developing countries where sophisticated equipment and highly skilled labor is not easily available. Construction may be done by local contractors in small towns. Waste stabilization ponds work well in nearly all environments and can treat most types of wastewater. They are particularly well-suited for tropical and subtropical countries because the intensity of the sunlight and temperature are key factors for the efficiency of the removal processes. Ponds are used throughout the world. In many countries and regions ponds are the most widely used treatment process. For this reason, they are one of the processes recommended by WHO for the treatment of wastewater for reuse in agriculture and aquaculture, especially because of their effectiveness in removing nematodes (worms) and helminth eggs. Ponds cannot achieve very high efficiencies in the removal of organic matter, and usually have low capacities for removing nitrogen and phosphorus. The effluent usually has high concentrations of suspended solids, resulting from algal production in the ponds. Therefore, ponds are not a suitable technology in areas where stringent discharge standards exist, unless additional stages of post treatment are included. Since ponds require large areas, they may not be practical in proximity to towns where land is expensive. A suitable topography and a suitable soil structure are also desired, in order to reduce construction costs. Operation and maintenance Regarding operation and maintenance, the tasks performed by the operational staff are very simple and do not require special skills. Additionally, there is no energy consumption for aeration, no need of heavy equipment maintenance and no frequent sludge removal, sludge treatment and disposal. Ponds require very little maintenance, since there is no heavy electric or mechanical equipment that requires attention. The only routine maintenance needed is on the preliminary treatment (cleaning of screens and removal of sand), routine checking of pipes, weirs and other hydraulic structures, and removal of unwanted vegetation growth in embankments. Sludge removal Sludge accumulates inside the ponds. It needs to be removed only in the interval of several years. This is an important advantage of the system. However, when removal is necessary, it is usually an expensive and labor-intensive operation. Removal is more frequent in anaerobic ponds (every few years), because of their smaller volume and lower capacity to store the sludge, compared with facultative ponds. In facultative ponds, sludge removal may be necessary only in intervals around 15 to 25 years. In maturation ponds, sludge accumulation is