The city has broken ground on the nation’s first grease-to-diesel conversion facility at the John Oliver Memorial Sewer Plant — a project that promises not only a remedy for the plague of sewer grease that has terrorized Oliver’s native England with airliner-size fatbergs, but a revenue source for Danbury in the form of biodiesel sales. Construction of the $5.5 million grease-to-diesel conversion facility is part of a larger $100 million wastewater treatment plant upgrade that began last year. Danbury’s partners are REA Resource Recovery Systems and Richard Parnas, professor emeritus in UConn’s Department of Chemical and Biomolecular Engineering, whose patented reactor mixes refined brown grease with methanol to produce the biodiesel. Production could begin late this year. Earlier this month, the City Council backed the application to the federal Advanced Research Projects Agency-Energy. If the city is successful, the grant could lead to a larger award for a solar power system capable of meeting the electrical needs of the entire John Oliver Sewer Plant.