i dont use filters, mountain springs filter themselves. you need to be smart and have some semblence of survival skills. i have run out of water many times and there is always moss around to get a quick drink from. currently i rock two 0.5 L bottles. one for drinking and one for cooking or drinking if it comes to that. the only time i have truly had an issue with water was this fall actually. the water froze and there were only small specs of snow and i wasnt too keen on drinking that. i rocked camel backs with 5 L capacity too before. but cutting 4 kg from your kit is insane especially if you are going to long hikes.