A KWSB official recently said, “Chlorination carried out at the plant killed the germs that the unfiltered water might be carrying. Though we lack complete filtration capacity, we do chlorinate the entire water and that improves its quality.” This is, technically, an incorrect comment.

Chlorination will not be effective if the turbidity of water is more than 3 ntu (nephelometric turbidity units). This is because the suspended particles protect microorganisms, which embed themselves within the particles, to escape the disinfection. Simply stated, since raw waters, supply water to Karachi water treatment plants, are turbid (turbidity greater than 15 ntu), it is useless to waste chlorine on such waters. For optimum chlorine efficiency, the turbidity in water should be 1.5 ntu.

Chlorination of drinking water is not that simple. A number of parameters impact the chlorination process.

The generally accepted value of residual chlorine of 0.5 mg/L (milligrams per liter) is required to be maintained in the distribution system up to the last customer. In Karachi, the water supply is intermittent and the water pipe systems leak. When the water supply is switched off, the pressure will drop and contaminated water will enter the distribution system through the leaks in the pipes.

The contamination that enters the pipelines will consume the residual chlorine and the net result will be zero residual chlorine further down the distribution system. The water agency in Karachi must note this point and undertake remedial measures. One such measure is the rechlorination (booster chlorination) in the distribution system.

The World Health Organisation (2008) gives the guideline value of residual chlorine as: “For effective disinfection, there should be a residual concentration of free chlorine of 0.5 mg/L, after at least 30 minutes contact time at pH less than 8.0.”

F. H. Mughal
Karachi

Published in Dawn, January 17th, 2017