Waste stabilization pond
Waste stabilization ponds are used worldwide for wastewater treatment and are especially suitable for developing countries that have warm climates.
Waste stabilization ponds involve natural treatment processes which take time because removal rates are slow.
[4][5] 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.
In ponds, the most important group of microorganisms are bacteria, which utilize most of the organic matter from the wastewater, but also consume oxygen.
Atmospheric oxygen is also dissolved into the liquid, which assists in maintaining an aerobic layer on the top of the pond surface.
Pathogens are inactivated as a result of a complex interaction of mechanisms that involve pH (the pH value in ponds is high because of algal photosynthesis), temperature, ultraviolet radiation present in the sunlight that reaches the pond surface and photooxidative reactions taking advantage of high dissolved oxygen concentrations.
In that case, the final pond effluent may be in compliance with World Health Organization guidelines for irrigating with treated wastewaster (or "reclaimed water").
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.
Depending on loading and climatic conditions, these ponds are able to remove between half to two thirds of the influent BOD.
[14] A large fraction of the settled solids will accumulate close to the point where wastewater enters the pond.
The surface area is important because it allows atmospheric oxygen to dissolve and sunlight radiation to penetrate the water.
[citation needed] Several types of invertebrates are present in the ponds where they control the population of algae, which then settles to the bottom.
Higher efficiencies are difficult to achieve because the effluent contains high concentrations of particulate organic matter, in the form of algae, naturally produced during treatment.
These ponds are only included in the treatment line when high efficiencies of pathogen removal are required, either for discharge of the treated effluent in surface water bodies, or for use for irrigation or aquaculture.
They are usually used after facultative ponds, but may also follow other treatment processes, such as upflow anaerobic sludge blanket (UASB) reactors.
The pH values are high because of intense photosynthesis, and ultraviolet radiation penetration takes place in the upper layers.
Given the high surface area of the maturation ponds, protozoan cysts and helminth eggs are also removed, with sedimentation as the main mechanism.
[citation needed] Very high pathogen removal efficiencies may be achieved, depending on several factors: temperature, hydraulic retention time (the amount of time the liquid remains in the system - from entrance to exit), the number of ponds in the series, the presence of baffles and the depth of ponds.
[15] 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.
[citation needed] Waste stabilization ponds work well in nearly all environments and can treat most types of wastewater.
[6] 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.
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.
Therefore, ponds are not a suitable technology in areas where stringent discharge standards exist, unless additional stages of post treatment are included.
In the second alternative, when the pond is left in operation during desludging, the removed sludge will be wet and will require further drying.
[6] 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.
[citation needed] Waste stabilization ponds are one of the cheapest wastewater treatment processes in terms of operation and maintenance.
[6] The following types of water and wastewater infrastructure may superficially resemble waste stabilization ponds, but are not the same:[citation needed]
