If we parse the title of this paper we end up with two words that may or may not be related. “Sustainable,” in the context used here means “…relating to, or being a method of harvesting or using a resource so that the resource is not depleted or permanently damaged.” A utility (again, in this context) means “A commodity or service, such as electricity, water, or public transportation that is provided by a public utility.”
Given the above definitions, I would say that a sustainable utility provides a commodity or service using methods so that resources are not depleted or permanently damaged.