Sedimentation is one of the earliest unit operations used in water or wastewater treatment (Reynolds, 1982). The sedimentation process, is removal of heavy settleable from turbid water sources to lessen the solids on treatment plant processes (Willis, 2005). The principals of sedimentation are the same for basins used in either water or wastewater treatment: the equipment and operational methods are also similar (Reynolds, 1982). The sedimentation process removes many particles including clay and silt based turbidity, natural organic matter, and other associated impurities. These impurities include microbial contaminants, toxic metals, synthetic organic chemicals, iron, manganese and humic substances. Humic substances come from soil are produced within natural water and sediments by chemical and biological processes such as the decay of vegetation. Removal of humic substances from drinking water is desirable since they form disinfection byproducts when chlorine is added to the water (URL 11).
Figure 4.13. Schematic view of sedimentation basin (URL 12)
After flocculation, the water and floc moves slowly through large basins known as sedimentation or settling basins (Figure 4.13). The water moves very slowly through these basins due to their large size. This allows the floc to settle to the bottom of the basin. The floc that falls to the bottom of the basins is collected into a hopper by large rotating scrapers where it is removed several times daily by the plant operators. Clear water above the floc layer (referred to as treatment residuals) flows out of the sedimentation basin and to the filters. Removal of particles in the sedimentation basin improves the operation of the filters that comprises the next treatment process after sedimentation.
Figure 4.13. Schematic view of sedimentation basin (URL 12)
Sedimentation in potable water treatment generally follows a step of chemical coagulation and flocculation, which allows grouping particles together into flocs of a bigger size. This increases the settling speed of suspended solids and allows settling colloids (URL 13).
Sedimentation is a solid-liquid separation utilizing gravitational settling to remove suspended solids. In water treatment its main applications are (Reynolds, 1982):
Most sedimentation basins used in treatment are the horizontal flow type in rectangular, square or circular design. Both long, rectangular basins and circular basins are commonly used; the choice is based on local conditions, economics, and personal preference. Basins were originally designed to store sludge for several months and were periodically taken out of service for manual cleaning by flushing. Most basins are recently designed to be cleaned with mechanical equipment on a continuous or frequent schedule (Willis, 2005). There are a variety of designs for sedimentation tanks available. These include (URL12):
The following factors influencing the sedimentation process: density and size of suspended particles, water temperature, turbulence, stability of flow, bottom scour and flocculation (URL 14):
Filtration, which is a unit operation of separating solids from fluids, is the only one of the three water purification methods that is capable of removing chlorine, chlorine byproducts, and VOCs from drinking water (Figure 4.14). Chlorine and VOCs are the most dangerous and threatening contaminants of municipally treated drinking water. Besides the removal of these dangerous chemicals, water filters also extract from drinking water the chlorine-resistant protozoa giardia and cryptosporidium. These protozoa have plagued the water treatment industry for several decades and have caused a number of epidemics of severe gastrointestinal disease, contracted through drinking contaminated water (URL 15; Sincero and Sincero, 2003e). The purpose of filtration is to remove the particulates suspended in water by passing the water through a layer of porous material. Larger particulates are retained by straining and sedimentation, while colloidal matter is retained by adsorption, or coagulation and sedimentation. Biological interactions occur only when the water passes very slowly through the porous mass (Chen et al., 2005).
Figure 4.14. Filtration mechanism used in practice (URL 16)
Once the floc has settled to the bottom of the water supply, the clear water on top will pass through filters of varying compositions (sand, gravel, and charcoal) and pore sizes, in order to remove dissolved particles, such as dust, parasites, bacteria, viruses, and chemicals (URL 17). There are three basic terms used to describe the method of applying the motive force used in filtration systems -vacuum filtration, pressure filtration, and gravity filtration (Chen et al., 2005). Filters may be classified as gravity, pressure, or vacuum filters. Gravity filters are filters that rely on the pull of gravity to create a pressure differential to force the water through the filter. On the other hand, pressure and vacuum filters are filters that rely on applying some mechanical means to create the pressure differential necessary to force the water through the filter. The filtration medium may be made of perforated plates, septum of woven materials, or of granular materials such as sand. Thus, according to the medium used, filters may also be classified as perforated plate, woven septum, or granular filters. The filtration medium of the microstrainer mentioned above is of perforated plate. The filter media used in plate-and-frame presses and vacuum filters are of woven materials (Sincero and Sincero, 2003e).
Filtration systems treat water by passing it through granular media, e.g., sand, which removes the contaminants. Their effectiveness varies greatly, but these systems may be used to improve turbidity and color concerns, as well as to treat Giardia and Cryptosporidium, bacteria, and viruses (URL 18). Conventional filtration first utilizes a pretreatment chemical coagulant, such as iron or aluminum salts, which is added to the source water. The mixture is then slowly stirred to induce tiny suspended particles to aggregate to form larger and more easily removable clots, or “flocs.” (URL 18). These systems next employ a sedimentation step. In this process particles in the water, including the floc created by flocculation, are allowed to settle out of the water naturally by gravity’s pull. These contaminants gather on the bottom of the system as a “sludge” that is periodically removed (URL 18). Once these processes are complete, water is passed through filters so that any remaining particles will physically attach themselves to filter material. The suspended particles are destabilized by the coagulant and thus attach more readily to the filter material (URL 18). Conventional filtration, like other filtration systems, results in significant improvement of a wide variety of source waters. It is best employed on sources with constant flow and low levels of algae -which can clog filter systems (URL 18). Coagulation chemicals require expert handling to achieve the desired results, so trained personnel are necessary to manage filtration treatment facilities (URL 18).
Filtration systems treat water by passing it through granular media, e.g., sand, that remove the contaminants. Filtration effectiveness varies greatly, but these systems may be used to improve turbidity and color concerns, as well as to treat Giardia and Cryptosporidium, bacteria, and viruses (URL 18). Direct filtration first utilizes a chemical coagulant, such as iron or aluminum salts, which is added to the source water. The mixture is then slowly stirred to induce tiny suspended particles to aggregate to form larger and more easily removable clots, or “flocs” (URL 18). Once these processes are complete, source water is passed through filters so that any remaining particles attach themselves to the filter material. The suspended particles are destabilized by the coagulant and thus attach more readily to the filter (URL 18) Conventional filtration processes use sedimentation to allow particulates to settle out of water for removal. Direct filtration eliminates this step and allows the filter material itself to do the work of straining contaminants (URL 18). Direct filtration is a relatively simple filtration process, and it is economically attractive. The system results in significant improvement of source water quality -but it is best employed on relatively high quality source waters, with constant flows and low turbidity. High algae levels, in particular, may clog filtration systems (URL 18).
Diatomaceous earth filtration is used to physically remove particulates, which are simply strained from source water. The process is effective at removing Giardia, Cryptosporidium, algae, and, depending on the grade, some bacteria and viruses (URL 18). This system’s filter consists of a cake of diatomaceous earth, a floury, chalky substance made of the crushed, fossilized remains of one-celled marine life forms called diatoms (URL 18). Water is passed through a diatomaceous earth filter system by pumps that either force pressurized water through the cake from the source inlet, or use vacuum suction to pull it through from the outlet side (URL 18). Unlike many other forms of filtration, coagulation chemicals are usually not used to enhance the agglomeration of contaminant particles. Because of this limitation, diatomaceous earth filtration is best-suited to higher-quality source water that is devoid of inorganic contaminants (URL 18).
Slow sand filtration can effectively remove the microorganisms that cause waterborne disease -including protozoa like Giardia and Cryptosporidium, as well as bacteria and viruses- a capability that was first demonstrated by plunging disease rates in the European cities that pioneered the treatment (URL 18). Water treated by these systems is allowed to slowly pass through a bed of sand some 2 to 4 feet (0.6 to 1.2 meters) deep. En route, a combination of physical and biological processes filters the water and removes contaminants (URL 18). After repeated use, the sand bed becomes host to a multitude of bacteria, algae, protozoan, rotifers, copepods, and aquatic worms. These microorganisms assist the filtration process by removing contaminants, though they may be slowed by water temperatures below 10 °C. Sand that hosts these organisms is said to be “ripened,” and is preferable to clean or new sand. It may take several weeks or months to ripen sand, depending on water contents and temperature. The process eventually clogs the sand bed and slows flow rates to the point that it must be unclogged, typically by reversing the flow, or “backwashing”. (URL 18). Slow sand filtration systems may not be able to accommodate chlorinated water because chlorine can have a detrimental impact on the filter’s microbiological community. Therefore, water to be disinfected with chlorine may be treated in storage facilities after passing through the filtration process (URL 18). Storage also helps to add flexibility to a system’s water output. Slow-acting sand filter systems cannot handle increased water volumes in times of peak demand, nor should they be run at less than optimal flows during periods of lower demand (URL 18). Slow sand systems work well only on source water that is low in turbidity and algae levels, and without color contamination. These systems struggle particularly with high algae or clay content -which can clog sand beds. Nutrient-rich source water, on the other hand, may aid the cleansing action of slow sand filters by boosting their biological component (URL 18). Slow sand systems generally are simple, require little maintenance, and have low operating costs (URL 18).
Filtration systems treat water by passing it through porous materials that remove and retain contaminants (URL 18). Bag and cartridge filters are simple and easy-to-operate systems that use a woven bag or a cartridge with a wound filament filter to physically strain microbes and sediment from source water as it is passed through the filter medium (URL 18). These systems are effective against Giardia cysts, but not are sufficient to eliminate bacteria, viruses, or chemicals. Thus, they are most appropriate for higher quality source waters and those with limited turbidity (URL 18). Bag and cartridge technology is developing rapidly and is tailored for use in small scale treatment facilities. Such systems also deliver ease of operation and maintenance, with little skill required on the part of the operator. Costs are variable depending on how often the filters must be changed (URL 18). Like many other filters, cartridges quickly become fouled by water that is high in particulates -so low turbidity water is preferred. Alternatively, “roughing filters” that use sand, mesh screens, cartridges, and other substances to physically remove larger particulates may pretreat water (URL 18). Filter materials must be changed periodically, more often when source water is high in particulates (URL 18). With repeated use of bag and cartridge systems, microbes may grow on filters, though this problem can be tempered by the use of a disinfectant. Disinfectants may also be required if water testing reveals that source water virus removal is necessary (URL 18).
Ceramic filters are typically shaped like a flowerpot or a bowl and are impregnated with tiny, colloidal silver particles as a disinfectant and to prevent bacterial growth in the filter. Laboratory testing has shown that, if designed and produced correctly, these devices can remove or inactivate almost all bacteria and protozoan parasites. Its effectiveness against viruses is unknown (URL 18). Cleaning and maintenance of the filter is critical; so like other low-cost point of use systems, it is best combined with an educational program about safe storage, filter cleaning, and other recommended practices (URL 18). The advantages of ceramic filters are their ease of use, long life (if not broken), and fairly low cost. Disadvantages include possible recontamination of stored water since there is no chlorine residual and a relatively low flow rate-typically 1 to 2 L/h (URL 18).
Slow sand systems have recently been adapted for point-of-use systems, especially in developing countries. In this context they are generally known as “biosand” filters (URL 18). Most commonly, a biosand filter takes the form of a container a little less than a meter tall and perhaps 30 cm in width and depth, filled with sand. The biologically active layer, which takes a week or two to fully develop, is maintained by keeping the water level above the top of the sand. As with slow sand filters, this bioactive layer helps to filter, adsorb, destroy, or inactivate pathogens. A porous plate is usually located above the sand to prevent disturbance to the bioactive layer when water is added. Users simply pour water into the top of the apparatus, and collect treated water from the outlet (URL 18). In laboratory and field testing, biosand filters have removed nearly all protozoa, and most bacteria. Their performance with viruses is not well established (URL 18). The apparatus can be built using concrete -a commonly available and relatively inexpensive material. Maintenance is fairly simple, usually consisting of agitating the upper surface of the sand once a month or so and manually collecting the suspended material. The cost of upkeep is quite low, since there are few or no parts to replace (URL 18).