In a UK emergency the first choice is always stored or bottled water. If your tap water might be contaminated, follow your water company's instructions; a boil water notice means boiling (DWI). A filter is for when those run out and you have to use water from a water butt, stream or river.
The guidance below is from the US Centers for Disease Control and Prevention (CDC), a US government agency that publishes detailed advice on portable treatment. UK official sites do not give this level of detail.
Boiling is the most reliable
The CDC says boiling is the best way to kill germs in water. If you can't boil, the next best is to filter, then disinfect the filtered water.
What the labels mean
- 1 micron or smaller (absolute), or certified to NSF 53 or 58 for cyst reduction: removes parasites such as Giardia and Cryptosporidium, but not viruses or all bacteria.
- 0.3 micron or smaller (absolute): removes bacteria and parasites, but not viruses.
- Reverse osmosis: removes parasites, bacteria and viruses (and salt).
- The CDC figures are for absolute pore size: check the label says so.
The CDC says most portable filters will remove parasites, but not viruses or bacteria. So read the label carefully.
What filters don't do
- Most portable filters do not remove viruses, such as norovirus. To deal with viruses, boil the water or treat it with chlorine-based tablets after filtering.
- Most portable filters are made for germs, not chemicals. Some include activated carbon, which can improve taste and reduce some chemicals: check what the maker claims. Don't rely on a portable filter for water that may hold fuel or chemicals, such as floodwater.
- Chlorine and iodine tablets kill bacteria and viruses but do not work well on parasites. That is why filtering then disinfecting covers most germs (CDC).
Practical tips
- Pre-filter cloudy water through a clean cloth, and let silt settle first. Tablets work best in clear water.
- Keep "dirty" and "clean" containers separate and labelled.
- Follow the maker's instructions for flow, cleaning and replacing the filter.
- Treated water still goes off: keep it covered and use it soon.
- Water filter
Filters remove bugs and dirt, not chemicals. Boil or use tablets if unsure- Budget Personal straw water filter · Search Amazon UK (Ad)
- Moderate Gravity bag or filter water bottle · Search Amazon UK (Ad)
- Ultimate Premium gravity water filter system (ceramic or hollow fibre) · Search Amazon UK (Ad)
Purification tablets are a cheap stop-gap. OrCore Prep's Tier 1 list includes Aquatabs (1 tablet per litre, clear water only) until you buy a filter.