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"Stuff grows on pipes." That's how one drinking water expert sums up the National Risk Management Research Laboratory's (NRMRL's) innovative research into the pipe-wall dynamics of the distribution systems that carry finished water from treatment plant to consumer tap.

Although millions of miles of distribution pipes transport drinking water worldwide, surprisingly little is known about pipeline interactions among treatment disinfectants, chemical contaminants, and pipe deposits—the "stuff" that coats pipe walls. To better understand these interactions, NRMRL scientists and engineers have created a model water distribution system at the Environmental Protection Agency's (EPA) Test and Evaluation Facility in Cincinnati, Ohio. The model, called the Distribution System Simulator (DSS), is unique in the United States for its testing capabilities.

Background

Clean, safe drinking water depends on a complex set of interactions among water sources, purification treatments, and distribution systems. Consider just a few:

• Source water may contain high levels of calcium carbonate, iron, or manganese, creating "hard" water. If the hardness is not removed at the water treatment plant, biofilm and scale can form on the inner walls of the distribution pipes. Biofilm is made of layers of microorganisms.
• The formation of biofilm is further dependent on the type of pipes, whether iron, polyvinyl chloride (PVC), or concrete-lined. In turn, biofilm and scale can affect the life span of the pipes, thus playing a role in the cost of the distribution system infrastructure.
• Disinfectants (typically chlorine) introduced during the treatment process may react with organic carbons naturally present in the source water to form unwanted disinfection by-products (DBPs).
• Biofilm and scale can decrease concentrations of disinfectants. They can also cause taste and odor problems and harbor disease-causing microorganisms or chemical pollutants, possibly releasing them into drinking water in distribution systems.

NRMRL's DSS allows researchers to study many of these complex interactions.

The "Loops"

The DSS unit is designed to simulate the continuous flow conditions found in a typical distribution system. Five individual 75-foot lengths of 6-inch iron pipes and one 75-foot length of PVC pipe are arranged in a pipe-loop configuration. Above the loop units, two 1,500-gallon reservoir tanks simulate stored water as in a distribution infrastructure system. The loops can be operated individually or collectively in a variety of experimental configurations. Two 2,500-gallon stainless steel holding tanks are used for pre-mixing test water with varying properties (e.g., chloramines versus chlorine).

The model is operated by a computerized data acquisition system that controls experimental variables, including pH, temperature, chlorine, flow, and chemical addition. Biofilm samples can be collected from 18 points in the pipelines and from a 100-gallon recirculation/mix tank.

Current research includes experiments on flow rate, organic carbon concentrations, DBP formation in both iron and PVC pipes, and biofilm sampling methods. Future research, done in collaboration with the EPA's Office of Water and regional offices, will assess the risk of contaminants that can escape many treatment processes intact and the ways in which they interact over time with biofilm and pipe walls.

By evaluating the physical, chemical, and biological phenomena that occur in drinking water distribution systems, the DSS unit serves EPA's mission to protect human health and the environment.

Contact: Patricia Schultz, NRMRL Office of Public Affairs, 513-569-7966.
For more information, go to http://www.epa.gov/nrmrl/wswrd/dw/dsr.html.