and photooxidative reactions taking advantage of high dissolved oxygen concentrations. Protozoan pathogens are present in the wastewater in the form of cysts or oocysts. Helminths (worms) are present in the form of eggs. The protozoan and helminth pathogens can be removed by the mechanism of sedimentation. Very high removal efficiencies may be achieved, especially if maturation ponds are part of the treatment system. In that case, the final pond effluent may be in compliance with World Health Organization guidelines for irrigating with treated wastewaster (or "reclaimed water"). However, sludge (sediment) from the ponds may be heavily contaminated with helminth eggs, which may survive even after several years of storing the sludge inside of the pond. Types Waste stabilization ponds consist of man-made basins comprising a single or several series of anaerobic, facultative or maturation ponds. The presence or absence of oxygen varies with the three different types of ponds, used in sequence. Anaerobic waste stabilization ponds have very little dissolved oxygen, thus anaerobic conditions prevail. The second type of pond, facultative stabilization ponds, sustain an aerobic surface habitat above an anaerobic benthic habitat. Maturation ponds offer aerobic conditions throughout, from the surface to the bottom. The main configurations of pond systems are: Facultative pond only; Anaerobic pond followed by a facultative pond; Facultative pond followed by maturation ponds in series; Anaerobic pond followed by a facultative pond followed by maturation ponds in series. If an anaerobic pond is present, part of the suspended solids from the wastewater settles, thus removing the organic matter (BOD) contributed by these solids. Additionally, some of the dissolved organic matter is removed by anaerobic digestion. During the second stage in the facultative pond, most of the remaining BOD is removed mainly by the heterotrophic bacteria that receive oxygen from the photosynthesis undertaken by algae. The main function of the tertiary stage in maturation ponds is the removal of pathogens, although it may also assist in nutrient reduction (i.e. nitrogen). However, nitrogen fixation by algae living in stabilization pond systems may increase nitrogen levels in stabilization pond effluent. Anaerobic ponds Anaerobic ponds receive raw wastewater. They have a smaller surface area compared to facultative ponds and are also deeper (usually 3.0 to 5.0 m). The depth decreases the influence of oxygen production by photosynthesis, leading to anaerobic conditions. Depending on loading and climatic conditions, these ponds are able to remove between half to two thirds of