growth may block sunlight from penetrating into the pond. This decreases the potential for photosynthesis to contribute oxygen to the pond. In the treatment of sewage, systems composed of anaerobic ponds followed by facultative ponds usually have overall BOD removal efficiencies between 75 and 85%. Higher efficiencies are difficult to achieve because the effluent contains high concentrations of particulate organic matter, in the form of algae, naturally produced during treatment. The sludge comprising the sediment layer in the pond undergoes anaerobic digestion, and may accumulate for several years without needing removal. Maturation ponds Some additional removal of organic matter and other pollutants may be achieved in maturation ponds. These ponds are only included in the treatment line when high efficiencies of pathogen removal are required, either for discharge of the treated effluent in surface water bodies, or for use for irrigation or aquaculture. They are usually used after facultative ponds, but may also follow other treatment processes, such as upflow anaerobic sludge blanket (UASB) reactors. They could also be placed after an activated sludge process. Maturation ponds must be shallow (around 1.0 m depth or less) with a great surface area so that more oxygen can dissolve into the water giving the bacteria enough oxygen to properly function. Shallow ponds benefit from high photosynthetic activity arising from the penetration of solar radiation. The pH values are high because of intense photosynthesis, and ultraviolet radiation penetration takes place in the upper layers. Both of these factors promote the removal of pathogenic bacteria and viruses. Given the high surface area of the maturation ponds, protozoan cysts and helminth eggs are also removed, with sedimentation as the main mechanism. Sludge accumulation is very low in maturation ponds. Very high pathogen removal efficiencies may be achieved, depending on several factors: temperature, hydraulic retention time (the amount of time the liquid remains in the system - from entrance to exit), the number of ponds in the series, the presence of baffles and the depth of ponds. Maturation ponds may be used in combination with a rainwater reservoir to form an ecological, self-purifying irrigation reservoir. Application and suitability Waste stabilization ponds are very efficient in their primary objective of removing organic matter and, under some conditions, pathogenic organisms. Their design criteria have changed very little over the years. Ponds are simple to design, build, operate and maintain, which is very important in remote areas and in