Household Water Treatment in Nepal: Boiling, Piyush, SODIS, Filters, RO

Diarrhoeal disease remains one of the most common preventable illnesses in Nepal, and unsafe drinking water is a leading cause. Piped supplies are often intermittent and can be recontaminated in the distribution network, jar (bottled) water is not always reliable, and shallow tube wells across the Terai can carry both faecal bacteria and naturally occurring arsenic. Because a safe piped supply is not universal, treating and safely storing water in the home, known as Household Water Treatment and Safe Storage (HWTS), is a core public-health strategy promoted by the Ministry of Water Supply and partners such as ENPHO (Environment and Public Health Organization) and WaterAid.

No single method is best for every situation. The right choice depends on what is wrong with your water. Microbial contamination (bacteria, viruses and protozoa from faecal pollution) is the most widespread hazard and can be dealt with cheaply by boiling, chlorination or sunlight. Turbidity (cloudiness from suspended silt) needs filtration or settling first, because cloudy water resists both chlorine and sunlight. Chemical contamination, above all arsenic in Terai groundwater, cannot be removed by boiling or chlorine at all and needs a specific filter.

This article compares the five methods most used in Nepali homes, boiling, Piyush chlorine drops, SODIS (solar disinfection), ceramic and biosand filters, and RO-UV units, so you can pick the one that matches your water source and budget. Nepal's National Drinking Water Quality Standards set the reference limits used throughout: zero E. coli per 100 ml, turbidity below 5 NTU (10 NTU as a relaxed limit), and an interim arsenic limit of 0.05 mg/L (50 micrograms per litre), against the stricter WHO guideline of 0.01 mg/L.

Boiling is the oldest and most trusted way to make water microbiologically safe, and it works against every disease-causing microbe: bacteria such as those causing cholera and typhoid, viruses such as hepatitis A and rotavirus, and hardy protozoa such as Giardia and Cryptosporidium. Guidance from the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC) is to bring clear water to a rolling boil for one minute at low and moderate altitude, and for three minutes above about 2,000 metres, because water boils at a lower temperature in the thin air of Nepal's hills and mountains.

The main advantage of boiling is that it needs no dosing skill and no special product, only a heat source. Its limits are equally important to understand. Boiling does not remove arsenic, other heavy metals, salts or most chemicals; in fact evaporation slightly concentrates them, so boiling arsenic-contaminated tube-well water makes it no safer. Boiled water also has no residual protection, so it must be cooled and stored in a clean, covered container and used within a day or two to avoid recontamination. If the water is cloudy, filter or let it settle first.

The recurring cost is the fuel: low with firewood or biogas, but comparatively expensive with LP gas. Boiling is best thought of as the fallback that always works for microbial safety, especially during outbreaks or after floods, when the water is clear and the contamination is biological rather than chemical.

Piyush is a 0.5% sodium hypochlorite (chlorine) solution developed by ENPHO and sold widely in pharmacies and shops in a small dropper bottle. It is Nepal's best-known household chlorination product and the answer most people are looking for when they search for 'piyush water purifier use'. The standard instruction is to add about 3 drops of Piyush per litre of clear water, mix, and wait at least 30 minutes before drinking; a single 60 ml bottle treats roughly 400 litres, which makes it one of the cheapest options per litre. ENPHO also markets Piyush Plus and 1% chlorine solutions for larger volumes and institutional use.

Chlorination reliably kills bacteria and viruses and gives Piyush a decisive advantage over boiling and SODIS: it leaves a small residual of chlorine in the water that protects against recontamination during storage. That makes it well suited to household jars and to emergency response after earthquakes and floods, when safe storage is as much a problem as initial treatment. Its main weaknesses are a faint chlorine taste and smell that some dislike, and reduced effectiveness against Cryptosporidium, a chlorine-tolerant protozoan.

Two rules matter for safe use. First, chlorine works only in reasonably clear water, so turbid water must be filtered or settled first and may need a longer contact time or a double dose. Second, like boiling, chlorine does nothing to arsenic or other dissolved chemicals. Use the correct dose, do not overdose, and keep the bottle out of reach of children.

SODIS (Solar Water Disinfection) is a near-zero-cost method promoted in Nepal by ENPHO and coordinated internationally by the Swiss aquatic research institute Eawag. Clear water is poured into clean, transparent PET plastic bottles, which are then laid on their side in direct sunlight. The combination of ultraviolet-A radiation and heat inactivates bacteria, viruses and protozoa. The rule of thumb is at least 6 hours of strong sunshine on a sunny or partly cloudy day, extended to two consecutive days (up to 48 hours) when the sky is overcast.

SODIS costs nothing but the bottles and works well for individual and small-household drinking-water needs, which makes it valuable for low-income and remote communities. Bottles should be colourless, transparent, scratch-free, no larger than about 1.5-2 litres, and cleaned with labels removed so light can pass through. The biggest limitation is that the water must be clear: SODIS does not work in turbid water, which shields microbes from the sunlight, so cloudy water must be filtered or settled first.

SODIS also depends on the weather and produces only modest volumes at a time, so it is less practical during the monsoon or for large families. Like boiling and chlorine, it removes no arsenic or chemical contaminants. Used correctly on clear water in good sun, however, it is a genuinely effective and sustainable option, and one of the answers people are looking for when searching 'sodis nepal'.

Filters physically strain contaminants out of water and are the go-to solution when the problem is turbidity or, in the Terai, arsenic. Ceramic candle filters (the common two-pot units sold across Nepal) trap suspended silt, bacteria, protozoa and helminths in their microscopic pores, and improve taste, smell and colour; candles impregnated with colloidal silver improve pathogen removal. Their limits are that they are less reliable against viruses, which are smaller than bacteria, and they do not remove arsenic. Candles must be cleaned regularly and replaced when worn.

Biosand filters are concrete or plastic columns of sand and gravel with a biological layer that removes turbidity, most bacteria and some viruses. The Kanchan Arsenic Filter (KAF), developed by MIT and ENPHO specifically for Nepal, adds a bed of iron nails above the sand: rust from the corroding iron binds arsenic so it can be filtered out. Field studies in Nepal reported the KAF removing roughly 85-90% of arsenic, 90-95% of iron, 80-95% of turbidity and 85-99% of total coliforms at a design flow of about 15 litres per hour. The traditional three-gagri (three-pitcher) filter works on a similar iron-oxidation principle.

Filters have the advantage of treating larger volumes with no fuel, no chemicals and no waiting, and an iron-based filter is one of the only affordable household options that actually removes arsenic. The trade-off is that most everyday filters (ceramic, biosand) do not guarantee removal of viruses. Best practice, endorsed in HWTS guidance, is to combine a filter with a disinfection step, for example filter the turbid or arsenic-bearing water first, then chlorinate or boil the clear output to be sure of microbial safety.

Electric RO (reverse osmosis) and UV (ultraviolet) purifiers are increasingly common in urban homes and offices in Kathmandu and other cities. A UV lamp gives a reliable kill step against bacteria and viruses but removes no chemicals, metals or dissolved solids. Reverse osmosis pushes water through a fine semi-permeable membrane and removes almost everything, including arsenic, other heavy metals, salts and total dissolved solids (TDS), so a combined RO-UV unit gives both chemical and microbial protection at the tap. This is the most capable option for tube-well water that is both arsenic-affected and microbially unsafe.

The drawbacks are cost and practicality. RO-UV units carry a high purchase price and ongoing costs for membranes, filters, electricity and servicing, and a UV lamp only works while it is powered, a real limitation given Nepal's history of load-shedding. RO also wastes a significant share of the input water as reject brine and strips out beneficial minerals, and both technologies need reliable running water and maintenance to keep working. Membranes and UV lamps that are not serviced can silently stop protecting the water.

For most rural and low-income households, RO-UV is neither affordable nor necessary, as boiling, Piyush or SODIS handle the microbial risk far more cheaply. RO becomes worth its cost mainly where the raw water genuinely contains arsenic or high dissolved solids that cheaper methods cannot touch.

Start by identifying what is wrong with your water. If it is a piped or spring supply that is clear but may be microbially contaminated, any disinfection method works: boiling is the surest, Piyush is the cheapest and gives storage protection, and SODIS is free where there is good sun. If the water is cloudy or muddy, treat the turbidity first with a ceramic or biosand filter (or let it settle and decant), then disinfect the clear water by boiling or chlorination, because chlorine and sunlight do not penetrate turbid water.

If your source is a Terai tube well, the decisive question is arsenic. Boiling, Piyush and SODIS do nothing for arsenic, so get the well tested; where arsenic exceeds the 0.05 mg/L Nepal limit, use a Kanchan-type iron/biosand filter or an RO unit, and still disinfect the output for microbial safety. A useful general principle from HWTS guidance is the 'multi-barrier' approach: combine a physical step (filtration) with a disinfection step (boiling, chlorine, UV or sun), and always finish with safe storage in a clean, covered, narrow-mouthed vessel with a tap or ladle so treated water is not recontaminated by hands or cups.

In short: clear microbially-risky water, boil, Piyush or SODIS; cloudy water, filter then disinfect; arsenic-affected tube-well water, iron filter or RO plus disinfection. Choosing by source rather than by fashion is the safest and most economical way to purify water at home in Nepal.