Zaid K. Chowdhury, Christopher P. Hill, Ferdous Mahmood, Michael J. Sclimenti, R. Scott Summers, and Christopher Valenti

OBJECTIVES:

The objectives of this project included the following:

1. Characterize disinfection by-products (DBP)/DBP precursors in consecutive systems (CS)

2. Understand impacts of wholesale system (WS) treatment and disinfection practices and available water age management strategies on DBP levels in CS

3. Evaluate/recommend DBP control strategies and treatment technologies for CS

4. Develop guidelines for negotiating water quality requirements between WS and CS

5. Provide guidance to allow CS and WS to work together to identify effective DBP control strategies

6. Develop/discuss tools to allow CS to evaluate storage facility operations, more effective operating strategies, and inlet/outlet pipe designs to improve water quality

BACKGROUND:

Consecutive systems (CS) serve nearly one-third of the water customers in the United States. One challenge facing CS is that, although they are responsible for achieving compliance with the Stage 2 Disinfectants and Disinfection By-products Rule (DBPR), in many cases they must rely on their wholesale system (WS) service provider to provide water that allows them to comply with the rule. The focus of this project was to identify strategies to assist CS in complying with the Stage 2 DBPR.

HIGHLIGHTS:

1. Many WS and CS lack a good understanding of water age and flow patterns in their distribution system. Coordination of WS and CS monitoring programs is one strategy to provide a better understanding of the impacts of treatment and distribution operations on water quality.

2. Optimization of CS distribution system operation can provide water quality benefits and may, in some instances, help to achieve compliance with the Stage 2 DBPR.

3. Often, water entering the CS distribution system is sufficiently high in DBP concentrations, so some modification in treatment or distribution system operation by the WS may be necessary.

APPROACH:

This project began with a survey of WS and CS water quality, treatment, and operational practices. Based on the survey data, several WS and CS were chosen for supplemental sampling to better characterize DBP growth from the water treatment plant through the CS distribution system. In addition to supplemental monitoring, hold studies were conducted in the laboratory and a number of utilities were selected for bench-scale evaluation. The storage facility operational and design data provided by some WS and CS were used to develop a spreadsheet tool that can be used to conduct a general evaluation of storage tank mixing and water quality.

RESULTS/FINDINGS:

Historical water quality data may be insufficient to adequately characterize DBP formation in WS and their subsequent CS. Coordination of WS and CS monitoring will help to better characterize DBP formation and aid in developing strategies to reduce DBP concentration in both systems.

CS should take steps to optimize distribution system operations, including evaluation of the impact of storage facility design and operation on water age. Optimization of distribution operations may enable CS to comply with the Stage 2 DBPR or may reduce levels sufficiently that the cost of treatment improvements or other strategies by the WS are reduced.

It was observed that, on average, approximately 70 percent of DBP formation occurs at the water treatment plant. Therefore, distribution system optimization by the CS may not be sufficient to achieve compliance with the Stage 2 DBPR in some cases. In such cases, optimization of the WS distribution system, enhanced precursor removal, or alternative disinfection strategies by the WS may be necessary for the CS to achieve compliance. CS and WS are urged to work together to identify a mutually agreeable compliance strategy. If an agreement cannot be reached, then the CS will need to evaluate treatment strategies.

IMPACT:

This research helps to better characterize the relative formation of DBPs in WS and CS and identifies effective strategies to reduce those concentrations. It provides suggestions regarding how to identify acceptable DBP level goals for CS entry points and strategies for CS to reduce DBP concentrations sufficiently to achieve compliance with the Stage 2 DBPR. The report discusses development of coordinated DBP monitoring programs to assist in characterization of DBP formation in WS and CS, and provides recommendations regarding negotiations between CS and WS.

MULTIMEDIA:

The project includes an Excel-based program on CD-ROM to evaluate mixing characteristics in distribution storage facilities.

RESEARCH PARTNER:

U.S. Environmental Protection Agency

PARTICIPANTS:

Thirty-nine utilities participated in this study.