The problem with PLA-based plastic is that it requires high temperatures and moist conditions for a relatively long period to actually degrade and turn into compost. Theoretically it is possible to compost it along with all other compostables (food leftovers, garden wastes), but PLA takes much longer to fully decompose than 'normal' organic waste and it seems most municipal composting centers don't bother doing this. Instead they filter out any plastics or large objects and either dump them in landfills or incinerate them,
There are 2 things you can do:
- Throw it in the 'general' or 'compostables' waste bin. In both cases, the plastic either gets either dumped in a landfill which would be unfortunate, or it gets incinerated in which case it may be useful as an energy source.
- Try and compost it yourself using hot composting. However, proper hot composting isn't easy as you need to monitor the temperatures in your compost pile and turn the materials regularly.
Whatever you do, do not dispose of it along with other plastics. If compostable plastics end up being recycled along with other plastics, the quality of the recycled plastic will be impacted severely.
If you'd like to know more about this there are two related questions on this website: