First off, why are we boiling water right now? We've all seen the news about the broken water main in Weston. Since the rupture was expelling 8 million gallons of treated water into the Charles, the MWRA needed to divert water from untreated reservoirs to maintain service. However, since pond water is now coming from the taps, there is a slight risk of getting sick from the natural contaminants. Of concern are Giardia, and Cryptosporidium and to a much lesser level common bacterias such as e. coli.
Giardia (Beaver Fever) and Crypto are not anything to laugh at, they are protozoans that cause gastro-intestinal issues in their hosts, most commonly diarrhea. Transmission is normally fecal to oral - which brings us back to the boil order. Mammals of all types serve as hosts, and the MWRA usually treats our water aggressively before sending it on to our taps in order to kill these contaminants. There are some relatively easy methods to safely treat our own water until the MWRA gets the system back together.
Hikers have been "enjoying" the effects of Giardia and Crypto for years and have come up with some great tech to prevent infections. I believe personally that the mechanical filtration available on the market is portable enough and inexpensive enough that everyone should have at least one unit in the home.
This is what I have, it's a light, inexpensive mechanical filter. Works easily, and the water it outputs is very clean tasting. It uses a filter with pores of .03 microns to capture contaminants - Giardia and Crypto are larger than the filter pores. The filter in this unit can treat 200 gallons, and costs about $80. For an effective cost of $0.40 per gallon for 200 gallons (Without fistfights at BJ's!), and new filters cost less than $40 (for a cost of $0.30 per gallon for 400 gallons.). This unit is small in size, has its own carrying bag, and weighs in at a paltry 11 oz. - so it's easy to keep around at home or on vacation. This is usually on hand at REI.
This is truly an amazing product, a portable "pen" that treats your water with UV light and effectively destroys problem causing microbes. This is another lightweight, easily usable and portable item, however it doesn't use a filter, but a lamp. The adventurer weighs in at a hefty 3.6oz., and will treat 50 qt. per pair of disposable batteries, for a cost of $8.00 per gallon for the first set of batteries, but that cost drops to $4.32 a gallon for the next 50 qts. For comparison, to the Hiker Pro above, 200 gallons will cost you around $1.10 per gallon. (The numbers are based on an assumption of $8 per pair of batteries.) The high cost offsets the value of the highly portable, and easy to use unit that is extremely effective.
There are two chemical treatments that I've used before, and would use again. Iodine tabs and chlorine bleach can effectively purify water, and chlorine is used by the MWRA at the treatment facilities.
Chlorine bleach treatment of water is fairly simple - add unscented household chlorine bleach to a large container of water, and allow it to sit for a bit. The resultant water is safe, but tends to taste "off". A trick that hikers use to combat that is to add some powdered drink mix to increase the palatability of the water. I will say this, chlorine isn't something to play around, and is highly toxic. When used correctly it is a great tool for sterilizing and sanitizing. It is important to be sure you have chlorine bleach, do NOT use non-chlorine bleach, or scented chlorine bleach to treat water. Most household chlorine bleach is 5.25% chlorine and potent enough for two drops to properly treat 1 qt. of water. 8 drops, or 1/8 tsp. will treat a gallon of water. The EPA site I've linked below has a table showing the amounts to use for different concentrations. Once mixed the water must stand for at least a half hour for it to work. Turbid water or extremely cold water will require a higher concentration of chlorine.
Iodine tablets are an easy alternative to bleach for a chemical treatment. The Potable Aqua tablets are the only ones I've tried, and while the taste isn't pleasant, it is decent enough once masked. The tabs come in 50 tablet bottles, and two tablets will treat 1 qt. of water. As with the chlorine, the treated water must sit for at least 30 minutes to take effect. One thing to note though, iodine is NOT effective against Cryptosporidium. The Potable Aqua tablets will last up to 4 years unopened, or 1 year opened, and has a cost of about $1.08 per treated gallon of water.
Boil, baby, boil!
Boiling is a safe and easy alternative to the above mentioned treatments. However, it is predicated on having enough available fuel to bring a large volume of water to a rolling boil for a minute or so, difficult if utility services are interrupted. The only real negative to boiling water is that the end product is HOT and tastes funky due to a lack of oxygen in the water. That's easily fixed by splashing it around in a clean pitcher and throwing it into the fridge for a few hours. This is really the best way to treat large amounts of water in the home, and any of your homebrewing friends should be able to hook you up with a large pot to use for boiling.
This document from the US EPA is a good rundown of how to treat water during an emergency. It covers boiling, home filtering of turbid water, and chemical treatment.
If In Doubt
If in doubt about how to handle this whole situation, don't sweat it. Grab a homebrewer, or a hiker. Beer brewing began as a method to preserve grain and purify water. We know how to fix what ales you. Hikers have also been dealing with these contaminants for years, it's old hat at this point. So, relax and don't sweat the small stuff.