National Public Radio: Cows are big, they eat a lot, and, well, they produce a lot of manure. All this waste can have a negative impact on water and the environment, and some industrial dairies are working to make their facilities greener. One way to keep cow manure from polluting the environment is to use a system called a digestor, in which the manure is funneled into a big vat, where it decomposes, producing methane that’s then used to create electricity. The digestor at Holsum Elm Dairy in Wisconsin …