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 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 facultative pond at parque fluminense treatment plant in ceara, brazil 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 very low. sludge removal, also called desludging, may be done in two basic ways: (i) interrupting the operation of the pond for desludging or (ii) keeping the pond in operation while desludging. in the first case, the influent wastewater to the pond to be desludged is closed. afterwards, the pond is drained and the bottom sludge is left for open drying for several weeks. during this period, the wastewater to be treated needs to be diverted to other ponds in the system. after the sludge has dried, its removal may be done manually (very laborious in large ponds) or mechanically using tractors or mechanical scrapers. in the second alternative, when the pond is left in operation during desludging, the removed sludge will be wet and will require further drying. this is undertaken outside the pond. sludge removal can be by suction and pumping using vacuum trucks (only for small ponds), dredging, pumping from a raft or involving other mechanical equipment. in either case, the amount of sludge to be removed is very high, considering its accumulation over a period of years. this process is very laborious, expensive and requires careful planning. costs in the selection of a wastewater treatment process, besides the technical aspects that are relevant to each alternative, also cost factors play a very important role. the latter can be basically divided into (i) construction costs and (ii) operation and maintenance costs. waste stabilization ponds are usually considered a cheap alternative in terms of construction costs. however, the final costs will depend essentially on the size of ponds, presence of maturation ponds in the process configuration, topography, soil conditions, groundwater level and land cost. because all these elements are site-specific, it is difficult to generalize overall construction costs. in most cases these will be lower compared with other wastewater treatment alternatives. depending on the specific situation of the area, construction costs can increase and level up with other technologies. waste stabilization ponds are one of the cheapest wastewater treatment processes in terms of operation and maintenance. comparison with other infrastructure the following types of water and wastewater infrastructure may superficially resemble waste stabilization ponds, but are not the same: aerated lagoons rely upon mechanical aerators to provide oxygen to stabilize organic matter. aerated lagoons may be used as the first stage in the treatment line instead of anaerobic ponds to limit release of malodorous gases, but with energy and maintenance requirements causing higher operating costs. constructed wetlands are designed to improve water quality by supporting rooted vegetation arranged to physically remove solids and particulate material while removing soluble nutrients in the water by uptake into plant tissue and supplying oxygen to the water to reduce bod. retention basins are used to manage stormwater runoff to prevent flooding and downstream erosion, and improve water quality in an adjacent river, stream, lake or bay. detention basins are similar but are "dry ponds" designed to temporarily hold runoff as a flood control measure. infiltration basins are ponds designed to percolate their contents into underlying permeable soils. settling basins are designed to separate solids from wastewaters without provision for treatment of those solids.