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DRINKING WATER TREATMENT ANDWATER SECURITY
C. P. Gerba, K. A. Reynolds, and I. L. Pepper
Rivers, streams, lakes, and aquifers are all potential sources of potable water. In the United States, all water obtained from surface sources must be filtered and disinfected to protect against the threat of microbiological contaminants. Such treatment of surface waters also improves values such as taste, color, and odor. In addition, groundwater under the direct influence of surface waters such as nearby rivers must be treated as if it were a surface water supply. In many cases however, groundwater needs either no treatment or only disinfection before use as drinking water. This is because soil itself acts as a filter to remove pathogenic micro organisms, decreasing their chances of contaminating drinking water supplies. At first, slow sand filtration was the only means employed for purifying public water supplies. Then, when Louis Pasteur and Robert Koch developed the Germ Theory of Disease in the 1870s, things began to change quickly. In1881, Koch demonstrated in the laboratory that chlorine could kill bacteria. Following an outbreak of typhoid fever in London, continuous chlorination of a public water supply was used for the first time in 1905 (Montgomery,1985). There gular use of disinfection in the United States began in Chicago in 1908. The application of modern water treatment processes had a major impact on water-transmitted diseases such as typhoid in the United States (see also Chapter 11).The following sections describe conventional water treat-ment that is practiced in the public sector (e.g., municipal water supplies).
28.1 WATER TREATMENT PROCESSES
Modern water treatment processes provide barriers, or lines of defense, between the consumer and waterborne disease. These barriers, when implemented as a succession of treatment processes, are known collectively as a treatment process train (Figure 28.1). The simplest treatment process train, known as chlorination, consists of a single treatment process, disinfection by chlorination (Figure 28.1a). The treatment process train known as filtration, entails chlorination followed by filtration through sand or coal, which removes particulate matter from the water and reduces turbidity (Figure 28.1b). At the next level of treatment, in-line filtration, a coagulant is added prior to filtration (Figure28.1c). Coagulation alters the physical and chemical state of dissolved and suspended solids and facilitates their removal by filtration. More conservative water treatment plants add a flocculation (stirring) step before filtration, which enhances the agglomeration of particles and further improves there moval efficiency in a treatment process train called direct filtration (Figure. 28.1d). In direct filtration, disinfection is enhanced by adding chlorine (or an alternative disinfectant, such as chlorine dioxide or ozone) at both the beginning and end of the process train. The most common treatment process train for surface water supplies, known as conventional treatment, consists of disinfection, coagulation, flocculation, sedimentation , filtration, and disinfection (Figure 28.1e).
As already mentioned, coagulation involves the addition of chemicals to facilitate the removal of dissolved and suspended solids by sedimentation and filtration. The most common primary coagulants are hydrolyzing metal salts, most notably alum [Al2(SO4)3 14H2O], ferric sulfate[Fe2(SO4)3], and ferric chloride (FeCl3). Additional chemicals that may be added to enhance coagulation are charged organic molecules called polyelectrolytes; these include high-molecular-weight polyacrylamides, dimethyldially-ammonium chloride, polyamines, and starch. These chemicals ensure the aggregation of the suspended solids during the next treatment step, flocculation. Some times polyelectrolytes (usually polyacrylamides) are added after flocculation and sedimentation as an aid in the filtration step.
Coagulation can also remove dissolved organic an dinorganic compounds. Hydrolyzing metal salts added to the water may react with the organic matter to form a precipitate,or they may form aluminum hydroxide or ferric hydroxidefloc particles on which the organic molecules adsorb. Theorganic substances are then removed by sedimentation and filtration, or filtration alone if direct filtration or in-line filtration is used. Adsorption and precipitation also remove inorganic substances.
Flocculation is a purely physical process in which the treated water is gently stirred to increase interparticle collisions, thus promoting the formation of large particles. After adequate flocculation, most of the aggregates settle out during the 1 to 2 hours of sedimentation. Microorganisms are entrapped or adsorbed to the suspended particles and removed during sedimentation (Figure 28.2).
Sedimentation is another purely physical process, involving the gravitational settling of suspended particles that are denser than water. The resulting effluent is then subjected to rapid filtration to separate out solids that are still suspended in the water. Rapid filters typically consist of 50–75 cm of sand and/or anthracite having a diameter between 0.5 and 1.0 mm (Figure 28.2). Particles are removed as water is filtered through the medium at rates of 4–24 L/min/10 dm2. Filters need to be backwashed on are gular basis to remove the buildup of suspended matter. This backwash water may also contain significant concentrations of pathogens removed by the filtration process. Rapid filtration is commonly used in the United States. Another method, slow sand filtration, is also used. Employed primarily in the United Kingdom and Europe, this method operates at low filtration rates without the use of coagulation. Slow sand filters contain a layer of sand(60–120 cm deep) supported by a gravel layer (30–50cm deep). The hydraulic loading rate is between 0.04 and 0.4 m/h. The buildup of a biologically active layer, called as chmutzdecke, occurs during the operation of a slow sandfilter. This eventually leads to head loss across the filter, requiring removing or scraping the top layer of sand. Factors that influence pathogen removal by filtration are shown in Table 28.1.
Taken together, coagulation, flocculation, sedimentation, and filtration effectively remove many contaminants as shown in Table 28.2. Equally important, they reduce turbidity, yielding water of good clarity and hence enhanced disinfection efficiency. If not removed by such methods, particles may harbor microorganisms and make final disinfection more difficult. Filtration is an especially important barrier in the removal of the protozoan parasites Giardialamblia and Cryptosporidium. The cysts and oocysts of these organisms are very resistant to inactivation by disinfectants, so disinfection alone cannot be relied on to prevent waterborne illness. Because of their larger size, Giardia and Cryptosporidium are removed effectively by filtration. Conversely, because of their smaller size, viruses and bacteria can pass through the filtration process. Removal of viruses by filtration and coagulation depends on their attachment to particles (adsorption), which is dependent on the surface charge of the virus. This is related to the isoelectric point (the pH at which the virus has no charge) and is both strain and type dependent. The variations in surface properties have been used to explain why different types of viruses are removed with different efficiencies by coagulation and filtration. Thus, disinfection remains the ultimate barrier to these microorganisms.
28.2 DISINFECTION
Disinfection plays a critical role in the removal of pathogenic microorganisms from drinking water. The proper application of disinfectants is critical to kill pathogenic organisms
Generally, disinfection is accomplished through the addition of an oxidant. Chlorine is by far the most common disinfectant used to treat drinking water, but other oxidants, such as chloramines, chlorine dioxide, and even ozone, are also used (Figure 28.3).
Inactivation of microorganisms is a gradual process that involves a series of physicochemical and biochemical steps. In an effort to predict the outcome of disinfection, various models have been developed on the basis of experimental data. The principal disinfection theory used today is still the Chick-Watson Model, which expresses the rate of inactivation of microorganisms by a first-order chemical reaction.
Nt/No=e-kt (Eq.28.1)
or
1n Nt/No=-kt (Eq.28.2)
where:
Ne : number of microorganisms at time 0,
Nt : number of microorganisms at time t
k : decay constant (1/time)
t : time
The logarithm of the survival rate (Nt/No) plots as a straight line versus time (Figure 28.4). Unfortunately, laboratory and field data often deviate from first-order kinetics. Shoulder curves may result from clumps of organisms or multiple hits of critical sites before inactivation. Curves of this type are common in disinfection of coli form bacteria by chloramines(Montgomery, 1985). The tailing-off curve, often seen with many disinfectants, may be explained by the survival of a resistant subpopulation as a result of protection by interfering substances (suspended matter in water), clumping, or genetically conferred resistance.
In water applications, disinfectant effectiveness can be expressed as C～t, where:
C disinfectant concentration t time required to inactivate a certain percentage of the population under specific conditions (pH and temperature)Typically, a level of 99% inactivation is used when comparing C~ t values. In general, the lower the C～t value, the more effective the disinfectant. The C～t method allows ageneral comparison of the effectiveness of various disinfectants on different microbial agents (Tables 28.3 through28.6). It is used by the drinking water industry to determine how much disinfectant must be applied during treatment to achieve a given reduction in pathogenic microorganisms. C· t values for chlorine for a variety of pathogenic microorganisms are shown in Table 28.3. The order of resistance to chlorine and most other disinfectants used to treat water is protozoan cysts viruses vegetative bacteria. To obtain the proper C～t, contact chambers (Figure 28.5) are used to retain the water in channels before entering the drinking water distribution system or sewage discharge.
28.3 FACTORS AFFECTING DISINFECTANTS
Numerous factors determine the effectiveness and/or rate of kill of a given microorganism. Temperature has a major effect, because it controls the rate of chemical reactions. Thus , as temperature increases, the rate of kill with a chemical disinfectant increases. The pH can affect the ionization of the disinfectant and the viability of the organism. Most waterborne organisms are adversely affected by pH levels below 3 and above 10. In the case of halogens such as chlorine, pH controls the amount of HOCL (hypochlorous acid) and OCl (hypochlorite) in solution. HOCl is more effective than OCl in the disinfection of micro organisms. With chlorine, the C· t increases with pH. Attachment of organisms to surfaces or particulate matter in water such as clays and organic detritus aids in the resistance of microorganisms to disinfection. Particulate matter may interfere by either acting chemically to react with the disinfectant, thus neutralizing the action of the disinfectant, or physically shielding the organism from the disinfectant (Stewart and Olson, 1996).
Repeated exposure of bacteria and viruses to chlorine appears to result in selection for greater resistance (Bates etal., 1977; Haas and Morrison, 1981). However, the enhanced resistance has not been great enough to overcome concentrations of chlorine applied in practice.
28.4 HALOGENS
28.4.1 Chlorine
Chlorine and its compounds are the most commonly used disinfectants for treating drinking and wastewater (Figure28.6). Chlorine is a strong oxidizing agent that, when added as a gas to water, forms a mixture of hypochlorous acid (HOCl) and hydrochloric acids.
Cl2 + H2O=HOCl + HCl (Eq.28.3)
In dilute solutions, little Cl2 exists in solution. The disinfectant’s action is associated with the HOCl formed.
Hypochlorous acid dissociates as follows:
HOCl?H+ + OCl- (Eq.28.4)
The preparation of hypochlorous acid and OCl (hypochorite ion) depends on the pH of the water (Figure28.7). The amount of HOCl is greater at neutral and lower pH levels, resulting in greater disinfection ability of chlorine at these pH levels. Chlorine as HOCl or OCl is defined as free available chlorine. HOCl combines with ammonia and organic compounds to form what is referred to as combined chlorine. The reactions of chlorine with ammonia and nitrogen-containing organic substances are of great importance in water disinfection. These reactions result in the formation of monochloramine, dichloramine, trichloramine, etc.
Such products retain some of the disinfecting power of hypochlorous acid, but are much less effective at a given concentration than chlorine.
Free chlorine is quite efficient in inactivating pathogenic microorganisms. In drinking water treatment, 1mg/1 or less for about 30 minutes is generally sufficient to significantly reduce bacterial numbers. The presence of interfering substances in wastewater reduces the disinfection efficacy of chlorine, and relatively high concentrations of chlorine (20–40 mg/l) are required (Bitton, 1999). Enteric viruses and protozoan parasites are more resistant to chlorine than bacteria and can be found in secondary wastewater effluents after normal disinfection practices. Cryptosporidium is extremely resistant to chlorine. A chlorine concentration of 80 mg/l is necessary to cause 90% inactivation following a 90-minute contact time (Korich et al.,1990). Chloramines are much less efficient than free chlorine (about 50 times less efficient) in inactivation of viruses.
Bacterial inactivation by chlorine is primarily caused by impairment of physiological functions associated with the bacterial cell membrane. Chlorine may inactivate viruses by interaction with either the viral capsid proteins or the nucleic acid (Thurman and Gerba, 1988).
28.4.2 Chloramines
Inorganic chloramines are produced by combining chlorine and ammonia (NH4) for drinking water disinfection. The species of chloramines formed (see Equations 28.5 through 28.7) depend on a number of factors, including the ratio of chlorine to ammonia-nitrogen, chlorine dose, temperature,and pH. Up to a chlorine-to-ammonia mass ratio of 5, the predominant product formed is monochloramine, which demonstrates greater disinfection capability than other forms, i.e., dichloramine and trichloramine. Chloramines are used to disinfect drinking water by some utilities in the United States, but because they are slow acting, they have mainly been used as secondary disinfectants when a residualin the distribution system is desired. For example, when ozone is used to treat drinking water, no residual disinfectant remains. Because bacterial growth may occur after ozonation of tap water, chloramines are added to prevent re grow thin the distribution system. In addition, chloramines have been found to be more effective in controlling bio film micro organisms on the surfaces of pipes in drinking water distribution systems because they interact poorly with capsular bacterial polysaccharides (Le Chevallier et al., 1990).
Because of the occurrence of ammonia in sewage effluents, most of the chlorine added is converted to chloramines. This demand on the chlorine must be met before free chlorine is available for disinfection. As chlorine is added, the residual reaches a peak (formation of mostly monochloramine) and then decreases to a minimum called the break-point (Figure 28.8). At the breakpoint, the chloramine is oxidized to nitrogen gas in a complex series of reactions summarized in Equation 28.8.
Addition of chlorine beyond the breakpoint ensures the existence of a free available chlorine residual.
28.4.3 Chlorine Dioxide
Chlorine dioxide is an oxidizing agent that is extremelysoluble in water (five times more than chlorine) and, unlikechlorine, does not react with ammonia or organiccompounds to form trihalomethane, which is potentiallycarcinogenic. Therefore it has received attention for use as adrinking water disinfectant. Chlorine dioxide must be generated on site because it cannot be stored. It is generated fromthe reaction of chlorine gas with sodium chlorite:
Chlorine dioxide does not hydrolyze in water but exists as a dissolved gas. Studies have demonstrated that chlorine dioxide is as effective as or more effective in inactivating bacteria and viruses in water than chlorine (Table 28.4). As is the case with chlorine, chlorine dioxide inactivates microorganisms by denaturation of the sulfyhydryl groups contained in proteins, in hibition of protein synthesis, denaturation of nucleic acid, and impairment of permeability control (Stewart and Olson, 1996).
28.4.4 Ozone
Ozone (O3), a powerful oxidizing agent, can be produced bypassing an electric discharge through a stream of air or oxygen. Ozone is more expensive than chlorination to apply to drinking water, but it has increased in popularity as a disinfectant because it does not produce trihalomethanes or other chlorinated by products, which are suspected carcinogens. However, aldehydes and bromates may be produced by ozonation and may have adverse health effects. Because ozone does not leave any residual in water, ozone treatment is usually followed by chlorination or addition of chloramines. This is necessary to prevent regrowth of bacteria because ozone breaks down complex organic compounds present in water into simpler ones thatserve as substrates for growth in the water distribution system. The effectiveness of ozone as a disinfectant is not influenced by pH and ammonia. Ozone is a much more powerful oxidant than chlorine(Tables 28.3 and 28.6). Ozone appears to inactivate bacteria by the same mechanisms as chlorine-based disinfection: bydisruption of membrane permeability (Stewart and Olson,1996), impairment of enzyme function and/or protein integrityby oxidation of sulfyhydryl groups, and nucleic acid denaturation. Cryptosporidium oocysts can be inactivated by ozone, but a C· t of 1–3 is required. Viral inactivation may proceed by breakup of the capsid proteins into subunits, resulting in re-lease of the RNA, which can subsequently be damaged.
28.4.5 Ultraviolet Light
The use of ultraviolet disinfection of water and wastewater has seen increased popularity because it is not known to produce carcinogenic or toxic byproducts, or taste and odor problems. Also, there is no need to handle or store toxic chemicals. A wavelength of 254 nm is most effective against microorganisms because this is the wavelength absorbed by nucleic acids(Figure 28.9). Unfortunately, it has several disadvantages, including higher costs than halogens, no disinfectant residual, difficulty in determining the UV dose, maintenance and cleaning of UV lamps, and potential photo reactivation of some enteric bacteria (Bitton, 1999) (Figure 28.10). However, advances in UV technology are providing lower cost, more efficient lamps, and more reliable equipment. These advances have aided in the commercial application of UV for water treatment in the pharmaceutical, cosmetic, beverage, and electronic industries in addition to municipal water and wastewater application. Microbial inactivation is proportional to the UV dose, which is expressed in microwatt-seconds per square centimeter ( W-s/cm2) orUV dose=I·t (Eq.28.10)
Where:
I: W/cm2
t: exposure time
In most disinfection studies, it has been observed that the logarithm of the surviving fraction of organisms is nearly linear when it is plotted against the dose, where dose is