Purifiers that work without electricity

A long power cut takes the purifier out along with the lights. RO stops, UV stops — what's left is whatever never needed electricity to begin with, and in most Indian homes during a multi-hour or multi-day outage that comes down to two real options: a gravity filter on the counter, or a pot of water on the stove.

The two don't do the same job, even though a ceramic candle filter and a UF gravity unit look nearly identical side by side. Boiling protects differently again, with its own gap. Here's what each one actually does — and doesn't — once the power's been out a while.

Why "no electricity" rules out more than it sounds like

Reverse osmosis needs a pump to force water through its membrane at pressure — no pump, no RO, which is why a genuinely non-electric RO purifier doesn't exist. UV needs continuous power to keep its lamp lit, and even while running it only disinfects water passing the lamp at that instant; it does nothing for water already sitting in a tank, and nothing at all once the power's gone.

That leaves two mechanisms that were never electric. Gravity-fed filtration pushes water through a membrane or ceramic shell using nothing but its own weight — an elevated tank does the work a pump would otherwise do. Boiling disinfects with heat, from a stove rather than a plug point. Both keep working through an outage; they just don't cover the same half of the problem.

Gravity-fed UF: the modern default

Ultrafiltration (UF) uses a hollow-fibre membrane with pores around 0.01–0.1 microns — fine enough to strain out bacteria, cysts, and turbidity, too coarse to touch anything actually dissolved in the water. Fill the upper chamber, gravity pulls water through the membrane, clean water collects below. No pump, no lamp, nothing to switch on.

It's sufficient on its own when two things are true: your water's TDS is already within BIS's acceptable limit of 500 mg/L, so there's nothing dissolved that needs removing, and the risk is microbial rather than chemical — municipal supply with occasional contamination, monsoon runoff, a tanker top-up of uncertain quality. Fluoride, nitrate, arsenic, and excess hardness pass straight through a UF membrane exactly as they would through a candle filter or a length of pipe; none of that is what UF is built to catch.

Maintenance is light: rinse the membrane on the manufacturer's schedule, replace it every year or two depending on use. There's no scrubbing routine to track, and no equivalent of a candle filter thinning with every clean.

Candle filters: solid on bacteria, weak on viruses

A ceramic candle filter is the older gravity technology — a porous ceramic shell, often wrapped around an activated-carbon core, suspended between two chambers. Water sits above, seeps through the ceramic wall, and drips out filtered below. Many candles are silver-impregnated, which slows bacterial regrowth on the surface between cleanings but filters nothing by itself.

The clearest test of Indian household units, published in the International Journal of Environmental Health Research (Clasen & Menon, 2007), found ceramic candle filters cut bacteria by more than 4 logs — over 99.99%, genuinely good. Against a virus surrogate (MS2 bacteriophage) and a 3-micron microsphere standing in for protozoan cysts, reduction fell well short: under 3.4 and 2.6 logs respectively. The microsphere is smaller than an actual Giardia or Cryptosporidium cyst, so even that weaker number is a best case, not a worst case. The physics explains why: ceramic pores are sized to catch bacteria — roughly 0.5 microns and up — but most waterborne viruses are an order of magnitude smaller and pass through the same pores water does.

None of this makes candle filters obsolete. They're cheap, widely stocked, and a reasonable choice for clear municipal or well water where bacteria is the main concern. But they need discipline UF doesn't: scrub the candle every two weeks — sooner if flow slows, a sign of a clogged surface — with a soft brush and plain water, no soap. Replace it every 6–12 months regardless of appearance, because each scrub thins the ceramic wall, and a thinned wall stops reliably blocking bacteria long before it looks worn. Skip that schedule, which most households do, and a candle filter quietly becomes a filter in name only.

Boiling: full microbial coverage, only half the job

A rolling boil for one minute — three minutes above roughly 2,000 metres — kills bacteria, viruses, and protozoa alike, broader coverage than either gravity option, and it needs nothing but a stove and fuel. It does nothing for anything dissolved: fluoride, arsenic, nitrate, and hardness ride through unchanged, same as with UF or a candle filter.

What people skip is storage. Boiling only disinfects the water in the pot at that moment; a glass left uncovered, or ladled out with a hand-dipped cup, can pick up new contamination within hours. Store it in a covered container with a narrow neck or a tap — not a wide-mouth vessel someone can dip into — and keep it separate from water used for anything else. Boiling and storing safely are two different steps, and only the first tends to get remembered.

What actually works during a power cut

If your TDS is already reasonable and the risk is mainly microbial, a gravity-fed UF unit is the stronger everyday choice — better virus and cyst protection than a candle filter, without the scrubbing discipline that filter demands. Keep boiling as the fallback for water a filter can't be trusted with — a tanker of unknown origin, a monsoon advisory — and store it properly rather than in an open pot on the counter.

Where this breaks down is high TDS or a specific dissolved contaminant, like fluoride or arsenic in the source. No gravity filter and no amount of boiling touches that; only RO or a contaminant-specific treatment does, and both need power. If outages are frequent and your water genuinely needs that kind of treatment, the answer isn't a different non-electric filter — it's backup power sized to run the RO unit, with a gravity filter or boiling covering the hours backup doesn't reach.