By Jorn Madslien
Business reporter, BBC News, Mumbai
In Mumbai, trucks delivering water to those who can pay are common
The heat is scorching as the young woman knocks on the window of my taxi, though rather than begging for cash she points at my water bottle, then to her mouth.
In a city where clean water has become a commodity that is delivered to the highest bidder, the poor often have to go without.
Yet those who have money can easily get enough. In Mumbai's wealthy suburbs, large tankers delivering water are commonplace.
Every day more than 5,000 tankers deliver some 50 million litres of water to people who can pay, according to unofficial estimates cited by the newspaper Mumbai Mirror.
But even if the wealthy were to go without such top-up deliveries, there would probably not be enough water to go around.
Access to clean drinking water is limited for many in India
Mumbai's Brihanmumbai Municipal Corporation (BMC) delivers some 90 litres of water per day to the city's residents.
That is far short of the 135 litres of water the World Health Organization (WHO) says they require for their basic needs.
So in Mumbai, there is growing anger over the water shortages.
Last month, BMC hydraulic engineers' office was vandalised by activists. Over the weekend, a man died after violent clashes in the city between police and protestors who demanded better access to water for the disabled.
But the lack of access to clean water is by no means merely a problem facing those who live in India's biggest cities.
In the central Indian city of Bhopal, people who live in some of the slums pump drinking water from groundwater contaminated by industrial pollution. Children who live in the slum play by the filthy and rubbish-strewn river that runs past.
Head out into rural India, and three-quarters of the population does not have access to safe drinking water.
As the population continues to grow the problem is getting worse.
India's water needs are set to double over the next two decades, according to consultants McKinsey.
Production of rice, wheat and sugar is set to push up demand from Indian agriculture, the consultancy warns.
And the problem is growing, both in India, as well as in China, South Africa and Sao Paulo state in Brazil.
By 2030, the four areas will account for more than two fifths of the world's water demand, largely thanks to a sharp rise in food production, McKinsey says.
By then, demand for water will be 40% higher than it is currently, the consultancy predicts.
Many in India are looking to the industrialist Ratan Tata for a solution.
For many, filthy rivers provide their only source of water
There are high hopes that he has delivered after Tata Group launched a water purifier that helps curb the spread of water-borne diseases such as cholera, typhoid or diarrhoea.
Tata's purifier will cost less than 1,000 ($21.50; £13) rupees to buy, half the price of the popular Pureit purifier already being sold by Hindustan Unilever.
Tata Chemicals' managing director R. Mukundan insists its purifier is unique.
"It doesn't compete with any existing product."
Moreover, he says, "this is opening up a complete new market" - one that is huge and growing.
Although limited access to safe drinking water is a huge problem in India, this is a global problem that affects about a billion people, according to the WHO.
"For the vast majority... today's water crisis is not an issue of scarcity, but of access," the WHO says.