NASHVILLE (AP) - A federal judge has blocked the city of Franklin's efforts to countersue an environmental group that is suing the wealthy municipality over pollution from its wastewater treatment plant.

Franklin claims in court papers that a lawsuit by the Harpeth River Watershed Association constitutes a form of extortion. U.S. District Judge Kevin Sharp on Wednesday dismissed the city's claim and encouraged the parties to have some "adult conversation" to resolve their differences.

The wastewater treatment plant has a permit issued by the Tennessee Department of Environment and Conservation that allows it to discharge pollutants into the Harpeth River within certain limits. The permitting process is a component of the federal Clean Water Act. The act also allows citizens to file lawsuits to enforce its provisions when they believe regulators are not doing so.

The original lawsuit filed by the Harpeth River Watershed Association cla ims, among other things, that the city is polluting the river with overflows from its sewage system, in violation of the Clean Water Act.

In court on Wednesday, attorney Gary Cohen argued that Franklin's permit has provisions that go beyond the scope of the Clean Water Act and cannot be enforced with a citizen suit in federal court. Those claims can only be enforced in state court, he said.

U.S. Justice Department attorney Matthew Oakes addressed the issue as a friend of the court -- that is, the federal government is not a party to the case but was providing relevant information. Oakes argued that the watershed association's claims are well within the scope of the Clean Water Act.

He said the U.S. government does not normally participate in citizen lawsuits, but the claims raised by the city have the potential to undermine the federal permitting program.

Franklin's Cohen also argued that most of the watershed association's claims should be dismissed b ecause many of the city's sewage overflows happen on private property or that they never reach the river.

Sharp said he would hold off ruling on the city's motion to dismiss in order to give the two sides time to talk.

He said both sides want clean water. "Now is the perfect time to try to resolve this case," he said.