Fill a bucket with sand that drains into another with a commercially available gravity filter that drains into another bucket. Perfectly clean water. The sand is a primary filter that will expand the life of a gravity filter and is easily replaceable. The whole setup is cheap and gravity fed. Hell you could even run a gutter spout into it from your roof. https://www.amazon.com/gp/aw/d/B00XTJJIF2/ref=ox_sc_saved_image_7?smid=A2RJ63KAV55ZXI&psc=1 Edit: Make sure and run some water through the sand bucket before hooking it up to the second bucket. Also you can mess around with layers of all kinds of filter substrates in the sand bucket. For instance maybe charcoal on the bottom, sand in the middle, and fish tank filter ceramic on top. At that point the water would probably already be about 98% filtered before it even touches the 0.2micron filter in the second bucket.