Spending $10 billion a year would enable the world to reach a 2015 goal of improved sanitation in developing countries with huge spin-offs such as less poverty and better health, U.N. experts said on Thursday.

Marking the U.N.'s annual World Water Day on March 20, they said every dollar spent on improving sanitation -- ranging from digging latrines or building sewers -- would have $9 in benefits such as higher economic growth or lower hospital bills.

But the world is lagging on a goal set in 2002 of halving the proportion of people, estimated at 2.6 billion or 40 percent of the world population, with no access to sanitation by 2015.

"In many regions we are missing the goal," Zafar Adeel, head of the U.N. University's International Network on Environment and Health, told Reuters. Spending $10 billion a year would be sufficient to help the world reach the goal, U.N. data shows.

"But there can be huge human health, well-being and economic benefits," from investing in sanitation, Adeel added. Diarrhoeal disease is a leading cause of death and illness, killing 1.8 million people a year.

The goal for improved sanitation is part of a wider set of Millennium Development Goals such as halving poverty, improving access to drinking water, improving education and the rights of women by 2015.

"You cannot reach the goals on health and education -- you can't keep children in school -- if you don't have water and sanitation," said Clarissa Brocklehurst, head of water and sanitation at the U.N. children's fund UNICEF.

This year is the International Year of Sanitation.

WOMEN

Women spend far more time than men fetching water or looking after sick children. "Water and sanitation underlie so much of health, empowerment of women, poverty alleviation. They are key to all of the millennium development goals," Brocklehurst told Reuters.

"It's pretty badly off track," she said of the sanitation goal, which is lagging most in Africa. According to U.N. data, reaching the 2015 sanitation goal would add 3.2 billion annual working days worldwide.

About 200 million tonnes of human waste are discharged untreated into watercourses every year -- exposing people to bacteria, viruses and parasites.

"On a typical day in sub-Saharan Africa...half the hospital beds are occupied by people with faecal-borne diseases," a U.N. statement said.

Girls are more likely to stay at school with better sanitation. "More girls in school means higher rates of female literacy -- for every 10 percent increase in female literacy, a country's economy can grow by 0.3 percent," it said.

The World Conservation Union, grouping governments and conservation organisations, also urged better sanitation, saying that rivers such as the Yangtze in China were "cancerous" with pollution.

"Rivers are the lifeblood of the earth, and maintaining their vitality ensures the flow of water resources to the environment and communities," said Ger Bergkamp, head of the Union's Water Programme.