In the West it is a question of hygiene and environmental pollution.
Elsewhere, it can be black gold.
Human waste or humanure? In the first of three programmes, Natalie Wheen wades in with the experts in the bowels of LONDON, where modern sanitation began.
In much of the developing world, sewage is black gold.
Natalie Wheen visits CALCUTTA, where it is used to grow vegetables and rice.
The largest civilian project in the USA is Boston's sewage treatment plant - seven billion dollars' worth of cutting-edge technology.
But need it cost this much to save our seas? Natalie Wheen investigates.