SAN ANTONIO: The city of San Antonio unveiled a deal that will make it the first US city to harvest methane gas from human waste on a commercial scale and turn it into clean-burning fuel.

San Antonio residents produce about 140,000 tons a year of a substance gently referred to as "biosolids", which can be reprocessed into natural gas, said Steve Clouse, chief operating officer of the city's water system.

"You may call it something else," Clouse said, but for area utilities, the main byproduct of human waste - methane gas - will soon be converted into natural gas to burn in their power plants.

The city approved a deal on Tuesday where Massachusetts-based Ameresco Inc will convert the city's biosolids into natural gas, which could generate about 1.5 million cubic feet per day, he said.

Methane gas, which is a byproduct of human and organic waste, is a principal component of the natural gas used to fuel furnaces, power plants, and other combustion-based generators.

"The private vendor will come onto the facility, construct some gas cleaning systems, remove the moisture, remove the carbon dioxide content, and then sell that gas on the open market," Clouse said.

The gas will be sold to power generators, he said.

Some communities are using methane gas harvested from solid waste to power smaller facilities like sewage treatment plants, but San Antonio is the first to see large-scale conversion of methane gas from sewage into fuel for power generation, he said.

Following the agreement, more than 90 percent of materials flushed down the toilets and sinks of San Antonio will be recycled, he said. Liquid is now used for irrigation, many of the solids are made into compost, and now the methane gas will be recycled for power generation.

This is not the sole example where people are using human waste, farmers facing water shortages and escalating fertilizer costs in developing countries are using raw sewage to irrigate and fertilize nearly 49 million acres of cropland, according to a new report.

While the practice carries serious health risks for many, those dangers are eclipsed by the social and economic gains for poor urban farmers and consumers who need affordable food, the study authors say.

Nearly 200 million farmers in China, India, Vietnam, sub-Saharan Africa, and Latin America harvest grains and vegetables from fields that use untreated human waste. Ten per cent of the world's population relies on such foods, according to the World Health Organization (WHO).

"There is a large potential for wastewater agriculture to both help and hurt great numbers of urban consumers," said Liqa Raschid-Sally, who led the study published by the Sri Lanka-based International Water Management Institute and released this week at the World Water Week conference in Stockholm, Sweden.

The report focused on poor urban areas, where farms in or near cities supply relatively inexpensive food. Most of these operations draw irrigation water from local rivers or lakes. Unlike developed cities, however, these areas lack advanced water-treatment facilities, and rivers effectively become sewers.