My city's giant landfills have clay liners that redirect the leachate to a treatment plant specifically for the purpose. The leachate literally flows into the plant constantly, it's a huge amount.
I only know this because I worked there in my early 20s. It had the most horrifying smell like a fully fermented Diaper Genie poop-snake with strong sweet and sour notes that seemed to have flavour.
I got used to the smell at the regular waste water treatment plant, and at the dump in general, but the leachate plant was unbearable.