. >> we're in the process of deciding on a framework for the biogas team. the plan is for cusco to convert 2000 cubic meters of gas into electricity by the end of the year. >> the plant will soon be completely self-sufficient. the water company will save over 20,000 euros in electricity costs per month. the only question that remains is, how did the incas deal with sewage? >> this is the bedroom. the toilet, the bathroom, was probably adjacent. >> the bathroom? >> yes. we assume it was the bathroom on the basis of the floor plan and the ceramics we found. researchers have identified layers of organic material on pots with layers of sand between them. the incas probably used them as chamber pots. >> the cat litter principle. perhaps the incas valued their water supply too highly to befoul it. >> next week on "global ideas," we head to south africa, where agriculture and trade are key to the economy. every day, thousands of refrigerated lorries carrying perishable goods hit the roads. most of them use environmentally-toxic cooling agents, and they're also oft