Tacoma Water - McMillin Reservoir - Executive Summary
Tacoma Water supplies drinking water to customers in Tacoma and surrounding areas. For more than 85 years, Tacoma Water has used the Green River as its primary source of drinking water. The Green River source can supply a maximum of 72 million gallons per day (MGD). When water demand exceeds 72 MGD, Tacoma Water relies on ground water from in-town wells to make up the difference. The ground water supply has a maximum capacity of 50 MGD. Storage reservoirs throughout the water system provide added capacity to meet customer needs. |
Tacoma Water adds chlorine to the drinking water supply at six locations to protect public health. In an effort to minimize and eliminate the use of chlorine gas, Tacoma Water conducted an in-house chlorination assessment. Originally, Tacoma Water had 10 chlorination sites, all of which used chlorine gas. After evaluating these sites, Tacoma Water determined that four sites could be eliminated if changes were made at other facilities. Tacoma Water the
n evaluated if alternative chlorination systems, such as liquid or solid forms of chlorine could be used at the remaining six chlorination facilities. The liquid or solid forms of chlorine are safer than chlorine gas for both employees who handle chlorine and for neighbors who live near chlorination treatment facilities.
Tacoma Water first evaluated alternative chlorination chemicals at unstaffed locations, because of concerns about employee response time in the event of an accident. The evaluation determined that two facilities could use a solid form of chlorine and two facilities could be re-designed to use liquid chlorine (bleach). After Tacoma Water made these changes, only two chlorination facilities continue to use chlorine gas.
The two chlorination facilities, the Green River Headworks and McMillin Reservoir, use the largest quantities of chlorine and have employees on- the premises eight hours a day, every day of the week. These employees live in on-site housing at both l
ocations. Tacoma Water feels eliminating chlorine gas from eight of its original ten chlorination facilities has improved employee and public safety. The two facilities that remain with chlorine gas use the best available design to chlorinate.
Both the Headworks and McMillin chlorination systems are designed with redundant chlorine equipment to provide continuous chlorine feed. While one chlorinator is operating, the other is idle and can be manually switched if necessary. The two chlorinators are rotated on a weekly basis, so that both function properly. At the Headworks, four one-ton chlorine containers are needed to run the system, and at McMillin Reservoir, two one-ton chlorine containers are needed. Each location also has an equal number of standby tanks that provide for a safe automatic transition when the in-use bank of containers becomes empty.
Both Headworks and McMillin Reservoir chlorination systems are designed to operate in a safe manner. The chlorinator operate
s on a vacuum principle, that enables a safe transfer of chlorine from the container to the injection point. The chlorine containers are connected via a manifold to a vacuum regulator. A vacuum, created when water passes through a venturi, holds a safety pin open to allow chlorine to pass under vacuum to the injection point. Should the vacuum line fail, the loss of vacuum will cause the safety pin to close and isolate the chlorine containers. Other safety measures include having the chlorination system connected to a backup generator power. If there is loss of regular power, the system automatically switches to backup power to provide non-interrupted power to the chorinators, chlorine analyzer and chlorine leak detector.
Safety systems on the chlorinator at both Headworks and McMillin Reservoir continuously monitor the process. Three types of safety systems are present: a chlorine leak detector, chlorine residual analyzer and high/low vacuum alarms. Chlorine leak detectors a
re present in both the chlorine tank room and chlorinator control room. The leak detectors sound an alarm if chlorine gas is present. A chlorine residual analyzer monitors the chlorine feed and has high/low residual alarms. These alarms may indicate an abnormal functioning of the system. High/low vacuum indicators on the system indicate possible under-feeding of chlorine. All three safety systems have alarms transmitted locally, as well as to Tacoma Water's Water Control Center located in Tacoma. An operator is on duty at the Water Control Center 24 hours a day. At Headworks alarms are sent to on-site homes as well as to the control center.
In May 1999, Tacoma Water completed its year long audit and revision of both chlorination facilities as part of its OSHA Process Safety Management and EPA Risk Management Plan requirements. The audit addressed major sections such as operating and maintenance, employee training and emergency response. All operating procedures were updated,
all necessary maintenance tasks were identified and the preventative maintenance schedule was revised. The employee chlorine training program was revised to include more site-specific training for both Headworks and McMillin Reservoir. Emergency response plans for both the Headworks and McMillin Reservoir were updated. At both facilities, employees are trained to in the proper use of self-contained breathing apparatus (SCBA) and personal protective suits before beginning work to isolate and stop a chlorine leak.
Tacoma Water and local emergency response agencies will conduct a chlorine emergency response drill annually to evaluate response procedures. At Headworks, an employee is present every day for an eight-hour day shift. At other times, an employee who lives on-site is designated to immediately respond to any alarms. The alarms are connected to the on-site residences. In the unlikely event of a chlorine release, the Palmer Fire District #47, Enumclaw Fire Department and Ma
ple Valley Fire Department will evacuate nearby residents and others in the area for recreation. None of these fire departments has a team equipped to directly handle a chlorine release. Kent Fire Department has initiated to have an agreement with Palmer Fire District #47 to provide hazardous materials team assistance for the Headworks. Tacoma Water is also evaluating whether to install two sirens as another way to alert area residents downstream of the Headworks of a chlorine release.
The McMillin Reservoir facility has a minimum of one employee present every day for an eight-hour work shift. McMillin Reservoir staff are given on-site housing but are not required to be present during non-working hours. All alarms triggered in the chlorination room sound a horn at the chlorine plant and are also transmitted to the Water Control Center in Tacoma. During working hours, employees immediately respond and evaluate the alarm. Should a chlorine leak alarm sound during non-working hour
s, the Water Control Center first contacts McMillin Reservoir staff to respond. If they are not available, the control center operator contacts Pierce County Fire & Rescue and Tacoma Water staff at the utility's Enumclaw office. Pierce County Fire & Rescue has a hazardous materials team nearby, and can respond within 5-10 minutes.
Chlorine release modeling was conducted to determine the off site impact. Both worst and alternate case scenarios were modeled for both facilities and showed similar off site impacts. The worst-case model followed EPA's required guidelines, using the following criteria. The largest container (2,000 pounds) would be lost in a 10-minute period and assumes no mitigation to slow the release. (The release period of 10 minutes selected by EPA is not specific to chlorine or the chlorination system and is used for all chemicals listed in EPA's Risk Management Program.) The off site impact from EPA's worst-case scenario was 1.3 miles downwind of the release.
An alternate release scenario was modeled to determine a more practical off site consequence. It was assumed a flexible connector under pressure had failed, with the opening drawing chlorine gas for one-hour from a full tank. The maximum quantity released through this opening was determined to be 900 pounds during the 60-minute period. The release also would occur inside the chlorine building, dampening the effects of the release. The off site impact from the release was 0.1 mile downwind of the release.
Tacoma Water has used chlorine gas at both Headworks and McMillin Reservoir for more than 85 years with no serious problems. To continue this record, Tacoma Water recently updated its chlorine operations and preventative maintenance program, its chlorine safety program and re-affirmed coordination with local fire departments that would provide assistance if a release occurred. Tacoma Water also has a strong working relationship with its chlorine supplier to ensure chlorine conta
iners are maintained properly. Tacoma Water ensures chlorine containers are in good, working order by visiting the chlorine supplier's facilities and inspecting chlorine containers prior to use. The facility audit is providing guidance for changes to improve safety. These improvements include: annual chlorine emergency drills with fire departments, response by Pierce County Fire & Rescue to a chlorine leak alarm when no McMillin Reservoir staff is present, possible installation of two sirens near Headworks to alert residents of a chlorine release, and revision of the employee chlorine training program to have more site-specific training.