CASE STUDIES AND VAlUE OF RESEARCH
localized Treatment for DBPs
Mao Fang and Laura Jacobson, Las Vegas Valley Water District, nevada
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, total trihalomethanes (TTHMs) at several locations can approach and exceed the Stage 2
Disinfectants/Disinfection By-Products Rule (DBPR) TTHM limit of 80 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. The project
objective was to provide information on how well simple treatment processes might perform in the distribution system
for removing TTHM and 5 haloacetic acids (HAAs).
The LVVWD uses a water distribution system modeling approach to track and address water quality issues. To
address TTHM issues in its distribution system, the LVVWD has employed operating strategies to reduce water age
in its distribution system. A real-time based water age simulation program was developed to assist with day-to-day
operational planning activities. Using the program to evaluate water age results, the LVVWD successfully reduced water
age in problem areas through refinement of its daily operating strategies. Although the TTHM levels are well controlled
as a result, TTHM reduction through operating strategies may be vulnerable to changes in local water demand,
limitations in distribution network configuration, and potential variations in water temperature and organic matters.
The LVVWD further enhanced this approach by evaluating the concept of using decentralized or localized DBP
treatment as an alternative to typical centralized treatment methods such as chloramination. This concept has the
following unique characteristics and requirements:
• The treatment targets local areas where the distribution system may have a greater potential to form high DBPs.
• The treatment facilities are installed within the water distribution network.
• The treated water needs to meet local hydraulic conditions and operating criteria for water distribution.
• Adequate chlorine residual must be maintained or restored after treatment.
• Treated water may be mixed with the non-treated water after DBP treatment.
From 2004 to 2007, the LVVWD actively participated in a Water Research Foundation tailored collaboration project,
entitled Localized Treatment for Disinfection By-Products (2009, order #91254/project #3103). A pilot-scale study
of three different localized treatment technologies (air stripping, granular activated carbon, and biological activated
carbon) for reduction of DBPs was conducted. The project demonstrated that localized treatment of DBPs is a viable
approach to meeting the Stage 2 DBPR.
As a result of this project, the LVVWD focused on air stripping as the most cost effective technology for the LVVWD to
use in its water distribution system where HAA5 treatment was not required. The pilot testing results for air stripping
technology indicated that:
• Air stripping was effective for TTHM removal (75 to 85%) at air-to-water ratios of 40 to 45: 1.
• Air stripping did not remove a significant amount of chlorine (less than 10%).
• Water temperature differences of 6°C did not significantly affect TTHM removal.
• After air stripping treatment to reduce TTHM, TTHM will continue to form at about the same rate as before
• As expected, air stripping was not effective for HAA5 removal.