Waste stabilization pond

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) are ponds designed and built for wastewater treatment where biological processes remove pathogens from the wastewater. Natural processes reduce the organic content, which is measured as biochemical oxygen demand (BOD). Stabilization ponds are man-made depressions confined by earthen structures. Liquid enters on one side of the stabilization pond (influent) and exits on the opposite side (effluent), after spending several days in the pond.

Waste stabilization ponds are used worldwide for wastewater treatment and are especially needed in 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. Waste stabilization ponds involve natural treatment processes which take time: conversation rates are slow. Therefore, larger areas are required than would be needed for other treatment processes.

The system may be comprised 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 has low enough levels of pathogenic organisms.

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 common in small towns.

Fundamentals

Organic material in wastewater is stabilized by the joint participation of several microorganisms living within the pond. This pond biome uses waste material as food and nutrients converted to 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. Bacteria consume oxygen while doing respiration which produces internal energy.

Algae are another essential group of microorganisms. They do not depend on the organic material from the influent; they undertake photosynthesis, which is important for the release of oxygen. In turn, the excess oxygen supports the respiration done by the aerobic organisms. Atmospheric oxygen is also dissolved into the liquid, which assists in maintaining and aerobic layer on the top of the pond surface. An oxygen concentration gradient becomes established in each pond. That includes an aerobic habitat at the surface down to a benthic zone of sediment being digested by anaerobic organisms.

Degree of aerobic conditions

The presence or absence of oxygen varies with the three different types of ponds, used in sequence. Anaerobic waste stabilization ponds have surface and environmental conditions with very limited ability to support aerobic organisms, 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.

Removal of pathogens

Pathogenic organisms can be efficiently removed in waste stabilization ponds. The process relies mostly on maturation ponds for removal of pathogens. However, some removal 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 (pH 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.

Protozoan pathogens are present in the wastewater in the form of cysts or oocysts, and helminths (worms) which are present in the form of eggs. The protozoan 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 line, in compliance with World Health Organization guidelines for irrigating with treated wastewaster. However, sludge (sediment) from the ponds may be heavily contaminated with helminth eggs, and may remain resistant even after several years.

Types

Waste stabilization pond

Waste stabilization ponds consist of man-made basins comprising a single or several series of anaerobic, facultative or maturation ponds. The main configurations are:

• Facultative pond;
• Anaerobic pond plus facultative pond;
• Facultative pond plus maturation ponds in series;
• Anaerobic pond plus facultative pond plus maturation ponds in series.

If an anaerobic pond is present, part of the suspended solids settle, removing some of the soluble element of organic matter (BOD). During the second stage in the facultative pond, most of the remaining BOD is removed through the coordinated activity of algae and heterotrophic bacteria. The main function of the tertiary treatment in maturation ponds is the removal of pathogens, although it may also assist in nutrient reduction (i.e. nitrogen). Nitrogen fixation by algae living in stabilization pond systems may increase nitrogen levels in stabilization pond effluent.

Maturation ponds

A maturation pond next to an irrigation reservoir, forming a self-purifying reservoir

Some additional removal of organic matter and other constituents may be achieved in maturation ponds. However, they are included in the treatment line when high efficiencies of pathogen removal are required, either for discharge of the treated effluent in water bodies, or for use for irrigation or aquaculture. They are usually used after facultative ponds, but may also follow other treatment processes, including upflow anaerobic sludge blanket (UASB) reactors or even more sophisticated processes, such as activated sludge. Maturation ponds are shallow (around 1.0 m depth), in order to maximize the aerobic conditions in the liquid column. Shallow ponds benefit from high photosynthetic activity arising from the penetration of solar radiation. The pH values are high because of intense photosynthesis, and ultraviolet radiation penetration takes place in the upper layers. Both of these factors promote the removal of pathogenic bacteria and viruses. Given the high surface area of the maturation ponds, protozoan cysts and helminth eggs are also removed, with sedimentation as the main mechanism.

Very high efficiencies may be achieved, depending on a combination of factors. The important factors are temperature, hydraulic retention, 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.

Maturation ponds may be used to stabilize effluent from mechanized secondary treatment. Maturation ponds receiving wastes with low BOD concentrations may be clear enough to accomplish pathogen reduction through exposure to ultraviolet solar radiation. Use of maturation ponds by waterfowl prevents complete elimination of pathogens. Birds moving from earlier ponds in the treatment sequence will carry pathogens to maturation ponds in their feces and on their feet and feathers. Maturation ponds may be used in combination with a rainwater reservoir to form an ecological, self-purifying irrigation reservoir or swimming pond. Maturation ponds should be saturated with dissolved oxygen throughout the entire pond and shallow enough for light to penetrate the entire depth of the pond. Maturation ponds must be shallow with a great surface area so that more oxygen can dissolve into the water giving the bacteria enough oxygen to properly function. Concentrations of phosphorus and nitrogen in some wastewaters may create eutrophic waste stabilization ponds supporting algae populations whose death sustains an anaerobic benthic zone more closely resembling a facultative pond than a maturation pond. Aerobic bacteria grow in the aerobic ponds and are able to decompose organic waste into oxidized products. With mosquitos being a problem for those who maintain the ponds the addition of mosquito fish and several other types of predatory fish is good in order to keep the bugs in check. They are primarily used for mosquito control as treatment ponds are fantastic breeding grounds for mosquitos. By adding the fish the ponds bug issues are controlled in a natural and environmentally friendly way without having to use harmful chemicals.

Anaerobic waste stabilization ponds

Main article: Anaerobic lagoon

Anaerobic ponds receive wastes with BOD loads capable of completely deoxygenating standing water at ambient conditions. 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, they are able to remove between 1/2 and 2/3 of the influent BOD. This significantly decreases the load of organic matter that goes to the facultative ponds, and thus decreases their size. Anaerobic stabilization ponds have the disadvantage of potentially releasing malodorous gases. This especially includes hydrogen sulfide (odor of rotten eggs) if the system suffers from 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 dissolves 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. Because anaerobic organisms can only thrive in suitable temperatures, anaerobic ponds are not of much use in temperate or cold climates.

Facultative waste stabilization ponds

Main article: Facultative lagoon

Facultative stabilization ponds that receive raw wastewater would be called primary facultative ponds. If receiving wastewater that has already been treated in anaerobic ponds, they would be called a secondary facultative ponds. Facultative stabilization ponds may also be used for treatment following other types of treatment processes. These could be upflow anaerobic sludge blanket (UASB) reactor, oxidation ditch or aerated lagoon. Compared with anaerobic ponds, facultative ponds are more shallow (1.5 to 2.5 m deep) and have much larger surface areas. The surface area is important in that it allows atmospheric oxygen to dissolve and sunlight radiation to penetrate the water, which allows photosynthetic activity which produces more oxygen.

Facultative stabilization ponds stratify with an aerobic surface layer and an anaerobic layer below the surface. The aerobic surface layer limits release of malodorous gas from the anaerobic benthic zone. Algae and c-Cyanobacteria typically grow in the aerobic zone and provide bacteria in the pond with plenty of oxygen during the daytime. However, algal photorespiration may consume oxygen during the dark hours. Stabilization ponds with large algal populations may show significant diurnal fluctuation in oxygen concentrations with a peak in the late afternoon, and a minimum at dawn.

Kinds of algae growing in treatment ponds include green, red and brown algae. In most ponds both bacteria and algae are needed in order to maximize the decomposition of organic matter and the conversation of other constituents. 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 of the organic matter in the wastewater.

Several types of invertebrates are present in the ponds where they maintain algae levels and help allow the sediment to settle on 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.

Areas with a consistently cool, but frost-free, climate may sustain facultative conditions in the first stabilization pond when treating lightly polluted water at low temperatures favorable to high concentrations of dissolved oxygen with low metabolic rates. Facultative pond stratification becomes unstable during cold weather increasing release of malodorous gas when water temperatures drop below 4 degrees Celsius (39 degrees Fahrenheit); and formation of ice on the pond surface will effectively prevent transfer of atmospheric oxygen to the pond biome. Stabilization ponds in climates with significant seasonal temperature variation may release malodorous gas during the season of rising temperatures as the pond biome consumes wastes accumulated during cold weather with increasing metabolic rates exceeding the atmospheric oxygen transfer rate at the pond surface.

Application and suitability

Waste stabilization ponds are very efficient in their primary objective of removing organic matter and, under some conditions, pathogenic organisms, even though design criteria has changed very little over the years. Ponds are simple to design, build and operate, which is extremely important in remote areas and in developing countries where sophisticated equipment isn't always available. Construction may be done by local contractors in small towns. There are few operational issues for the ponds themselves, since there is no heavy electric or mechanical equipment to require attention. The only maintenance needed is on the preliminary treatment, pipes, weirs and embankments. No energy is required (for aeration) and sludge is retained in the ponds for several years, with no need for removal.

However, it should be understood that ponds cannot achieve very high efficiencies in the removal of organic matter, and usually have low capacities for removing nitrogen and phosphorus. The effluent usually has high concentrations of suspended solids, resulting from algal production in the ponds. Therefore, if effluents with better quality are required, additional treatment stages or different treatment processes need to be incorporated. Since ponds require large areas, they may not be practical in proximity to towns where wastewater is produced. A suitable topography and a good soil are also desired, in order to reduce construction costs.

The ponds work well in nearly all environments and can treat most forms of wastewater produced. Pond depths usually range from between four and twenty feet deep. Depth is determined based on the type of bacteria and total amount of waste the system must treat. Stabilization ponds 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. However, ponds are used throughout the world. In many countries and regions ponds are the most widely used treatment process. Nevertheless, it should be pointed out that in some areas ponds cannot being adopted because of stringent discharge standards.

A current study (2016) by scientists from the Institute of Environmental Science & Research in New Zealand showed however that similar levels of removal of indicators and viruses can be achieved independent from sunlight, if the pH and dissolved oxygen levels remain elevated. It is also recommended by the WHO for the treatment of wastewater for reuse in agriculture and aquaculture, especially because of its effectiveness in removing nematodes (worms) and helminth eggs.

Suitable wastewaters characteristics

The wastewater temperature can be anywhere between 45–75 °F (7–24 °C), but the temperature of water entering the system usually does not affect the pond, because it is designed to handle a variety of temperatures.

The pH of the system is a huge factor in pond water characteristics with the water either being acidic 0.0–7.0 or basic 7.0–14.0. The pH levels are monitored to make sure that the water does not get too acidic and start to corrode metal pipes. It is most desirable for the water to be more basic so as to prevent corrosion from the acid.

Another set of characteristics is the contents of the water, like solids, suspended solids and dissolved solids. Dissolved solids are able to pass through most filters whereas suspended solids are just particles suspended in the water column easily caught in strainers or filters. Solids is just a general term for what would remain if all of the water was evaporated from a sample of the wastewater. To get into more technical terms colloidal solids are particles that are so small that they will not settle by gravity on their own. Settleable solids are pretty much self-explanatory as they are the solids that will settle on their own in a given period of time.

Use

Waste stabilization ponds are used for sewage treatment in many countries with warm climates, partiularly in developing countries. They are typically used in smaller towns where the cost of land is low. In cities where space is expensive activated sludge waste water treatment plants an be more suitable.

Many improvements have been made to improve their effectiveness and efficiency at turning harmful chemicals and sewage into less harmful forms of the previous. With ponds there is a variety of key elements to take care of in order to be properly maintained for the wastewater to be properly treated.

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, but the final costs will depend essentially on the size of ponds, presence of maturation ponds in the flowsheet, 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 specificities of the area, construction costs can increase and level up with other technologies.

Regarding operation and maintenance costs, the tasks performed by the operational staff are very simple, do not require special skills and can be performed by local people. Additionally, there is no energy consumption for aeration, no need of heavy equipment maintenance and no regular sludge treatment and disposal. All these factors lead to the fact that stabilization ponds are one of the cheapest wastewater treatment processes in terms of operation and maintenance.

Related wastewater treatment systems

The following types of wastewwater treatment systems may superficially resemble waste stabilization ponds, but have distinct differences:

• Aerated lagoons rely upon mechanical aerators to provide oxygen to stabilize high BOD wastes. Aerated lagoons may be used as a primary treatment stage instead of anaerobic stabilization to limit release of malodorous gas, 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 arrest 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.
• Detention basins 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.
• Lagoons are shallow bodies of water separated from a larger body of water by barrier islands or reefs.
• Natural pools hold water within an isolating membrane or membranes, forming swimming pools in which no chemicals or devices that disinfect or sterilize water are used, and all clarifying and purifying of the water is achieved through biological filters and plants rooted hydroponically in the system.
• Retention basins are wet ponds designed to temporarily hold runoff as a flood control measure.
• Settling basins are designed to separate solids from liquid wastewaters without provision for stabilization of those solids through biochemical oxidation.

See also

• Industrial wastewater treatment
• List of waste water treatment technologies
• Organisms used in water purification
• Water pollution

External links

• Stabilization pond pictures worldwide: Enter the keyword "pond" into search field of SuSanA photo database on Flickr