The issue of water purification in an urban bug out, likely on foot, from an office or entertainment venue, with only a small EDC/GHB and a 1-2 day walk home, is a particular interest of mine. Especially because I live, work and recreate in a metro area where this is my likeliest disaster scenario. What follows is my strategy: If I'm lucky enough to have access to my car kit, even if I cannot drive out, I'll have my General Ecology First Need XL Puifier. This unit will, according to EPA test ratings, remove bacteria, cysts, viruses and most chemicals. I believe, based on my own research, that this is as good as it gets for generally available civilian, backpack portable water purifiers.YMMV. If I'm limited to the small backpack or briefcase kits I carry, I go with a mixture of very small and lightweight components. Initial filtration is with coffee filters in a folding Survival Resources (SR) filter holder. Once I've got that in a SR 1 liter water bag, I'd add Katadyn water purifier tablets. Final stage is to run the pretreated water thru a Seychelles Advanced water bottle purifier or a Seychelles Advanced Water Purifier Straw. Both of the Seychelles units are very lightweight and compact. I believe that these set-ups, along with a four way silcock key, will allow me to scavenge enough clean water to get me home. I welcome everyone's comments. Constructive criticism can only prove or improve my preps. Cheers.