pollutants, and often includes proprietary flow controls.

Like all BMPs, a hydrodynamic separator requires occasional maintenance to continue normal operation, however this maintenance is minimal and cost effective. And, hydrodynamic separator technology has been developed to meet Phase I and Phase II pollutant removal requirements, making its use as a stand alone structure acceptable practice.

For further information, phone BaySaver, Inc. at 1-800-229-7283.

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