For people who are ''off mains'', a sewage installation is something that you know is there, but do not want to be constantly reminded about. If it is playing up, it can become a true nightmare. Some people are afraid to have a shower, or even use the W.C. for fear of  making matters worse.  This is especially true for the unfortunate people who are connected to a ''cesspool'' rather than a septic tank.  The difference between a septic tank and a cesspool lies in the fact that a cesspool is an enclosed tank with no outlet, and must be emptied every time it fills up. The liquid level needs to be constantly monitored, and the expense of emptying it every month by road tanker can be daunting.

Fortunately, in most cases, septic tanks and cesspools can be replaced with a modern packaged treatment plant. The result is similar to being connected to mains drainage. The end product of a properly installed and serviced unit is peace of mind.  Each unit is sized according to the dwelling it serves, and does not have any limit on the amount of wastewater it will treat.

Package treatment plants comes in three basic designs.

The first type relies upon a filter bed through which liquid sewage is trickled. The filter bed is home to millions of naturally occuring bacteria which feed on the nutrients in the sewage. Air rises through the filter bed by natural convection to provide oxygen for the bacteria. The process is the same as can be seen at a sewage works where wastewater is sprayed onto a circular bed of clinker.

The second type of plant does things in reverse by immersing the filter bed - commonly called the filter media - in the liquid sewage, and then blowing air through the liquid to provide oxygen for the bacteria. By this method the bacteria are in permanent contact with the sewage, so have a constant supply of food.

The third type of sewage plant is the one favoured by I.H.S. Systems for most domestic applications. Its grand title is ''Rotating Biological Contactor'' or, more simply, an ''RBC''. Its treatment process combines the advantages of the other two types of plant. The media comprises two or more banks of slowly rotating discs, colonised by naturally occuring bacteria. The discs are partially submerged in liquid sewage, so the bacteria alternately ''breath'' and feed on every revolution.

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