"At least in urban areas, waste water treatment is an activity suitable only for a carefully selected environment. 
A treatment plant built on the surface requires a large area of land surrounded by an adequate buffer zone. 
Many urban areas are so densely built up that finding a site acceptable to the town planners is an arduous and sometimes impossible process. 
Locating the treatment plant in a rock cavern is considerably easier. The limitations on underground construction are such that finding a suitable site is generally not a problem. It is not easy to detect the presence of an underground treatment plant, so local residents are easily sold on the merits of the "invisible" solution. 
In addition, land surplus to the requirements of the underground plant’s surface facilities can be made available for other purposes. The treatment plant can also be located centrally with regard to the sewerage system."