COLLEGE TOWNSHIP — Aerial maps of Centre Region blocks, 40 years apart, show thin, gray lines dotted with rectangles — neighborhoods that have spread to a thick, red line on the photos.

The red line represents the region’s growth boundary, about 32 square miles within the region agreed upon for development by the six municipalities — State College and College, Ferguson, Halfmoon, Harris and Patton townships.

The boundary was created in 1976 by officials who wanted to “support vibrant development” but also preserve rural land, said senior planner D.J. Liggett.

She and other regional planning officials explained the boundary — which corresponds with the area in which development is served by public water and sewer lines — at a public workshop Wednesday night.

Elected and planning officials from the region’s municipalities attended to discuss related growth issues, and heard a status of the region’s growth capacity based on a recently-completed report.

The issue of growth and corresponding sewer service has dominated Council of Governments discussion for the last year or so, after a development plan for more than 400 homes in Halfmoon Township included a plan for an advanced wastewater treatment system.

The 2006 agreement between the region’s municipalities allows only on-lot disposal systems for sewage outside the designated growth boundary. Developers can request a sewer line extension outside the growth boundary, but that involves a lengthy, regional approval process, and rarely happens.

None of Halfmoon Township exists within that line, and supervisors opposed the advanced plan. But it raised the issue of how to deal with technology more advanced than the region’s 6-year-old growth agreement.

Mark Stevenson, Halfmoon Board of Supervisors chairman, said he was concerned about the township’s liability, should it allow the technology and it later fails or the developer goes bankrupt and can’t maintain it.

“It’s the township that (the state Department of Environmental Protection) looks to,” he said. “There were enough administrative problems that I felt comfortable opposing it.”

Jim May, director of the Centre Regional Planning Agency, said a regional growth boundary is unusual, with only a few in the nation. But here, it seems to have worked to limit development and preserve farms and other open land.

From 2004 to 2011, 90 percent of the region’s new building permits were inside the boundary. “It really works,” May said.

That’s especially true for Ferguson and Harris townships. In Ferguson, 80 percent of the land remains agricultural. In Harris, 90 percent is agricultural or forest.

At issue now is how to continue balancing growth with the region’s wastewater treatment capacity, through the University Area Joint Authority. There are restrictions, due to how much discharge Spring Creek can receive.

May said the report shows the region has enough land to accommodate projected growth through 2040, but that maximum Spring Creek discharge will be reached before that time.

“Change is inevitable,” he said. With about 34,000 dwelling units in the region, as of the 2010 census, the report shows capacity for almost 13,000 more. By comparison, there are about that many in the borough alone.

Senior planner Eric Vorwald detailed the potentially tricky dance to ensure the wastewater flowing to UAJA doesn’t exceed capacity.

With its current equipment, UAJA can treat up to nine million gallons a day, and now treats half of that. Spring Creek can take only six million gallons per day in discharge.

“That number will never change, as far as we can tell,” Vorwald said.

That leads to UAJA’s beneficial reuse program. Water is treated to a level of high purity, and sold to area customers for use, such as heating and cooling and for laundry. That program can handle up to three million gallons, though UAJA has the equipment now to treat one million.

UAJA could expand to regularly treat more wastewater, and create the maximum of high-purity water, but that will cost tens of millions of dollars and require most customers for the reuse water.

For now, UAJA can operate with the region’s current level of development.

In the meantime, officials must consider how to address managing the region’s growth, such as strengthening the process that considers extending sewer service and studying the impacts of advanced wastewater treatment technologies.

Council of Governments committees will continue to discuss the issue, with the help of the report, to determine future steps.

Jessica VanderKolk can be reached at 235-3910. Follow her on Twitter @jVanReporter