You Should Never Dump Dairy Down The Drain. Here's Why
For everything there's a season, including the milk you drink. Whenever you have to buy milk, the first thing you look at is the expiration date, because you know a gallon could go in a couple of days, or it might still be in the fridge the following week, seal unbroken. It happens all the time. You buy a gallon of milk and for some, odd inexplicable reason, it turns into a busy week that leads to takeout and little, to no cooking. When you do finally get around to popping off the plastic top, it smells sour. Your first instinct might be to pour the milk down the kitchen sink drain, but resist the temptation.
Opened and refrigerated, milk can last for seven days. Unopened and refrigerated, milk can last up to seven days beyond its "best by" date. Its shelf life depends on several factors, which include how it is stored, how it was processed, how long it is exposed to light, and heat, and the carton date. Milk should be stored at, or below 40 degrees Fahrenheit (via Eat by Date).
But why not dump it down the drain when it's no longer good and starts to smell?
What you can do with expired milk
Dumping milk down the drain can be harmful to the environment and impact it in ways you never imagined. How? Milk apparently has a high oxygen demand, which is particularly harmful to ecosystems. Bacteria that feed off the discarded milk use up the oxygen, leaving fish and other small organisms with less. Disposing of milk this way can ultimately suffocate water life (via DS Smith). This is true of other dairy products like yogurt and cheese.
While your little gallon doesn't match the volume of what a farm produces, we all have to do our part. So, what can you do with your expired milk? Turns out plenty. If you find yourself in a cooking pinch for butter, yogurt, or sour cream, you can substitute any one of these with sour milk. You can use your expired milk for facials to create smoother, firmer skin. You can even use it to water your plants. Just create a ratio of half milk and half water. The calcium will help grow stronger plants (via Babamail).