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