thumb|upright=1.6|Schematic of the three main types of waste stabilization ponds (WSPs): (1) anaerobic, (2) facultative and (3) aerobic (maturation), each with different treatment and design characteristics
Waste stabilization ponds (WSPs or stabilization ponds or waste stabilization lagoons) are ponds designed and built for wastewater treatment to reduce the organic content and remove pathogens from wastewater. They are man-made depressions confined by earthen structures. Wastewater or "influent" enters on one side of the waste stabilization pond and exits on the other side as "effluent", after spending several days in the pond, during which treatment processes take place.
Waste stabilization ponds are used worldwide for wastewater treatment and are especially suitable for developing countries that have warm climates. They are frequently used to treat sewage and industrial effluents, but may also be used for treatment of municipal run-off or stormwater. The system may consist of a single pond or several ponds in a series, each pond playing a different role in the removal of pollutants. After treatment, the effluent may be returned to surface water or reused as irrigation water (or reclaimed water) if the effluent meets the required effluent standards (e.g. sufficiently low levels of pathogens).
Waste stabilization ponds involve natural treatment processes which take time because removal rates are slow. Therefore, larger areas are required than for other treatment processes with external energy inputs. Waste stabilization ponds described here use no aerators. High-performance lagoon technology that does use aerators has much more in common with the activated sludge process. Such aerated lagoons use less area than is needed for traditional stabilization ponds and are also common in small towns.
Fundamentals
thumb|Waste stabilization pond at Grand [[Agadir, Morocco (Station M’zar)]]
thumb|Effluent from a high rate [[algae 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
thumb|upright=1.6|Main configurations of waste stabilization pond systems
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
thumb|Facultative pond in [[Ruai, Kenya]]
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.