Wastewater is a combination of water-carried wastes removed from residences and institutions, waste created by commercial and industrial activity, water from the ground, and surface water (including storm water). Wastewater sources are generally categorized as municipal, agricultural, or industrial. Municipal wastes are from residential, commercial, and institutional activities, and waste from street drainage or runoff. Commercial and institutional activities that create waste include hospitals, clinics, department stores, offices, and public recreations, to name just a few. The contaminants in wastewater are suspended solids, nutrients, biodegradable organics, pathogens, heavy metals, refractory organics, and dissolved inorganic solids. Refractory organics include agricultural pesticides, surfactants, and phenols, which tend to resist conventional wastewater treatment methods. Heavy metals usually come from commercial and industrial activities. Inorganic solids, such as calcium, sodium, and sulfate are found in domestic water supplies. Biodegradable organics are composed of proteins, carbohydrates, and fats, which destabilize natural oxygen in the ecosystem, especially if they are discharged into lakes and stagnant waters before being treated.
Generally there are two major treatment methods used to treat sewage—stabilization ponds and advanced wastewater treatment methods. Advanced wastewater treatments require capital-intensive units, often aided mechanically with concrete channels, tanks, and other devices (including screens, grit chambers, settling tanks, thickeners, aeration tanks, digesters, and other unit processes). In this method, chemicals are used to remove pathogens.
Rural Sanitation
Rural sanitation is very important to eliminate waterborne diseases that are transmitted through the fecal-oral cycle. Effective rural waste sanitation breaks this cycle at the source and greatly reduces pathogen intake. However, rural sanitation by itself cannot solve the waterborne-related problems unless it is accompanied by hygiene education and a clean, safe water supply. However, handling rural domestic wastes is usually much easier than handling urban wastes. People in rural areas are quite dispersed, and they do not require complex sewage networks or drainage pipes. Solid wastes, such as ashes from cooking, dung, and other refuse are usually biodegradable and are used in agricultural fields. The rural sanitation problem is mainly related to handling excreta and other non-biodegradable refuses. However, in the rural areas of many developing countries, non-biodegradable wastes are very rare and any waste that does occur (such as dry cell batteries) can be handled by the villagers with a simple program of health education. Therefore, this section will discuss how to handle excreta in rural villages. In most rural areas, a simple pit latrine or some type of composite latrine is used to handle the excreta. There are number of modified designs available on the market for pit latrines and composite pits. In a pit latrine, a hole is dug, a timber or concrete slab is placed over the hole, and a shed and roof are placed around the slab for privacy. However, there is a problem with odor and flies, which a simple cover can control.
Municipal Wastewater Treatment
In many developing countries, municipal wastes are the main public health concerns because there is no single individual responsible for these wastes except the city administration.
Municipal waste management is always a big responsibility for city administrations, especially in developing countries, due to a lack of infrastructure, finances, and know-how. However, in developed countries, waste management is well organized and is mostly privatized or leased;
every individual household, commercial center, and institution pays for the service. In this
section, various treatment methods and their drawbacks will be discussed. However, before
discussing the wastewater treatment methods, it is important to understand the characteristics of the sewage (i.e., physical, chemical, and biological).
The characteristics of sewage indicate the quality of the wastewater. The physical characteristic is the level of suspended solids: the presence of various chemicals and microbiological pollutants. The biological characteristic is the amount of oxygen required to oxidize the various organic chemicals. The oxygen demand is expressed either as a chemical oxygen demand (COD) or a biochemical oxygen demand (BOD), or total organic carbon (TOC). The measure for BOD is expressed as BOD5 to relate to the measure of biodegradable organic matter contained in the sewage, and COD is approximately 1.5 times the BOD5.
The BOD is usually measured by keeping a sample of sewage at 20°C for five days and calculating the amount of oxygen used to oxidize the organics. The COD is measured by boiling the sewage with an acid dichromate solution, which converts most of the organics to carbon dioxide and water. The chemical characteristic of sewage is the presence of organic and inorganic constituents, nutrients, and toxic chemical contaminants. Sewage quality is normally expressed in terms of its BOD. The strength of the BOD reflects the type of sewer and the lifestyle of the people because the BOD comes from feces, urine, and sludge. For example, BOD values of 400–800 mg/l are common in cities and towns of developing countries; in such areas, raw sewage contains approximately 40 g of BOD per person per day. In this case, if the per capita water consumption of the community is 100 l/person/day, the sewage will contain 400 mg/l of BOD (i.e. (40 x 103)/100). Similarly, if the water consumption is lower, the BOD will be higher. However, if the sewage passes through a septic tank or some kind of settling tank (e.g., aqua-privy), approximately half of its BOD will be lost. Night soil (sewage not diluted with sludge) will clearly have a high BOD because it has no sludge (it contains only feces and urine). In such cases, the BOD of night soil may be as high as 30,000 mg/l (30 g of BOD/day and 1 l/day of liquid is contributed by each person). According to Mara (1977), the strength of the BOD is categorized as weak (up to 200 mg/l), medium (350 mg/l), strong (500 mg/l), and very strong (above 750 mg/l).
In wastewater treatment, contaminants are removed by physical, chemical, and biological means and the treatment methods are usually classified as physical, chemical, and biological processes (Metcalf and Eddy, Inc. 1979, and Steel and McGhee 1979). The physical wastewater treatment process applies physical forces. Typical physical processes are screening, mixing, flocculation, sedimentation, flotation, and filtration. Chemical treatment processes remove or convert the contaminants by adding chemicals or through chemical reactions. The most common examples used in chemical wastewater treatment are precipitation, gas transfer, adsorption, and disinfection. Chemical precipitation, for example, is accomplished by producing a chemical precipitate, which will settle at the end.
A biological treatment is used primarily to remove the biodegradable organic substances (colloidal or dissolved) in wastewater. Basically, these substances are converted into gases that can escape to the atmosphere or into biological cell tissues that can be removed by settling.
Biological treatment is also used to remove pathogens and nitrogen from wastewater. In most cases, wastewater can be treated biologically.
The four major groups of biological treatment processes are aerobic, anaerobic, anoxic (the process by which nitrate is converted biologically into nitrogen gas in the absence of oxygen), or a combination of the three. The principal applications for these processes are removing carbonaceous organic matter (measured in BOD, COD, or in TOC), nitrification, denitrification, or stabilization. The most common wastewater treatment method used in many regions with hot to moderate climate regions is a stabilization pond, which is discussed in the next section. Other emerging technologies will be discussed in later sections.
Stabilization Ponds
Stabilization ponds are a suitable treatment technology because they are also very effective at removing pathogens (WHO 1987). Stabilization ponds consist of a series of ponds into which the sewage flows. Treatment occurs through natural physical, chemical, or biological processes and no extra energy is required except the sun. Such treatment methods are the cheapest and simplest of all the treatment technologies and are capable of providing a very high-quality effluent. Ponds are very easy to maintain and require no routine operation. They can absorb both hydraulic and organic disturbances and can treat a wide variety of domestic and industrial wastes. The system can be flexible and can be expanded with little investment. Stabilization ponds can also be used to convert the emitted gases into useful energy. The biogas produced from the biological processes can be collected and used to produce energy (either electricity or heat or both). The biggest disadvantage of stabilization ponds is that they take up a lot of space. There are basically four types of wastewater stabilization ponds: anaerobic ponds, facultative ponds, maturation ponds, and a high-rated pond, which is also called an aerated lagoon or an oxidation ditch. All four types of ponds are discussed below. In practice, the first three types of ponds are basically joined in series and can have two or three stages. If one stage of treatment is used, the pond will normally be anaerobic or facultative. However, in general, a secondary pond for additional aerobic biological treatment should follow an anaerobic pond.
Anaerobic Ponds
Anaerobic ponds are basically open septic tanks used for pre-treating large volumes of strong wastes. Anaerobic digestion involves the decomposition of organic and inorganic matter in the absence of molecular oxygen. In anaerobic ponds, anaerobic digestion and settling will take place, and a thick scum usually develops on the surface. Retention times typically vary from 1–4 days, and the preferred pond depth is 2–4 m. Odor can be avoided by controlling the volumetric load of the BOD (not more than 400 g/m3/day) and the concentration of sulfate ion in the raw waste (not higher than 100 mg/l). According to Cairncross and Feachem (1983), at 20°C temperatures, 50% of the BOD can be removed after one-day retention, and 70% of BOD can be removed after a five-day retention period.
There are two types of anaerobic suspended-growth processes used for treating wastewater: anaerobic digestion and anaerobic contact. Between the two, the anaerobic digestion process is the most effective method for stabilizing organic materials and biological solids. It is also one of the oldest processes used to stabilize sludge. During the process, the organic material contained in mixtures of primary settled and biological sludge in anaerobic conditions is biologically converted into methane (CH4) and carbon dioxide (CO2). Diluted organic wastes can also be treated anaerobically. The process is carried out in an airtight tank; sludge needs to be supplied continuously or intermittently and retained in the tank for varying periods of time, depending on the quality of the sludge and the surrounding geographical conditions, such as the ambient temperature. If digesters are used in an area where the ambient temperature is very low, such as in Canada and Northern Europe, half of the energy goes for heating and half for electrical energy (mostly for pumping but also for ventilation). However, if the wastewater treatment plant does not have a digester, heating is not required.
Facultative Ponds
Facultative ponds are a combination of aerobic, anaerobic, and facultative bacteria. Facultative processes are biological treatment processes in which the organisms are indifferent to the presence of dissolved oxygen (these organisms are known as facultative microorganisms). There are three zones in facultative ponds: (1) a surface zone where aerobic bacteria and algae exist; (2) an anaerobic bottom zone in which accumulated solids are actively decomposed by anaerobic bacteria; and (3) an intermediate zone, which is partly aerobic and partly anaerobic, in which the decomposition of organic wastes is carried out by facultative bacteria.
The facultative pond is usually the largest pond in the system, and, in the absence of pretreatment in anaerobic ponds, the wastewater flows first to this pond. On the upper layers of the pond, oxidation of organic matter takes place with the oxygen being provided by photosynthesizing algae. Sludge accumulates and digests anaerobically at the base of the pond so that sludge removal is required every 10–20 years. According to Mara (1976), the depth of the pond suggested is a compromise between the effect of excessive anaerobic activity in deeper ponds and the risk of vegetation in shallow ponds. The area is generally calculated based on the surface BOD loading rate, and this depends on the amount of sewage flow rate, sunlight, the BOD of the influent, and the ambient temperature.
Maturation Ponds
Maturation ponds are wholly aerobic and are responsible for the final stage of the BOD removal, reducing the fecal bacteria and viruses. Generally, two or more maturation ponds must follow a facultative pond. As a rule of thumb, three maturation ponds are used with a retention time of five days and depths of 1–1.5 m. The retention time decreases as the number of maturation ponds increases, and increasing the retention time will also provide a greater chance of microbiological purification. In a warm climate, maturation ponds can remove 95% of fecal coliforms with a retention time of five days. Maturation ponds can also provide the best environment for fish farming.
The biological processes involved in maturation ponds are similar to other aerobic suspended growth processes. Residential biological solids are endogenously respired, and ammonia is converted to nitrate using the oxygen supplied from the surface reaction and from algae. As with all biological nitrification systems, the efficiency of (low-rate) ponds decreases as the wastewater temperature increases. Normally, secondary treatment in maturation ponds will eliminate the need to disinfect effluents intended for agricultural reuse. However, to provide a reliably nitrified effluent that is low in BOD and suspended solids, an efficient and reliable effluent-treatment process is required.
Aerobic Stabilization Ponds
Aerobic stabilization ponds are large, shallow earthen basins that are used to treat wastewater by natural processes involving algae and bacteria. In aerobic ponds, the oxygen is supplied by natural surface aeration and by algae photosynthesis. The bacteria in the aerobic degradation of organic matter use the oxygen released by the algae through photosynthesis. The algae in turn, use the nutrients and CO2 released in this degradation. The main function of aerobic stabilization ponds is to further purify the effluent.
Aerated Lagoons/Oxidation Ditches
These kinds of ponds are also called “high-rate” stabilization ponds because the treatment approach is to speed up the conversion of organic wastes into algae by using a motorized aeration system. Aerated lagoons (ponds) evolved from facultative stabilization ponds when surface aerators were installed to overcome the odors from organically overloaded ponds. If a facultative pond is too small, or if toxic substances or lack of sunlight prevent the algae from adequately photosynthesizing, the BOD will exceed the oxygen supply and the pond will turn anaerobic. In that case, it may require extra oxygen to be supplied by mechanical means. Such a method is called mechanical aeration or an aerated lagoon. When motor-driven surface aerators provide the oxygen, the lagoon develops a flocculated suspension of bacterial cells. These bacterial cells convert from organic solids to form sludge, and this sludge must be removed before the effluent is discharged or reused. Therefore, maturation ponds generally follow aerated lagoons. Four days is a typical retention time and will remove 85% to 90% of the BOD. Bacterial reduction is poor, but this problem can be solved by a sufficient number of maturation ponds. Normally, the recommended depth of an aerated lagoon is between 3–4 m, with banked slopes of 1:2 (Cairncross and Feachem 1983). The banks and bottom must be protected from erosion caused by the turbulence of the aerators.
In general, oxidation ditches are very similar to aerated lagoons; the only difference is the layout and the fact that most of the sludge is recirculated. Wastes are circulated around a 1–2-m-deep oval channel at a velocity of about 0.3–0.4 m/s (Cairncross and Feachem 1983). The velocity and the aeration is provided by rotating cylindrical brushes pushing the effluent forward while at the same time providing intense turbulence. In such a method, effluent from the ditch is settled into a secondary sedimentation tank and more than 95% of the sludge from the tank is returned to the ditch. Such an approach produces a much richer concentration of bacterial flocks than would be produced in an aerated lagoon. This facilitates shorter retention times (1–3 days) and causes the sludge to be aerated for much longer periods (20–30 days) (Cairncross and Feachem 1983). Such a method helps produce a highly mineralized excess sludge that can be dried on sludge-drying beds without further digestion. BOD reduction using an oxidation ditch approach is usually good, but, like the aerated lagoons, bacterial removal is poor. However, as with aeration lagoons, maturation ponds are used for further purification.
Generally there are two major treatment methods used to treat sewage—stabilization ponds and advanced wastewater treatment methods. Advanced wastewater treatments require capital-intensive units, often aided mechanically with concrete channels, tanks, and other devices (including screens, grit chambers, settling tanks, thickeners, aeration tanks, digesters, and other unit processes). In this method, chemicals are used to remove pathogens.
Rural Sanitation
Rural sanitation is very important to eliminate waterborne diseases that are transmitted through the fecal-oral cycle. Effective rural waste sanitation breaks this cycle at the source and greatly reduces pathogen intake. However, rural sanitation by itself cannot solve the waterborne-related problems unless it is accompanied by hygiene education and a clean, safe water supply. However, handling rural domestic wastes is usually much easier than handling urban wastes. People in rural areas are quite dispersed, and they do not require complex sewage networks or drainage pipes. Solid wastes, such as ashes from cooking, dung, and other refuse are usually biodegradable and are used in agricultural fields. The rural sanitation problem is mainly related to handling excreta and other non-biodegradable refuses. However, in the rural areas of many developing countries, non-biodegradable wastes are very rare and any waste that does occur (such as dry cell batteries) can be handled by the villagers with a simple program of health education. Therefore, this section will discuss how to handle excreta in rural villages. In most rural areas, a simple pit latrine or some type of composite latrine is used to handle the excreta. There are number of modified designs available on the market for pit latrines and composite pits. In a pit latrine, a hole is dug, a timber or concrete slab is placed over the hole, and a shed and roof are placed around the slab for privacy. However, there is a problem with odor and flies, which a simple cover can control.
Municipal Wastewater Treatment
In many developing countries, municipal wastes are the main public health concerns because there is no single individual responsible for these wastes except the city administration.
Municipal waste management is always a big responsibility for city administrations, especially in developing countries, due to a lack of infrastructure, finances, and know-how. However, in developed countries, waste management is well organized and is mostly privatized or leased;
every individual household, commercial center, and institution pays for the service. In this
section, various treatment methods and their drawbacks will be discussed. However, before
discussing the wastewater treatment methods, it is important to understand the characteristics of the sewage (i.e., physical, chemical, and biological).
The characteristics of sewage indicate the quality of the wastewater. The physical characteristic is the level of suspended solids: the presence of various chemicals and microbiological pollutants. The biological characteristic is the amount of oxygen required to oxidize the various organic chemicals. The oxygen demand is expressed either as a chemical oxygen demand (COD) or a biochemical oxygen demand (BOD), or total organic carbon (TOC). The measure for BOD is expressed as BOD5 to relate to the measure of biodegradable organic matter contained in the sewage, and COD is approximately 1.5 times the BOD5.
The BOD is usually measured by keeping a sample of sewage at 20°C for five days and calculating the amount of oxygen used to oxidize the organics. The COD is measured by boiling the sewage with an acid dichromate solution, which converts most of the organics to carbon dioxide and water. The chemical characteristic of sewage is the presence of organic and inorganic constituents, nutrients, and toxic chemical contaminants. Sewage quality is normally expressed in terms of its BOD. The strength of the BOD reflects the type of sewer and the lifestyle of the people because the BOD comes from feces, urine, and sludge. For example, BOD values of 400–800 mg/l are common in cities and towns of developing countries; in such areas, raw sewage contains approximately 40 g of BOD per person per day. In this case, if the per capita water consumption of the community is 100 l/person/day, the sewage will contain 400 mg/l of BOD (i.e. (40 x 103)/100). Similarly, if the water consumption is lower, the BOD will be higher. However, if the sewage passes through a septic tank or some kind of settling tank (e.g., aqua-privy), approximately half of its BOD will be lost. Night soil (sewage not diluted with sludge) will clearly have a high BOD because it has no sludge (it contains only feces and urine). In such cases, the BOD of night soil may be as high as 30,000 mg/l (30 g of BOD/day and 1 l/day of liquid is contributed by each person). According to Mara (1977), the strength of the BOD is categorized as weak (up to 200 mg/l), medium (350 mg/l), strong (500 mg/l), and very strong (above 750 mg/l).
In wastewater treatment, contaminants are removed by physical, chemical, and biological means and the treatment methods are usually classified as physical, chemical, and biological processes (Metcalf and Eddy, Inc. 1979, and Steel and McGhee 1979). The physical wastewater treatment process applies physical forces. Typical physical processes are screening, mixing, flocculation, sedimentation, flotation, and filtration. Chemical treatment processes remove or convert the contaminants by adding chemicals or through chemical reactions. The most common examples used in chemical wastewater treatment are precipitation, gas transfer, adsorption, and disinfection. Chemical precipitation, for example, is accomplished by producing a chemical precipitate, which will settle at the end.
A biological treatment is used primarily to remove the biodegradable organic substances (colloidal or dissolved) in wastewater. Basically, these substances are converted into gases that can escape to the atmosphere or into biological cell tissues that can be removed by settling.
Biological treatment is also used to remove pathogens and nitrogen from wastewater. In most cases, wastewater can be treated biologically.
The four major groups of biological treatment processes are aerobic, anaerobic, anoxic (the process by which nitrate is converted biologically into nitrogen gas in the absence of oxygen), or a combination of the three. The principal applications for these processes are removing carbonaceous organic matter (measured in BOD, COD, or in TOC), nitrification, denitrification, or stabilization. The most common wastewater treatment method used in many regions with hot to moderate climate regions is a stabilization pond, which is discussed in the next section. Other emerging technologies will be discussed in later sections.
Stabilization Ponds
Stabilization ponds are a suitable treatment technology because they are also very effective at removing pathogens (WHO 1987). Stabilization ponds consist of a series of ponds into which the sewage flows. Treatment occurs through natural physical, chemical, or biological processes and no extra energy is required except the sun. Such treatment methods are the cheapest and simplest of all the treatment technologies and are capable of providing a very high-quality effluent. Ponds are very easy to maintain and require no routine operation. They can absorb both hydraulic and organic disturbances and can treat a wide variety of domestic and industrial wastes. The system can be flexible and can be expanded with little investment. Stabilization ponds can also be used to convert the emitted gases into useful energy. The biogas produced from the biological processes can be collected and used to produce energy (either electricity or heat or both). The biggest disadvantage of stabilization ponds is that they take up a lot of space. There are basically four types of wastewater stabilization ponds: anaerobic ponds, facultative ponds, maturation ponds, and a high-rated pond, which is also called an aerated lagoon or an oxidation ditch. All four types of ponds are discussed below. In practice, the first three types of ponds are basically joined in series and can have two or three stages. If one stage of treatment is used, the pond will normally be anaerobic or facultative. However, in general, a secondary pond for additional aerobic biological treatment should follow an anaerobic pond.
Anaerobic Ponds
Anaerobic ponds are basically open septic tanks used for pre-treating large volumes of strong wastes. Anaerobic digestion involves the decomposition of organic and inorganic matter in the absence of molecular oxygen. In anaerobic ponds, anaerobic digestion and settling will take place, and a thick scum usually develops on the surface. Retention times typically vary from 1–4 days, and the preferred pond depth is 2–4 m. Odor can be avoided by controlling the volumetric load of the BOD (not more than 400 g/m3/day) and the concentration of sulfate ion in the raw waste (not higher than 100 mg/l). According to Cairncross and Feachem (1983), at 20°C temperatures, 50% of the BOD can be removed after one-day retention, and 70% of BOD can be removed after a five-day retention period.
There are two types of anaerobic suspended-growth processes used for treating wastewater: anaerobic digestion and anaerobic contact. Between the two, the anaerobic digestion process is the most effective method for stabilizing organic materials and biological solids. It is also one of the oldest processes used to stabilize sludge. During the process, the organic material contained in mixtures of primary settled and biological sludge in anaerobic conditions is biologically converted into methane (CH4) and carbon dioxide (CO2). Diluted organic wastes can also be treated anaerobically. The process is carried out in an airtight tank; sludge needs to be supplied continuously or intermittently and retained in the tank for varying periods of time, depending on the quality of the sludge and the surrounding geographical conditions, such as the ambient temperature. If digesters are used in an area where the ambient temperature is very low, such as in Canada and Northern Europe, half of the energy goes for heating and half for electrical energy (mostly for pumping but also for ventilation). However, if the wastewater treatment plant does not have a digester, heating is not required.
Facultative Ponds
Facultative ponds are a combination of aerobic, anaerobic, and facultative bacteria. Facultative processes are biological treatment processes in which the organisms are indifferent to the presence of dissolved oxygen (these organisms are known as facultative microorganisms). There are three zones in facultative ponds: (1) a surface zone where aerobic bacteria and algae exist; (2) an anaerobic bottom zone in which accumulated solids are actively decomposed by anaerobic bacteria; and (3) an intermediate zone, which is partly aerobic and partly anaerobic, in which the decomposition of organic wastes is carried out by facultative bacteria.
The facultative pond is usually the largest pond in the system, and, in the absence of pretreatment in anaerobic ponds, the wastewater flows first to this pond. On the upper layers of the pond, oxidation of organic matter takes place with the oxygen being provided by photosynthesizing algae. Sludge accumulates and digests anaerobically at the base of the pond so that sludge removal is required every 10–20 years. According to Mara (1976), the depth of the pond suggested is a compromise between the effect of excessive anaerobic activity in deeper ponds and the risk of vegetation in shallow ponds. The area is generally calculated based on the surface BOD loading rate, and this depends on the amount of sewage flow rate, sunlight, the BOD of the influent, and the ambient temperature.
Maturation Ponds
Maturation ponds are wholly aerobic and are responsible for the final stage of the BOD removal, reducing the fecal bacteria and viruses. Generally, two or more maturation ponds must follow a facultative pond. As a rule of thumb, three maturation ponds are used with a retention time of five days and depths of 1–1.5 m. The retention time decreases as the number of maturation ponds increases, and increasing the retention time will also provide a greater chance of microbiological purification. In a warm climate, maturation ponds can remove 95% of fecal coliforms with a retention time of five days. Maturation ponds can also provide the best environment for fish farming.
The biological processes involved in maturation ponds are similar to other aerobic suspended growth processes. Residential biological solids are endogenously respired, and ammonia is converted to nitrate using the oxygen supplied from the surface reaction and from algae. As with all biological nitrification systems, the efficiency of (low-rate) ponds decreases as the wastewater temperature increases. Normally, secondary treatment in maturation ponds will eliminate the need to disinfect effluents intended for agricultural reuse. However, to provide a reliably nitrified effluent that is low in BOD and suspended solids, an efficient and reliable effluent-treatment process is required.
Aerobic Stabilization Ponds
Aerobic stabilization ponds are large, shallow earthen basins that are used to treat wastewater by natural processes involving algae and bacteria. In aerobic ponds, the oxygen is supplied by natural surface aeration and by algae photosynthesis. The bacteria in the aerobic degradation of organic matter use the oxygen released by the algae through photosynthesis. The algae in turn, use the nutrients and CO2 released in this degradation. The main function of aerobic stabilization ponds is to further purify the effluent.
Aerated Lagoons/Oxidation Ditches
These kinds of ponds are also called “high-rate” stabilization ponds because the treatment approach is to speed up the conversion of organic wastes into algae by using a motorized aeration system. Aerated lagoons (ponds) evolved from facultative stabilization ponds when surface aerators were installed to overcome the odors from organically overloaded ponds. If a facultative pond is too small, or if toxic substances or lack of sunlight prevent the algae from adequately photosynthesizing, the BOD will exceed the oxygen supply and the pond will turn anaerobic. In that case, it may require extra oxygen to be supplied by mechanical means. Such a method is called mechanical aeration or an aerated lagoon. When motor-driven surface aerators provide the oxygen, the lagoon develops a flocculated suspension of bacterial cells. These bacterial cells convert from organic solids to form sludge, and this sludge must be removed before the effluent is discharged or reused. Therefore, maturation ponds generally follow aerated lagoons. Four days is a typical retention time and will remove 85% to 90% of the BOD. Bacterial reduction is poor, but this problem can be solved by a sufficient number of maturation ponds. Normally, the recommended depth of an aerated lagoon is between 3–4 m, with banked slopes of 1:2 (Cairncross and Feachem 1983). The banks and bottom must be protected from erosion caused by the turbulence of the aerators.
In general, oxidation ditches are very similar to aerated lagoons; the only difference is the layout and the fact that most of the sludge is recirculated. Wastes are circulated around a 1–2-m-deep oval channel at a velocity of about 0.3–0.4 m/s (Cairncross and Feachem 1983). The velocity and the aeration is provided by rotating cylindrical brushes pushing the effluent forward while at the same time providing intense turbulence. In such a method, effluent from the ditch is settled into a secondary sedimentation tank and more than 95% of the sludge from the tank is returned to the ditch. Such an approach produces a much richer concentration of bacterial flocks than would be produced in an aerated lagoon. This facilitates shorter retention times (1–3 days) and causes the sludge to be aerated for much longer periods (20–30 days) (Cairncross and Feachem 1983). Such a method helps produce a highly mineralized excess sludge that can be dried on sludge-drying beds without further digestion. BOD reduction using an oxidation ditch approach is usually good, but, like the aerated lagoons, bacterial removal is poor. However, as with aeration lagoons, maturation ponds are used for further purification.
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