Stormwater Treatment: A Look at Various Methods, Hydrodynamic Separators

July 10, 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

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

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

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