Many countries used to send their dirtiest recycling to China, where cheap labor sorted through the mess extracting valuables and dumping the rest. Plastic waste is a particular problem. But in 2017, China severely tightened rules for taking contaminated trash.
The Recycling Partnership in the US tested a program that educates citizens on what materials had value to recyclers.
They found that what worked best was sending a mailing that educated residents on the rules, then following up by enforcing those rules uncompromisingly. This included putting a team of inspectors out to evaluate each recycling bin and leave Oops cards on bins they rejected for collection.