A worker at a recycling plant was attacked and left with a broken jaw after a row broke out over rubbish placed in the wrong container.
The council employee was assaulted after making a spot check on somebody's waste at Household Waste Recycling Centre in Sharston, Manchester.
Greater Manchester Combined Authority said police were investigating.
The worker was taken to hospital and later discharged but may need surgery, GMCA added.
His jaw was broken in two places during the 19 December attack.
Councillor Andrew Western said it was "appalling" that hundreds of staff at recycling centres "regularly experience verbal abuse and threats of physical violence" every year.
He added: "Unfortunately there are people who use waste recycling centres illegally, when they are designed specifically for residents to dispose solely of domestic waste."