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.Peña Varón, M., Mara, D. (2004). Stabilisation ponds. IRC International Water and Sanitation Centre. Delft, The Netherlands. 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
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.
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.
Removal of pathogens Pathogens can be efficiently removed in waste stabilization ponds. The process relies mostly on maturation ponds for removal of pathogens, although some removal also takes place in the other ponds of the system. The higher the number of ponds in the series, the more efficient the pathogen removal.
Removal of pathogenic bacteria and viruses occurs mainly by inactivation. 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.Verbyla, M., von Sperling, M., Ynoussa, M. (2017). Waste Stabilization Ponds. In: J.B. Rose and B. Jiménez-Cisneros, (eds) Global Water Pathogens Project. (C. Haas, J. Mihelcic and M. Verbyla) (eds). Part 4. Management of Risk from Excreta and Wastewater). Michigan State University, E. Lansing, MI, UNESCO.
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. Very high removal efficiencies may be achieved, especially if maturation ponds are part of the treatment system. In that case, the final pond effluent may be in compliance with World Health Organization guidelines for irrigating with treated wastewaster (or "reclaimed water").WHO (2006). WHO Guidelines for the Safe Use of Wastewater, Excreta and Greywater - Volume IV: Excreta and greywater use in agriculture. World Health Organization (WHO), Geneva, Switzerland 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.Stewart M. Oakley, Department of Civil Engineering, California State University: The Need for Wastewater Treatment in Latin America: A Case Study of the Use of Wastewater Stabilization Ponds in Honduras , Small Flows Quarterly Juried Article, Spring 2005, Volume 6, Number 2
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:USEPA (2011) Principles of Design and Operations of Wastewater Treatment Pond Systems for Plant Operators, Engineers, and Managers. EPA/600/R-10/088. United States Environmental Protection Agency 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). However, nitrogen fixation by algae living in stabilization pond systems may increase nitrogen levels in stabilization pond effluent.
Anaerobic ponds
Anaerobic ponds receive raw wastewater. They have a smaller surface area compared to facultative ponds and are also deeper (usually 3.0 to 5.0 m). The depth decreases the influence of oxygen production by photosynthesis, leading to anaerobic conditions. Depending on loading and climatic conditions, these ponds are able to remove between half to two thirds of the influent BOD. This significantly decreases the load of organic matter that goes to the facultative ponds, and thus decreases their required size. Anaerobic stabilization ponds have the disadvantage of potentially releasing malodorous gases. This especially includes hydrogen sulfide with an odor of rotten eggs, if the system has operational problems.
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. A large fraction of the settled solids will accumulate close to the point where wastewater enters the pond. Therefore, anaerobic ponds must be designed to be far deeper than either aerobic or facultative ponds. The depth decreases the oxygen levels so anaerobic bacteria can efficiently digest the waste. Anaerobic ponds contain anaerobic organisms which are able to break down complex organic waste into basic compounds that are less harmful to the environment.Cincinnati, O. (2010, June 29). NEPIS document display. 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. Algae produce oxygen (photosynthesis) and also consume oxygen (respiration), but they leave an excess of oxygen that can then be used by aerobic bacteria for respiration and for the processes of oxidation (or stabilization) of the organic matter in the wastewater.
Several types of invertebrates are present in the ponds where they control the population of algae, which then settles to the bottom. Heavy algal growth may block sunlight from penetrating into the pond. This decreases the potential for photosynthesis to contribute oxygen to the pond.