kerryedwards, the container that you refer to as a cistern is actually a grease trap. A cistern is a tank for the storage of clean water or rain water. If the grease trap is still functional after cleaning the tank and discharge pipe, I suggest that you maintain the plumbing system as it is. It matters not whether you are served by a septic system or a centralized sewer system, for Fat, Oil, and Grease (FOG) are detrimental to the treatment processes involved in either of these sewage treatment systems.