what is the procedure that greyhound buses use for disposing of waste in sanitary tanks?

It is not dumped on the road, ever! It works on the same principle as a portable toilet like you would find at a construction site or any other function where portable units are needed from time to time. It had a chemical based fluid in it and when the holding tank gets full, it is emptied through a hose into the sewer system, just like flushing a toilet. This is usually done after every trip the bus makes.

Greyhound Waste Disposal

I don't know the real answer, but I personally think it'd work just like a boat. They unload the tank after a trip when they return the bus to the station.

But if they actually do release it onto the road... heck, I sure wouldn't want to be behind one of those buses.

Its held in a black water tank on the bus. Its then disposed of when they fill the gas tank it's off loaded in the sewer just as if you flushed your toilet. We have an RVand we have to do it all the time.

while you are in the front of the mcdonald's eating a quarter pounder with cheese, the bus is out back pumping raw sewage into the grease vats at mcdee's. which is in turn recycled to make french fries.

It is dumped out of the bus.

at some point the tank is emptied, by pump, into a receptacle.

this gets done at McDonalds restaurants as the emptying becomes periodically needed.

the refuse is pumped through the back door into the vat marked 'special sauce'

they use a really long straw....