Treating sewage usually involves a combination of different methods to separate out solids, deal with pathogenic (disease carrying) organisms, and remove nutrients.
The first stage (primary treatment) is the separation of most of the solids from the liquid effluent, usually by some kind of settlement chamber. The most common example is the septic tank. These are a robust and economic option if used properly. Most of the solid material will either sink (as sludge) or float to the top (as crust), leaving the effluent between to flow out to a secondary treatment stage.
A common secondary treatment system is a leachfield. This is a system of perforated pipes laid in underground gravel trenches. They are unsuitable for clayey soils or areas with a high water table. The liquid effluent percolates through the gravel, where solids are removed and digested by micro-organisms, leaving the liquid clean enough to filter into groundwater.
Vertical flow reed beds are another kind of secondary treatment. The effluent from a septic tank (or similar) is percolated through a tank containing layers of sand and gravel planted with reeds. The reeds help bacteria to break down the pollutants and make the beds attractive to wildlife. Horizontal flow reed beds are usually used as a third stage of treatment where very high quality effluent is required.
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