Bruce A. Johnson, Joseph C. Lin, David Rexing, Mao Fang, John Chan, Laura Jacobsen, and Patricia Sampson

OBJECTIVES:

This objective of this study was to examine how localized treatment of disinfection by-products (DBPs) was a viable approach to meeting the Stage 2 Disinfectants and Disinfection By-Products Rule (Stage 2 DBPR).

BACKGROUND:

The Las Vegas Valley Water District (LVVWD) provides water to about 1.2 million people. Water age can be 12 to 15 days at the extremities of the distribution system because of the size of the area served. Due to the long detention time in the distribution system, trihalomethanes (TTHM) at several locations can approach and exceed the Stage 2 DBPR TTHM limit of 80 micrograms per liter (µg/L). It would be extremely expensive and of little value to treat for DBPs at a centralized location because of the size of the central treatment plants. This tailored collaboration project was initiated with LVVWD, South Central Connecticut Regional Water Authority, and the City of Phoenix to provide information on how well simple treatment processes might perform in the distribution system for removing TTHM and haloacetic acids (HAA5).

HIGHLIGHTS:

For the three treatments used at the pilot plant, air stripping was effective at removing TTHM, biologically active carbon (BAC) was effective at removing HAA5, and granular activated carbon had short-term removal of both TTHM and HAA5. Even a blend of 25 percent treated water and 75 percent reservoir water was able stay below the Stage 2 DBPR limits after 72 hours of rechlorination. Cost models show a savings of $18 million in capital cost and $4 million/year operations and maintenance cost for a 5-mgd localized treatment plant with BAC treatment compared with 50 mgd at a central plant with BAC treatment.

APPROACH:

The research team conducted a literature review of existing treatments for DBP removal and existing models for DBP reformation, determined which carbon to use for the pilot plant study, operated a pilot plant to examine treatment effectiveness, examined DBP reformation through simulated distribution system testing, formed a DBP model that has both DBP treatment and reformation, incorporated a cost model, and reviewed other important issues (e.g., hydraulic modeling, regulatory issues).

RESULTS/FINDINGS:

Based on pilot testing at an extremity of the LVVWD distribution system, localized treatment for removal of DBP can be an effective alternative for meeting the requirements of the Stage 2 DBPR. After 1 year of pilot plant testing, the following conclusions can be drawn:

• Air stripping consistently removed 75 to 85 percent of TTHM.

• Air stripping had no effect on HAA5.

• GAC was found to be effective in removing both TTHM and HAA5 over a limited number of bed volumes.

• GAC removed more than 90 percent of TTHM for the first 10,000 bed volumes. Thereafter, removal efficiency decreased to about 20 percent after 35,000 bed volumes.

• There was almost complete removal of HAA5 up to 10,000 bed volumes. After 10,000 bed volumes, HAA5 removal efficiency decreased to 70 percent while the columns were in transition from GAC to BAC. After 25,000 bed volumes, the transition appeared to be complete as evidenced by a return to 100 percent HAA5 removal.

• Removal of chlorine residual or addition of phosphorous before to the GAC columns had no apparent impact on the rate of conversion from GAC to BAC.

• All chlorine residual was removed within the first 2 minutes of EBCT making it inevitable that GAC would be converted to BAC unless the media is replaced.

• The conversion of GAC to BAC had no impact on TTHM removal.

• Backwashing had no impact on the performance of BAC for HAA5 removal.

• If TTHM and HAA5 both exceed or are close to exceeding the Stage 2 DBPR limits, air stripping followed by GAC/BAC may be considered.

• During pilot plant testing, use of GAC/BAC resulted in 90 percent TTHM removal and 100 percent HAA5 removal through 45,000 bed volumes.

• Pilot plant testing indicated that localized treatment for DBP can be very effective in reducing both TTHM and HAA5 in the distribution system.

Based on simulated distribution system (SDS) testing on water, DBPs will continue to form after localized treatment and rechlorination at about the same rate as before treatment. But, with the level of treatment previously discussed, the formation potential for up to 360 hours can be kept below Stage 2 DBPR limits.

A cost model was developed to estimate the cost of localized treatment. The cost model was linked with the treatment and reformation models for ease of use. A utility can use this cost model to estimate the economic viability of hot spot treatment and to compare costs of localized treatment with centralized treatment.

IMPACT:

Use of the treatment/reformation/cost model will help a utility determine which approach they will take to meet the Stage 2 DBPR. Significant savings may be available for a utility that is in need of treating only a portion of its distribution system. If the utility chooses to go the route of localized treatment, regulatory issues will arise, such as release of TTHM or HAA5 into the atmosphere or rechlorination of distribution system water.

MULTIMEDIA:

The report includes a CD-ROM containing the Localized Treatment Evaluation Model.

RESERCH PARTNERS:

• Las Vegas Valley Water District

• South Central Connecticut Regional Water Authority

• City of Phoenix Water Services Department