Access to safe drinking water is essential for health and is a basic human right. Water is essential to sustain life and a satisfactory (adequate, safe and accessible) supply should be available to all. Diseases related to contamination of drinking water constitute a major burden on health. Safe drinking water is defined as that water that does not represent any significant risk to health over a lifetime of consumption. Those at greatest risk of waterborne disease are infants and young children, people who are debilitated or living under unsanitary conditions and the elderly.

Drinking water contamination can occur from

•	Microbes
•	Chemical contamination
•	Exposure to naturally occurring radionuclides
•	Objectional odors and testes

Advice for travelers

•	Avoid consumption or use of unsafe water (even when brushing teeth) if you are unsure about water quality.
•	Avoid unpasteurized juices and ice made from untreated water.
•	Avoid salads or other uncooked meals that may have been washed or prepared with unsafe water.
•	Drink water that you have boiled, filtered and/or treated with chlorine or iodine and stored in clean containers.
•	Consume ice only if it is known to be of drinking water quality.
•	Drink bottled water if it is known to be safe; carbonated bottled beverages (water & sodas) only from sealed, tamper-proof containers and pasteurized/canned juices and pasteurized milk.
•	Drink coffee and tea made from boiled water and served and stored in clean containers.

Ways to treat water
The simplest ways of removing microbes from water are boiling, disinfection and filtration.

Boiling water:- Bringing water to a rolling boil (prolonged and vigorous boiling is not required) is the most effective way to kill disease causing pathogens. The hot water should be allowed to cool down on its own without the addition of ice. If water for boiling is to be clarified, this should be done before boiling.

Disinfection:- Chemical disinfection is effective for killing bacteria, some viruses and some protozoa (does not kill cryptosporidium oocysts). Chlorine or Iodine are mostly used for disinfection. After disinfection, a carbon (charcoal) filter may be used to remove excess chlorine or iodine. If water is turbid, it should be clarified before disinfection. Clarification includes filtration, settling and decanting.

Filtration:- Various types of filters are available such as portable filters, ceramic filters and some carbon block filters. The filter's pore size rating must be 1 µm (absolute) or less to ensure removal of cryptosporidium oocysts. Most filters do not remove viruses.

Desalinated water:- Increasing brackish or salty water which is otherwise unacceptable for human consumption is being desalinated to provide drinking water. Water sources used are estuarine water, coastal water, sea water, brackish inland water. These waters may contain hazards not encountered in fresh water systems. These include diverse harmful algae, certain free living bacteria (esp. Vibrio species), Chemicals such as boron and bromide. These can be removed by distillation and reverse, osmosis. Desalinated water is hard and bland, flavorless and thus is commonly treated by adding chemical constituents such as calcium and magnesium carbonate with carbon dioxide. Desalinated waters are commonly blended with small volumes of more mineral rich waters to improve their acceptability.

Problems with desalinated water:

•	More susceptible to contamination with petroleum hydrocarbons.
•	It is a manufactured product and contains lower than usual concentrations of other ions commonly found in water, some of which are essential elements.
•	It is a manufactured product and contains lower than usual concentrations of other ions commonly found in water, some of which are essential elements.

Waterborne infections
Water contaminated with human or animal feces can lead to infection. The pathogens that are transmitted through contaminated through contaminated drinking water.

Bacteria	:	Camylobacter jejuni E. coli Legionella Pseudomona aeruginosa Salmonella (typhi and non typhoidal) Shigella Vibrio Yersinia enterocolitis
Viruses	:	Adenovirus Enterovirus Hepatitis A virus Hepatitis E virus Narovirus & Sapovirus Rotavirus
Protozoa	:	Acanthamoeba Cryptosporidium Cyclospora Entamoebia histolytica Giardia intestinalis Naegleria focoleri Toxoplasma gondii
Helminths	:	Dracunculus medinensis (Guinea worm) Schistosoma

Of these, legionella, non-tuberculous mycobacteria, adenovirus can be transmitted through aerosols whereas acanthamoeba, aeromonas, non-tuberculous mycobacteria, leptospira, pseudomonas aeruginosa and schistosoma are transmitted through bathing and contact with mucus membranes, abraded skin or eyes. Rest all are transmitted through ingestion.

Balantidium coli, certain helminthes (Fasciola, Fasciolopsis, Echinococcus, Spirometra, Ascaris, Trichuris, Toxocara, Necator, Ancylostoma, Strongyloides and Taenia solium) are transmitted through food contaminated with faeces.

Microbial growth in water
After leaving the body of their host, most pathogens lose viability and the ability to infect. However common waterborne pathogens and parasites are those that have high infectivity and either can proliferate in water or possess high resistance to decay outside the body. Viruses, cysts, oocysts, ova are unable to multiply in water.

Processes to reduce bacteria, viruses and protozoa in water include, filtration coagulation, sedimentation, flocculation, lime softening, membrane filtration-ultrafiltration, Nanofiltration and reverse osmosis, disinfection by chlorine, ozone treatment and UV irradiation. Of these backside infiltration, membrane filtration with ultrafiltration, nanofiltration and reverse osmosis, result in removal of pathogens from 99% - 100%. UV irradiation results in 99% inactivation of bacteria, viruses and giardia and cryptosporidium. Ion exchange does not remove any pathogens. Disinfection does not remove cryptosporidium and has limitations against some viruses. It is also unsatisfactory against pathogens within flocks or particles.

For drinking water either directly intended for drinking or in distribution system E. coli or thermotolerant coli form bacteria should not be detectable in any 100 ml sample.

Chemical hazards in drinking water
There are many chemicals that may occur in drinking water, however only a few are of immediate health concern in any given circumstance. Exposure to high levels of fluoride which occurs naturally can lead to mottling of teeth and skeletal florosis. Similarly arsenic may occur naturally can increases risk of cancer and skin lesions. Other naturally occurring chemicals include uranium, selenium. Presence of nitrates and nitrites in water (arise from excessive application of fertilizers) has been associated with methemoglobinemia especially in bottle fed infants. Acidic or hard water can erode lead in lead pipes and increase lead levels in drinking water.

Treatment consists of chlorination, filtration, aeration, chemical coagulation, granular activated carbon treatment, ion exchange, ozonation, advanced oxidation processes and membrane treatment.

 Radionuclide exposure in drinking water
It is very small under normal circumstances but may become contaminated by accidental release of radioactive substances in environment.

Further Reading:
Guidelines for Drinking-water Quality. World Health Organization. Geneva, 2006.

Last Updated: 1^st August 2009