Are you using boards view or list view? And what do you mean by filter out completed tasks?
If you’re using list view I recommend that you simply check off the completed tasks and view your project by incomplete tasks only.
I face a similar challenge in my planning project. I use the same Community planning project each episode (an Asana measurement of time, about 6 months), but that means my completed tasks pile up. I just view my project by incomplete tasks and thus my completed tasks are hidden away.
You could also get creative with an advanced search of some kind. https://asana.com/guide/help/fundamentals/search
You might also get some suggestions in this blog post about boards https://blog.asana.com/2016/11/introducing-boards/
Does this help? Let me know if you have follow up questions.