The Guppyfriend washing bag is the first practical solution to prevent microplastic pollution from washing clothes with elastane or synthetic clothes.
Please dispose of the collected microfibres in the residual waste. It is best to collect and dispose of the collected fibres in a closed container. This reduces the chance of the small and very light synthetic fibres being blown away during waste collection and to end up in the environment.
Microfibres do not belong in recyclable waste (yellow bin, yellow bag, green dot) because they cannot be recycled.
By the way: In Germany, residual waste is used to generate electricity and does not end up in landfills. The exhaust gases from waste incineration are filtered and 99% of them are non-toxic. If you don't live in Germany, you can check the Waste Atlas here to see what happens to the waste in your region.