Wastewater Sources and Treatment

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.

Water Resources & Water Quality

Water is a fundamental part of life, and for years everyone took it for granted. Recently, governments and concerned individuals worried that water resources were finite and could be lost to contamination or sudden loss, or from the pressure of large-farm irrigation that would create physical and chemical stresses.
Water Resources
Apart from its day-to-day use for drinking, irrigation, and marine life, water is used for many applications. It is used as a solvent (water dissolves more substances in greater quantities than any other liquid), for heating spaces (except for liquid ammonia, water has the highest heat transfer capacity, and is better suited for heating buildings), and for its ability to conduct electricity through dissociation, when acid is added (e.g., in automobile batteries). Therefore, it is necessary to understand the existence of water. For example, the presence of underground water depends not only on the creation of the storage facilities (between rocks, clays, and permeable soils) but also on nature’s ability to keep them supplied. We all know that there can be an abundance of water in one area or scarcity in other. To understand why water is present, we need to know the reasons for the uneven distribution of precipitation over the earth’s surface and the processes involved in the movement of water from place to place. In principle, the total volume of water on this planet is finite and constant, but the uneven distribution of water on the earth’s surface is due to hydrological cycle and weather patterns.

In principle, it is solar energy that causes the uneven distribution of water on the earth’s surface.
The water itself serves as a thermal energy storage medium, which determines the three parameters of climate: air temperature, air pressure, and precipitation. When solar radiation strikes the earth’s surface, the earth is heated. At the Equator, there is a net heat gain, while at the poles; there is a net heat loss. Through the movement of the oceans and the atmosphere, the surplus heat moves toward the poles. The cool air is heated when it reaches the earth’s surface and rises back into the atmosphere, while the warm rising air that contains water through evaporation eventually cools and falls back to earth as rain. Water from the sea evaporates to form clouds, returns to the earth as precipitation, and via streams, rivers, and infiltration, returns to the sea. This process is called the hydrologic cycle. This cycle creates certain weather patterns so that one location is dry while another location is wet. Therefore, the sources of water vary from one locality to another. However, the availability of groundwater depends not only on the hydrologic cycle and weather pattern. It also depends upon the formation of aquifer systems. The formation of aquifers can be from weathering, erosion, glacial deposits, sedimentary rocks, alluvial aquifers, and/or igneous and metamorphic rock aquifers. There are two main water sources: surface water and groundwater. Surface water sources are rivers, streams, man-made ponds or reservoirs, lakes, and seas. Streams are generally seasonal; depending on the size and tributaries, river-water sources can be seasonal or year round. Seasonal water sources require man-made dams or reservoirs for water supply and irrigation purposes. However, water resources from year-round rivers or lakes do not require such storage.
Generally, surface waters require treatment for domestic water supply, and this will be discussed in the following chapters.
Groundwater resources are formed when the surface is over-saturated and the excess water filters down. The depth of the soil water zone varies from about 1 meter (m) to 9 m (3 feet (ft) to 30 ft).
Water is also lost by transpiration and evaporation. Soil undergoes wide variations in moisture content—from complete saturation to a total lack of moisture. Water is held in the soil by molecular or capillary attraction, acting against the force of gravity. Molecular attraction holds water in a thin film on the surface of each soil particle. Capillary attraction holds water in the smallest spaces between soil particles. Water begins to percolate downward under the force of gravity when the water-holding capacity of the capillary forces is exceeded. The region immediately below the soil water zone is called the intermediate zone. Most water in this zone will move downward, has no in-situ use, and cannot be recovered. There is a capillary fringe at the bottom of the intermediate zone where groundwater is drawn upward by capillary forces. Depending on the kind of aquifer, water may migrate upward more than 3 m (Driscoll 1986).
Well-sorted, fine sediments are most effective at holding water and are often completely saturated within the capillary fringe zone; coarse sediments are not as effective in holding water. The groundwater table lies at the very bottom of the capillary zone. Generally, subsurface water used for domestic purposes and irrigation is pumped from below the groundwater table. However, groundwater can also be springs or artesian wells, where water is forced from the aquifer by compaction caused by the weight of overlying sediments or a well that derives its water from a confined aquifer in which the water level is above the ground surface. In such cases, groundwater is capped at the surface (at the eye of the spring or artesian well).
Groundwater found in shallow wells can generally be extracted using hand pumps or with a simple pulley and bucket. Such wells can be dug by hand or bored using earth augers. There are three main types of earth augers: large-diameter bucket augers, solid-stem augers, and hollow stem augers. Large-diameter bucket augers are most commonly used to drill up to about 45 m (150-ft) deep and up to 1.2 m (48–inch [in]) diameter wells. Solid-stem augers can drill up to 35 m (120 ft) deep and up to 600 millimeters (mm) (24 in) in diameter. The most common depth for hollow-stem augers in stable formations are 35 m (120 ft) for a 150 mm (6 in) diameter hollow stem auger and about 12 m deep for a 300 mm (12-in) diameter hollow-stem auger (Driscoll 1986). Generally, deep wells are drilled using drilling machines. There are several types of drilling methods, depending on the geologic formation and the depth and diameter of the well.
Particular drilling methods become dominant in certain areas because they are most effective in penetrating the local aquifers and thus offer cost advantages. Some of the most common drilling machines are cable tool drills, direct- and reverse-circulation rotary drills, and air drills. Rotary drilling machines are mostly used to reach greater depths and to increase drilling speeds.
Normally drilling fluids (air, clean water, and mixtures of special-purpose materials) are essential for efficient rotary drilling. However, direct rotary drilling is very expensive because drill bits are costly and drilling rigs require a high level of maintenance. Reverse circulation drilling is generally most successful in soft sedimentary rocks and unconsolidated sand and gravel where the static water level is 3 m (10 ft) or more below ground level. This drilling method is the least expensive for drilling large-diameter holes in unconsolidated formations.
Springs are commonly found at the foot of mountains. Mountains are also sources of streams, and many streams flow into rivers. When streams and rivers flow over a flat area, the surrounding area will generally have good underground water because water soaks into the aquifer. Such areas are generally good for shallow wells. Although surface and rainwater infiltration are the main sources for enriching underground water sources, water also flows underground through fractured rocks and aquifers, depending on the hydrological formations of the ground. The best aquifers are coarse sand and gravel, limestone openings, sandstone, or fractured rocks, and aquifers, such as clay, silt, and solid metamorphic rock like marble have very minimal water penetration. An aquifer on the surface of the ground, having a reasonable depth and followed by a layer of impermeable materials (e.g., solid rock, silt, or clay), is considered to be a good catchments area for underground water. Therefore, a detailed geological survey should be made before drilling. More than 40% of the wells drilled in developing countries for domestic water supply are abandoned due to lack of sufficient water.

Raw Water Quality
The source of the water determines its characteristics. Generally, surface water is exposed to contamination due to human, animal, and industrial activities upstream. Surface water can be contaminated with both pathogenic and non-pathogenic organisms and suspended solid particles from precipitation or runoffs. On the other hand, groundwater is usually clear and odorless. Groundwater does not usually contain suspended solid particles or bacteria or organic matter, but does usually contain dissolved mineral ions (minerals are generally dissolved in water and the term total dissolved solids (TDS) refers to them). The type and concentration of these dissolved minerals can affect how the groundwater can be used. If certain minerals are present in excessive amounts, certain types of treatment may be necessary to change or remove the dissolved mineral before using the groundwater for its intended purpose. However, studies show that moderate TDS levels have some health benefits. Although groundwater may not have bacteria, there is a risk of contamination, especially for shallow wells, from human and animal activities in the area. Contaminants can seep into the ground from the top of the borehole. Therefore, the area surrounding the borehole should have proper drainage to keep it dry, and the borehole should be properly capped.
The water quality level varies, depending on the intended purposes. Water used for irrigation can be very low quality, as long as it is not salty, which might burn the soil and crops. On the other hand, drinking water should fulfill the water quality standard guidelines set by national governments and the World Health Organization (WHO). Sources of contaminants are characterized as physical, chemical, bacteriological, and radiological. The WHO has guidelines for five categories of contaminants for drinking water:
1. Microbiological and biological standards (microorganisms and other organisms)
2. Inorganic constituents that pose health risks (arsenic, cadmium, nitrate, lead, and sodium)
3. Organic constituents (benzene, phenols, dichlorodiphenyltrichloroethane (DDT), and others)
4. Aesthetic guidelines (odor, taste, hardness, and color)
5. Radioactivity guidelines (mostly for groundwater).

The needs of clean water

The availability of clean drinking water is a development issue faced by billions of people in the developing and near-developed world. Development organizations continually site the lack of access to clean water and sanitation as the leading cause of death amongst children in rural areas. The scale of this problem is immense, as are its solutions. The global nature of this issue opens the door for the application of communal solutions.

Like energy, the need for clean water is increasing rapidly as supplies of traditional resources continue to diminish due to overuse, waste, and pollution. Unlike energy, the ability to harness local resources to produce water is not possible. However, we do have the capability to use local energy resources to gain access to water supplies that would otherwise be unavailable and then ensure that this water is safe for human consumption. Most water is located underground in deep aquifers, in surface lakes, rivers, and streams or in the ocean. Technologies exist to make use of all of these water sources but in many cases, the absence of available, inexpensive energy makes their use expensive, time consuming, and potentially dangerous. The proper application of any number of energy options available today can make gaining access to this water a reality in many areas not previously considered.

Absolutely pure water is not found in nature. Water evaporates into the atmosphere, condenses, and when it falls back to the ground, the water contains dissolved gases including oxygen, carbon dioxide, bacteria, and dust absorbed from the air. Once the water hits the ground, it picks up many more organic and non-organic chemicals, microorganisms, and organisms as it make its way into streams and rivers. Some of the rainfall percolates into the soil, loosening suspended silt and bacteria. There is also the danger of contamination by radioactive isotopes in the groundwater. Contaminants from the ground’s surface include municipal, industrial, and agricultural wastes. These wastes wash into rivers and also infiltrate the groundwater. Depending on the intended use, all of these contaminants need some kind of treatment.
Generally, wastewater requires a certain level of treatment before it can come into contact with the surface or with groundwater. Similarly, domestic water should be clean and safe to drink.