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Space-saving devices to treat runoff Readers of this magazine understand the operation and effectiveness of hydrodynamic separator systems. Whether the units are in place as part of a treatment train or as an individual best management practice (BMP), the separators are an effective method of capturing floatables, oily residues, and sediment and preventing these pollutants from entering waterways. In recent years, more of the units are being installed as municipalities struggle to meet stringent requirements and even the expectations of the public. Because the systems are installed underground, they are invisible to the public and require less space than BMPs such as detention ponds.
Connecticut Sand The swirl chamber of the Vortechs unit enhances settling of heavy sediment, while floatables are retained in the first chamber. For most situations, the Vortechs operates at low flow rates. For a larger storm, the swirl rate increases to force sedimentation. Even during high flow times, the pollutants are retained in the chambers. Dobmeier notes that the city didn’t want to put the units inline because of the chance of blocking the flow. Because the units were fairly new at the time, Dobmeier and others were not sure about the possibility that they would become clogged with trash and debris. They placed the units on the bypass flow where the first flush of any two-year storm could be captured. Like most pollutants, the first flush carries the heaviest load of sediment. When a storm event occurs, stormwater rushes through catch basin inserts with sumps, so some sediment is captured there. Then the water flows into the Vortechs units. The units chosen have 5,000- and 7,000-gallon capacities. Dobmeier says that if the city had to size for a 10-year storm, the 10,000-gallon-capacity units would probably be required and the cost would have been too high. The city chose areas that contribute the highest levels of pollutants and planned for the units to handle a two-year storm at those points. One 5,000-gallon unit captures runoff from a busy road that has a steep incline. The road is sanded heavily during snowstorms because of the steep grade, so there is a heavy load of sediment washed off of it even with regular street sweeping. After years of observation, Dobmeier reports that the unit has worked well and has kept tons of sediment out of the Connecticut River. Initially, the city had scheduled yearly maintenance for the units. But recently maintenance has been rescheduled to take place every other year because of the units’ large capacity and other measures that have reduced trash and sediment, such as street sweeping and the catch basin inserts. By observation, Dobmeier knows that the main pollutant trapped by the hydrodynamic separators has been sediment with some trash and large debris. In one place, crews did find quite a bit of oil, probably from someone dumping it after removing it from a vehicle. Products by other manufacturers were considered. However, Dobmeier says that some of the others did not have as large a capacity as the Vortechs units, and some were so deep they would have required new equipment for maintenance. Minnesota Rivers Long Island Sound Protection
Just about every kind of stormwater treatment device has been considered, and many have been put into operation. Municipalities around Long Island Sound have installed Smart Sponge filter technology, manufactured by AbTech Industries Inc. of Scottsdale, AZ, to control bacterial contamination on beaches. Backer says that vault-type BMPs have been especially effective at handling sediment in runoff. Some cities are also operating hydrodynamic separators to get rid of trash, sediment, and oil. Nassau County, NY, rivers and streams also drain into Long Island Sound. Ken Arnold, unit head for water and wastewater for the county, says that three Stormceptor separators have been installed and the county is preparing to install two more. The Stormceptor system is a prefabricated device, generally of precast concrete, with two chambers—a treatment chamber and a flow bypass chamber. The bypass chamber can pass water through the system without treatment during storms of heavy rainfall. A fiberglass Stormceptor is also available. Sea Cliff Avenue Park is in a residential area in Nassau County. Five years ago, the park was being reconstructed; at about the same time, county officials saw a demonstration of a Stormceptor unit. Hempstead Harbor is the receiving water for runoff from the region, and sediment levels had been high. The decision was made to install a Stormceptor STC 2400 to filter the stormwater runoff. A Stormceptor 7200 was placed in a road in Glen Cove. The area is very hilly and has to be sanded heavily in winter. The hills also experience erosion, contributing to the sediment problem.
The Woodmere golf course also installed a Stormceptor unit to help reduce the maintenance required for its detention ponds. Two ponds on the course detain the water until the overflow reaches surface waters. Floatables and trash had been a big problem, requiring frequent maintenance of the ponds. Installation of the separator has reduced the amount of trash in the ponds, and the course has been able to decrease maintenance man-hours. Intervals for dredging the pond to remove sediment can also be stretched, saving money. Arnold reports that all of these units have been working well. Maintenance was a challenge at the beginning: The unit at Sea Cliff was not cleaned for two years because no vacuum truck was available with long enough extensions. A new truck with the required capability has since been acquired, and maintenance now occurs once a year. The Glen Cove unit is checked more often, especially in the fall, because of a heavy load of leaf debris. Arnold says his job is made easier by the geologic fact that the county has great infiltratable soil. The area sits on a big sandbar, so outflows from hydrodynamic separators infiltrate the soil easily. However, the state has only recently accepted the units as BMPs for redevelopment projects. The state now approves products by some manufacturers but has been reluctant to do so in the past. Maryland Manholes and Florida Streams
Tomasini notes that the installation is fairly straightforward because the units fit into a standard manhole. The BaySaver uses two manholes, one primary and one storage. An advantage to this design is that the units are lightweight and can be installed with medium-sized equipment, not requiring large excavators or cranes. The City of Pensacola, FL, had a plan to clean up bays, streams, and estuaries in the city and the surrounding Escambia County. Over a three-year period, the city installed 32 BaySaver units; the water quality has improved markedly. Texas Projects The first task was to delineate the runoff areas that needed to be addressed. Fifteen to 20 stormwater inlets were scattered through the area. The concentration of pollutants fluctuated with the number of cars parked there, so a plan was needed to handle a maximum amount of runoff. The main pollutant concerns were automotive fluids, but sediment and trash were also a problem. The multi-compartment capability of the StormTrooper worked well for the application. The heavy solids settle to the bottom, and oils and greases remain in the chamber. Absorbent inserts collect the oils and are replaced during maintenance. During high water flows, a control manhole diverts the extra stormwater. The first flush flow rates were from 0.07 to 0.79 cubic feet per second. The size of the unit needed was calculated from these flow rates. Most of the StormTrooper units chosen were the SwST-05. In one larger area, a SwST-10, with a flow rate capacity of 1.45 cubic feet per second, was needed. A total of 10 StormTrooper units was installed at the Auto Pound, four inline units and six inlets. A big challenge with this project was that no “as built” surveys were available; placement of the units had to be flexible in case the chosen site proved unsuitable. The best time for maintenance is three to four days after a rain. The protocol written by BDS Technologies outlines a step-by-step maintenance program. Depths of oil and sediment are measured with a Sludge Judge, an instrument manufactured by Pollard Water of New Hyde Park, NY. BDS also recommended a monitoring program for the effluent, measuring total suspended solids, hydrocarbons, aluminum, iron, and lead. The units allow Auto Pound to meet the requirements of Section M-Automobile Salvage Yards of the Texas Commission on Environmental Quality. The City of Dallas has installed other StormTrooper units at vehicle service centers that support police, public works, streets, and water departments. It is considering similar systems for the firefighter training center. The StormTrooper comes in both cylindrical and rectangular models and capacities from 500 to 15,000 gallons. Freddie Guerra of BDS Technologies says that one of the biggest pluses of the StormTrooper system is the flexibility. Inlet and outlet holes can be placed on opposite or the same sides, depending on the site. California Stormwater Requirements The design included three Stormceptor models to treat the runoff from 162 acres. Two STC 16000 units were connected inline, side by side on a 54-inch-diameter storm drain. Another STC 7200 was placed at another location. New Jersey Company Tests performed by an engineering company showed significant removal of fine grain sediment, meeting the recommended 80% removal rate. Hydroguard has a circular design and comes in five sizes, ranging from 4 to 12 feet in diameter and 5 to 8.5 feet in depth. The units are maintained by vacuum truck. A Less-Expensive Alternative When buried, these chambers receive stormwater runoff as it percolates the soil. Bacteria form a biomat at the bottom of the arch. These bacteria are present in soil, and their population increases dramatically when their feedstock, such as oil, is available. As the stormwater infiltrates the soil, the bacteria consume the oil, releasing only water into the water table. Robert Maestro of HydroLogic says the StormChamber is a less-expensive but effective method for stormwater treatment. The StormChamber can be installed in any stormwater structure and have the capacity comparable to a 48-inch stormwater pipe. According to Maestro, the chambers have 60% more capacity than other plastic stormwater chambers. A sediment chamber can be added to remove that constituent of stormwater. Typical installation involves two chambers at a cost of $400 each. The StormChamber works in all soils. Maestro says that it will even work better in clay soils, which have higher organic levels and moisture content, providing a better environment for the biomat. Most of these experts agreed on the challenges they face in installing and maintaining these large hydrodynamic separators. The most difficult ones are:
Advantages of hydrodynamic units are:
Whatever challenge you are facing in stormwater treatment, you can find a system that will work. Knowing the characteristics of these systems, the advantages and disadvantages will give you the information to decide what’s right for your area. Roberta Baxter specializes in science and technology topics. SW October 2007 |
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