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My apartment in Kathmandu, where I lived for five years, had a toilet that looked very much like the one in my house in California. Nicer even; it was pastel porcelain and had dual flush.

But although flush toilets in Nepal and the rest of South Asia may work quite well, sewer systems have not kept pace. My toilet and all the others in my Kathmandu neighborhood were connected to pipes that carried the contents of toilets away from our residences and straight into a small river a half-mile away. Stray dogs lapped the water and children played nearby.

The rivers of the Indian subcontinent flow clean and clear from the Himalaya, then become little more than sewers as they run through major cities in the plains. New Delhi’s Yamuna River receives roughly half of the largely untreated sewage of a metropolis of 17 million. The Ganges, the holiest of Hindu rivers, is fouled by raw sewage from tens of millions of people as it flows 1,500 miles from the western Himalaya to the Bay of Bengal.

A movement is gaining momentum to do something about this major environmental and public health problem in South Asia and the developing world. The solution, many experts say, is not to invest in western-style flush toilets and centralized sewage systems but rather to develop toilets and decentralized waste-treatment technologies that use far less water. The latest development in this field is the decision by the Bill & Melinda Gates Foundation to invest $40 million in prize money and financial support to groups working on new toilet technology.