Water: Collect, Store, Filter

Water is the gap in most home preparation. People stock tins, buy a torch, and put four bottles of supermarket still under the stairs. Then a main bursts two streets away, or the pumping station goes down with the power cut, and the four bottles last until lunchtime. This course fills that gap, and it's the piece the off the grid project needs first.

Start with what you use now: about 140 litres per person per day in the UK, roughly a fifth of it flushed away. Nobody stores that, and the number is there to show how far you're going to fall. The survival end is much smaller. The World Health Organisation puts the drinking minimum at 2.5 to 3 litres per person per day: water into a body, no washing or cooking. The gov.uk Prepare campaign uses that figure and adds the honest one, 10 litres a head, covering basic cooking and hygiene too. That figure isn't invented. Under the Security and Emergency Measures Direction, water companies must plan to get 10 litres a head to you within 24 hours of a supply failure and every 24 hours after, rising to 20 litres once an outage passes five days.

You need both figures. Three litres a head is what must be in the cupboard tonight; 10 litres a head is what a bearable week looks like, and most of that extra seven can come from a butt, a bath run full at the first warning, or a bowser at the end of the street. Two weeks of the must-have tier for four people is 168 litres, or seven 25-litre containers. Add more for pregnancy, illness, medical equipment, formula feeding or animals, and if any of that applies, get on your water company's Priority Services Register while nothing is wrong.

Tap water arrives with a residual disinfectant in it, which is why storing it works at all. Fill clean food-grade containers from the cold tap, seal them, keep them cool and dark, and the water stays safe to drink for around six months. Rotate at six months regardless: old water on the garden, rinse, refill. Date every container in marker pen, because you will not remember. If stored water turns cloudy or smells off, tip it and start again.

Not milk bottles, which is the most common bad advice online. Milk protein and fat cling to the inside where no rinse reaches and feed bacteria in whatever goes in next, and the bottles are made for one trip and split at the seams under a stack. Juice bottles the same. Commercially bottled water in its own sealed bottle is fine and comes dated.

Rainwater harvesting is legal in the UK. No permit, no licence, no notification to put a butt under a downpipe and use the water on the garden. Worth saying plainly, because much of what you'll read online is American, where a few states have historically restricted collection under prior appropriation water law. The UK boundaries sit elsewhere: abstracting more than 20 cubic metres a day from a river or the ground needs an Environment Agency licence, and anything joined to the house plumbing falls under the water fittings regulations. Rain off your own roof into a tank is yours.

The yield is better than people expect. Multiply roof plan area in square metres by annual rainfall in metres, then knock off about 20% for what evaporates or blows away. The UK averages around 1,160mm a year and even the drier east of England clears 600mm, so a 40 square metre roof gathers roughly 22,000 litres a year. Rain doesn't arrive on a schedule, though, so storage volume rather than roof area is what limits you. Fit a first flush diverter: the opening half millimetre of a storm washes off bird droppings, moss and grit, about 20 litres on that roof, and it's the dirtiest water you'll collect all week. Gutters clear, mesh over the inlet, lid sealed.

Then the roof, which decides what the water is good for. Never drink water off a roof treated with moss killer, or one with bitumen or asphalt shingles, lead flashing or lead paint, asbestos cement sheets, or zinc-galvanised flashing. Those leach into runoff and no domestic filter removes dissolved metals or biocides. The Environment Agency's guidance on domestic rainwater harvesting covers non-potable uses only: flushing, the washing machine, the garden. Treat that as the default. Clean glazed tile or slate, with a diverter, sediment removal and then boiling, is drinkable in an emergency, and it's still the water you turn to third, after your store and after anything the water company delivers.

Sediment first. Let the water stand so the heavy material drops out, then pour off the top through cloth or a coffee filter. You're removing turbidity, not pathogens. Skip it and your filter clogs and your chemical dose gets spent neutralising mud instead of microbes.

Purification second. The step that kills things: boiling, tablets or UV. A mechanical filter does part of this and cannot finish it, so treat filtering as advanced sediment removal and always follow it with a real kill step.

Storage third. Treated water goes into a clean, covered, food-grade container and gets used within days. Pouring purified water back into the open bucket it came from undoes the whole chain, and that's how people who do everything else right still get ill.

Boiling is the gold standard and the default. Bring it to a rolling boil; WHO's position is that reaching a rolling boil kills pathogenic bacteria, viruses and protozoa, and the widely used public health advice is a rolling boil held for one minute (three above 1,000 metres of altitude, which in Britain concerns nobody). It deals with every biological threat including Cryptosporidium. It does nothing about chemicals, nitrates or heavy metals, and by evaporating water it slightly concentrates them. It costs fuel and time and it's still right more often than not. A boil notice means exactly this.

Chemical tablets are the lightweight backstop. Aquatabs and similar NaDCC tablets act in about half an hour on bacteria and viruses. What they don't reliably handle is Cryptosporidium, whose oocysts are notoriously chlorine-resistant. Chlorine dioxide tablets do, at the price of up to four hours of contact in cold water, out of sunlight. Both need clear water to start with, both leave a taste, and neither touches chemicals or metals.

Gravity and ceramic filters (Doulton and British Berkefeld candles and their like) are the comfortable everyday option: no power, no fuel, litres an hour, and a carbon core that improves taste and reduces chlorine and some lead. They remove bacteria, protozoa and sediment. They are not virus purifiers, and a hairline crack you can't see means untreated water going straight through.

Hollow fibre filters (Sawyer, LifeStraw) filter to 0.1 micron and remove bacteria and protozoa to very high log reductions for £25 to £40 and almost no weight. Viruses, dissolved chemicals and heavy metals go straight through. Freezing destroys them invisibly: one left in a cold shed over winter can look perfect and filter nothing.

UV pens inactivate bacteria, viruses and protozoa in clear water in about a minute. They need batteries, they need properly clear water because shadow protects pathogens, and they break. A fine second string, a poor only string.

Greywater does two jobs in a shortage: watering the garden and flushing the loo. Neither involves drinking it, and that separation is absolute. Different containers, marked, stored apart. Bath, shower, basin and washing machine rinse water count; kitchen sink water doesn't, because fats and food residue make it behave like sewage.

Use eco or bio detergent, skip the bleach and the softener, and apply it with a watering can at soil level rather than a sprinkler, never onto salad leaves or anything eaten raw. Alternated with rainwater so salts don't build up, it's a useful drought supply for the veg patch. The hard limit is time: untreated greywater must not be stored beyond 24 hours, per BS 8525, because it goes from unpleasant to a real health risk inside a day. Use it or tip it. To flush, pour a bucket straight into the pan, briskly, not into the cistern. A full bath flushes a household for a day with no power and no mains.

Private supplies. A borehole, well or spring serving your house is a private water supply, and it's regulated. In England the Private Water Supplies (England) Regulations 2016, as amended in 2018, put the duty on your local authority: they risk assess supplies in their area, review that at least every five years, and set the sampling regime. Wales, Scotland and Northern Ireland have their own equivalents. Single dwellings used only for domestic purposes are the main exception from mandatory monitoring, which is exactly why you should ask for a test rather than assume. Drilling is a specialist job.

Anything joined to the house pipework. The Water Supply (Water Fittings) Regulations 1999 exist to stop contaminated water getting back into the public main, and they class rainwater and greywater as fluid category 5, the highest risk band. No rainwater or greywater pipe may connect directly to the drinking water supply; any mains top-up to a tank needs a type AA or AB air gap, a physical break the water falls through; and installing a rainwater harvesting system is notifiable, so your water company must be told beforehand. Contravening the regulations is an offence. Use a WaterSafe approved contractor. Butts, jerrycans, filters and kettles you can do this weekend; pipework into the house is a plumber's job.

Clean water on tap is about 170 years old, and the argument that produced it was won in one London street. Through the first half of the nineteenth century London drew its water from the Thames, downstream of its own sewers, and cholera killed in the tens of thousands. The Metropolis Water Act 1852 required water taken from the Thames to be filtered and moved the intakes above Teddington Lock: the first legal quality standards for a public supply anywhere.

Two years later came the Broad Street pump. A cholera outbreak in Soho killed around 500 people in ten days. John Snow, a physician who thought cholera was waterborne when almost everyone believed it travelled through bad air, plotted the deaths on a map, found them clustered around one public pump on Broad Street (now Broadwick Street), and persuaded the parish to take the handle off. The outbreak was already declining, and the honest version credits the map and the argument rather than the handle, but it became the founding case of epidemiology. Disinfection followed: bleach on a water main in Maidstone in 1897 after typhoid, then Lincoln in 1905, the first place in Britain to chlorinate routinely. Worth remembering while you stand over a pan waiting for a rolling boil. You're doing by hand what a country spent a century learning to do for everyone.

The project is a plan you can point at, not a shopping list. By the end of a weekend you'll have your household's numbers, two weeks of drinking water stored and dated, a butt on a downpipe with a diverter, and one batch of collected water run all the way through filtering and purification, so you know how long it takes and what it tastes like.

The obvious next step is the wider kit: water is one line on the list in Build a 72-Hour Emergency Kit, and this course is the long version of that line.