Netting System Helps With CSO Control

Dec. 1, 2002
The small but densely populated community of Ridgefield Park, NJ, is located on the Hackensack River. It is part of an urban concentration that includes New York, Newark, and Jersey City.
The Bilco Company's H-20 aluminum doors provide access to the in-line storm water netting system.
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The small but densely populated community of Ridgefield Park, NJ, is located on the Hackensack River. It is part of an urban concentration that includes New York, Newark, and Jersey City. This is a double-edged sword, as nearness to these areas provides a wealth of cultural and economic opportunity. However, it can also present problems, one of which is environmental protection.

The town's Environmental Commission was formed in 1996 "to study and make recommendations concerning open space preservation, water resources management, air pollution control, solid waste management, noise control, soil and landscape protection, environmental appearance, marine resources, and the protection of flora and fauna in the village." As its mission states, one of the Commission's concerns is management of the water supply.

Ridgefield Park is one of about a thousand communities across the United States with a combined sewage system, meaning that a single set of pipes is used to contain and direct both stormwater and sewage flows. During a wet weather event when flows are increased by stormwater, sheer volume makes it likely that overflows will occur and be dumped into the nearby water supply; in Ridgefield Park's case, the Hackensack River. While built-in overflow points ease the total flow going to the treatment plant, the pollution created by this process becomes an issue to the local community as well as neighboring areas downstream whose water resources are thus tainted.

The inner structure of the Netting TrashTrap® uses knitted mesh nets anchored into the combined sewer system where outflows are fed into the receiving water body.
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Floatables are a particularly hazardous and unsightly contaminant resulting from outfalls in a combined sewage system. Floatables include street litter and other debris that enter the combined sewage system during a wet weather event and are transported through the system into the nearby water source.

For a waterfront area, a large amount of business rests upon aesthetics. New Jersey's beaches are a substantial source of tourist income; likewise, a vast deal of money is to be made on the waterfront in commercial and residential development. None of these can flourish on a beachfront littered with unsightly and unhealthy floatables build-up. Likewise, the U.S. Environmental Protection Agency and the New Jersey Department of Environmental Protection have developed regulations requiring that communities with combined sewer system take steps to eliminate or minimize pollution caused by combined sewer overflows.

With all of this in mind, Ridgefield Park's officials worked with consulting engineers to explore different floatables control options, including: stationary, rotary, or mechanical screens; dynamic separators; or netting.

Screening is an effective tool against contamination by floatables, and is an attractive option because it does not require the investment of new subsystems; however, it can be expensive. Dynamic separators are excellent for the task, but also cost a great deal. These systems are best used in water treatment facilities rather than for local applications. Netting is a less expensive option, which is attractive unto itself. Of course, cost does vary based on the type of installation necessary for the application, but, it still weighs in at considerably less than other systems. Beyond the price tag, it is recognized by the EPA as an acceptable solution for floatables control.

According to the Assistant Superintendent of Public Works, Mike Monroe, the town's final two choices were netting and screening. In the end, netting was chosen based on cost relative to benefits. Superintendent of Public Works Alan O'Grady notes that the cost of screening was nearly twice the cost of netting for the application.

Fresh Creek Technologies of New Jersey was chosen as the provider. Its Netting TrashTrap® system comes in three models (in-line, end-of-pipe, and floating) which can withstand flow velocities of up to five feet per second and can be scaled to filter floatables of a half inch size.

Sizing and determination of installation type is based on the site in which the netting is required: peak flow volume that the system must transmit, peak velocity which the system will experience, and floatables volume anticipated during the maximum wet weather event. Nets are anchored into the combined sewer system at strategic locations where outflows are fed into the receiving water body. The nets trap floatables during overflows, keeping them from entering the receiving water.

Ongoing maintenance of the system involves replacement and disposal of the nets. Because the system works by using the energy of the overflow, no power is required, and there are no moving parts to wear out. The EPA paper which addresses the system's use affirmed that it can provide removal efficiency of greater than 90 percent for trash and floatables when properly operated and maintained.

For Ridgefield Park, where the product would need to be placed within the existing sewer system, six in-line installations were chosen. These systems cannot be viewed from above ground, and are installed in line with the outfall pipe. It's underground location is also ideal for public safety and health. The system is comprised of multiple parts:

• Disposable, knitted mesh nets
• Stainless steel frame to hold the nets
• Drain grating
• Precast concrete system chamber
• Access door and access hatch
• Secondary overflow screen

The knotless, knitted mesh nets, with standard sizes rated for 500 pounds or 25 cubic feet of captured pollutants, are tested independently for strength and performance to ASTM standards. The frame is used to hold the net in place and provide support during high-velocity flows. The precast concrete chamber minimizes site work and makes maintenance simpler. The secondary overflow screen and grating act as their names imply.

Because the installation itself is underground, access is necessary to safely retrieve and replace the nets. While Fresh Creek offers maintenance on its units, many customers, such as Ridgefield Park, choose to undertake maintenance as a part of their local combined sewage system maintenance program. The principal goal for Fresh Creek is to ensure that access is as safe and simple as possible.

Fresh Creek's in-line systems use The Bilco Company's H-20 aluminum door for access. The large door, measuring 5 feet by 10 feet, is designed to withstand the weight of vehicular or pedestrian traffic, while its electrostatically coated springs, forged aluminum hinges, and composite spring tubes allow the door to remain durable in corrosive environments. The door's compression spring operators also maintain controlled opening and closing motions for ease of operation, as well as ensure the user's safety while accessing the nets.

The in-line system was one of six installations made in Ridgefield Park nearly three years ago. The regular maintenance of the site includes replacement of the nets by the town's CSO patrol, on a scheduled basis and after significant wet weather events. Each time the nets are replaced, the nets are weighed to determine the amount of floatables that would have remained in the water supply.

According to O'Grady, 30 tons of debris were pulled from the nets in six locations in the town just last year. This is the same amount would have been discharged into the town's water resources, and would have gone downstream into neighboring waters. In terms of qualitative improvements to the town since netting was installed, Monroe states that the town's water is visibly cleaner and clearer since the installations. O'Grady notes that the nets are the town's first line of defense against floatables, and that they have been pleased with the product's performance.

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