I have a gravity filter as well and just use the dirty water bag then filter that water ( by gravity) into a collapsible 5gl container with a spicket. We just kind of constantly keep refilling the bag every now and again and before you know it we have a full 5gl of clean water to use. By myself, I only use my Aqua-Mirra drops. I'm always by a lake, creek or river though. I have a 3ltr platypus that I will fill up and 15 min I have clean water to drink .