In addition to some of our favorite stories from 2015, we examine the pros and cons of fertilizers. And, what we flush down the toilet could become energy.
One expert says current technologies could transform organic waste into enough natural gas to replace half the diesel fuel used in the US transportation sector.
As the world's topsoil erodes and drinking water stores dwindle, developed countries could be forced to recycle more human waste to grow food. How safe is it?