Waste stabilization pond

, Morocco (Station M’zar) pond and two maturation ponds in Attaouia, Morocco. Note the green color, caused by algae. Concept of stabilization Sewage and many types of industrial wastewaters contain organic matter. If wastewater is discharged untreated into surface water bodies (for instance, rivers and lakes), their organic matter serves as food for microorganisms living in the surface waters. These organisms use the organic matter for energy generation for their growth and reproduction. This is done via their respiration, in which they convert the organic matter into carbon dioxide and water. These stable components do not cause water pollution problems. Therefore this is frequently called "stabilization" of the organic matter. However, these organisms use oxygen in their respiration, thus reducing the oxygen concentration in the surface waters. This is one of the main water pollution problems, which may affect the surface water biota, including fish. Waste stabilization ponds reproduce these biological phenomena before they take place in the receiving surface water and cause the pollution problems due to oxygen consumption. The ponds receive wastewater, and, by natural processes similar to those that take place in the surface waters, carry out stabilization of the organic matter inside them, as part of the treatment. This is why they received the name of waste stabilization ponds. Microorganisms The reactions take place by the joint participation of several microorganisms living within the pond. The organic matter is measured as biochemical oxygen demand (BOD). BOD values in the pond effluent are lower than in the influent, reflecting the removal of organic matter. This pond biome uses organic matter from the wastewater as food. Nutrients are converted to cell material and energy for life processes including reproduction and growth of living cells. Some of these living cells will be consumed by organisms at higher trophic levels within the pond. In ponds, the most important group of microorganisms are bacteria, which utilize most of the organic matter from the wastewater, but also consume oxygen. Algae are another essential group of microorganisms. They do not depend on the organic material from the influent. Instead, they undertake photosynthesis, in which they produce the organic matter for their own consumption and, very importantly here, they release oxygen. The excess oxygen released supports the respiration done by the aerobic organisms in the pond. Atmospheric oxygen is also dissolved into the liquid, which assists in maintaining an aerobic layer on the top of the pond surface. Oxygen levels The oxygen concentration varies in the liquid column: Close to the surface, concentrations are high and support the growth of aerobic organisms. Close to the pond bottom, sunlight penetration is low, and thus photosynthetic activity is reduced. This causes oxygen concentrations to be low there. Finally, inside the sediments in the bottom layer, there is no oxygen at all. Here, organic matter is removed by digestion undertaken by anaerobic organisms. 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. 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). The first pond biome in a series of stabilization ponds digests the putrescible solids suspended in the wastewater being treated. Anaerobic ponds allow solids to settle down at the bottom as sludge. This settling removes a portion of the particulate organic material. Because anaerobic organisms can only thrive in warm temperatures, anaerobic ponds are not suitable in temperate or cold climates. Sludge accumulates at the bottom of the anaerobic ponds and needs to be removed every few years. Facultative ponds Facultative stabilization ponds that receive raw wastewater are called primary facultative ponds. If they are receiving wastewater that has already been treated in anaerobic ponds, they are called secondary facultative ponds. Facultative stabilization ponds may also be used for treatment following other types of treatment processes such as upflow anaerobic sludge blanket (UASB) reactors, oxidation ditches or aerated lagoons. Compared with anaerobic ponds, facultative ponds are shallower (1.5 to 2.5 m deep) and have much larger surface areas. The surface area is important because it allows atmospheric oxygen to dissolve and sunlight radiation to penetrate the water. This allows for photosynthetic activity to occur which produces more oxygen. In most ponds both bacteria and algae are needed in order to maximize the decomposition of organic matter and the removal of other pollutants. 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. == 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. 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 ==

, 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. 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. ==See also==