SINCE last May the brain-eating amoeba or naegleria fowleri has caused the death of three persons. As reported in this paper (July 3), at a meeting held by the Karachi commissioner, it was informed that 41 per cent of water being supplied to the city was found with an insufficient amount of chlorine. A total of 44 of the 1,445 samples had no chlorine at all; 546 samples had less than 0.25 ppm, a value which the KWSB calls incorrect as a standard.

There is no such thing as a standard for residual chlorine in water distribution systems. 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 emg/l after at least 30 minutes contact time at pH <8.0.

While the Karachi commissioner deserves a pat on the back for taking interest, he should note that the issue of chlorination of drinking water is not that simple. A number of parameters impact the chlorination process.

Chlorination will not be effective if the turbidity of water is more than 3 ntu (nephelometric turbidity units). This is because suspended particles protect microorganisms, which hide themselves within the particles to escape the disinfection. Karachi drinking water is quite often turbid.

The disinfection process is also affected by the pH value of water (pH is a measure of the degree of the acidity or the alkalinity of a solution, is defined as the negative logarithm of the hydrogen ion concentration. The optimum range of pH for chlorine disinfection is between 6.5 and 7.5. The effectiveness of chlorine disinfection reduces by a factor of between 3 and 6, when pH increases from 6 to 9. Chlorine disinfection is poor when the pH is above 9.

Temperature of water impacts disinfection efficiency. Temperatures above 20°C are suitable for disinfection. When temperature drops below 20°C, the disinfection efficiency reduces. For every 10°C drop in temperature, below 20°C, the efficiency of disinfection reduces by 50 - 60 per cent. Near 0°C disinfection efficiency becomes very poor.

Disinfection efficiency is significantly impacted by the contact time. Contact time is defined as the period available for the interaction between the disinfectant and constituents in the water. It is an important parameter in the design of disinfection systems for water treatment.

The minimum contact time for chlorination should be 30 minutes, at 20°C, to ensure effective disinfection. In case of Karachi, this means that there must be the contact time of 30 minutes of chlorine with water, before the water reaches ‘the first consumer’. If the temperature drops below 20°C, the contact time should be increased. Contact time increases from 30 to 60 minutes if the temperature drops from 20°C to 10°C.

The generally accepted value of residual chlorine of 0.5 mg/l 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 Indus, a major source of water for Karachi’s water treatment plants, receives large-scale untreated municipal and industrial wastewaters.

This is addition to the diffused sources of agricultural flows, containing agrochemicals. As a result, the Indus River supports microbiological communities that include pathogens.

A rational programme of watershed management can reduce the discharge of pollutants in the Indus, but Karachi water agency never undertook Indus River watershed management initiatives. Current and emerging organisms of concern in Karachi’s raw water are Giardia lamblia and Cryptosporidium.

A major issue is the use of standard disinfectants of required strength. Substandard chemicals are widely available in Karachi. Substandard disinfectants will cause rapid residual chlorine decay in the distribution system.

F.H. MUGHAL Karachi