“While over 80 percent of the people in Karachi boil water, many still suffer from health problems. Since there is no advanced water treatment at Karachi’s plants, all health-threatening contaminants in raw water escape treatment and end up in finished water. Conventional water treatment plants are ineffective in removing heavy metals, pesticides and agrochemicals.
Water treatment plants in Karachi and Hyderabad must have tertiary or advanced water treatment units in addition to the rapid-sand filtration system so that toxic contaminants, which are not removed by conventional water treatment plants, can be eliminated.”
Full article: http://www.dawn.com/2008/08/20/op.htm#3