In DPS RK Puram, of the 37 samples collected, 24 per cent were found to be unfit for drinking. In Panchshila Park, out of 17 samples collected, 41 per cent were unfit for drinking. 50 samples were collected in Defence colony and 50 per cent of these samples were not fit for drinking. And in Vasant Kunj, of the 105 samples collected, a whopping 89.5 per cent were not fit for drinking.

But why is it so? According to Mr H Subramaniam of everythingaboutwater.com, "There are certain government standards on drinking water that come under IS 10500-regarding colour, odour, hardness, turbidity (suspended solids in water), total dissolved solids (TDS), levels of fluoride, nitrates, iron and even chlorine along with microbiological impurities like bacteria and viruses that are present in water. If these are in excess of the permitted standards, then the water can be termed as `not fit for drinking'."

For example, says Mr Subramaniam, chlorine is used as a disinfectant in water to rid it of impurities. But free chlorine, which is chlorine in excess of the normal amount, can act as a carcinogenic agent. Also, high levels of fluoride-more than 1 gram per 1,000 litres of water (1 ppm)-can cause dental diseases, and excessive iron can lead to genetic disorders along with giving water a bad taste. Also, TDS which is more than 500 ppm can lead to gastro-intestinal problems. Results also showed that among the samples unfit for consumption TDS exceeding the recommended limit was 39 per cent in Defence Colony, 41 per cent in Panchshila Park and 22 per cent in DPS RK Puram. In Vasant Kunj, 89.5 per cent of the water samples unfit for drinking were found to have TDS exceeding the recommended limit. And microbial contamination was 11 per cent in Defence Colony, 2 per cent in DPS and 26 per cent in Vasant Kunj. Microbiological impurities like the presence of bacteria like ecoli in water can be removed through UVpurification, which uses a beam of light to deactivate the bacteria and viruses in water.

On the other hand, the UV purifier is ineffective in the case of dissolved particles/solids and hardness or saltiness of water. For such things, what is most effective is reverse osmosis-an advanced filtering process in which water is passed through a fine membrane at very high pressure which filters out the solids.