Through the '80s, he worked in Africa, preaching the Chinese way of waste conservation. In the 1990s, he worked in Cuba, using Chinese models to help Cuba set up a system to funnel pig manure into growing porcine food on farms after the Russians cut off exports of pig feed.

Meanwhile, the world's most populous nation scaled up a model used in farms. Originally used to keep humans from doing their business in pig troughs, today 40 million farm homes have a holding tank for human and animal waste that is partly sanitized by depriving the solids of oxygen. What's left is then converted to liquid fertilizer for the farms.

What's happening in Beijing is an industrialized, scaled-up version of that model, said Mang, who has lived in the capital for a decade. Across the city, which has seen its population double to 21 million in the past decade, the average annual amount of human waste processed will increase by 200 to 300 tons a day, said Zhang Jiang, general manager of Beijing Century Green Environmental Engineering & Technology Ltd, which operates night-soil treatment plants. Treating waste is set to be a growing area of business, Zhang said.

Other parts of the world are also making an effort to harness energy from fecal sludge. In recent years, the Bill & Melinda Gates Foundation invested $1.5 million in a project to explore biodiesel production from human waste in Ghana. It's also investing in family-level biogas units or septic tanks that process human waste in Thailand and India.

On a recent Wednesday, Mang took a group of visiting sanitation researchers and students to visit some of Beijing's private plants. Processing at one named Sijiqing - or "four seasons green" - begins on the ground floor of a two-story, red-tiled building. The odor of human excrement in the air gives away its true purpose: some 200 trucks unload 800 tons of fecal sludge every day.

Compost wing

A bright yellow truck parked inside channels the human waste through a pipe into a machine, where unrecyclable solid material like tissue paper and plastic bags is separated, explained Zhang Hui, the plant's manager.

The rest then goes for separation: the solid waste is propelled into a compost wing to ferment at 60 degrees Celsius for 10 days. The process kills harmful bacteria and ascaris eggs - parasitic roundworms that infect humans - and turns the excrement into rich fertilizers for trees and vegetables. The liquid material is routed into tanks to generate biogas, and eventually pumped to bigger water-treatment plants. Phone calls to the Beijing mayor's office and the city commission in charge of environmental sanitation management weren't answered.

Born into a fruit-farming family near Frankfurt, Mang's father earned additional income as a truck driver, often transporting manure and straw. As a baby, Mang's mother would leave him on top of manure heaps wrapped in a blanket as it was "always soft and warm." He reckons that's one reason he's never been squeamish about fecal matter.