ABSTRACT
Facultative ponds are of two types: primary facultative ponds which receive raw wastewater (after preliminary treatment – Chapter 8 and see Figure 9.1), and secondary facultative ponds which receive settled wastewater (usually the effluent from anaerobic ponds). They are designed for BOD (biochemical oxygen demand) removal on the basis of a relatively low surface BOD loading in the range 100–400 kg/ha day (see equation 11.1 below) to permit the development of a healthy algal population, as the oxygen for BOD removal by the pond bacteria is mostly generated by algal photosynthesis. Facultative ponds are coloured dark green as a result of the large numbers of micro-algae in them, although they may occasionally appear red or pink (especially when slightly overloaded) due to the presence of anaerobic purple sulphide-oxidizing photosynthetic bacteria (see ‘Purple ponds’ later in this chapter). The algae that tend to predominate in the turbid waters of facultative ponds (Table 11.1) belong to motile genera (such as Chlamydomonas, Pyrobotrys and Euglena) as these can optimize their vertical position in the pond water column in relation to incident light intensity more easily than non-motile forms (such as Chlorella, although this is also common in facultative ponds). The concentration of algae in a facultative pond depends on loading and temperature (see Figure 11.5 below), but is usually in the range 500–2000 chlorophyll a per litre.
