Disinfection Methods to Ensure Safe Drinking Water

Safe drinking water is one of the most basic requirements for human health, yet it remains out of reach for billions of people. According to the World Health Organization, at least 1.7 billion people use a drinking water source contaminated with faeces, and microbiologically unsafe water is responsible for approximately 505,000 diarrhoeal deaths every year. Diseases like cholera, typhoid, dysentery, and polio can all spread through contaminated water. This is where disinfection becomes critical. Water disinfection refers to the process of eliminating or inactivating harmful pathogens – bacteria, viruses, and protozoa – to make water safe for drinking. Three widely used methods for achieving this are boiling, chemical disinfection using chlorine, and solar water disinfection (SODIS). Each has its own mechanism, strengths, and limitations, and understanding them is essential for making informed decisions about water safety.

Why disinfection matters 🔗

Not all water that looks clean is safe to drink. Pathogens are invisible to the naked eye, and even clear water drawn from a river, well, or stored container can carry dangerous organisms. Research published in The Lancet estimates that unsafe water, sanitation, and hygiene collectively account for around 1.6 million deaths and over 100 million disability-adjusted life-years annually. The burden falls disproportionately on children and communities in low- and middle-income countries. Disinfection – whether done at the household level or through centralized municipal systems – is the last line of defence against waterborne illness. It does not remove chemical contaminants or heavy metals, but it effectively targets the biological threats that cause the majority of water-related disease.

Boiling: the simplest and most reliable method 🔗

Boiling is the oldest and most universally trusted method of water disinfection. It requires no chemicals, no special equipment, and no technical knowledge – just a heat source and a container. When water is brought to a full rolling boil, the heat destroys the cell structures of bacteria, viruses, and protozoa, making the water biologically safe to drink.

How to boil water correctly 🔗

Iowa State University Extension recommends boiling water at a rolling boil for one full minute, which is sufficient to kill all disease-causing microorganisms. At altitudes above 2,000 metres, water boils at a lower temperature, so the boiling time should be extended to three minutes to ensure complete pathogen kill. After boiling, the water should be allowed to cool in a covered container to prevent recontamination. If the flat taste of boiled water is undesirable, pouring it back and forth between two clean containers – a process called aeration – improves its palatability, as does adding a small pinch of salt.

Advantages and limitations of boiling 🔗

Boiling is 100% effective against biological pathogens when done correctly. It requires no added chemicals, leaves no residual taste concerns (beyond the flatness that aeration can fix), and is immediately available during emergencies when other supplies are unavailable. However, it has real limitations. It requires fuel – firewood, gas, or electricity – which may be scarce or costly in resource-limited settings. A study in ScienceDirect notes that the scarcity of fuels required for boiling is one of the primary reasons this method is underutilized even in communities that need it most. Boiling also does not provide any residual protection: once water cools and is handled, it can be recontaminated during storage or serving.

Chemical disinfection: chlorine 🔗

Chlorine-based disinfection is the backbone of municipal water treatment systems worldwide. It is cost-effective, scalable, and critically, it provides residual protection – meaning it continues killing pathogens in the water even after the initial treatment. This makes it particularly useful for water that needs to be stored before use.

How chlorine works 🔗

When chlorine is dissolved in water, it forms hypochlorous acid and hypochlorite ions, which attack and destroy the cell walls of bacteria, the protein coats of viruses, and the membranes of certain parasites. The CDC confirms that sodium hypochlorite – the active ingredient in common household bleach – has been used for over a century and is the primary disinfectant promoted by both the CDC and the World Health Organization. At appropriate concentrations and contact times, chlorine is effective against bacteria and viruses. However, some protozoa, particularly Cryptosporidium and Giardia cysts, are more resistant and may require longer contact times or combined filtration.

How to use chlorine for household water treatment 🔗

The most accessible form of chlorine disinfection for household use is regular, unscented liquid bleach containing 6% to 8.25% sodium hypochlorite. The US EPA recommends adding 6 to 8 drops of bleach per gallon of water, doubling the amount if the water is cloudy, coloured, or very cold. After adding chlorine, the water should be mixed and left to stand for at least 30 minutes. It should have a faint chlorine smell – if it does not, the dose should be repeated and the water left for another 15 minutes before use. Chlorine tablets containing pre-measured doses are also widely available and are particularly convenient for travellers and emergency preparedness kits.

Limitations of chlorine disinfection 🔗

Chlorine disinfection has a few important constraints. Turbid or cloudy water reduces its effectiveness because suspended particles shield pathogens from the disinfectant. Water should always be clarified by settling or filtering before chlorine is added. Chlorine also imparts a taste and odour that some people find unpleasant, which can reduce willingness to use treated water. Household bleach has a relatively short shelf life – it loses potency within about 6 months – so it should be rotated regularly to ensure adequate disinfecting strength. Finally, certain parasites like Cryptosporidium have a protective outer shell that makes them resistant to standard chlorine concentrations; combining chlorination with filtration provides better overall protection against these organisms.

Solar water disinfection (SODIS) 🔗

Solar water disinfection, commonly known as SODIS, is a low-cost, chemical-free method of making water safe using sunlight. It is especially relevant in tropical and subtropical regions where fuel is scarce but sunshine is abundant. The method works through a combination of UV-A radiation from sunlight and heat, which together inactivate pathogens in the water.

How SODIS works 🔗

Research published in PMC explains that UV-A radiation from sunlight is lethal against bacteria and viruses, while UV-B radiation is additionally active against protozoa. When the water temperature rises above 45°C, a synergistic effect between UV radiation and heat significantly accelerates pathogen inactivation, reducing the exposure time needed. The SODIS procedure is straightforward: water with turbidity no higher than 30 NTU is filled into clear 2-litre PET bottles and placed in direct sunlight. On sunny days, 6 hours of exposure is sufficient; on cloudy days, the bottles must be exposed for up to 48 hours. SODIS should not be used during continuous rainfall, as the solar radiation dose will be insufficient.

Checking water clarity before SODIS 🔗

A simple field test for water clarity is the newspaper test. Place a full bottle upright on a newspaper headline and look down through the bottle opening. If the letters are readable through the water, it is clear enough for SODIS. If not, the water is too turbid and must be pre-filtered before treatment. This quick check ensures that solar radiation can penetrate the water adequately to inactivate pathogens throughout the bottle.

Acceptance and scope of SODIS 🔗

According to a study published in Scientific Reports, the SODIS process has been accepted by the WHO and recommended for low-to-middle-income countries and natural disaster scenarios. Field implementations in Kenya, Cameroon, Latin America, and Asia have demonstrated its effectiveness in real-world conditions. A case study in Tigray, Ethiopia found that diarrhoea prevalence among children under 5 dropped from 8.5% in week one to nearly zero by week 36 following SODIS adoption. The method is estimated to cost as little as $0.63 per person per year, making it one of the most economical water treatment options available.

Limitations of SODIS 🔗

SODIS has several practical constraints. It is weather-dependent and most effective in regions between latitudes 35°N and 35°S where strong sunlight is consistently available. The method cannot treat large volumes efficiently – standard procedures use 2-litre bottles – and it does not provide residual protection, meaning treated water should be consumed within 24 hours. SODIS also cannot remove chemical contaminants, heavy metals, or toxic factory waste from water; it only addresses biological pathogens. Turbid water further reduces effectiveness, requiring pre-filtration before solar treatment can work.

Comparing the three methods 🔗

Each disinfection method serves a different set of circumstances, and choosing the right one depends on what resources are available and what scale of treatment is needed.

Boiling is the most reliable method for killing all biological pathogens, including protozoa like Cryptosporidium that resist chlorine. It is ideal for emergency use when no other option is available, but its dependence on fuel makes it impractical for continuous daily use in resource-limited areas.

Chlorine disinfection is the method of choice for municipal water treatment and household use where ongoing protection is needed. Its key advantage is residual protection – it keeps water safe during storage. It is cost-effective and scalable, but requires access to chemical supplies and does not fully protect against chlorine-resistant protozoa without complementary filtration.

SODIS is the most sustainable option where sunshine is reliable and fuel or chemicals are unavailable. It is free beyond the cost of bottles, requires no consumables, and has been proven effective across dozens of countries. Its drawbacks are time requirements, volume limitations, and total weather dependence.

In practice, combining methods often provides the best protection. For example, pre-filtering turbid water before either chlorination or SODIS, or boiling water during disease outbreaks even in households that routinely use SODIS, adds an important layer of safety. The CDC’s Yellow Book on water disinfection advises that preventing waterborne illness depends on the combination of safe drinking water, proper hygiene, and adequate sanitation – no single method is sufficient on its own without good water handling practices.

Practical tips for safe water storage after disinfection 🔗

Disinfecting water is only half the task. Recontamination during storage is a serious and common risk. Treated water should always be stored in clean, covered containers with narrow openings to prevent hands or ladles from introducing new pathogens. Containers used for storage should be sanitized regularly. Chlorinated water maintains its residual protection only for a limited time, especially in warm conditions or open containers, so stored water should be used within 24 hours of treatment where possible. Boiled and SODIS-treated water, which have no residual protection at all, are especially vulnerable to recontamination and must be handled with clean hands and clean utensils at all times.

What do you think? Given the global scale of unsafe water access, which disinfection method do you believe holds the most promise for communities in resource-limited settings – and why? If you had to choose just one method for an emergency water supply situation, what factors would guide your decision?

References

1. https://www.who.int/news-room/fact-sheets/detail/drinking-water
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC10290941/
3. https://www.extension.iastate.edu/shelby/effective-methods-disinfect-drinking-water-boiling-vs-chemical-treatment
4. https://www.sciencedirect.com/science/article/abs/pii/S0048969724024008
5. https://www.cdc.gov/yellow-book/hcp/preparing-international-travelers/water-disinfection-for-travelers.html
6. https://www.epa.gov/ground-water-and-drinking-water/emergency-disinfection-drinking-water
7. https://theprovidentprepper.org/making-water-safe-to-drink-7-disinfection-techniques/
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC8201346/
9. https://www.nature.com/articles/s41598-022-23709-5
10. https://pubs.acs.org/doi/10.1021/acscatal.0c03325
11. https://wwwnc.cdc.gov/travel/yellowbook/2024/preparing/water-disinfection