the river flows in underergroud channels to ththe neighborhoodf san jeronimo at the other end of the city. from there, it f flows into a sewage treatment plant. it's the pride of cusco. it's been undergoing renovation since 2014. with the help of german consultants, it's being made more efficient. >> working in peru's most modern sewage treatment plant is very gratifying. it's also very challenging. every day we investigate new technologies and learn about how cities in other countries are implementing them -- things that are new to us in peru. >> most of peru's sewage ends up unfiltered in rivers, lakes, and the ocean. it's dangerously unhygienic and an environmental nightmare. >> human waste generate approximately 75 kilos of carbon dioxide per year per person. if it isn't treated, it could soon be 400. this plant allows us to reduce human-generated emissions to 20 kilos of carbon dioxide per year. >> you c can smell the improvement. the fermentation gas that accrues in sewage treatment plants is now filtered out more efficiently, so the stench is far less overwhelming. much to the