All medications should be disposed of separately to other waste. I'm not sure how it's handled worldwide, but my country allows disposal in community pharmacies.

Many antibiotics (e.g. pretty much all penicillins) don't get broken down significantly on passing through the body, so a major source of contamination is coming from human waste and inducing bacterial resistance in the bacteria in the wastewater system.

The real solution here is to reduce unnecessary antibiotic usage.