A NIOSH investigation of a fatal accident at a Pennsylvania wastewater treatment facility was reported. Two workers were killed when a light bulb broke when they lowered it into the manhole at the top of a digester, igniting combustible gas, probably methane, and causing an explosion that lifted the 30 ton floating cover on which the workers had been standing. The workers had been spraying water into about 8 feet of sludge remaining in the bottom of the digester during a drainage procedure. A pumping truck driver was also at the scene, pumping air into the sludge to loosen it. Both workers' bodies were retrieved by divers and showed cervical fracture as the cause of death. NIOSH recommended that the employer, the public works department of a borough, develop proper work procedures and train employees in safe maintenance procedures, and that it develop comprehensive policies and procedures for confined space entry. Issues that should be addressed prior to confined space entry included whether entry was necessary and a permit had been issued, whether the air quality was tested, whether the confined space was isolated and locked out from other systems, whether employees and supervisors were trained in using protective and emergency equipment and also for confined space entry, whether ventilation equipment was available and used, and whether the air quality was tested when the ventilation system was operating.