IS 10500 drinking water standard: is your tap water safe?

Whether the water comes from a municipal pipe, a society borewell or a tanker, the yardstick in India is the same. The IS 10500 drinking water standard, published by the Bureau of Indian Standards (BIS), sets the limits that water fit for drinking should meet, and water testing laboratories commonly report results against it.

What IS 10500:2012 is

IS 10500 is a specification: a list of parameters, each with a limit, that drinking water should meet. The 2012 edition is the second revision, and the text in common use carries later amendments, including Amendment No. 2 of 2018.

Some parameters describe how water looks and tastes, such as turbidity and pH. Some are minerals that occur naturally, such as hardness, chloride and fluoride. Some point to pollution, such as nitrate. The bacteriological tests cover the germs behind diarrhoea, typhoid and cholera.

Most chemical limits are given in milligrams per litre (mg/L). For water, one milligram per litre is roughly one part per million.

Acceptable and permissible limits

For many parameters the standard gives two numbers, and the difference between them matters.

• The acceptable limit is the value drinking water should meet. Water within all acceptable limits meets the standard.
• The permissible limit is a relaxation. It applies only when no alternate source of water is available. It does not mean the higher value is equally good; it means that, where there is no other source, the water may still be accepted up to that value.

For some parameters there is no relaxation at all. pH, nitrate and iron are examples: the acceptable limit is the only limit.

So a borewell report showing total dissolved solids above the acceptable limit is a reason to look for a better source or to treat the water. It is not a clean result, even if the figure sits below the permissible limit.

Key limits at a glance

The table below lists common parameters from the BIS text of IS 10500:2012. Where we have not given a permissible limit, read it in the standard itself.

The bacteriological rule is stricter than any of these. E. coli or thermotolerant coliform bacteria shall not be detectable in any 100 ml sample. Total coliform bacteria in treated water, both entering and within the distribution system, shall not be detectable in any 100 ml sample either. One positive result means the water needs attention.

What the main parameters tell you:

• Turbidity is cloudiness. Cloudy water can shelter germs from disinfection.
• TDS is the total of everything dissolved. High TDS tastes salty or bitter.
• Hardness comes from calcium and magnesium. It leaves white scale on taps, geysers and kettles, and makes soap lather poorly.
• Fluoride above the limit, taken over years, can mottle teeth and affect bones.
• Nitrate often comes from fertiliser or sewage reaching groundwater, and is a particular concern for infants.
• Iron gives a rusty colour, a metallic taste and brown stains on buckets and tiles.
• Free residual chlorine is chlorine left over after disinfection. Finding some at your tap shows the water was treated and still carries protection against germs picked up on the way.

This is a summary. The BIS text of IS 10500:2012 is the authority, and it lists many more parameters than are shown here.

What to test in a borewell and in a piped supply

The source decides which parameters are worth paying for.

Borewell and handpump water picks up minerals from the rock and soil it passes through. Ask for TDS, hardness, chloride, fluoride, nitrate and iron, along with a bacteriological test. Shallow bores near septic tanks or soak pits are more exposed to contamination. Some regions also have naturally occurring arsenic in groundwater, so ask the laboratory whether it is a concern in your area.

Piped municipal water is usually treated and chlorinated before it leaves the plant, so the question is what happens on the way to your tap. Free residual chlorine, turbidity and coliform bacteria are the useful tests. No chlorine at the tap can mean the water stood too long or picked up contamination through a leaking pipe.

Storage changes both. Water sitting in a sump or overhead tank loses its chlorine, and an uncleaned tank can grow bacteria. Take the sample from the tap you actually drink from. Society managers can test at the inlet and at a flat's tap: if the inlet is clean and the tap is not, the problem is inside the building.

How to get water tested

Use an accredited laboratory, or the water testing laboratory run by your state's public health engineering department. Before sampling, ask the lab for its containers and instructions. Bacteriological tests need a sterile bottle and should reach the lab quickly, so collect the sample only when you can take it in as the lab directs.

When the report arrives, check that it names the test method and compares each result with IS 10500. Groundwater quality can change with the seasons, so a repeat test after the monsoon is useful for a borewell.

What a water purifier can and cannot fix

A purifier is a treatment for a specific problem, and the test report should decide which kind you need.

• Reverse osmosis (RO) lowers dissolved salts, hardness, fluoride, nitrate and many metals. It sends part of the incoming water to the drain as reject, and on low-TDS municipal water it can remove more than needed.
• UV kills bacteria and viruses in clear water. It does nothing for dissolved salts, fluoride or nitrate, and it works poorly on cloudy water.
• Activated carbon removes chlorine taste and smell and some organic chemicals. It does not remove dissolved salts or reliably remove germs.
• Sediment filters catch sand, silt and rust particles, and protect the stages after them.

No purifier fixes a dirty storage tank, a cracked supply pipe or a contaminated borewell; it only treats what reaches it. An unserviced unit can itself become a source of bacteria. And a handheld TDS meter measures only dissolved solids, so a low TDS reading says nothing about germs. The glossary explains TDS, turbidity and residual chlorine in more detail.