Aeration is an optional part of the water treatment process using scrubbing action and oxidation to remove or modify constituents of the water. Aerators work by increasing the amount of surface area of air coming in contact with water. This may be achieved by passing air through water, as in an air diffusion aerator. In contrast, many aerators pass water through air, as in spray nozzle, cone tray, cascade, and coke tray aerators. Finally, forced draft aerators both pass air through water and water through air (URL 1). Aeration is a unit process in which air and water are brought into intimate contact. Turbulence increases the aeration of flowing streams The contact time and the ratio of air to water must be sufficient for effective removal of the unwanted gas. Aeration is also an effective method of bacteria control. Aeration as a water treatment practice is used for the following operations:
Aeration removes or modifies the constituents of water using two methods -scrubbing action and oxidation (Figure 4.1). Scrubbing action is caused by turbulence that results when the water and air mix together. The scrubbing action physically removes gases from solution in the water, allowing them to escape into the surrounding air. In the picture above, carbon dioxide and hydrogen sulfide are shown being removed by scrubbing action. Scrubbing action will remove tastes and odors from water if the problem is caused by relatively volatile gases and organic compounds (URL 3).
Figure 4.1. Aeration steps (URL 3)
Oxidation is the other process through which aeration purifies water. Oxidation is the addition of oxygen, the removal of hydrogen, or the removal of electrons from an element or compound. When air is mixed with water, some impurities in the water, such as iron and manganese, become oxidized. Once oxidized, these chemicals fall out of solution and become suspended in the water. The suspended material can then be removed later in the treatment process through filtration (URL 3).
There are several different methods used to aerate water, but all either involve passing water through air or air through water. Water can be exposed to air by spraying or by distributing it in such a way that small particles or thin sheets of water come in contact with the air. Water can also aerated by pumping large volumes of air through the water (URL3). The method of aeration to be used depends on which materials on the water are to be removed. The chemical characteristics of the water to be treated also influence which treatment method is used. Finally, each method has a different efficiency. In general, pumping water through air is much more energy efficient than pumping air through water. Different types of aeration and other methods of treatment should all be compared to determine the most efficient and practical method of treatment in each case (URL 3). Methods of aeration can be classified into four general categories (Figure 4.2, 4.3) (Dyksen, 2005; URL 4). (1) Waterfall aeration uses the desorption principle and accomplishes gas transfer by causing water to break into drops of thin films, increasing contact surface area between the air and water. There are five common types of waterfall aerators: (a) Spray Aerators have been used in water treatment for many years for iron oxidation and stripping VOCs and dissolved gases. Spray aerator installation generally consists of fixed nozzles or a pipe grid located over an open-top tank. The nozzles spray fine water droplets into the surrounding air, creating the air-water interface necessary for contaminant transfer. The configuration of pipe grid on top of open-top tank is preferable for stripping applications. Spray aerators require a large installation area and pose operating problems during freezing weather; furthermore, for effective application they require supplemental air exchange. (b) Cascade aerators consist of a series of steps that allows water to fall in thin layers from one level to another, where aeration is accomplished in the splash zones. The exposure time of air to water is increased by increasing number of steps, and the area-volume ratio is improved by adding baffles to produce turbulence. The major operating problems include corrosion and slime and algae buildup. (c) A multi-tray or slat tray aerator consists of series of trays equipped with slats or perforated or wire-meshed bottoms. Water is distributed over the trays and is allowed to fall from each tray onto the collection basin at the base. Air is forced or induced to flow perpendicular to water path. Often coke, stone, or ceramic balls of size 2 to 6 in. are used to improve efficiency of gas exchange. There are typically 3-9 trays spaced 12-30 inches apart in multi-tray aerators. Multi-tray aerators are analogous to cooling towers and require adequate ventilation. They provide excellent oxygen absorption and carbon dioxide, ammonia, and hydrogen sulfide removal. (d) Cone aerators are primarily used to oxidize manganese and iron as a pretreatment measure. The design of the aerator is similar to cascade aerators, with stacked pans arranged such that water fills top pan and cascades down to each succeeding pan. (e) Packed tower or air strippers are capable of removing from water fuel, solvents and volatile SOCs, VOCs, ammonia, hydrogen sulfide, and carbon dioxide. A packed column consists of a cylindrical tower, a packing material (usually plastic, steel or ceramic) and a centrifugal blower. The contaminated feed water is pumped into the top of the tower and blowers are used to introduce air countercurrent through the bottom of the tower. The large surface area provided by the packing material allows more liquid-gas transfer compared with other air stripping methods. The number of stages is determined by Henry’s Law constant and the tower loading rate. A most common problem with packed tower aerators is fouling of packing material with solids, resulting in loss of plant capacity, efficiency, and increased pressure drop. (2) Bubble aerators consist of a rectangular concrete tank in which perforated pipes, porous diffuser tubes or plates are installed. Compressed air is forced through these pipes to produce air bubbles that rise through water, producing turbulence resulting in effective water-air mixing. Bubble aerators are effective in removing carbon dioxide, VOCs, gasoline components, hydrogen sulfide, methane, and radon from contaminated groundwater. They are mostly used by small water systems with low flow rates. (3) Mechanical aerators employ motor driven impellers alone or in combination with air injection devices. They are also installed at water reservoirs to control taste and odor. The mechanical aerators can be installed as surface aerators (on the float) or submerged (below the water level) aerators. They are normally used for increasing dissolved oxygen levels, but they can also remove certain contaminants. (4) Pressure aerators are typically used for oxidizing iron and manganese. There are two basic types of pressure aerators in which either compressed air is injected directly in pressurized pipeline or water is sprayed into the top of a closed tank while the tank is continuously supplied with compressed air.
Figure 4.2. Nozzles for sprays and units for aeration or stripping: (a-d) nozzle types;(e) inclined apron that may be studded with riffle plates; (f) perforated plates; (g) spray tower; and (h) cascade (Sincero and Sincero, 2003a)
Figure 4.3. Aeration units: (a) turbine aerator with an air sparger; (b) porous ceramic diffuser; and (c) surface aerator (Sincero and Sincero, 2003a)
The effectiveness of aeration depends upon the aeration method selected, Henry's law constant of contaminant, design factors such as air to water ratio, flow and loading rate, available area of mass transfer, temperature, pH and algae production. As Henry's law constant increases, design factors such as air to water ratio becomes less critical for effective removal (Dyksen, 2005; URL 4).
Aeration typically raises the dissolved oxygen content of the raw water. In most cases, this is beneficial since a greater concentration of dissolved oxygen in the water can remove a flat taste. However, too much oxygen in the water can cause a variety of problems resulting from the water becoming supersaturated. Supersaturated water can cause corrosion (the gradual decomposition of metal surfaces) and sedimentation problems. In addition, air binding occurs when excess oxygen comes out of solution in the filter, resulting in air bubbles, which harm both the filtration and backwash process (URL 3). Aeration can also cause other problems unrelated to the supersaturated water. Aeration can be a very energy-intensive treatment method, which can result in overuse of energy. In addition, aeration of water can promote algal growth in the water and can clog filters (URL 3).
Adsorption is a phase transfer process that is widely used in practice to remove substances from fluid phases (gases or liquids). It can also be observed as natural process in different environmental compartments. The most general definition describes adsorption as an enrichment of chemical species from a fluid phase on the surface of a liquid or a solid. In water treatment, adsorption has been proved as an efficient removal process for a multiplicity of solutes. Here, molecules or ions are removed from the aqueous solution by adsorption onto solid surfaces. In adsorption theory, the basic terms shown in Figure 4.4 are used. The solid material that provides the surface for adsorption is referred to as adsorbent; the species that will be adsorbed are named adsorbate. By changing the properties of the liquid phase (e.g. concentration, temperature, pH) adsorbed species can be released from the surface and transferred back into the liquid phase. This reverse process is referred to as desorption (Worch, 2012).
Figure 4.4. Basic terms of adsorption (Worch, 2012).
Adsorption processes are widely used in water treatment. Table 4.1 gives an overview of typical application fields and treatment objectives. Depending on the adsorbent type applied, organic substances as well as inorganic ions can be removed from the aqueous phase (Worch, 2012). Activated carbon is the most important engineered adsorbent applied in water treatment. It is widely used to remove organic substances from different types of water such as drinking water, wastewater, groundwater, landfill leachate, swimming-pool water, and aquarium water. Other adsorbents are less often applied. Their application is restricted to special adsorbates or types of water (Worch, 2012).
Table 4.1. Adsorption processes in water treatment (Worch, 2012).
Application field | Objective | Adsorbent |
---|---|---|
Drinking water treatment |
Removal of dissolved organic matter Removal of organic micropollutants Removal of arsenic |
Activated carbon Activated carbon Aluminum oxide, Iron hydroxide |
Urban wastewater treatment |
Removal of phosphate Removal of micropollutants |
Aluminum oxide, Iron hydroxide Activated carbon |
Industrial wastewater treatment | Removal or recycling of specific chemicals |
Activated carbon, polymeric adsorbents |
Swimming-pool water treatment |
Removal of organic substances |
Activated carbon |
Groundwater remediation |
Removal of organic substances |
Activated carbon |
Treatment of landfill leachate |
Removal of organic substances |
Activated carbon |
Aquarium water treatment | Removal of organic substances | Activated carbon |
Activated carbon is an adsorbent that is widely used in water treatment, advanced water treatment, and the treatment of certain organic industrial wastewaters because it adsorbs a wide variety of organic compounds and also economically feasible. It is generally used in granular form either in batch, column (both fixed bed and countercurrent bed), or fluidized-bed operations, with fixed bed columns being the most common. Occasionally, activated carbon is used powdered form and is not recovered for regeneration: however, such application is usually limited to water treatment where amounts of carbon used are not appreciable. Adsorbents other than activated carbon are used to lesser extent in environmental engineering (Reynolds, 1982).
For nearly 100 years, adsorption processes with activated carbon as adsorbent have been used in drinking water treatment to remove organic solutes. At the beginning, taste and odor compounds were the main target solutes, whereas later the application of activated carbon was proved to be efficient for removal of a wide range of further organic micropollutants, such as phenols, chlorinated hydrocarbons, pesticides, pharmaceuticals, personal care products, corrosion inhibitors, and so on. Since natural organic matter (NOM, measured as dissolved organic carbon, DOC) is present in all raw waters and often not totally removed by upstream processes, it is always adsorbed together with the organic micropollutants. Since activated carbon is not very selective in view of the adsorption of organic substances, the competitive NOM adsorption and the resulting capacity loss for micropollutants cannot be avoided. The competition effect is often relatively strong not least due to the different concentration levels of DOC and micropollutants. The typical DOC concentrations in raw waters are in the lower mg/L range, whereas the concentrations of organic micropollutants are in the ng/L or ?g/L range. On the other hand, the NOM removal also has a positive aspect. NOM is known as a precursor for the formation of disinfection by-products (DBPs) during the final disinfection with chlorine or chlorine dioxide. Therefore, removal of NOM during the adsorption process helps to reduce the formation of DBPs. Activated carbon is applied as powdered activated carbon (PAC) in slurry reactors or as granular activated carbon (GAC) in fixed-bed adsorbers. The particle sizes of powdered activated carbons are in the medium ?m range, whereas the GAC particles have diameters in the lower mm range. In recent years, the problem of arsenic in drinking water has increasingly attracted public and scientific interest. As a consequence, a number of water works, in particular in areas with high geogenic arsenic concentrations in groundwater and surface water have to upgrade their technologies by introducing an additional arsenic removal process. Adsorption processes with oxidic adsorbents such as ferric hydroxide or aluminum oxide have been proved to remove arsenate very efficiently. The same adsorbents are also expected to remove anionic uranium and selenium species (Worch, 2012).
A wide variety of membrane processes can be categorized according to driving force, membrane type and configuration and removal capabilities and mechanisms. Membrane processes in the drinking water are used for desalination, softening and color, particle, microbial and natural organic materials removal that foul water’s taste and taint its clarity and other purposes (Bergman, 2005; URL 5). Water treatment membranes are thin sheets of material that are able to separate contaminants based on properties such as size or charge. Water passes through a membrane; but depending on their size, larger particles, microorganisms, and other contaminants are separated out (URL 5). Some of these systems are pressure driven, depending on water pressure to separate the particles based on size. Microfiltration (MF) employs the largest pore size, and can remove sand, silt, clay, algae, bacteria, Giardia, and Cryptosporidium. Ultrafiltration (UF) can also remove viruses. Nanofiltration (NF) systems provide nearly complete protection against viruses, remove most organic contaminants, and can reduce hardness in water. Electrodialysis (ED) and Reverse osmosis (RO) systems are dense membranes that remove almost all inorganic contaminants and all but the smallest organic molecules (URL 5). The lower limits of ED-RO, NF, UF, and MF particle rejection in potable water are 0.0001 mm, 0.001 mm, 0.01 mm, and 0.1 µm, respectively. However, the size range for each process is broad (Figure 4.5) (Taylor and Hong, 2000).
Figure 4.5. Removal capability of membrane systems (Flynn, 2009)
Electrodialysis combines membrane technology with the application of electrical current, to separate contaminants based on charge. Unlike other membrane processes, the source water never passes through the membranes during electrodialysis. It is not used as much in large water treatment facilities as some of the other technologies described here. Rather, it is mostly used for medical and laboratory applications that need ultrapure water (URL 5). Membranes, which especially reverse osmosis and nanofiltration, can be a good option for small-scale water treatment systems faced with a wide variety of contaminants. However, they often produce larger quantities of wastewater (or “concentrate”) than most other treatment systems -as much as 15 percent of the total treated water volume- and can become clogged with clay or organic materials if particle-rich source water is not filtered first (URL 5). Pressure-driven membrane processes can be designed for Cross-Flow or Dead-End operating mode (Figure 4.6).
Figure 4.6. Membrane flow configurations (Walsh and Gagnon, 2006)
The advancement of membrane technology over the past ten years has resulted in an economically viable drinking water treatment solution for both large- and small-scale applications. From a fundamental perspective, membrane technology is based on the principle that these systems act as a physical, size- exclusion barrier to contaminants present in raw water feedstreams. Low-pressure membranes, microfiltration (MF) and ultrafiltration (UF), effectively remove suspended or colloidal particles via a sieving mechanism based on the size of the membrane pores relative to that of the particulate matter (USEPA, 2003). Dissolved substances that are smaller in dimension than the pores in a MF or UF membrane surface will pass through the surface of these membranes. As presented in Figure 4.7, UF membrane filtration with a rated pore size of 0.01 to 0.1 ?m can effectively remove particulate matter and microorganisms via a size exclusion mechanism. However, based on the principle of pore size exclusion, dissolved material present in the feed water or wastewater streams (i.e., viruses, DOC and soluble inorganics) may not be effectively removed (Walsh and Gagnon, 2006).
Figure 4.7. Conceptual drawing of UF membrane pore sieving mechanism (Walsh, 2005)
Ion exchange processes are widely used in water and wastewater treatment to remove objectionable ionic contaminants. The first ion exchange processes used inorganic zeolites mined from natural deposits as a cation exchanger for water softening. Modern synthetic polymer-based exchange media are today used as cation and anion exchangers. Ion exchange is also used in many specific applications, for example, to reduce or remove potentially harmful ionic contaminants from potable water supplies in chemical processes and for product purification and recovery for specialty separations, such as chromatographic separations by size, valance charge, and as catalysts (Gottlieb, 2005). Ion exchange removes unwanted ions from a raw water by transferring them to a solid material, called an ion exchanger, which accepts them while giving back an equivalent number of a desirable species stored on the ion exchanger skeleton (Kemmer, 1988). Certain ions in the solution are preferentially sorbed by the ion exchanger solid, and because electroneutrality must be maintained, the exchanger solid releases replacement ions back into the solution. The reactions are stoichiometric and reversible and obey the law of mass action (Reynolds, 1982). Ion exchange is the exchange of ions from one phase to another. In water treatment, the exchange of ions occurs between the solid phase of the ion exchanger and influent water. In a water softener, a cation exchange resin operating in the sodium form exchanges sodium ions for an equivalent amount of calcium and magnesium ions are removed from the raw water by the resin, and an equivalent number of sodium ions are added to the water from the resin (Gottlieb, 2005). The ion exchangers used in water conditioning are skeleton like structures having many ion exchange sites, as shown in Figure 4.8. The insoluble plastic skeleton is an enormously large ion that is electrically charged to hold ions of opposite charge. The ion exchanger has a limited capacity for storage of ions on its skeleton, called its exchange capacity; because of this, the ion exchanger eventually becomes depleted of its desirable ions and saturated with unwanted ions (Kemmer, 1988). The softener is similar in design to a pressure filter, with resins in place of the filter media (Figure 4.8).
Figure 4.8. Left image: Model of a cation exchanger (Kemmer, 1998),Right image: Ion exchange softener (URL 6).
Two types of ion exchange materials are used: the cation exchange material and the anion exchange material. The cation exchange material exchanges cations, while the anion exchange material exchanges anions. The insoluble part of the exchange material is called the host (Sincero and Sincero, 2003b).
Units for expressing resin capacity are equivalents per liter (eq/L), milliequivalents per millilitre (meq/mL), kilograms per cubic foot (kg/ft3), and grams per liter (g/L). Generally, chemists use the first two units; the last two are practical units used by designers and system owners (Flynn, 2009).
It is important to keep in mind that the ion exchange process only works with ions. Substances that do not ionize in water are not removed by ion exchange. Each type of ion exchange resin exhibits an order of preference for various ions. This can be stated quantitatively through selectivity coefficients. Each ion pair has a unique selectivity value for each ion exchange resin. The higher the selectivity coefficient, the higher the relative affinity of the ion for the resin. The higher the affinity, the easier it is to load the ion, and conversely the more difficult it is to remove during regeneration (Gottlieb, 2005). An ion exchanger tends to prefer: 1) ions of higher valence, 2) ions with a small solvated volume, 3) ions with greater ability to polarize, 4) ions that react strongly with the ion exchange sites of the exchanger solid, 5) ions that participate least with other ions to form complexes. For the usual cation exchangers, the preference series for the most common cations is as follows (Reynolds, 1982):
Ba+2>Pb+2>Sr+2>Ni+2>Cd+2>Cu+2>Co+2>Zn+2>Mg+2>Ag+1>Cs+1>K+1>NH4+1>Na+1>H+1
For the usual anion exchangers, the preference series for the most common anions is as follows (Reynolds, 1982):
SO4-2>I-1>NO3-1>CrO4-2>Br-1>Cl-1>OH-1
The operating performance, capacity, and leakage data for ion exchange resins for the common ions found in water are usually provided by resin supplier (Gottlieb, 2005).
higher the affinity, the easier it is to load the ion, and conversely the more difficult it is to remove during regeneration (Gottlieb, 2005).
Ion exchange resins have a finite capacity. When this capacity is used up, the resins are exhausted and leakage of the unwanted ions increases. The exhausted resin can be regenerated with a salt, acid, or base solution containing the ion whose “form” the resin will be operated in. This is passed through the resin bed in sufficient quantity and at a sufficiently high concentration to reverse the exchange, desorb, and replace the previously exchanged ions from the resin with the ions from the regenerant solution. The most common regenerant used in potable water applications of ion exchange is sodium chloride. It is used softening, dealkalization, barium, radium, uranium, selenium, arsenic, and nitrate removal (Gottlieb, 2005).
The first commercially used ion exchange materials were naturally occurring porous sands that were commonly called zeolites. Zeolites were the first ion exchangers used to soften waters; however, they have been almost completely replaced in recent years by synthetic organic exchange resins which have a much higher ion exchange capacity. Synthetic cation exchange resins are polymeric materials that have reactive groups, such as sulfonic, phenolic, and carboxylic, that are ionisable, and may be charged with exchangeable cations. Also, synthetic anion exchange resins are available that have ionisable groups, such as the quarternary ammonium or amine groups, which may be charged with exchangeable anions (Reynolds, 1982). Figure 4.9 shows the schematics of the unit operations of ion exchange. Figure 4.9.a shows a cation exchanger and Figure 4.9.b shows an anion exchanger. In both units, the influent is introduced at the top of the vessel. The bed of ion exchanger materials would be inside the vessels, where, as the water to be treated passes through, exchange of ions takes place. This exchange of ions is the chemical reaction of the unit process of ion exchange; the mere physical passing through of the water with the attendant head loss and pumping consideration is the unit operation of ion exchange (Sincero and Sincero, 2003b).
Figure 4.9. Unit operations of ion exchange (Sincero and Sincero, 2003b)
Coagulation and flocculation may be broadly described as chemical and physical processes that mix coagulating chemicals and flocculation aids with water. The overall purpose is to form particles large enough to be removed by the subsequent settling or filtration processes. Particles in source water that can be removed by coagulation, flocculation, sedimentation, and filtration include colloids, suspended material, bacteria, and other organisms. The size of these particles may vary by several orders of magnitude. Some dissolved material can also be removed through the formation of particles in the coagulation and flocculation processes (Delphos and Wesner, 2005). The processes of coagulation and flocculation are employed to separate suspended solids from water whenever their natural subsidence rates are too slow to provide effective clarification (Figure 4.10). Water clarification, lime softening, sludge thickening, and dewatering depend on correct application of the theories ofcoagulation and flocculation for their success (Flynn, 2009).
Figure 4.10. Physical-chemical process involved in coagulation-flocculation (URL 7)
Coagulation is the destabilization of colloids by neutralizing the forces that keep them apart. This is generally accomplished by adding chemical coagulants and applying mixing energy. Aluminum salts, iron salts, or polyelectrolytes are the chemicals usually used. Initially small flocs join, creating larger, settleable agglomerates. The destabilization step is coagulation (charge neutralization); the floc-building stage is flocculation (Flynn, 2009). The water treatment literature sometimes makes a distinction between the terms “coagulant” and “flocculant.” When this distinction is made, a coagulant is a chemical used to initially destabilize the suspension and is typically added in the rapid-mix process. In most cases, a flocculant is used after the addition of a coagulant; its purpose is to enhance floc formation and to increase the strength of the floc structure. It is sometimes called a “coagulant aid.” Flocculants are often used to increase filter performance (they may be called “filter aids” in this context) and to increase the efficiency of a sludge dewatering process.(Letterman et al., 1999).
All waters, especially surface waters, contain both dissolved and suspended particles. Coagulation and flocculation processes are used to separate the suspended solids portion from the water. The suspended particles vary considerably in source, composition charge, particle size, shape, and density. Correct application of coagulation and flocculation processes and selection of the coagulants depend upon understanding the interaction between these factors. The small particles are stabilized (kept in suspension) by the action of physical forces on the particles themselves. One of the forces playing a dominant role in stabilization results from the surface charge present on the particles. Most solids suspended in water possess a negative charge and, since they have the same type of surface charge, repel each other when they come close together. Therefore, they will remain in suspension rather than clump together and settle out of the water (URL 8).
Coagulation and flocculation occur in successive steps intended to overcome the forces stabilizing the suspended particles, allowing particle collision and growth of floc. If step one is incomplete, the following step will be unsuccessful (URL 8) (Figure 4.11)
Figure 4.11. Mechanism of coagulation (a) and flocculation (b) (URL7)
Coagulation is a complex process, involving many reactions and mass transfer steps. As practiced in water treatment the process is essentially three separate and sequential steps: coagulant formation, particle destabilization, and interparticle collisions. Coagulant formation, particle destabilization, and coagulant-NOM interaction typically occur during and immediately after chemical dispersal in rapid mixing; interparticle collisions that cause aggregate (floc) formation begin during rapid mixing but usually occur predominantly in the flocculation process. For example, using the aluminum sulfate salt known as alum [Al2(SO4)3.14H2O] in coagulation involves formation of an assortment of chemical species, called aluminum hydrolysis products, that cause coagulation. These species are formed during and after the time the alum is mixed with the water to be treated. Coagulants are sometimes formed (or partially formed) prior to their addition to the rapid-mixing units (Letterman et al., 1999).
Following the first step of coagulation, a second process called flocculation occurs. Flocculation, a gentle mixing stage, increases the particle size from submicroscopic microfloc to visible suspended particles. The microflocs are brought into contact with each other through the process of slow mixing. Collisions of the microfloc particles cause them to bond to produce larger, visible flocs called pinflocs. The floc size continues to build through additional collisions and interaction with inorganic polymers formed by the coagulant or with organic polymers added. Macroflocs are formed. High molecular weight polymers, called coagulant aids, may be added during this step to help bridge, bind, and strengthen the floc, add weight, and increase settling rate. Once the floc has reached it optimum size and strength, the water is ready for the sedimentation process. Design contact times for flocculation range from 15 or 20 minutes to an hour or more (URL 8).
Coagulation reactions ocur rapidly, probably taking less than one second (Delphos and Wesner, 2005). Historically, metal coagulants have been most widely used in water clarification (Flynn, 2009). The most commonly used coagulants are:
Difficulties with settling often occur because of flocs that are slow-settling and are easily fragmented by the hydraulic shear in the settling basin. For these reasons, coagulant aids are normally used (Sincero and Sincero, 2003c).
Typical additivies used for coagulant aids are (Sincero and Sincero, 2003c; Delphos and Wesner, 2005):
These additives have been used as coagulant aids in conjunction with iron and alum primary coagulants in treating waters containing high color, low turbidity, and low mineral content (Sincero and Sincero, 2003c).
In practice, irrespective of what coagulant or coagulant aid is used, the optimum dose and pH are determined by a jar test. This consists of four to six beakers (such as 1000 ml in volume) filled with the raw water into which varying amounts of dose are administered. Each beaker is provided with a variable-speed stirrer capable of operating from 0 to 100 rpm. (Sincero and Sincero, 2003c). Upon introduction of the dose, the contents are rapidly mixed at a speed of about 60 to 80 rpm for a period of one minute and then allowed to flocculate at a speed of 30 rpm for a period of 15 minutes. After the stirring is stopped, the nature and settling characteristics of the flocs are observed and recorded qualitatively as poor, fair, good, or excellent. A hazy sample denotes poor coagulation; a properly coagulated sample is manifested by well-formed flocs that settle rapidly with clear water between flocs. The lowest dose of chemicals and pH that produce the desired flocs and clarity represents the optimum. This optimum is then used as the dose in the actual operation of the plant . (Sincero and Sincero, 2003c).
The jar test is used to identify the most adapted mix of chemical compounds and concentrations for coagulation-flocculation. It is a batch test consisting of using several identical jars containing the same volume and concentration of feed, which are charged simultaneously with six different doses of a potentially effective coagulant. The six jars can be stirred simultaneously at known speeds. The treated feed samples are mixed rapidly and then slowly and then allowed to settle. These 3 stages are an approximation of the sequences based on the large scale plants of rapid mix, coagulation-flocculation, and settling basins. At the end of the settling period, test samples are drawn from the jars and turbidity of supernatant liquid is measured. A plot of turbidity against coagulant dose gives an indication of the optimum dosage (i.e. the minimum amount required to give acceptable clarification). The criteria thus obtained from a bench jar test are the quality of resultant floc and the clarity of the supernatant liquid after settling. The design of the full-scale plant process is then done based on the bench-scale selection of chemicals and their concentrations (URL 7).
Oxidation-reduction (redox) reactions form the basis for many water treatment processes addressing a wide range of water quality objectives. These may include removal of iron, manganese, sulphur, color, tastes, odor, and synthetic organics (herbicides and pesticides) (Hesby, 2005). A redox reaction consists of two half-reactions: One of the half-reactions involves a loss of electrons, and it is defined as oxidation. The other half-reaction, involving the gain of electrons, is defined as reduction. Chemical species serving as potential electron acceptors are regarded as oxidants. Those functioning as potential electron donors are known as reductants (Shammas et al., 2005). An oxidation and a reduction reaction must always be coupled because free electrons can not exist in solution and electrons must be conserved (Hesby, 2005). Oxidizing agents, or oxidants, used in water treatment include chlorine, chlorine dioxide, permanganate, oxygen, and ozone. The appropriate oxidant for achieving a specific water quality objective depends on a number of factors, including raw water quality, specific contaminants, and local chemical and power costs (Hesby, 2005). Chemical oxidation is a process involving the transfer of electrons from an oxidizing reagent to the chemical species being oxidized. In water and wastewater engineering, chemical oxidation serves the purpose of converting putrescible pollutant substances to innocuous or stabilized products. Chemical oxidation processes take place in natural waters and serve as an important mechanism in the natural self-purification of surface waters. Oxidative removal of dissolved iron and sulfide pollutants in aerated waters is a prominent example. The degradation of organic waste materials represents an even more important phenomenon associated with natural water self-purification. It is well known that the efficacy of natural water organic oxidations is due to the presence of microorganisms, which serve to catalyze a highly effective utilization of dissolved oxygen as an oxidant. In fact, such microorganism-catalyzed processes have been optimized and developed into the various forms of so-called “biological processes” in high concentration organic waste treatment applications (Shammas et al., 2005).
Putrescible substances are known to comprise the most frequently occurring classes of pollutants in natural water systems. These substances have a most objectionable effect on water quality in that their decomposition often causes a depletion of dissolved oxygen in water. Dissolved oxygen is, in turn, essential to the existence of upper trophic aquatic organisms and is widely accepted as a most important indicator of the quality of a water system or its state of pollution. An analysis of oxygen balance in the aquatic environment shows that oxygen transfer from the atmosphere normally constitutes the most important oxygen source, whereas pollutional material consumption constitutes the major sinks. Chemical reactions giving rise to such consumption of oxygen are known as oxidation processes (Shammas et al., 2005). To account for proper materials balance, every chemical change of a specific nature must be accompanied by a process of opposite effect. Thus, the actual consumption of molecular oxygen is more properly termed as reduction, while the accompanying degradation of putrescible pollutants is defined as oxidation. In other words, oxidation and reduction must occur as coupled processes. The most functionally acceptable definition of oxidation–reduction is given in terms of electron transfer between reacting species (Shammas et al., 2005).
Hydrogen sulfide is commonly found in well water, where it results in a distinctive rotten egg odor. As the water passes through the ground, it comes in contact with sulfates. If the water is highly mineralized or contains products of decomposition, these minerals and other substances will react with the sulfates and change them to hydrogen sulfide (H2S). Surface waters rarely have hydrogen sulfide problems since the water is naturally aerated as it runs through streams (URL 3). Hydrogen sulfide gas turns into hydrosulfuric acid when it dissolves in water. The acid is weak but highly corrosive, eating up electrical contacts, causing a slight odor, and resulting in black water complaints. Water containing hydosulfuric acid will become very dark after remaining in the water lines for a few hours. The black water is most often noticed when flushing a fire hydrant (URL 3). The presence of larger quantities of hydrogen sulfide can be readily noted by odor. The disagreeable rotten egg odor is very characteristic of this gas and unless it is removed or reduced, the smell results in many complaints. As a result, even though H2S gas in water is not injurious to people, it is usually removed when present (URL 3). There are three methods used for the removal of hydrogen sulfide. If there is a heavy concentration of the gas, the water should be aerated, allowing most of the gas to escape into the air. Aeration of hydrogen sulfide requires that the pH of the water first be lowered to 6 or less, and then the gas can be scrubbed away by aeration. The remaining gas (or lower concentrations of the gas) can be oxidized by chlorine. Alternatively, ozone can be used to convert hydrogen sulfide to sulfurous acid, but ozone is also corrosive so it may cause as many problems as it solves (URL 3).
Both iron (Fe) and manganese (Mn) are minerals, which can be found in water supplies. The minerals cause stains on on porcelain plumbing fixtures and laundry and cause coffee or tea to be cloudy and unpalatable. In addition, they can cause diarrhea (URL 3). Water containing iron and manganese will be clear when first discharged from a well. Upon exposure to air for several hours, the minerals oxidize and colored water results. The presence of oxidized iron causes water to be red and results in stains of the same color. Mn is a dark brown mineral and the resulting stains are dark brown or black (URL 3). The usual treatment to remove Fe and Mn from water is to oxidize the minerals as rapidly as possible and then to remove the oxidized material through filtration. Mn oxidizes and discolors water at a slower rate than iron, which affects the treatment method used for each mineral. In addition, pH affects the rate of oxidation for both minerals, so it is often necessary to change the pH of the water during treatment (URL 3). Dissolved Fe and Mn are normally in the reduced state (Fe II, Mn II) and can be removed by oxidizing to Fe III and Mn IV, where they will precipitate as Fe(OH)3 and Mn(OH)2. Precipitates are subsequently removed in sedimentation and/or filtration steps. Several oxidants are available for this process, namely, chlorine dioxide, ozone and potassium permanganate. They are also removed through conventional lime softening treatment (Hesby, 2005) In some cases the oxidation is accomplished entirely by the addition of chemicals. In other cases the water is first aerated, then an alkali is added to complete oxidation. The alkali optimizes the pH and uses the oxygen in the air to oxidize the iron and manganese. At the same time, the alkali reduces the carbon dioxide concentration in the water (URL 3).
Most objectionable tastes and odors that occur in raw water, which particulary those of organic nature, can be mitigated by judicious application of a preoxidant. Surface waters in particular are prone to taste odor problems from presence of algae, other odor causing organisms and decaying vegetation (Hesby, 2005). The most well known and common odor causing compounds associated with algae, methyllisoborneol (MIB) and geosmin. Both are produced by actinomycetes and various blue-green algae and are particularly resistant to oxidation. Particulary tough applications may require both oxidation and adsorption step to lower tastes and odors to acceptable levels (Hesby, 2005).
Softening is the term given to the process of removing ions that interfere with the use of soap. These ions are called hardness ions due to the presence of multivalent cations, mostly calcium and magnesium. In natural waters, other ions that may be present to cause hardness but not insignificant amounts are Fe2+, Mn2+, Sr2+, Ba2+ and Al3+ (Sincero and Sincero, 2003d). Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water (URL 9). Hard waters are those waters that contain these hardness ions in excessive amounts (Sincero and Sincero, 2003d). The resulting soft water is more compatible with soap and extends the lifetime of plumbing. Water softening is usually achieved using lime softening or ion-exchange resins (URL 9).
Hardness is generally expresses in terms of equivalent miligrams per liter of calcium carbonate. Total hardness is usually defined as simply the sum of magnesium and calcium hardness in mg CaCO3/L. Total hardness can also be differentiated into carbonate and noncarbonate hardness. Carbonate hardness is the portion of total hardness present in the form of bicarbonate salts [Ca(HCO3)2 and Mg(CO3)2] and carbonate compounds (CaCO3 and MgCO3). Noncarbonate hardness is the portion of calcium and magnesium present as noncarbonate salts, such as calcium sulfate (CaSO4), calcium chloride (CaCl2), magnesium sulfate (MgSO4), and magnesium chloride(MgCl2) (Horsley et al., 2005). The following lists the general classification of hard waters (Sincero and Sincero, 2003c; Gottlieb, 2005): Soft <50 mg/L as CaCO3 Moderately hard 75–150 mg/L as CaCO3 Hard 150–300 mg/L as CaCO3 Very hard > 300 mg/L as CaCO3 A very soft water has a slimy feel. For example, rainwater, which is exceedingly soft, is slimy when used with soap. For this reason, hardness in water used for domestic purposes is not completely removed. Hardness is normally removed to the level of 75 to 120 mg/L as CaCO3 (Sincero and Sincero, 2003d).
The presence of certain metal ions like Ca and Mg principally as bicarbonates, chlorides, and sulfates in water causes a variety of problems (URL 9).
Potential benefits of softening water at a central treatment plant include the following (Horsley et al., 2005):
The most common means for removing water hardness rely on ion-exchange polymers or reverse osmosis. Other approaches include precipitation methods and sequestration by the addition of chelating agents. Devices, which supposedly use magnetism or electrolysis as a water softening technique, claim to inhibit scale buildup without actually removing hardness ions from the water. Such devices have been marketed to consumers since the early 20th century, but are fraudulent (URL 9).
In practice, two types of plants are generally used for chemical precipitation hardness removal: One type uses a sludge blanket contact mechanism to facilitate the precipitation reaction. The second type consists of a flash mix, a flocculation basin, and a sedimentation basin. The former is called a solids-contact clarifier. The latter arrangement of flash mix, flocculation, and sedimentation were discussed in previous chapters on unit operations. A solids-contact clarifier is shown in Figure 4.12. The chemicals are introduced into the primary mixing and reaction zone. Here, the fresh reactants are mixed by the swirling action generated by the rotor impeller and also mixed with a return sludge that are introduced under the hood from the clarification zone. The purpose of the return sludge is to provide nuclei that are important for the initiation of the chemical reaction. The mixture then flows up through the sludge blanket where secondary reaction and mixing occur. The reaction products then overflow into the clarification zone, where the clarified water is separated out by sedimentation of the reaction product solids. The clarified water finally overflows into the effluent discharge. The settled sludge from the clarification is drawn off through the sludge discharge pipe (Sincero and Sincero, 2003d).
Figure 4.12. Solids-contact clarifier (Courtesy of Infilco Degremont, Inc.) (URL 10).
The chemicals soda ash and lime may be used for the removal of hardness caused by calcium and magnesium. Thus, the lime–soda process is used. This process, as mentioned, uses lime (CaO) and soda ash (Na2CO3). As the name of the process implies, two possible sets of chemical reactions are involved: the reactions of lime and the reactions of soda ash. To understand more fully what really is happening in the process, it is important to discuss these chemical reactions. Let us begin by discussing the lime reactions. CaO first reacts with water to form slaked lime, before reacting with the bicarbonate. (Sincero and Sincero, 2003d) Two types of solids are produced: Mg(OH)2 and CaCO3 and that the added calcium ion from the lime that would have produced an added hardness to the water has been removed as CaCO3. Although the hardness ions have been precipitated out, the resulting solids, however, pose a problem of disposal in water softening plants. Magnesium, whether in the form of the carbonate or noncarbonate hardness, is always removed in the form of the hydroxide. Thus, to remove the total magnesium hardness, more lime is added to satisfy the overall stoichiometric requirements for both the carbonates and noncarbonates (Sincero and Sincero, 2003d). The implementation of the lime-soda process should be such that as much magnesium as possible is left unremoved and rely only on the removal of calcium to meet the desired treated water hardness. If the desired hardness level is not met by the removal of only the calcium ions, then removal of the magnesium may be initiated. This will entail the use of lime followed by the possible addition of soda ash to remove the resulting noncarbonate hardness of calcium. As noted before, the calcium ion is removed in the form of CaCO3. This is the reason for the use of the second chemical known as soda ash for the removal of the noncarbonate hardness of calcium. It is worth repeating that soda ash is used for two purposes only: to remove the original calcium noncarbonate hardness in the raw water and to remove the by-product calcium noncarbonate hardness that results from the precipitation of the noncarbonate hardness of magnesium. It is also important to remember that by using lime the carbonate hardness of magnesium does not produce any calcium noncarbonate hardness. It is only the noncarbonate magnesium that is capable of producing the byproduct calcium noncarbonate hardness when lime is used (Sincero and Sincero, 2003d).
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).
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