When my scout troop is teaching the Emergency Preparedness merit badge, we often do an exercise on bulk treatment of water beyond what is practical to filter with the portable filters we take on backpacking trips. We use alum as a flocculant, then sand to filter, then battery-powered UV sterilization (while noting that leaving clear water out in full sun for a few hours will accomplish the same dose as stirring a UV lamp through a bucket for a minute or two). Alum isn't exactly a common household item these days and the UV lamp certainly isn't, but it does make for a cheap and compact kit that can treat a lot of water with relatively little time and effort.