A chemical used to disinfect water and sewage could kill thousands in the Dallas-Fort Worth area if it is accidentally released.

Photos by WILLIAM DESHAZER/DMN

Water plant supervisor Carl Rabe of Mesquite does a check every two hours at the Bachman Water Treatment Plant to monitor the amount of chlorine being distributed. The plant uses about a railcar of chlorine a month.

The Department of Homeland Security, environmental activists and the railroads have all raised concerns about the lethal consequences of an incident involving chlorine gas.

"We can no longer continue to risk the lives of millions of Americans by using, transporting and storing highly toxic chemicals when there are safer alternatives commercially available," the Association of American Railroads, which represents the major freight lines, said recently.

Many cities have switched to safer alternatives, but Dallas has been slow to adopt them.

Water utility officials in the Dallas-Fort Worth area say they have studied alternatives, but it would be difficult and expensive to convert to them because of the millions of gallons of water that are treated each day. Construction alone could cost more than $10 million.

Blame it on being a hot, dry, flat and big metropolitan area.

"We have probably some of the largest facilities anywhere in the country," said Charles Stringer, an assistant director for Dallas Water Utilities. "We're in a semiarid region and our per capita uses are higher."

A 2007 report by the Center for American Progress in Washington, D.C., surveyed waterworks nationwide and found that 37 plants still receive chlorine by rail. Seven of those are in the Dallas-Fort Worth area. Five others in Dallas County receive smaller amounts of chlorine by truck.

The report noted that plants in many populated areas – including Washington, Philadelphia and suburban Los Angeles – had switched to safer alternatives such as industrial bleach or ultraviolet light.

Though the new techniques were more expensive than chlorine and capital construction costs were high, the plan's author, Paul Orum, found that plants had made up much of the difference by not having to spend as much on emergency planning, security and keeping up with as many regulations.

"The most they were spending was $1.50" per person served by a plant each year, he said. "Many of them were spending 20, 30, 60 cents per customer per year. It's affordable for these facilities to switch over to something that does not put people in such danger."

Families living near Dallas-area plants said that's not a bad price for more safety.

"You spend like $1.50 on a Pepsi and a candy bar at the convenience store," said Rachel Melara, who lives in a subdivision across from Garland's Rowlett Creek sewage treatment plant. "So if that's what it takes, I think that would be a great idea. Think about what we pay for gas."

Since the early 1900s, chlorine has been a cheap and reliable disinfectant. It was key to eliminating such diseases as typhoid and cholera in the U.S.

Pressure to find alternatives has mounted since 9/11 – and after chlorine railcar accidents suffocated three people outside San Antonio in 2004 and nine in South Carolina in 2005.

But there are drawbacks besides cost.

Drinking water plants are required by law to have a "residual," which keeps the water clean as it travels through the system. UV light, which makes the bacteria unable to reproduce, does not produce a residual, and water plants must add another chemical. So UV light is mainly used to disinfect sewage.

Industrial bleach, diluted chlorine, does provide a residual. But it requires about nine times as much storage space as chlorine and degrades faster, especially in hot climates.

That's one of the reasons converting to bleach is unfeasible, Mr. Stringer said.

Dallas Water Utilities – which treats water and sewage for the city and several suburbs – just completed switching its drinking water plants from chlorine to ozone last year. That's allowed the city to reduce its chlorine shipments, but it still needs to keep large amounts of chlorine on site for the residual.

The utility has studied UV several times for its sewage plants but decided not to switch. And it is in the early stages of reviewing another option, reducing the amount of chlorine that is stored or transported by generating it on site.

The North Texas Municipal Water District, which serves the western and northern suburbs, is evaluating UV as its sewage plants come up for expansion or renovation, but it has no timetable.

The Trinity River Authority, which runs a sewage plant in Grand Prairie, expects to convert from chlorine to a safer alternative by the end of 2010.

Water and sewage plants typically have been built in remote areas. The stench of sewage was enough to keep people away. But in the last 20 to 30 years, new technology has reduced odors. And as the metropolitan area expands, houses have crept closer to both types of plants.

Ken and Kimberly Dortch didn't realize the Eastside Water Treatment Plant stored up to 200 tons of chlorine when they moved into the Stone Canyon subdivision in Sunnyvale in March.

They had studied everything from school ratings to classroom size to dollars spent per student. Mr. Dortch, a Dallas police lieutenant, has received training on a chlorine accident, but he said he didn't think about the chemicals stored at the plant.

"We were looking for obviously the school district, looking for low crime, looking for a safe neighborhood, a quiet area, but still close proximity to Dallas," Ms. Dortch said. "And I guess that was just one of those invisible things that we really didn't consider."