the influent BOD. This significantly decreases the load of organic matter that goes to the facultative ponds, and thus decreases their required size. Anaerobic stabilization ponds have the disadvantage of potentially releasing malodorous gases. This especially includes hydrogen sulfide with an odor of rotten eggs, if the system has operational problems. The first pond biome in a series of stabilization ponds digests the putrescible solids suspended in the wastewater being treated. Anaerobic ponds allow solids to settle down at the bottom as sludge. This settling removes a portion of the particulate organic material. A large fraction of the settled solids will accumulate close to the point where wastewater enters the pond. Therefore, anaerobic ponds must be designed to be far deeper than either aerobic or facultative ponds. The depth decreases the oxygen levels so anaerobic bacteria can efficiently digest the waste. Anaerobic ponds contain anaerobic organisms which are able to break down complex organic waste into basic compounds that are less harmful to the environment. Because anaerobic organisms can only thrive in warm temperatures, anaerobic ponds are not suitable in temperate or cold climates. Sludge accumulates at the bottom of the anaerobic ponds and needs to be removed every few years. Facultative ponds Facultative stabilization ponds that receive raw wastewater are called primary facultative ponds. If they are receiving wastewater that has already been treated in anaerobic ponds, they are called secondary facultative ponds. Facultative stabilization ponds may also be used for treatment following other types of treatment processes such as upflow anaerobic sludge blanket (UASB) reactors, oxidation ditches or aerated lagoons. Compared with anaerobic ponds, facultative ponds are shallower (1.5 to 2.5 m deep) and have much larger surface areas. The surface area is important because it allows atmospheric oxygen to dissolve and sunlight radiation to penetrate the water. This allows for photosynthetic activity to occur which produces more oxygen. In most ponds both bacteria and algae are needed in order to maximize the decomposition of organic matter and the removal of other pollutants. Algae produce oxygen (photosynthesis) and also consume oxygen (respiration), but they leave an excess of oxygen that can then be used by aerobic bacteria for respiration and for the processes of oxidation (or stabilization) of the organic matter in the wastewater. Several types of invertebrates are present in the ponds where they control the population of algae, which then settles to the bottom. Heavy algal