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  The most common of these systems is the traditional fosse and gravel / sand bed (although other filter materials are available). Proven to be effective in purifying waste water without requiring any electrical input, these systems are ideal for properties that are not inhabited all year round or where the environmental impact is important. The systems work by collecting all waste water in the “fosse toutes eaux” where it is separated and the lighter suspended solids are digested by aerobic bacteria and the heavier sludge waste digested by anaerobic bacteria. As this usually only results in a reduction of pollutants, some germs and pathogens still remain and these could pose a real threat of contamination or disease if released directly into waterways. As such the effluent then needs to pass through a filtration process where the remaining waste and toxic particles are removed before the resulting liquid is either allowed to soak away or be siphoned off. However, there are problems with these systems. A sizeable amount of land is required for the filter bed, land that can then not be built upon. They can be prone to producing odours and need to emptied by a licensed waste collector on a regular basis. Additionally the sand (or other material) in the filter needs to be changed regularly and this could prove expensive. The size of the system also means that installation costs can be high. They are also prone to flooding and often require a pumping system to overcome land lie difficulties. As such these systems should not be used where land is limited, where there is a high water table, where the soil is of a water retaining nature or where there is a problematic terrain.