How the drainage system works
There are several domestic drainage systems but each of them can be broken down into five separate sections. When waste water leaves an appliance of any sort, it will go immediately through a 'waste trap' -a 1800bend containing a water seal which fills the trap whenever the waste pipe empties. this keeps drain smells out of the room and prevents insects and the like from entering the home. With WCs it also makes self-cleansing easier. WC traps are cast as an integral part of the WC pan, but on other appliances they are separate, and are attached to the outlet pipe by a large retaining nut.
From the trap, waste water enters a branch pipe which leads to the main vertical drainage 'stack'. This takes it below ground level to the first underground section of the drainage system where it flows through at least one inspection chamber (covered with a manhole cover) and into the public sewer, which is usually situated underneath the road. The sewer is provided by the public health authority and it is their responsibility to remove all waste running into it.
Often rainwater from the roof is fed into the drainage system to flow into the public sewer But some authorities provide a separate street drain for it or insist on the provision of soakaways (pits filled with rubble and gravel which allow the water to soak into the surrounding earth) near the house. Tanks and cisterns rarely overflow, but when they do they discharge clean water, so it's not necessary for the overflow pipes to be located over a drain.
The water can fall directly onto the ground. The cost of laying public sewers in rural areas means that the waste from many houses in these parts flows into a cess pool or septic tank. These are specially constructed pits for storing effluent (and in the case of a septic tank, for breaking it down into harmless matter). Both of these require periodic pumping out, cess pools much more often as 'they store all the waste. if you're buying a house with one of these systems, check how often this has to be done, who does it and how much you may have to pay.