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Snipped this but there is much more at the link
Septic Tanks A septic tank is similar to a cesspool, but has the important difference that it only stores the solid material and allows partly treated liquid (effluent) to run away from the tank, either to further treatment, or to a system of underground land drains.
The raw sewage enters the septic tank via an inlet pipe, which slows down the velocity of the sewage and does not disturb the surface as it enters the tank. As the water slows, it cannot carry so much solid material in suspension and a sludge drops to the bottom of the tank. Due to the very slow movement through the tank, a scum (known as the crust) will form on the top surface of the tank. This allows the growth of anaerobic (oxygen hating) bacteria. These bacteria set about digesting or breaking down the solid material at the bottom of the tank, and turn it into liquid form, which then leaves the tank via the outlet. Not all the solid material will be broken down by this action so that over a period of time there is a build-up of sludge which has to be pumped out by a licensed contractor every 12 months depending upon usage.
http://www.sweeptech.co.uk/septic-t...d-treatment-plant-installation-and-servicing/
Septic Tanks A septic tank is similar to a cesspool, but has the important difference that it only stores the solid material and allows partly treated liquid (effluent) to run away from the tank, either to further treatment, or to a system of underground land drains.
The raw sewage enters the septic tank via an inlet pipe, which slows down the velocity of the sewage and does not disturb the surface as it enters the tank. As the water slows, it cannot carry so much solid material in suspension and a sludge drops to the bottom of the tank. Due to the very slow movement through the tank, a scum (known as the crust) will form on the top surface of the tank. This allows the growth of anaerobic (oxygen hating) bacteria. These bacteria set about digesting or breaking down the solid material at the bottom of the tank, and turn it into liquid form, which then leaves the tank via the outlet. Not all the solid material will be broken down by this action so that over a period of time there is a build-up of sludge which has to be pumped out by a licensed contractor every 12 months depending upon usage.
http://www.sweeptech.co.uk/septic-t...d-treatment-plant-installation-and-servicing/