ANDOVER - Millions of gallons of sewage from Andover residents is causing a $4 million problem near Lawrence's new high school.

Part of the two-mile-long pipe that carries anywhere from 3 million to 15 million gallons of untreated sewage a day from Andover to the Greater Lawrence Sanitary District treatment facility is corroding.

The deteriorating pipe, called the Lawrence interceptor, is in the area around the new Lawrence High School. It will cost Andover sewer users about $4 million to fix it.

While the pipe's not in immediate danger of collapsing and leaking untreated sewage, it's getting there, and the problem has to be taken care of, Andover's Public Works Director Jack Petkus said.

"It's certainly something that we have to deal with," Petkus said. "Should that pipe fail, all sewerage has got to go somewhere. Wherever it fails, that's where it's going to come out of the ground.

"And if you have five and a half million gallons of sewerage coming out somewhere, you're going to cause an environmental problem."

About 40 percent of Andover households are hooked up to sewer. Nearly all of the sewage generated by those residents flows through the Lawrence interceptor to the Greater Lawrence Sanitary District treatment facility on Charles Street in Lawrence.

If that sewage were to spill out, it would cause a public health hazard in the area, as well as threaten the Merrimack River, which is the water source for Lawrence, Methuen, Tewksbury and Lowell, according to the state Department of Environmental Protection spokesman Joe Ferson.

The part of the pipe that's corroding hasn't had any repairs since it was installed 40 years ago. Since then, large amounts of hydrogen sulfide in the sewage - the main ingredient in a stink bomb - have reacted with the water in the pipe to create sulfuric acid.

That acid is eating away the concrete in the pipe so badly that the steel reinforcement is the only thing in several parts holding back the sewage.

Town officials found the problem when they had to move part of the pipe a few years ago for the construction of the new Lawrence High School.

Petkus' plan is to reline the pipe and inspect it every two years to monitor its health.

He has proposed the project as part of the town's capital improvement program for the next budget year. That means that while the $4 million cost of fixing the pipe will only be assessed to sewer users, the project must be approved by all of Andover's voters at Town Meeting in April.

Andover owns the rights to the land in Lawrence on which the pipeline sits, so no approval in Lawrence would be necessary for the project.

Town Manager Reginald "Buzz" Stapczynski is looking at requests for projects made by all of the town's departments and finalizing those he'll recommend for Town Meeting. Stapczynski said the work on the Lawrence interceptor is one he'll definitely recommend "because of the compelling nature of the request," Stapczynski said, adding that he's seen the pipe and some of the deterioration.

Petkus agreed.

"It's my recommendation (that) it will be done as quickly as possible," Petkus said. "If you don't do it, you're skating on some pretty thin ice I think."

The Greater Lawrence Sanitary treatment facility treats 52 million gallons of sewage a day generated by Andover, Lawrence, North Andover, Methuen and Salem, N.H., residents.