Florida's Miami-Dade County has proposed a $1.5 billion, 15-year plan to reconstruct its aging sewage-treatment plants, pipes and pump stations in a bid to avoid a federal lawsuit and heavy fines for violating the Clean Water Act. The plan includes replacement or repair of a portion of the 7,500-mile network of sewer lines and a $555 million rebuilding of a wastewater-treatment plant on Virginia Key.