Eh, I think you're a little paranoid about the bathroom tap, but perhaps you know something I don't.

Split this into 2 situations: 1) Every day 2) Emergencies.

For every day, just get a Pur pitcher and the municipal water from the tap once a day - first thing when you arrive at the office. <shrug> you could use a Brita, but IMHO the Pur is better. Don't leave the filtered water standing out at ambient over a weekend - the municipal treatment chemicals are stripped out by the filter, so there is no residual microbiotic inhibition.

Since neither the Pur nor the Brita can claim to reduce bacteria or virus, if you are concerned about those there are a couple of things that that you could do (call these stage 0 and stage 2, with the pitcher stage 1):

Stage 0: Stash a bottle of plain bleach and a small non-metallic cup at work. Before filling the pitcher, a) run the tap for 10 seconds and turn it off; b) pour some raw bleach into the cup and hold the cup on the faucet so the bleach covers the spigot to a depth of about an inch (depends on faucet style) and hold it there as long as you don't feel like an idiot - 10 to 60 seconds; c) dump the bleach, run the tap a second or two, and fill the filter. This addresses any occult gross cross contamination by hands, mouths, and dish rags.

Stage 1 - water drips thru your Pur pitcher filter.

Stage 2 - add treatment chemical to the filtered water and let sit 15 minutes. Hardly matters what you use at this point, so pick something economical - cheapest would be (you guessed it) a measured amount of the bleach from stage 0.

Personally, I would skip stage 0 and 2 and just use the Pur pitcher; YMMV.

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Situation 2 - Emergencies. You probably already have this covered... perhaps you might add an Ex Stream or Ex Stream XR to your desk drawer.

HTH give you some useful ideas for your circumstances.

Tom