Technology aids Gwinnett County I/I assessment

An aging sewer system is literally falling apart. Rapid development hastens the sewers’ deterioration. Worsening incidents of pollution increase the likelihood of heavy fines for failure to comply with environmental regulations.

In metropolitan Atlanta, such bleak facts of life are ascribed to the city of Atlanta – not its suburbs.

And while suburban Gwinnett County, Ga., does not face such a crisis, officials there are leery that if they don’t act quickly to get a handle on their own problems, the county’s rapid growth in recent years could foretell serious trouble ahead.

Consequently, the county has implemented an infiltration/inflow control program to control costs and improve efficiency. Gwinnett’s department of public utilities, which manages the wastewater collection system, oversees the program and also has developed a GIS for wastewater.

Much of the county’s collection system is relatively new, but the occurrence of I/I has become an increasing concern during the later fall and winter months. For example, a spike in wastewater flows occurred countywide in the fall of 1995, heightening concerns about I/I’s effect on the system.

Average annual flows in 1995 were 31.54 mgd. However, flows peaked at 35.13 mgd in October following an unusually early onset of rains and increased groundwater levels caused by the remnants of tropical storm Opal. Monitoring into the early months of 1996 continued to indicate increasing levels of I/I.

The average flows in the first quarter of 1996 were 37.19 mgd. As winter rains subsided, flows to the treatment plants decreased dramatically.

Increased I/I levels encroach upon the treatment plant capacity that Gwinnett needs to accommodate its new customers. Gwinnett officials do not want the presence of I/I to hinder the county’s economic boom. As is true throughout the country, the allocation of capacity in the wastewater system is one of the most important considerations in the development of property.

Providing treatment capacity for county residents who want to get off their existing septic tank systems and onto the county’s sewer system is another principal concern for the Gwinnett officials.

“Large areas of the county are served by existing septic tank systems,” says George Heckman, director of Gwinnett’s department of public utilities. “When these tanks fail to function, they can contribute to deg-radation of stream and surface water quality. Our desire is to maximize available treatment capacity to accommodate both new development and existing development served by septic tanks. I/I reduction is a way that we can address both needs as well as improve surface water quality.”

The costs of transporting and treating I/I flows are yet another concern to Gwinnett officials, especially when these flows are taking up capacity that should be reserved for ratepayers. Although no one pays for I/I directly, all users ultimately pay because the increased costs must be spread to all system users.

County officials initially attempted to ferret out I/I problems with existing staff. Public utilities employees inspected manholes in areas suspected to be the source of the problem.

However, although leaking defects were found, no major leaks were uncovered that alone could explain the increase of flows. It quickly became apparent that a coordinated effort would be required to effectively identify and remove excess I/I, as the limited resources of the county staff were insufficient.

The I/I program’s immediate short-term goal is making the most effective use of existing capacity. The long-term goals are to obtain the most cost-effective service for the system users, reduce the potential for sanitary sewer overflows and develop a strategy to minimize future I/I problems.

Deciding that contracting for services with an I/I specialist would be the most effective way to tackle the problem, the county selected Wheaton, Ill.-based RJN Group, an engineering and information technology firm, and Marietta, Ga.-based Mayes, Sudderth and Etheredge, an architectural and engineering firm.

Gwinnett County’s contract contains provisions for a pilot area for sewer system rehabilitation. If authorized by the county, this pilot area will be used to provide a benchmark for predicting program results.

Initially, a comprehensive flow monitoring program was conducted countywide to pinpoint the extent and severity of I/I in the Gwinnett system. A total of 100 temporary flow meters were used to establish a baseline of flow from December 1996 through March 1997. Permanent in-line flow meters were installed for long-term system management.

Two drainage basins – the Yellow River/Sweetwater Creek Basin and the Beaver Ruin Basin – have been thoroughly evaluated.

The intensive survey includes an above-ground reconnaissance survey to verify mapping and detect manhole defects. Additionally, where flow data analyses indicate severe infiltration, full-descent manhole inspections will be undertaken. The worst lines will be visually inspected with specialized television equipment.

Rainfall simulation techniques, along with smoke testing and dyed water flooding, were employed to study the collection system response to wet weather.

Smoke testing involves introducing a nontoxic test smoke into the sewer and observing for smoke outletting from the ground, drainage system structures, cleanouts, downspouts or other sources, which could be the origin of surface water gaining entrance to the sanitary sewer.

Smoke sources are confirmed and quantified by introduction of dyed water. The dyed water can be observed in the receiving sewer, and an estimated flow rate determined.

Gwinnett County’s GIS system will benefit from the project because coordinate data has been collected and will be uploaded to the county GIS system to provide accurate information for a sewer layer.

The project is conceived as the initiation of a multi-year effort to identify and remove extraneous I/I flows. The county hopes to remove as many I/I sources as possible at a modest cost.