It's the largest purchase order by any company in the U.S.: Delivery giant UPS plans to buy a whopping 170-million-gallon equivalent of biogas converted from waste sources from provider Clean Energy over the next seven years to power its fleet of natural gas trucks, the company announced on Wednesday. The contract is a big deal because it's the largest of its kind, ever, in the United States. It also highlights how waste gas, also called renewable natural gas, is moving from a niche product into a more widely available zero-emission fuel source for private and public fleet operators.