Tossing that plastic water bottle in the blue bin? Good for you.

Just don't forget the cap.

Because, according to new guidance from the Association of Plastic Recyclers (APR), caps and bottles not only get along just fine in the same bin, there's a big demand for both.

What's this, you say? Mom told you to separate the caps from the bottles! Well, let's face it. Mom was probably willing to work a little harder to recycle than today's easy-peasy youth.

Besides, as recently as last year, recycling program managers had been sending a decidedly different message about caps.

"Just about any plastic can be recycled," Signe Gilson of Seattle-based CleanScapes told Scientific American. "But when two types are mixed, one contaminates the other, reducing the value of the material or requiring resources to separate them before processing."

The bottom line? Recycling processes change. Despite the new guidance from the Association of Plastic Recyclers — which represents 90 percent of the post-consumer plastic processing capacity in North America — it's a smart idea to check your local recycling rules first.

Changing rules

We've actually come a long way since even last year. The way plastic is collected and the technology used to process it has changed dramatically. What once was an unhappy union in the blue bin — caps are made from different type of plastic than the bottles — is now a marriage made in recycling heaven.

"In the past the plastics recycling industry was not able to effectively recycle bottles with caps on, so the message to remove the cap was created." APR explains on its website. But times have changed.

Here's how the great melting pot at the local plant works: The bottles, cap and all, are ground into flake. A special "float/sink" process takes it from there — essentially, as 911 Metallurgist explains, "particles of lower specific gravity float on the surface of the medium, while the particles of higher specific gravity sink to the bottom."

In other words, PET, the material bottles are made from, floats, while the heavy stuff in the cap — high density polyethylene (HDPE) and polypropylene (PP) — sink to the bottom.

Thus, both kinds of plastic are separated in a kind of bath before going on to their next lives.

Water bottles and caps at recycling plant Tiny caps tend to get lost in the recycling system unless they're screwed on tightly to the bottles they were born with. (Photo: Alba_alioth/Shutterstock)

And the thing is, those caps — and the high-density plastic they're crafted from — are actually in great demand worldwide, according to APR.

So what happens if you lose the cap? Can a bottle still go to the hereafter without its polyethylene crown? Sure, but those little caps may not find their way through the system. Due to their size, these hard plastic nuggets can get waylaid in the system and improperly sorted.

"It can be confirmed that plastic caps should be left on plastic bottles for recycling," the U.K.-based Recycling Of Used Plastics Limited chimes in. "This reduces the potential for the cap to be littered separately, and when attached to the bottle it also allows the cap (as well as the attached neck ring) to pass through the sorting facility and get to a plastic bottle reprocessor."

Most importantly, keeping cap and bottle together until the very end satisfies an important requirement for recycling programs to work: It has to be as easy as possible.

Consumers will generally follow the path of least resistance. A seemingly trivial thing like removing a bottle cap is one extra step — and depressingly enough, it can be a deal-breaker for someone wavering between the blue bin and the regular trash bin.

Yes, it really does boil down to the tiniest of efforts. As APR notes on its website, participation in recycling programs tends to flag when people are asked to do too much.

So go ahead and do a little less. Leave the cap on the bottle. And you just might end up doing a lot more for our planet.

Editor's note: This story has been updated since publication in early March 2019.

Why you shouldn't separate the cap from a plastic bottle to recycle it
Plastic bottles and caps should now be kept together when you recycle them. The reason? It's easier to sort them that way.