Calcutta: There's nothing wrong in its drinking water, the civic body has said. But the level of residual chlorine in potable water has gone up manifold.

Ever since nine south Calcutta wards reported a diarrhoea outbreak on February 9, the civic body increased the chlorine dose as a preventive measure.

But its own tests have now shown the level of residual chlorine is more than double the permissible limit.

Chlorine kills harmful microbes in water but the dose has to be right, a public health expert said.

The standard permissible limit of residual chlorine is 0.2 to 0.5 part for every million of treated water, an engineer of the civic body's water supply department said.

A test of a water sample in Baghajatin on Monday showed the count had reached two parts. "It's alarming and has to be reduced," the engineer said. "It must have happened because of improper dosage."

Drinking water with high levels of chlorine can damage internal organs, a doctor said. It can damage the skin leading to itching, dry skin and other problems, the doctor said.

"Chlorine acts as a protection against microbes," a public health expert said.

After the outbreak was reported, the civic body's water supply department tested water samples from the affected wards that include Baghajatin, Patuli, Kasba, Dhakuria, Nonadanga and Santoshpur.

Chlorine is first added when water enters the treatment plant. It is again added in the course of treatment, the water supply department engineer said.

The treated water is sent to booster pumping stations, where chlorine is again added. "Since the stations are some distance away, the residual chlorine content decreases as the water travels through pipelines," he said. "So, it is again added at the stations."

The test for residual chlorine is done on the spot while collecting water samples. "We collected the sample in a test tube and mixed ortho-tolidine reagent. The water turned dark yellow, indicating the high level of residual chlorine," an engineer said.

The intensity of the colour is the marker.

The engineer said they were given a "model sample" where the water had the right amount of residual chlorine. "We matched all the samples with the model sample. In one sample I found the colour darker. It was so dark that I can say without doubt that the level is at least two parts."