Members of the Clean Rivers Campaign and other local groups asked Allegheny County Sanitary Authority executives Wednesday to consider including trees and other "green infrastructure" in their plans to fix the region's aging stormwater and sewage management systems.

Alcosan executives, however, said that request might be out of their control.

As many as 10 billion gallons of untreated sewer water, which can carry bacteria, flow into the Pittsburgh area's rivers and creeks each year, and Alcosan executives signed a consent decree several years ago agreeing to submit in 2013 a plan to correct the problems and to implement the plan by 2026.

The current plans Alcosan is considering would use either basins or tunnels to help with the run-off.

The plans would cost more than $3 billion and possibly close to $8 billion, according to numbers the authority discussed Wednesday at a meeting on the South Side. The federal Environmental Protection Agency estimated that any project costing more than $2 billion would constitute a burden to taxpayers.

Members of the Clean Rivers Campaign and several religious and environmental organizations announced their request for green infrastructure before Alcosan's meeting.

Barney Oursler, a coordinator for Clean Rivers, said he and fellow campaign members would like to see executives incorporate more trees into business districts and to consider using materials that will soak up water when repaving roads or constructing new sidewalks.

Ideally, these additions should soak up extra water, helping with a portion of the runoff, he said.

Mr. Oursler said he does not know how much the additions would cost because the closest estimates come from projects completed in cities that were flatter than Pittsburgh.

Still, he thinks the project is worth pursuing.

"The biggest public investment in our lifetime ought to give us the biggest public response," he said as supporters gathered before the meeting. "Do we want to leave our children and grandchildren with big pipes or do we want to leave them with rainforests?"

Alcosan executive director Arletta Scott Williams said several times during the 21/2-hour meeting that Alcosan does not have the authority to force municipalities, many of which own their own sewer lines, to do anything.

"What we are not able to do is create a physical structure that we do not have responsibility for," Ms. Scott Williams said, explaining why the group could not plant trees and construct the permeable sidewalks. "We issue a bond, we have to own it at the end of the day."

Mr. Oursler said he recognizes the authority's limitations but hopes that executives will be more creative in the future, possibly forming partnerships with the municipalities to encourage them to install green infrastructure.

During the meeting, which 136 people attended, people peppered Ms. Scott Williams and Alcosan director of engineering and construction David Borneman with questions about whether they could restructure their rates so property owners that contribute to the largest amount of runoff would pay higher bills.

Ms. Scott Williams said Alcosan does not currently have the authority to restructure rates in that way but that it is researching ways to possibly restructure rates in the future.