ABSTRACT
Anaerobic ponds (Figure 10.1) are usually the first type of pond used in a series of ponds (Chapter 9). They are 2–5 m deep and receive such a high organic loading (usually >100 g BOD/m3 day, equivalent to >3000 kg/ha day for a depth of 3 m) that they contain no dissolved oxygen and no algae, although occasionally a thin film of Chlamydomonas may be present at the surface. They function much like open septic tanks, and their primary function is BOD (biochemical oxygen demand) removal. Anaerobic ponds work extremely well: a properly designed and not significantly underloaded anaerobic pond will achieve >60 per cent BOD removal at 20°C. Retention times are short: for wastewater with a BOD of ≤300 mg/l, for example, 1 day is sufficient at a temperature of 20°C.
