Your best bet is to get a Big Berkey filter, made in England, get a jug of pond water and run it through.Yeah i've been googling around but a lot need sunlight to make 'em go, so on dull days they'd be scuppered.
Others are big contraptions like moonshine stills which you have to light a fire underneath, but in a SHTF world in a city making fire might not be easy.
Add a bit of chlorine bleach to it also, look in here for the amount to use.
Scupper is a naval term, right? to sink?
Link for a Big Berkey, it is what I use everyday.