A light-emitting device includes a group III nitride semiconductor layer of a multilayer structure consisting of a group III nitride semiconductor having a major surface defined by a nonpolar plane or a semipolar plane and having at least an n-type layer and a p-type layer. We also describe a method of measuring one or both of the food content and the biomass content of a fluid of a waste water treatment plant, the method comprising determining a value for one or both of food content and biomass content from a change in pres sure. The hub and optional ribs are machine finished from the raw vaned diffuser to yield the finished vaned diffuser.