addition of one of the Sawyer .1 micron filters would strain out the critters, but not virus or anything dissolved in solution, but not much does.. possibly clear glass aquarium type container or coil of clear tubing to get some UV help... might try a final activated charcoal can (aquarium supply house)for halogens....

first volume dump
biomass slow sand
UV exposure
Sawyer
charcoal

might have something...