There are different types. The simplest is a single chamber tank, others have successive chambers where, in stages, the sewage is settled and digested by bacteria. All the different types are emptied periodically by a tanker, but most of the time only the first chamber is emptied in multi-chamber tanks. The multi-chamber ones most often have an outlet from the final chamber and the effluent runs into ground soak areas and in some cases into watercourses, lakes or the sea (depending on local regulations). Single chamber tanks rarely have an outlet to anywhere these days and therefore usually have to be emptied of the entire contents before they are full to overflowing. The latest approach is to have mini treatment plants installed, some of which are smaller than a multi-chamber septic tank, and these are often certified as producing effluent that can be discharged directly onto agricultural land and/or into watercourses.