Please do not dispose of anti-freeze down an outside drain - if you must, flush it down the toilet if you are on mains drains with plenty of water to dilute it, it's poisonous to humans and animals and attractive to animals as it tastes sweet. If you are on a septic tank take it to your local recycling centre.
There is a busy thread on here from many years back, and plenty of advice on Google. The consensus seems to be the toilet route, but if you have a suitable space, it seems you could pour the contents into a small pit, cover it and let it degrade naturally. Glycol is chemically similar to glycerol and other natural carbohydrates, so is not a lethal poison.
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