Coolant CAN be disposed of down the drain, but it's better to take it to a reclamation site if there is one in your area.
From a state web site:
Antifreeze is used as an engine coolant and contains either the chemical ethylene glycol or propylene glycol. Ethylene glycol is very toxic (one teaspoon to one ounce may be fatal to an adult) and is attractive to animals and small children due to it's sweet taste. Propylene glycol is less toxic and lacks a sweet taste. Disposed of improperly, antifreeze poses a potential health hazard and may pollute groundwater, surface water, and (potentially) drinking water supplies. Do not mix used antifreeze with engine oil or other wastes.
Dilute and Flush
Flushing used antifreeze down the drain is the least-preferred disposal option, but it may be the only option available in some areas. Your wastewater treatment facility should be contacted to ensure that they can accept the waste prior to using this method.
Although antifreeze is biodegradable, households on septic systems should NOT dispose of antifreeze down the drain. If your home is connected to a septic tank, give this product to someone who is connected to a municipal sewer system. Even in a diluted form, antifreeze can overwhelm and destroy the bacterial action in your septic system and drain field.
Procedures include:
Provide adequate ventilation by opening windows and doors and/or turning on a room fan.
Wear protective clothing such as chemical resistant gloves, eye protection and long sleeves.
Carefully pour about 1 gallon of diluted antifreeze down the sink, flushing with large amounts of water. Failure to dilute antifreeze may damage pipes or other parts of your plumbing.
Continue disposing in 1 gallon batches until completely gone. Don't mix products, and wait several hours between disposing of different types of products.
The best time to do this is during the working day when the sewage system is in full use so that this material passes through the municipal sewage system faster.
These procedures are intended for small quantities of used antifreeze from a household only.
