Our shop uses 3 methods to deal with used coolant.
1) A coolant recycler, which consists of a centrifuge and filtering stages. The used, filtered coolant is then mixed with water and new concentrate.
2) An evaporator (as HU suggested) is used for the really rank stuff. The liquid evaporates out and the sludge is hauled away. Cheaper than disposing of liquid coolant. Less volume.
3) The pit is well, a pit. Nasty old coolant is dumped into the pit. When the pit is full, a waste removal firm empties it. Big $$$. Only used if the evaporator is full and the coolant can't be recycled.
Ideally, coolant should be changed every week. This is not an ideal world, and production pressures may keep the coolant in the machine for a month or more.
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Diplomacy is the art of saying "Nice doggie" until you can find a rock. - Will Rogers