Whenever we share recycling advice here on SBS Science, many commenters focus on an item that's probably amongst the most confusing - pizza boxes. Some are adamant that you should not put these in recycling whatever you do, while others are surprised that a large packaging item made entirely out of cardboard should be destined anywhere but the recycling bin.
So, which one is it? Let's take a closer look at the issue and clear the air once and for all.
The reason why you would want to avoid putting a food-stained cardboard item in recycling is all to do with that dreaded foe of resource recovery, contamination. When cardboard and paper items are sent to a recycling plant, after sorting they are finely shredded and mixed with water and various chemicals, such as surfactants and sodium silicate, in order to produce a slurry of pulp. The pulp is a good source material - and it can undergo a process of ink removal, and can also be easily filtered for contaminants such as bits of sticky tape and staples.
Grease, however, is a nastier contaminant - it gets mixed in with the pulp, lowering the quality of the recovered fibre, and may even lead to ruining a whole batch of potentially reusable paper. This is why any cardboard and paper item you throw out into your kerbside recycling bin should be as clean as possible.
With that in mind, here's how you should go about throwing out your pizza boxes and similar cardboard food packaging (fancy burgers and cakes these days come in cardboard, too).
Absolutely no food scraps, uneaten crusts, hunks of cheese, extra packaging, garlic bread wrappers, napkins. The box has to be completely, utterly empty.
Assess the damage
Sometimes pizza comes with an extra layer of waxed paper underneath, and the grease gets trapped on that, leaving you with a relatively clean box which most certainly can be recycled. But sometimes that extra layer of pepperoni creates an oil-soaked mess, which has to go into trash (or compost if you have access to that).
Tear off the lid
Even if the bottom of the box is definitely ruined with fat (better on the cardboard than your arteries, right?), check the lid for stains - often the top part of a pizza box remains pristine, which makes it perfectly fine to recycle.
Ask your council
According to photos on the Planet Ark recycling tips website, the pizza box on the right can be recycled, because the grease stains are minimal. However, as with every recycling approach, you're always advised to double-check everything with your local council.
Most councils these days have recycling advice on their websites, explaining precisely what you can do with a variety of items, pizza boxes included. Because of the high risk of contamination, some councils flatly reject pizza boxes, so you're better off following that advice instead of just listening to us.