The balcony is too small to have a compost there. But if I had a compost, I would put in my daily coffee grounds. So I decided to see whether I can reuse the coffee filter even without composting it first.
I cut out a piece of a used coffee filter roughly to the size of a saucer. Parts of it were still sprinkled with coffee grounds. I then put garden cress seeds on it and watered the whole thing carefully.
If the cress develops that would not only mean fresh green edible leaves in winter, but also having grown them from what is actually garbage. Admittedly, garden cress is usually rather undemanding, but I am not sure whether the low ph value of the coffee works for the cress. If you think using old coffee filters is too disgusting, you can also grow cress on a piece of paper towel.
Updates on how the experiment works are to follow.