One solution is to start a 1000 year landfill pipeline. Plastic is usually inert and harmless in a landfill and will slowly decompose. Landfills >> Rivers
Source for the fact that plastic will decompose in isolated environements (dry, no sun light, constant temperature, no radiations, only plastic) ? Because else you will have to engineer a system that risk leaking into the environnement (dirty water, particles, vapors, etc.).
Just a reminder that 1000 years is a crazy long time to maintain a landfill when we do not seem to be able to contain some waste for merely 30 years without it becoming a superfund site... https://cumulis.epa.gov/supercpad/cursites/csitinfo.cfm?id=0...
A TON of people have an irrational gut reaction against landfills in general and the concept of burying plastic in particular. There is absolutely no shortage of landfill space, but due to some mafia fuckery in the 80s (MOBRO 4000: https://slatestarcodex.com/2019/01/01/what-happened-to-90s-e...) a ton of people think we're on the verge of running out of space.
If you try to open a new landfill you'll be against a ton of people protesting you.
So we're not out of space in theory but in practice it's problematic.
The same goes for sewage treatment plants or basically any infrastructure at all. A city where everybody is happy cannot exist.