ABSTRACT
Waste stabilization ponds (WSP) (Figure 9.1) are large shallow basins enclosed by earth embankments in which raw wastewater is treated by entirely natural processes involving both algae and bacteria. Since these processes are unaided by wastewater treatment engineers (who merely allocate a properly dimensioned place for their occurrence) the rate of oxidation is slower, and as a result hydraulic retention times are longer than in conventional wastewater treatment (eg electromechanical treatment processes such as activated sludge) _ retention times in WSP are measured in days rather than in hours. They are without doubt the most important method of wastewater treatment in developing countries where sufficient land is normally available and where the temperature is most favourable for their operation. Indeed they are so advantageous (see below) that a very good case has to be made for not using them.
