How best to provide clean water and sanitation to the world's cities and their expanding slums is the overriding theme of World Water Week, which opened Monday in Stockholm, dpa reported. "More than 800 million people live in slums where water-related diseases such as diarrhoea, malaria and cholera have devastating effects on the livelihood of families and the economies of their countries," said Anders Berntell, head of the Stockholm International Water Institute, co-hosts of the conference. Berntell reminded delegates about how ancient city civilizations along the Indus river were able to provide clean water, public baths and sanitation to their inhabitants. The lack of such facilities in today's cities is, therefore, a matter of "political will and priorities ... since the solutions existed 3,500 years ago," he said. Berntnell also warned that unless there is a change in policy, water demand could in a few decades outstrip supply, giving rise to further tension between rural and urban areas. According to UN Habitat, a United Nations agency that works with housing, about half the world's population currently lives in urban areas. However, that proportion is set to reach two-thirds by 2050. Meanwhile, the World Wildlife Fund (WWF), a pressure group, has just published a report highlighting the strain on water resources placed by a growing number of megacities - cities with more than 10 million inhabitants - in Asia, Africa and Latin America. Sheela Patel of the organization Shack-Slum Dwellers International, called on officials to listen to the needs of the urban poor when designing water solutions. Joan Clos of UN Habitat, for his part, warned that since slums cannot simply be bulldozed away, there should be "dialogue" between city planners and slum dwellers on how to "upgrade the slums." Another speaker at the conference opening was US researcher Steven Carpenter, winner of the 2011 Stockholm Water Prize for his research on lake ecosystems. The annual 150,000-dollar award was created in 1990 to recognize achievements in water science, water management, water action and awareness building.