Ambient temperatures in hot-climate countries are conducive to these anaerobic reactions and expected BOD5 removals for different retention times in treating sewage have been given by Mara (1976) as shown in Table 2. More recently, Gambrill et al. (1986) have suggested conservative removals of BOD5 in anaerobic ponds as 40% below 10°C, at a design loading of 100 g/m3d, and 60% above 20°C, at a design loading of 300 g/m3d, with linear interpolation for operating temperature between 10 and 20°C.
Higher removal rates are possible with industrial wastes, particularly those containing significant quantities of organic settleable solids. Of course, other environmental conditions in the ponds, particularly pH, must be suitable for the anaerobic microorganisms bringing about the breakdown of BOD. Table 2. BOD removals in Anaerobic Ponds loaded at 250 g (Mara 1976) BOD5/m3d. Retention Time (days)| BOD5 removal %| 1| 50| 2. 5| 60| 5| 70| Anaerobic ponds are normally designed on the basis of a temperature dependent empirical value for the permissible organic loading rate.
Land requirements will be lowest if the maximum possible BOD loading can be applied. The upper limit of the volumetric BOD loading is determined by odor emissions and minimum pH threshold value at which the anaerobic decomposition processes cease to work. The maximum BOD loading rate acceptable to avoid odor nuisance was discussed earlier in section 3. 1. However, the effect of pH must be taken into consideration. Concentrations of H2S, which is the sulphur form responsible for odors, increases sharply as the pH drops below 7. , phenomenon which may occur if an anaerobic pond is heavily loaded or overloaded (based on a BOD loading rate criterion). Sulphide may also impede methane production in anaerobic ponds if occurring at excess concentrations. The presence of heavy metals will lead to insolubilisation of sulphides (e. g. iron sulphides). Since methanogenesis is the rate-limiting factor in anaerobic metabolism, products from the preceding acidogenesis reaction may accumulate and lead to a pH decrease.