When Sediment Control Goes Underground

July 1, 2001
Talk about a tough job of removing solids from a postconstruction stormwater treatment structure: Joe Cardello, principal engineer with Maguire Group, a consulting firm in Providence, RI, tells of the time he pulled off the cover of a catch basin, peered inside, and was dumbfounded by what he saw–a bicycle.“It was too big to pull out through the cover or the curb inlet,” he recalls. “To remove it, we had to slice up the bike with a cutting torch. No one can figure out how it got in there.”Admittedly this bizarre incident exaggerates the challenge of removing sediment and other suspended solids from runoff once a site is developed. But it does illustrate a key point: If an underground system isn’t inspected and maintained properly, suspended solids, trash, and other contaminants can gum up the works, reducing the system’s effectiveness and threatening the quality of water downstream.Fortunately the types of pollutants typically found in stormwater running off developed sites with impervious surfaces, such as roads, parking lots, and vehicle maintenance areas, are usually quite a bit smaller than a bicycle. They’re more like sediment, floating oils and grease, and suspended heavy metals. Still, these contaminants can quickly add up to a significant amount. Gordon England, a project manager with Creech Engineers Inc. in Melbourne, FL, was the lead engineer for the Brevard County, FL, stormwater management program. He reports a large, continuous deflective separator stormwater treatment unit there might collect as much as 15,000-20,000 lb. of sediment and other debris in a less than a year, depending on rainfall and ground surface conditions. Without proper treatment, of course, stormwater laden with such materials can pollute streams, lakes, and near-shore waters.England sums up the task facing project owners and designers in removing sediment and other pollutants in postconstruction stormwater runoff: “The challenge is to do it so that you are not blocking the drainage pipe and causing upstream flooding. You have to have some kind of trap or treatment system that allows water to flow continually, even during heavy rains, while at the same time pulling sediment and other materials out of the water.”One way to do this is to treat runoff on the surface of a site, using sediment ponds and basins. In fact, where space permits, that’s usually the more economical route. The Maguire Group often uses a three-stage pond approach–a sediment basin, a wetland, and a micropool–to slow runoff enough for suspended sediment and other solids to drop out or to infiltrate into the soil before stormwater leaves the site.If there’s no room on the surface, then Cardello and his colleagues go underground. Depending on the type of land use, size of the excavation, and number of buried utilities, this can be much more expensive than constructing a sediment pond. But it might be the only practical alternative for treating runoff. Responding to Regulations
Various types of underground stormwater treatment systems have been developed within the past decade, and the technology continues to evolve. “The US Environmental Protection Agency has described runoff as the largest remaining unaddressed source of surface-water pollution,” points out Fran Tighe, vice president of sales and marketing for Vortechnics, which makes underground stormwater treatment systems. “The stormwater treatment industry is still in its infancy. It’s at the stage where wastewater treatment was 50 years or so ago. However, the field of stormwater treatment is growing rapidly, and the technology is changing more quickly than the wastewater field did. Public expectations for water quality are increasing, and government regulations are getting tighter. So solutions to stormwater treatment problems are becoming more sophisticated, and we expect that the market for these systems will continue to grow for some time.”For the most part, the market for underground stormwater treatment systems is driven by regulations, such as the National Pollutant Discharge Elimination System permits and EPA’s total maximum daily load requirements. In Maryland, for example, owners of stormwater treatment systems are presently required to remove pollutants from the first half-inch of runoff from an impervious area. “Studies have shown that this first half-inch is the dirtiest portion of the runoff,” states Jay Beatty, environmental engineer with the Montgomery County, MD, Department of Permitting Services. “This year, the state will tighten the rules to require treating the first inch of runoff.”
Increases in the amount of runoff treated is being driven by studies showing which storm events in urban areas have the most detrimental impact on the quality of receiving waters, says Walt Stein, manager of project development for CDS Technologies Inc. He reports that a 1990 study by George Chang with the City of Austin, TX, found that a half-inch rain event washed off only 40% of the pollutant load. He notes that monitoring in Portland, OR, revealed that devices sized to treat the city’s water-quality design storm of 0.83 in. would have treated only 8-26% of the pollutant load from three catchments. What’s more, Stein adds, analysis of Nationwide Urban Runoff Program data by Robert Pitt, professor of civil and environmental engineering at the University of Alabama, shows that significantly larger events generate the largest mass of pollutants.Tighter environmental protection regulations are prompting city and county officials to retrofit stormwater systems to clean up water pollution, and so is the public. Web sites on the Internet inform citizens about their local watersheds and the impact that stormwater running off their neighbor’s property will have on the watershed in which they live. Brevard County in Florida includes the Indian River, a national protected estuary. “People here tend to be very environmentally conscious,” England says. “When they see trash and pollutants running from pipes into canals, they often call the county to clean it up, not just for the aesthetics, but to protect manatees, fishing, and other environmental concerns.”Usually underground stormwater treatment systems are not designed to control sediment at active construction sites. They are key components of a storm-drain system, however, and are often installed at the same time. “During construction, it’s important to block off or bypass inlets to these water-quality control devices,” stresses Beatty. “Either that or delay installing them until the latest possible time.”Typically these systems are installed in urban areas as stand-alone devices to capture sediment and other pollutants in runoff from commercial and industrial properties, transportation and distribution centers, highways, and other developed sites. They can also be installed as part of a treatment train approach, notes Mark Smith, national sales manager for Stormceptor, which manufactures a system that uses gravity to separate sediment, other solids, and free oil from stormwater.“For example, our system can be installed as an upstream treatment to make sediment ponds, wetlands, infiltration systems, and other conventional BMPs [best management practices] more effective,” he explains. “This extends the maintenance interval of conventional structures and may prevent contamination of water resources in the event of total petroleum hydrocarbon spills.” Underground stormwater treatment systems, Smith adds, also eliminate the need for catch-basin sediment traps and reduce the frequency of scheduled sewer flushing.New development projects aren’t the only places where underground stormwater treatment systems are installed. In Maryland, a state long recognized as a national leader in stormwater management, probably no county is stricter about controlling pollution from stormwater than Montgomery County. Aging shopping centers and office buildings are being demolished to make way for new buildings. Even though many of these sites have been covered with impervious surfaces for decades, the county requires all such redevelopment to meet current stormwater management rules.“You can argue that redevelopment doesn’t increase the amount of runoff or pollutant load from these sites,” Beatty says. “Our view is that, as a water resources planning agency, we have to anticipate more and more redevelopment in the county throughout the coming decades and now is the time to provide for stormwater management. This offers the opportunity to introduce water-quality controls in older areas and better technology to older facilities.”Cardello notes that underground stormwater treatment systems also have a place in brownfields where soils are contaminated. “Usually you want to minimize the amount of soil being moved on those sites. You can install an underground stormwater treatment system, which is relatively small, without disturbing nearly as much ground as you would digging a big settling pond.”In many cases, one unit may be enough to treat stormwater runoff from a given site. The StormFilter system features an underground vault containing cartridges with filters. “Often we put in one central vault at the lowest point and all the catch basins and drains empty into it,” describes Brendan Fitzpatrick of Stormwater Management, which manufactures the unit. “But if the site sits on a hillside with water running in different directions and we don’t want to pipe all the water to a central location, we might install a number of smaller underground boxes, each with fewer filters, at the individual catch basins.”Filters and HydrodynamicsTwo basic choices of underground stormwater treatment systems are available. A filtration unit stores water temporarily by releasing it at a slower rate than it enters the system. Various media trap and filter out pollutants during this storage period. A sufficient change in elevation between incoming and outgoing runoff pathways is required for proper operation. “On a development site, the existing storm drain might not be deep enough to accommodate the necessary change in elevation, which may be 4 to 7 feet,” Beatty points out.Filtration systems are common in Montgomery County. One type is a relatively new device that uses a sand filter to separate pollutants from runoff. It consists of a concrete storage vault, typically about 50 ft. long, 10 ft. wide, and 10 ft. deep, with a 2-ft.-thick bed of sand spread over the bottom. Runoff from impervious areas flows into the structure, and sediment and other pollutants, such as nutrients and metals, are filtered out as gravity pulls down the water through the sand.“We’re still building a track record with this type of system,” Beatty says. “After about three months, the top layer of sand may become clogged with finer grits and is skimmed off. This rejuvenates the filter. The entire bed of sand will probably have to be replaced after two or three years of use at a commercial site.”Another type of underground filtration system used in the county is a large underground concrete vault in which runoff is piped through canister filters before being discharged. Beatty reports these systems are easier to maintain than the sand-filter units.The other basic choice in underground treatment systems is a hydrodynamic or flow-through structure, which uses enhanced gravity separation and engineered flow controls.“They are not as expensive as filtration systems,” Beatty observes. “They are relatively easy to install and can fit among utilities where installation of a filtration system might require relocating large pipes and gas lines. However, hydrodynamic structures may not remove as many pollutants as filtration systems can.”
Design Considerations
Deciding which of the various underground stormwater treatment systems fits your site means zeroing in on the types of pollutants to be removed. Some are designed to remove sediment but might not control trash and floating debris. Others will filter out hydrocarbons but not sediment. In some cases, a combination of different types might be necessary. Some agencies help narrow the search by limiting the selection to those on an approved list. Site conditions may dictate a unit with either a horizontal design or a vertical design. For example, soil conditions or groundwater levels could favor a more shallow excavation and a horizontal unit.One of the trickiest design decisions involves sizing the unit. “There are no standard numbers,” says England. “Basically you analyze the drainage basin, pipe capacities, and expected flows, then you design a system to handle it.”One difficulty in designing a system is accurately estimating runoff flows and accumulation of sediment and other pollutants, because one site is seldom identical to any other and the number and intensity of storms can vary considerably from one year to the next. Stormwater treatment requirements and public expectations can also differ from one locality to another. Cardello describes some considerations involved in sizing a system to handle highway runoff that can reach flows of up to 125 cfs in Rhode Island. There a stormwater treatment system is required to remove at least 80% of total suspended solids from a design storm.“To reduce maintenance costs, we like a system big enough to achieve that amount of contaminant removal on an annual basis,” he explains. “That limits the need to clean out the collected solids to just once a year rather than three or four times annually. However, that also means the sump must be big enough to handle the sediment from the entire year’s runoff. Several commercial systems are designed so that if the sediment storage capacity is reached, it will bypass the entire flow. That’s especially critical with highways because we can’t have runoff backing up and flooding roads.”When it comes to designing systems, Smith of Stormceptor notes the distinction between a design event and annual performance of a device. “While a water-quality volume or peak flow from a design storm may be a simple concept to grasp, it is a poor substitute for BMP-specific criteria. Flow-related criteria only address the volume of water treated and not the expected performance.” As a result, he reports, his company developed a computer simulation model for its system based on the USEPA Storm Water Management Model Version 4.3.“The model takes into account the drainage area, local rainfall, buildup/wash-off characteristics, particle-size distribution, and settling calculations to estimate suspended sediment captured,” Smith explains. “The model then calculates the percent of annual runoff treated, not bypassed, with different bypass flow rates.”As for effectiveness of the various systems, Beatty for one is waiting for more definitive findings. “Studies show that underground filtration facilities generally perform reasonably well in removing pollutants. But I’d like to see a good, independent study of all the proprietary hydrodynamic structures. Each one, it seems, claims to be the best.”“Many subsurface structures are relatively new, and we’re just starting to get data about their actual effectiveness,” adds Cardello.Maintaining Efficiency
Regardless of the effectiveness of any one system, all will suffer a drop in performance if not properly maintained. Excess accumulation of sediment and other pollutants can cause surface structures, such as sediment ponds, to short-circuit treatment and cause subsurface systems to scour and lose sediment.Maintenance is important for both surface stormwater treatment systems and underground units. However, a full or clogged surface facility is easier to see than a subsurface setup. As a result, manufacturers are working to raise awareness about the importance of proper inspection and cleanout procedures.“Often with an underground system, it’s out of sight and out of mind,” Beatty says. “You can’t tell it needs maintenance unless you open the manhole and go down inside to examine the structure. If you clean out these units routinely, you can put off and reduce the cost of longer-term maintenance of them and downstream structures. Proper maintenance also plays a big role in extending the useful life of underground systems and any downstream surface treatment facilities.”Despite the value of sound maintenance, it’s often given a low priority if not outright neglected, say the experts.“Maintenance is the most overlooked aspect of underground stormwater treatment systems by owners and regulators,” states Smith. “Any stormwater treatment system will fail to work if not maintained properly.” One way his company responds to this challenge is by providing owners a manual that describes the operation and recommended maintenance procedures for its products. Stormceptor also offers a maintenance program to owners through licensed maintenance facilities throughout the United States. “We want to make it as easy as possible for owners to get rid of sediment and other pollutants collected by these systems,” Smith remarks.Assigning responsibility for maintenance is another way to help ensure that it gets done. “When we specify an underground stormwater treatment system, we include a maintenance program and a schedule for carrying it out, and we identify the entity responsible for the program,” Cardello explains. “If it’s a private project and the contract calls for a maintenance program, the regulatory authority can enforce that provision. However, if it’s a public project, a government agency is often reluctant to enforce against another one.”Obviously, inspecting and cleaning these systems costs money. That explains part of the problem, at least in the public sector. “Figure out what it costs to clean one structure and multiply that by the total number of units in a municipal system, and many communities don’t have the budget to keep up with a maintenance program,” says Cardello.England agrees. “Generally public works people are pretty overwhelmed by an almost daily crisis or emergency. If you assign them to clean all these devices in addition to their regular duties, the job tends to fall to the bottom of their priority list.”It’s a similar problem with regulatory agencies, notes Tighe. “Some regulators don’t have the resources to enforce maintenance procedures the way they would like to.”Because the pollutant load at a given site can change significantly from one time to another, an effective maintenance program begins with frequent inspections. That can help head off unexpectedly large accumulations of sediment and other materials.Cleanout Considerations
A number of manufacturers of underground stormwater treatment systems have responded to the maintenance challenge by designing their units to be as easy to maintain as possible. Many systems can be cleaned in several hours using a vacuum truck. Even then, though, there are limitations.“You want to be able to back up the truck to the access hole, drop in the hose, and suck up the material,” describes Cardello. In some cases, a machine equipped with a clamshell bucket can be used.CDS Technologies produces a system that uses a vortex to separate solids from the runoff. It can be cleaned out with a vacuum truck or by removing a sump basket. The largest basket, 10 ft. in diameter and 6 ft. deep, holds up to 10 tons of pollutants. The basket is emptied into a dump truck for disposal.The frequency of cleanouts varies with the system and the pollutant load. Manufacturer recommendations for a specific site can range from several times a year to no more than once every year or so.In designing underground treatment systems, Cardello and his colleagues specify cleaning them when the volume of collected material exceeds two-thirds of the unit’s storage capacity. “For economic reasons, we prefer an annual cleanout. But if we need two per year, then we like a spring cleaning to remove sand applied on roads and parking areas in the winter and a fall maintenance after trees have dropped their leaves.”McTighe Industries makes a gravity-type oil and water separator. “We recommend semiannual maintenance unless experience shows otherwise,” says Jeremy Morrison, the company’s engineering sales manager. “Due to the ability to monitor and maintain oil storage, periodic maintenance is recommended more for removal of rags, chunks of wood, and other solids than it is for oils.”To promote timely inspections and cleaning of their systems, several manufacturers are currently looking into offering their customers maintenance contracts.In some areas of the country, maintenance of underground stormwater treatment systems isn’t an option for owners–it’s a requirement. Montgomery County is one of those. The county holds a maintenance agreement and easement on each stormwater management facility that allows county offices to inspect the units. “It’s a legal covenant that requires owners of the facility to maintain it to our standards,” explains Beatty. “They can do the maintenance themselves or hire contractors who do this.”Similar to a number of other areas in Florida, Brevard County has a stormwater utility. Part of the money it collects is used to pay private contractors to clean out public stormwater treatment facilities, reports England. “It helps reduce the burden on the Public Works Department,” he says.As these and other efforts to reinforce the need to maintain underground stormwater treatment spread within the industry and among the public along with continued improvements in the technology itself, the quality of stormwater discharged into waterways can only get better.What’s Available?Here are some of the systems currently available for removing pollutants in runoff from developed sites:Baffle Box. Produced by Suntree Technologies Inc., this unit separates sediment and trash without head loss and features bypass filter separators and oil booms. It installs in-line with stormwater drain pipe, usually at an outfall to a lake or a pond. Water contaminated with sediment, foliage, and trash travels over oil collection booms and then over aluminum grills, where sediment falls through and is retained behind baffles. The grills trap grass clippings, branches, and other items. The company reports the unit has removed up to 46,000 lb. of sediment per month. Water flow can be stopped for cleaning if the unit is installed below water level or if base flow is present. The rounded bottom also aids cleaning.BaySaver Separation System. This gravity-driven system separates suspended solids, oils, and debris from stormwater runoff. It consists of two standard precast concrete manholes and a separator unit. Large sediments settle in the first manhole. Oils, floatable solids, and light suspended solids are diverted to the second manhole. In the storage manhole, oils and floatable solids rise to the top of the water column while lighter solids fall to the bottom and are stored off-line to prevent resuspension. One unit treats runoff from up to 8.5 impervious ac., with a treatment capacity of 11.1 cfs and a maximum bypass capacity of 50 cfs.Best Management Technologies. Best Management Technologies filter systems, designed for easy monitoring, handling, and disposal, allow runoff entering storm sewers to be filtered multiple times before exiting. They remove such pollutants as trash, sediment, heavy metals, oil, and grease. A variety of filter styles can be used in existing or new storm sewers. The system can be custom designed and used with other technologies. Units are made of noncorrosive, industrial-grade materials. Applications include commercial and industrial staging areas, public rights of way, corporation yards, salvage industries, and parking lots.CDS Technologies. Made of precast concrete and stainless steel, CDS Technologies’ units combine a nonblocking, nonmechanical screening system with swirl concentration (vortex) solid separation to remove suspended solids, fine sediment, plastics, paper, leaves, and other floatable materials as small 1 mm from runoff flows of 0.6—300 cfs. The units are equipped with a conventional oil baffle to control oil and grease and can screen dry-weather diversion flows before pumping them to a wastewater treatment facility. They can be cleaned using a vacuum truck, by removing a basket, or with an underflow pump.PetroPak. This unit, made by McTighe Industries, is designed to remove free oils and other lightweight material and solids as small as 20 microns from oil-water mixtures, while producing effluent quality of less than 10 ppm of free oil. The unit features a polypropylene matrix of oleophilic (oil-attracting) fibers layered from coarse to fine and enclosed in a stainless steel framework. These fibers attract droplets of oil too small for gravity-assisted separation. As these droplets collect and coalesce, they rise upward and separate from the water. The self-purging unit can be installed below or aboveground.PS International Oil-Water Separators. Gravity-enhanced PS International oil-water separators feature a single corrugated plate and multiple corrugated parallel plates to treat flow rates from 1 to 5,000 gpm. Installing multiple units in parallel will treat larger flows. Available polypropylene coalescer is designed to remove oil droplets as small as 20 microns. Two models can produce a free oil and grease effluent quality of 10 ppm or less. An integral solids interceptor can be used in high-solids loading situations. Other options include automatic water pump-out systems. Units are designed for easy removal of sludge.Stormceptor System. Stormceptor products, which include inline, inlet, and submerged units, use gravity to remove sediment, metals, nutrients, and hydrocarbons from stormwater runoff and store them for removal. The vertically oriented precast concrete units consist of a lower treatment chamber and a bypass chamber, which prevents resuspension and scouring of trapped pollutants during infrequent high-flow storm events. Pollutant holding capacities are available for up to 1,470 ft.3 of sediment and 3,055 gal. of oil.

StormFilter. Manufactured by Stormwater Management, this system uses cartridges filled with various site-specific media to remove sediment, oil and grease, soluble heavy metals, organics, and soluble nutrients. Precast, l