Code 7580 applies to employers engaged in all operations of a sewage disposal plant that can be operated by a municipal board or commission or a private company.  The plant can dispose of sewage by a number of methods, the most common of which are plain sedimentation, chemical coagulation, biological processes and various combinations of these three.  These processes kill bacteria and speed up decomposition so that raw sewage is not dumped into waterways or incinerated.  The extension of pipelines is contemplated by this classification as well as repair and maintenance on same.