This article says CNG.
http://www.phillyfirenews.com/2016/01/26/14992/
LNG is not pressurized to thousands of pounds of pressure like CNG.
My experience in the truck business indicates that sometimes maintenance procedures (especially in unionized municipal fleets) aren’t always the best. Also trash trucks often get stuck in the landfill and get pushed / dragged through the swarf by bulldozers or loaders.
So diesel may be a safer plan for situations like that. CNG seems pretty unforgiving.