I have been thinking about this some recently. No one seems willing to tell us how effective these Brita type filters are at removing things but that doesn't mean they are not effective. I think the facts are that they do not filter out the bugs that make us sick so companies are very leery of making promises about what they can do lest they be sued when someone uses it as their sole water treatment.

The fact is activated carbon is very effective at bonding to and neutralizing a lot of pollutants in water that might not make you sick right away but could over a long period of drinking them. If you ingest poison one of the more common treatments is a little capsule of activated carbon. How can that little capsule be effective at saving my life when I directly ingest poison but an entire tube of it be worthless for treating mildly polluted water?

With CLO2 I think the proof is in the taste. If much of the CLO2 taste is removed with carbon treatment, then it is probably a pretty effective treatment.

A more serious issue came up over at BPL: that of blue-green algae aka cyanobacteria. Standard water treatments will kill cyanobacteria but none of them (filter, clo2, steripen, boiling) will remove the toxins they produce. If you get cyanobacterial poisoning, the treatment is activated carbon, but I can find no guidance on using the activated carbon to treat the water before you drink it.

So... I suspect those little brita filters could be very useful but I have no direct information at just how useful they are.