Septic tanks are available in either traditional onion shape or low profile cylindrical shape tanks. The low profile tanks are designed for areas with high water table or hard rock conditions and can possibly be installed in pea-shingle alone. The tanks are tested and certified to EN12566-1, but for discharge into a land drainage system not ditch or watercourse. No power is needed for this type of tank.
Septic tanks can only be discharged into a percolation drainage system (implemented using modular crates or perforated pipes) as set out in building regulations. These tanks cannot be discharged into a river, watercourse or ditch (only sewage treatment plants can be discharged into a watercourse etc.). Septic tanks are not suitable for areas of the country where the soil is of a heavy clay constitution. Septic tanks need to be regularly de-sludged to prevent the solids entering the drainage field. A porosity test to ensure the soil is suitable installing a septic tank must be undertaken prior to installation. It is possible that septic tanks can be fitted with a filter which further filters the suspended solids and prevents the percolation drainage field from blocking.
Septic tanks up to 20,000 litres can be produced and as guide to size a septic tank, you start with a base figure of 2000 litres then add 150 litres per person.
Shallow profile or low profile two stage septic tanks have become more popular in recent years, very often they are installed in areas of high water table or hard rock where the normal spherical tank with its deep profile is not a practical solution with the shallow tank being easier and more economical to install.
These tanks are also available manufactured from rotationally moulded polyethylene. This strong design enable the tank to be back-filled with suitable pea shingle/gravel instead of concrete to save costs in installation, time and labour.
Cesspools are a covered, water-tight tank that are used for storing sewage. They have no outlet and no treatment is involved and rely upon road transport for the removal of the raw sewage. No treatment is involved.