Northampton Public Works officials are trying to figure out what caused a multiple system failure that led to the release of raw sewage during Tuesday night's heavy rainstorms.  DPW Superintendent Ned Huntley told city councilors Thursday night that the spill occurred around 9 p.m.. on Tuesday, when power went out at the wastewater treatment plant, and the backup generator failed to kick on.  Huntley says the plant was put back online immediately after personnel arrived at 6:00 the next morning.  He estimates between a half-million and a million gallons of sewage may have spilled into the river.  The EPA and officials in neighboring states were immediately notified.