Slow sand filtration can be very effective to treat municipal treatment in a tertiary step. The application of slow sand filtration (SSF) is feasible on a biologically pre-treated wastewater. The mechanisms for water impurities removal are mechanical filtration, enhanced biological activity on surfaces and adsorption. These features make SSF very attractive for advanced treatment of effluents. The key parameters of the SSF are the depth of the sand filter and the effective size of the sand. The effective size of the sand is usually 0.15 – 0.4 mm and the depth between 0.6 to 1.0 m. The filter doesn’t need to be backwashed regularly, it is easy to remove the first layer on the surface and to replace it with new material. The used material can be recycled or disposed. The SSF is able to remove significantly bacteria so that the effluent is almost hygienically safe.

Figure 7: Slow sand filtration system (source: National Drinking Water Clearinghouse)

Table 2 gives an overview of the common technologies and their characteristics

Table 2: Wastewater technologies and their characteristics (Wendland, 2003)

The basic technologies applied in developed countries, primarily activated sludge process, are not able to meet common hygienic standard. The effluent of these plants contains still many pathogens which are usually highly diluted in the receiving waters so that they don´t harm. Only the filtration, physical and chemical technologies can meet the hygienic standards mostly required for water reuse.