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
Technology
Climate
Ground space demand
Energy costs
Capital and operation costs
Technical knowledge for operation and maintenance
Hygienic Quality in the effluent
Activate sludge plant (SBR)
Good biological activity in warm climate, evaporation in warm and dry climate
low
high
High capital costs, lower operation and maintenance costs
Elimination by factor 10-100
Trickling filter, rotation disc contactor
Independent, usually built in house
medium
High capital, operation and maintenance costs
Anaerobic reactor
No evaporation problems, the warmer, the better the biological activity
Energy recovery
High capital costs but energy recovery of biogas
Constructed Wetland
Transpiration depends on the type of plants
Low capital, operation and maintenance costs
1 log elimination
Lagoons (aerated or natural)
High evaporation rate in dry climate
Low for natural, medium for aeration
> 3 log elimination for long residence time
Membrane reactor
Evaporation in warm and dry climate
very low
very high
Hygienically safe (UF)
Slow sand filtration
Hygienically almost safe
UV, Chlorine Ozone
Needs building
Low capital, high operation and maintenance costs
Hygienically safe
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.