A federal judge has refused to dismiss litigation asserting that sellers of fittings that connect iron pipes in pressurized municipal water and wastewater systems colluded to inflate prices.

U.S. District Judge Anne Thompson in Trenton ruled on Tuesday that the plaintiffs adequately alleged that the companies acted contrary to self-interests by increasing prices during waning demand, engaged in conspiratorial communications, and — based on their combined 90 percent market share in 2008 — had a motive to fix prices.