World Water: Stormwater Management April / May 2015 27

Stormwater Solutions

For decades, hydrodynamic separators (HDS) have been used as a primary

treatment device for stormwater, but does this technology still have a place in today’s world of low impact development (LID) and green infrastructure? For many owners, regulators, and engineers the answer is yes.

HDS systems effectively capture and retain a variety of stormwater pollutants and also are very easy to maintain. These two key benefits have resulted in new uses for HDS technologies, such as pretreating LID and green infrastructure practices as well as other land-based stormwater treatment systems.

What are hydrodynamic separators?HDS are flow-through stormwater management devices that remove sediment, trash, debris, and oil from stormwater runoff. HDS systems are belowground structures, typically made of concrete, with internal components that create a swirling vortex to enhance particle settling. The combination of swirling water and gravity separation causes floatables and larger debris to rise while sediment and other pollutants settle in a sump.

HDS systems often contain internal components designed to reduce water velocity to prevent turbulence, which decreases resuspension of previously captured pollutants. Some HDS systems, like Contech’s continuous deflection separation (CDS), incorporate screening technology to increase pollutant removal capability.

One of the primary benefits of HDS systems is their ease of maintenance. Pollutants are captured and retained in a single structure that can be easily cleaned using a vacuum truck.

HDS systems are used frequently to provide stormwater quality treatment in places where space is limited for conventional volume-based stormwater treat-ment practices – such as ponds, wetlands, and swales. HDS systems also are commonly used as end-of-pipe solutions, helping to meet total maximum daily load requirements by treating stormwater before it enters receiving waters.

Origins of hydrodynamic separatorsEarly HDS systems were commonly called oil and grit separators. These systems focused primarily on capturing sandy grit, oil, and occasionally floatables. These technologies were commonly used in parking lots for heavy and light industrial facilities, which remains a common use for HDS systems to this day.

In the early 1990s, newer applications developed as a result of Phase I National Pollutant Discharge Elimination System (NPDES) regulations, which required the removal of trash and debris. Research conducted in 1995 by Dave Shepp with the Metropolitan Washington Council of Governments showed that many HDS devices designed to meet Phase I NPDES requirements were undersized. At this time, HDS systems did not incorporate high-flow bypass mechanisms and were subject to occasional

blowouts during intense storms. At about the same time – mostly in areas where road sanding was common practice such as the northeast United States – emerging technology began incorporating flow controls and bypassing to avoid pollutant resuspension.

In these early days, the market was focused primarily on total suspended solids and not yet sophisticated enough to classify these emerging best management practices (BMP) and properly identify their true value – which was to target trash, debris, and settable solids. Upstream removal of these pollutants is useful to protect more sensitive BMPs in a treatment train or to provide minimal protection of receiving waters in situations where removal of finer pollutants is not practical.

Fortunately, more recent and sophisticated regulatory bodies have recognized the value of these devices and placed them in a category where value is added.

Hydrodynamic separators have long been used to remove sediment, trash, debris, and oil from stormwater runoff. Now, when coupled with green infrastructure, this traditional technology can pretreat stormwater to improve aesthetics while reducing maintenance time and costs associated with green practices, Hannah Schlachter and David Corr of Contech Engineered Solutions report.

Repurposing an original stormwater treatment technology

HDS ARE FLOW-THROUGH STORMWATER MANAGEMENT DEVICES THAT REMOVE SEDIMENT, TRASH, DEBRIS, AND OIL FROM STORMWATER RUNOFF.

Above: A Contech HDS system pretreats a rainwater harvesting system at the Dallas Arboretum in the US state of Texas. Image credit: Contech Engineered Solutions

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Stormwater Solutions

New uses for HDS: Pretreating LID and green infrastructure Recently, there is a push to implement more landscape-based stormwater controls that include vegetation and engineered soils designed to infiltrate, filter, and detain runoff. The popularity of landscape-based controls seems to be based, in part, on the perception that they need little to no maintenance. That is simply not true.

Think about what happens to a bioswale, a landscape-based stormwater control, after installation: Rain events occur, and the bioswale functions as designed. Stormwater flows to the bioswale from adjacent drainage areas and slowly infiltrates while depositing sediment, trash, and debris on the surface of the bioswale. Over time, trash and debris continue to accumulate at the site. As sediment deposits in different locations of the swale, the bioswale cross section begins to change. Eventually, the bioswale may no longer provide the design infiltration rate or storage volume.

While it depends on the location and amount of stormwater flowing to the bioswale, the US state of Oregon’s Department of Water Quality recommends that this practice be inspected seasonally and after every major rain event to avoid failure. The bioswale may need to be reshaped due to large sediment deposits. Its vegetation needs regular maintenance, and plantings need irrigation during dry spells.

Moreover, untrained landscape maintenance staff may use the “mow, blow, and go” method. A landscaper’s primary focus is aesthetics, not key stormwater design issues like infiltration rate, flow path of water through the practice, or storage capacity.

This illustrates why an increasing number of engineers are incorporating pretreatment components, such as HDS, into LID and green infrastructure designs. These systems can reduce maintenance time and costs. So while the addition of HDS pretreatment increases the upfront capital cost of the entire system, lifecycle costs will be reduced.

Another important benefit is improved aesthetics due to removal of unsightly trash and debris by the upstream HDS unit.

Newport Boulevard bioswaleIn 2006, the City of Newport Beach constructed a bioswale alongside Newport Boulevard in the US state

of California. The city designed the swale to treat an estimated 114,000 liters of urban runoff per day from the 160-hectare watershed before that runoff enters Newport Bay.

The bioswale proved effective in removing dissolved pollutants and bacteria. However, trash – such as cigarette butts, food packaging, cans and bottles, and plastic waste – was collecting in the bioswale. And some of that trash was making its way into Newport Harbor. This trash not only detracted from the aesthetics of the bioswale and the harbor, it also posed a threat to marine life and public health.

In 2012, the city applied for a Measure M Grant to address this issue. This grant is administered by the Orange County California Transportation Authority and is designed to mitigate more visible pollutants, such as litter and debris that collect in storm drains prior to being deposited in waterways and the ocean.

Orange County approved the grant application, and in December 2013, a Contech CDS hydrodynamic separator and a diversion structure were installed upstream of the bioswale to remove trash, decrease maintenance costs, and protect the ocean.

Humboldt River GreenwayThe Los Angeles River in the US state of California extends 84 kilometers through the region. The river has been encased in cement since the 1930s to protect the fast-growing city from life-threatening floods. The area is now the focus of an extensive region-wide program to revitalize the area and create green space while maintaining hydrologic and water quality requirements.

As part of these efforts, the Humboldt River Greenway project – a half-hectare park on a former industrial field – became one of many Los Angeles Bureau of Engineering projects targeted for change.

Along a corridor of Humbold Street, the project daylighted, or brought to the surface, an existing underground storm drain system by constructing a stormwater greenway with a stream ecosystem. The project includes an HDS system to capture trash, debris, and sediment before it enters the greenway.

The project enhances the water quality of the Los Angeles River and provides natural habitat with the planting of native plants. It also creates a communal recreation space, which promotes the idea that urban streams are a valuable resource to be enjoyed by the community.

Pretreating rainwater harvesting systemsPretreating harvested rainwater is essential to ensure the long-term operation of any rainwater harvesting system. Pretreating harvested water prior to storage protects downstream pumps, filters, and fixtures from damage or clogging, and it lowers cleaning and maintenance costs by keeping pollutants out of the cistern and mechanical system. It also reduces the amount of organic matter and biological oxygen demand in the cistern, decreasing the likelihood of creating anaerobic conditions and associated odors.

Traditional rainwater harvesting guidelines recommend the use of first-flush diverters that divert the first portion of a storm event away from the cistern. The theory is that the first portion of the storm likely contains a higher concentrationof pollutants.

The use of first-flush diverters has several drawbacks. Opinions vary on the contaminant loading of the first flush, and the term itself has different meanings in a conventional rainwater harvesting context compared to a stormwater context. Some stormwater research shows the potential for a seasonal first flush, but suggests pollutant loading during a storm

is associated more with rainfall intensity rather than timing.

Additionally, any pollutants in the water bypassed by a first-flush diverter still need to be treated in order to comply with local stormwater management regulations.

More importantly, diverting the first flush results in a significant reduction in the volume of harvested water. Many rainwater-harvesting guides recommend bypassing the first portion of a storm event, in the range of 1.3 millimeters. Depending on the rainfall patterns, this amount can easily equate to more than 10 percent of the annual runoff. This is likely a low estimate because it is based on total daily rainfall. However, rainfall often occurs multiple times per day and can create multiple first-flush volumes.

Because of these issues, many designers have turned to HDS systems as pretreatment for rainwater harvesting systems. HDS systems meet the pretreatment needs of rainwater harvesting systems, without the drawbacks of first-flush diverters. They eliminate the need for treating the diverted first flush, and capture 100 percent of the storm event.

Looking forwardIt may be surprising that one of the original stormwater treatment devices, and a concrete one at that, is quickly becoming the pretreatment system of choice for so many LID and green infra-structure projects, but it makes perfect sense. HDS systems have a proven track record of capturing and retaining pollutants, they are easy to maintain, and they help keep green infrastructure functioning properly andlooking great.

Authors’ NoteHannah Schlachter is the hydro-dynamic separation product manager for Contech Engineered Solutions, based in West Chester, Ohio, United States. Schlachter holds a bachelor’s degree in Civil Engineering and a Master’s degree in Environmental Engineering both from the University of Toledo in Ohio.

David Corr is the director of marketing for Contech Engineered Solutions Treatment Segment. Corr holds a bachelor’s degree in Business Administration from the University of Rhode Island andan MBA from the University of Dayton in Ohio.

A Contech CDS system helps keep the Humboldt River Greenway clean from trash and debris in Los Angeles, California, United States. Image credit: Contech Engineered Solutions

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