Facultative PondsThese facultative ponds exhibit aerobic conditions in the top surface of the pond due to the production of oxygen during photosynthesis by microalgae and anaerobic conditions in the bottom layers. Therefore, they are characterized by an upper aerobic zone, region with available free oxygen in the pond, and a lower anaerobic zone, with empty free oxygen area in lower depth. Active wastewater treatment processes occur in both levels of FP. These ponds are most commonly used to treat domestic sewage and other wide range of organic industrial effluent discharge, and it involves a complex relationship between other aerobic and anaerobic microbes and microalgae.According to Oswald (1991), the competency of the above concept is demon- strated by the following parameters:i. Growth rate of microalgaeii. Contaminant concentration in the mediumiii. Heavy metal concentration in the mediumiv. Vital percentage of removal of metals from the mediumv. Capital and operating costs.The major function of FP is to maintain the microalgae population within the basin with high-level heterotrophic activity supplied with required dissolved oxy- gen. This aids the decomposition and stabilization of organic matter at an optimum rate. Due to periodic odor issue, FPs are typically not designed to establish closer than 0.5–1 km from residential areas. Ideal site locations for FPs are open zones because it facilitates mixing by allowing maximum surface wind sweep over the pond. As long as the wastewater influent mixes rapidly within the pond supernatant, it is not important to consider the shape of the FP. These ponds are in the range of depth from 2 to 5 feet (0.7–1.5 m) with a freeboard of 3–5 feet (1.0–1.5 m) above high water mark level. This strategy is followed to accommodate any wave action. Typical loading rates for FPs are based on general climate zones (Table 16.2).