PHYSICAL - CHEMICAL TREATMENT OF WATER AND WASTEWATER - CHAPTER 17 (END) ppt

Disinfection Disinfection is a unit process involving reactions that render pathogenic organisms harmless. A companion unit process is sterilization. Sterilization refers to the killing of all organisms. Sterilization is not often practiced in the treatment of water and wastewater. Thus, this chapter will only discuss the unit process of disinfection. This discussion will include methods of disinfection, factors affecting disinfection, and the various disinfectants that have been used. Because chlorine is the most widely used disinfectant, its chemistry will be discussed at length. The design of chlorination unit operations equipment will also be discussed. The following disinfectants will also be specifically addressed: ozone and ultraviolet light. 17.1 METHODS OF DISINFECTION AND DISINFECTANT AGENTS USED Generally, two methods of disinfection are used: chemical and physical. The chem- ical methods, of course, use chemical agents, and the physical methods use physical agents. Historically, the most widely used chemical agent is chlorine. Other chemical agents that have been used include ozone, ClO 2 , the halogens bromine and iodine and bromine chloride, the metals copper and silver, KMnO 4 , phenol and phenolic com- pounds, alcohols, soaps and detergents, quaternary ammonium salts, hydrogen per- oxide, and various alkalis and acids. As a strong oxidant, ClO 2 is similar to ozone. (Ozone will be discussed specif- ically later in this chapter.) It does not form trihalomethanes that are disinfection by-products and suspected to be carcinogens. Also, ClO 2 is particularly effective in destroying phenolic compounds that often cause severe taste and odor problems when reacted with chlorine. Similar to the use of chlorine, it produces measurable residual disinfectants. ClO 2 is a gas and its contact with light causes it to photoox- idize, however. Thus, it must be generated on-site. Although its principal application has been in wastewater disinfection, chlorine dioxide has been used in potable water treatment for oxidizing manganese and iron and for the removal of taste and odor. Its probable conversion to chlorate, a substance toxic to humans, makes its use for potable water treatment questionable. The physical agents of disinfection that have been used include ultraviolet light (UV), electron beam, gamma-ray irradiation, sonification, and heat (Bryan, 1990; Kawakami et al., 1978; Hashimoto et al., 1980). Gamma rays are emitted from radioisotopes, such as cobalt-60, which, because of their penetrating power, have been used to disinfect water and wastewater. The electron beam uses an electron generator. A beam of these electrons is then directed into a flowing water or wastewater to be disinfected. For the method to be effective, the liquid must flow in thin layers. 17 TX249_frame_C17.fm Page 739 Friday, June 14, 2002 4:49 PM © 2003 by A. P. Sincero and G. A. Sincero 740 Several theories have been proposed as to its mechanics of disinfection, including the production of intermediates and free radicals as the beam hits the water. These intermediates and free radicals are very reactive and are thought to possess the disin- fecting power. In sonification, high-frequency ultrasonic sound waves are produced by a vibrating-disk generator. These waves rattle microorganisms and break them into small pieces. Ultraviolet light will be addressed specifically later in this chapter. In general, the effect of disinfectants is thought to occur as a result of damage to the cell wall, alteration of cell permeability, alteration of the protoplasm, and inhibition of enzymatic activities. Damage to the cell wall results in cell lysis and death. Some agents such as phenolic compounds and detergents alter the permeabil- ity of the cytoplasmic membrane. This causes the membrane to lose selectivity to substances and allow important nutrients such as phosphorus and nitrogen to escape the cell. Heat will coagulate protoplasm and acids and alkali will denature it causing alteration of the structure and producing a lethal effect on the microorganism. Finally, oxidants, such as chlorine, can cause the rearrangement of the structure of enzymes. This rearrangement will inhibit enzymatic activities. 17.2 FACTORS AFFECTING DISINFECTION The effectivity of disinfectants are affected by the following factors: time of contact between disinfectant and the microorganism and the intensity of the disinfectant, age of the microorganism, nature of the suspending liquid, and temperature. Each of these factors are discussed next. 17.2.1 T IME OF C ONTACT AND I NTENSITY OF D ISINFECTANT In the context of how we use the term, intensity refers to the intensive property of the disinfectant. Intensive properties, in turn, are those properties that are independent of the total mass or volume of the disinfectant. For example, concentrations are expressed as mass per unit volume ; the phrase “per unit volume” makes concentration indepen- dent of the total volume. Hence, concentration is an intensive property and it expresses the intensity of the disinfectant. Another intensive property is radiation from an ultra- violet light. This radiation is measured as power impinging upon a square unit of area . The “per unit area” here is analogous to the “per unit volume.” Thus, radiation is independent of total area and is, therefore, an intensive property that expresses the intensity of the radiation, which, in this case, is the intensity of radiation of the ultraviolet light. It is a universal fact that the time needed to kill a given percentage of microor- ganisms decreases as the intensity of the disinfectant increases, and the time needed to kill the same percentage of microorganisms increases as the intensity of the disinfectant decreases, therefore, the time to kill and the intensity are in inverse ratio to each other. Let the time be t and the intensity be I . Thus, mathematically, (17.1)t ∞ 1 I m TX249_frame_C17.fm Page 740 Friday, June 14, 2002 4:49 PM © 2003 by A. P. Sincero and G. A. Sincero Disinfection 741 Note: I has been raised to the power m , which is a constant. This is to make the relationship more general. Letting k be the proportionality constant, the equation becomes (17.2) In this equation, if I m is multiplied by t , and if I is expressed as the concentration of the disinfectant C in mg/L, the equation is the famous Ct concept with m equal to 1 and t in minutes. Ct values at given temperatures and pH are tabulated in Ct tables used by regulating authorities and by the U.S. Environmental Protection Agency. The time to kill t is synonymous with the time of inactivation of the microorganisms. The constants may be obtained from experimental data by converting the above equation first into an equation of a straight line. Taking the logarithms of both sides, (17.3) This equation is the equation of the straight line with y -intercept ln k and slope m . The constants may then be solved using experimental data. Assume n experimental data points, and divide them into two groups. Let there be l data points in the first group; the second group would have m − l data points. From analytic geometry, (17.4) Substituting Equation (17.4) into Equation (17.2) and solving for k produces (17.5) Having obtained m and k , the time t can be solved using Equation (17.2) from a knowledge of the value of I . This time is called the contact time for disinfection, and the intensity I is called the lethal dose . From Equation (17.2) any reasonable amount of dose is lethal when administered in a sufficient amount of contact time as calculated from the equation. We call Equation (17.2) the Universal Law of Disinfection. Example 17.1 It is desired to design a bromide chloride contact tank to be used to disinfect a secondary-treated sewage discharge. To determine the contact time, an experiment was conducted producing the following results: Contact Time (min/residual fecal coliforms) (No./100 mL) BrCl Dosage (mg/L) 15 30 60 3.6 10,000 4,000 600 15.0 800 410 200 47.0 450 200 90 t k I m = lnt lnk mlnI–= m ∑ l+1 n t i ln nl– ∑ 1 l t i ln l – ∑ l+1 n I i ln nl– ∑ 1 l I i ln l – –= k ∑ 1 l t i ln l m ∑ 1 l I i ln l    +exp= TX249_frame_C17.fm Page 741 Friday, June 14, 2002 4:49 PM © 2003 by A. P. Sincero and G. A. Sincero 742 Determine the contact time to be used in design, if it is desired to have a log 2 removal efficiency for fecal coliforms. Calculate the Ct value. The original concen- tration of fecal coliforms is 40,000 per/100 mL. Solution: The percentage corresponding to a log removal can be obtained by first assuming any original value of the concentration of the microorganisms x 1 , computing the next value x 2 based on the given log removal, and computing the corresponding percentage. Thus, let x 1 = 8888888. Then, Note: Any number could have been assumed for x 1 and the answer would still be 99. Thus, log 2 removal is equal to 99% removal or 99% inactivation. From the previous table, we produce the following table for the time to effect a 99% inactivation: BrCl Dosage (mg/L) 15 min % Inactivation 30 min % Inactivation 60 min % Inactivation 3.6 10,000 0.75 a 4000 0.90 600 0.985 15.0 800 0.98 410 0.98975 200 0.995 47.0 450 0.98875 200 0.995 90 0.99775 a 0.75 = (40,000 − 10,000 )/ 40,000 BrCl Dosage (mg/L) Time to 99% Inactivation (min) 3.6 61.76 a 15.0 31.43 b 47.0 18 8888888 x 2 log–log 2= and x 2 88888.88= % 8888888 88888.88– 8888888 99== 30 0.90 x 60– 60 30– 0.99 0.985– 0.985 0.90– = 60 0.985 x a 61.76= x 0.99 30 0.98975 x 30– 30 60– 0.99 0.98975– 0.98975 0.995– = x 0.99 x b 31.43= 60 0.995 TX249_frame_C17.fm Page 742 Friday, June 14, 2002 4:49 PM © 2003 by A. P. Sincero and G. A. Sincero Disinfection 743 Use a contact time of 30 minutes and find the corresponding lethal dosage. let l = 1; n = 3 Therefore, Therefore, 17.2.2 A GE OF THE MICROORGANISM The effectiveness of a disinfectant also depends upon the age of the microorganism. For example, young bacteria can easily be killed, while old bacteria are resistant. As the bacterium ages, a polysaccharide sheath is developed around the cell wall; this contributes to the resistance against disinfectants. For example, when using 2.0 mg/L of applied chlorine dosage, for bacterial cultures of about 10 days old, it takes 30 min of contact time to produce the same reduction as for young cultures of about one day old dosed with one minute of contact time. In the extreme case are the bacterial spores; they are, indeed, very resistant and many of the chemical disin- fectants normally used have little or no effect on them. BrCl Dosage (mg/L) Time to 99% Inactivation (min) ln I i ln t i 3.6 61.76 1.28 4.1233 15.0 31.43 2.708 3.4477 47.0 18 3.850 2.8904 t k I m ;= m ∑ l+1 n t i ln nl– ∑ 1 l t i ln l – ∑ l+1 n I i ln nl– ∑ 1 l I i ln l – ;–= k ∑ 1 l t 1 ln l m ∑ 1 l I i ln l    +exp= m 3.4477 2.8904+() 31– 4.1233() 1 – 2.708 3.850+() 31– 1.28 1 – – 6.3381 2 4.1233– 6.558 2 1.28– – 0.477=== k 4.1233() 1 0.477 1.28{}+exp 4.733[]exp 113.60=== t k I m ;30 113.60 I 0.477 ;== I 16.3= mg/L Ans Ct 16.3 30() 489 mg/L min Ans⋅== TX249_frame_C17.fm Page 743 Friday, June 14, 2002 4:49 PM © 2003 by A. P. Sincero and G. A. Sincero 17.2.3 NATURE OF THE SUSPENDING FLUID In addition to the time of contact and age of the microorganism, the nature of the suspending fluid also affects the effectiveness of a given disinfectant. For example, extraneous materials such ferrous, manganous, hydrogen sulfide, and nitrates react with applied chlorine before the chlorine can do its job of disinfecting. Also, the turbidities of the water reduces disinfectant effectiveness by shielding the microor- ganism. Hence, for most effective kills, the fluid should be free of turbidities. 17.2.4 EFFECT OF TEMPERATURE We have learned from previous chapters that equilibrium and reaction constants are affected by temperature. The length of time that a disinfection process proceeds is a function of the constants of the underlying reaction between the microorganism and the disinfectant; thus, it must also be a function of temperature. The variation of the contact time to effect a given percentage kill with respect to temperature can therefore be modeled by means of the Van’t Hoff equation. This equation was derived for the equilibrium constants in Chapter 11, which is reproduced next: (17.6) K T1 and K T2 are the equilibrium constants at temperatures T 1 and T 2 , respectively. is the standard enthalpy change of the reaction and R is the universal gas constant. If K T1 is replaced by contact time t T1 at temperature T1 and K T2 is replaced by contact time t T2 at temperature T2, the resulting equation would show that as the temperature increases, the contact time to kill the same percentage of microorgan- isms also increases. Of course, this is not true. Thus, the replacement should be the other way around. Doing this is the same as interchanging the places in the difference term between T 1 and T 2 inside the exp function. Thus, doing the interchanging, (17.7) Table 17.1 shows the standard enthalpy change as a function of pH for both aqueous chlorine and chloramines, and Table 17.2 shows the various possible values of the universal gas constant. Example 17.2 The contact time for a certain chlorination process at a pH of 7.0 and a temperature of 25°C is 30 min. What would be the contact time to effect the same percentage kill if the process is conducted at a temperature 18°C? Solution: K T 2 K T 1 = ∆H 298 o RT 1 T 2 T 2 T 1 –()exp ∆H 298 o t T 2 t T1 ∆H 298 o RT 1 T 2 T 1 T 2 –()exp= t T 2 t T 1 ∆H 298 o RT 1 T 2 T 1 T 2 –()exp= TX249_frame_C17.fm Page 744 Friday, June 14, 2002 4:49 PM © 2003 by A. P. Sincero and G. A. Sincero From Table 17.1, From Table 17.2, Therefore, TABLE 17.1 Standard Enthalpy Changes at 25°C Compound pH ∆ (J/mol) Aqueous chlorine 7.0 34,332 8.5 26,796 9.8 50,242 10.7 62,802 Chloramines 7.0 50,242 8.5 58,615 9.5 83,736 TABLE 17.2 Values and Units of R R Value R Units K Concentration Units Used ∆H° Units T Units 0.08205 — °K 8.315 °K 1.987 °K 82.05 — °K H 298 o L atm gmmole K ° gmmoles L J gmmole K ° gmmoles L J gmmole cal gmmole K ° gmmoles L cal gmmole atm.cm 3 gmmole K °⋅ gmmoles L ∆H 298 o 34,332= j/mol 34,332 N m/mol⋅= R 8.315 J gmmole K °⋅ 8.315 Nm⋅ gmmole K °⋅ == t T 2 = 30 34,332 8.315 298()291() 298 291–()exp = 30 0.333[]exp = 41.87 min Ans TX249_frame_C17.fm Page 745 Friday, June 14, 2002 4:49 PM © 2003 by A. P. Sincero and G. A. Sincero 17.3 OTHER DISINFECTION FORMULAS The literature reveals other disinfection formulas. These include Chick’s law for contact time, modifications of Chick’s law, and relationship between concentration of disinfec- tant and concentration of microorganisms reduced in a given percentage kill. Chick’s law and its modification called the Chick–Watson model, however, are not useful for- mulas, because they do not incorporate either the concentration of the disinfectant that is needed to kill the microorganisms or the incorporation of the concentration is incorrect. The relationship of the concentration of disinfectant and the concentration of the micro- organisms is also not a useful formula, since it does not incorporate the contact time required to kill the microorganisms. It must be noted that for a formula to be useful, it must incorporate both the concentration (intensity) of the disinfectant and the contact time corresponding to this concentration effecting a given percentage kill. For these reasons, these other disinfection formulas are not discussed in this book. The Chick–Watson model needs to be addressed further. Watson explicitly expressed the constant k in Chick’s law in terms of the concentration of disinfectant C as α C n , where α is an activation constant and n is another constant termed the constant of dilution. Chick’s Law, thus, became dN/dt = − α C n dt, where N is the concentration of microorganisms and t is time. Note that C is a function of time. When this equation was integrated, however, it was assumed constant, thus producing the famous Chick–Watson model, where N o is the initial concentration of microorganisms. Because the concentration C was assumed constant with time during integration, this equation is incorrect and, therefore, not used in this book. 17.4 CHLORINE DISINFECTANTS The first use of chlorine as a disinfectant in America was in New Jersey in the year 1908 (Leal, 1909). At that time George A. Johnson and John L. Leal chlorinated the water supply of Jersey City, NJ. The principal compounds of chlorine that are used in water and wastewater treatment are the molecular chlorine (Cl 2 ), calcium hypochlorite [Ca(OCL) 2 ], and sodium hypochlorite [NaOCl]. Sodium hypochlorite is ordinary bleach. Chlorine is a pale-green gas, which turns into a yellow-green liquid when pressurized. Both the aqueous and liquid chlorine react with water to form hydrated chlorine. Below 9.4°C, liquid chlorine forms the compound Cl 2 · 8H 2 O. Chlorine gas is supplied from liquid chlorine that is shipped in pressurized steel cylinders ranging in size from 45 kg and 68 kg to one tonne containers. It is also shipped in multiunit tank cars that can contain fifteen 1-tonne containers and tank cars having capacities of 15, 27, and 50 tonnes. In handling chlorine gas, the following points are important to consider: • Chlorine gas is very poisonous and corrosive. Therefore, adequate venti- lation should be provided. In the construction of the ventilation system, N N o ln α C n –= t TX249_frame_C17.fm Page 746 Friday, June 14, 2002 4:49 PM © 2003 by A. P. Sincero and G. A. Sincero the capturing hood vents should be placed at floor level, because the gas is heavier than air. • The storage area for chlorine should be walled off from the rest of the plant. There should be appropriate signs posted in front of the door and back of the building. Gas masks should be provided at all doors and exits should be provided with clearly visible signs. • Chlorine solutions are very corrosive and should therefore be transported in plastic pipes. • The use of calcium hypochlorite or sodium hypochlorite as opposed to chlorine gas should be carefully considered when using chlorination in plants located near residential areas. Accidental release of the gas could endanger the community. Normally, small plants that usually lack well- trained personnel, should not use gaseous chlorine for disinfection. Calcium hypochlorite is available in powder or granular forms and compressed tablets or pellets. Depending upon the source of the chemical, a wide variety of container sizes and shapes are available. Because it can oxidize other materials, calcium hypochlorite should be stored in a cool dry place and in corrosion-resistant containers. High-test calcium hypochlorite, HTH, contains about 70% chlorine. (Available chlo- rine will be defined later.) Sodium hypochlorite is available in solution form in strengths of 1.5 to 15% with 3% the usual maximum strength. The solution decomposes readily at high concentrations. Because it is also affected by heat and light, it must be stored in a cool dry place and in corrosion-resistant containers. The solution should be trans- ported in plastic pipes. Sodium hypochlorite can contain 5 to 15% available chlorine. 17.4.1 CHLORINE CHEMISTRY The chemistry of chlorine discussed in this section includes hydrolysis and optimum pH range of chlorination, expression of chlorine disinfectant concentration, reaction mediated by sunlight, reactions with inorganics, reactions with ammonia, reactions with organic nitrogen, breakpoint reaction, reactions with phenols, formation of trihalomethanes, acid generation, and available chlorine. Chlorine has the electronic configuration of [Ne]3s 2 3p 5 and is located in Group VIIA of the Periodic Table in the third period. [Ne] means that this element has the electronic configuration of the noble gas neon. The letters p and s refer to the p and s orbitals; the superscripts indicate the number of electrons that the orbitals contain. Thus, the p orbital contains 5 electrons and the s orbital contains two electrons, making a total of seven electrons in its valence shell. This means that the chlorine atom needs to acquire only one more electron to attain the neon configuration for stability. This makes chlorine a very good oxidizer. In fact, it is a characteristic of Group VIIA to attain a charge of −1 when the members of this group oxidizes other substances. The members of this group starting from the strongest oxidizer to the least are fluorine, chlorine, bromine, iodine, and astatine. This group forms the family of elements called the halogen family. All the chlorine disinfectants reduce to the chloride ion (Cl − ) when they oxidize other substances, which must, of course, be reducing substances. The chlorine starts TX249_frame_C17.fm Page 747 Friday, June 14, 2002 4:49 PM © 2003 by A. P. Sincero and G. A. Sincero with an oxidation state of zero and ends up with a −1; it only needs one reduction step. One the other hand, the hypochlorites start with oxidation states of +1 and end up with also a −1; thus, they need two reduction steps. Because the chlorine atom only needs one reduction step, while the hypochlorites need two, the chlorine atom is a stronger oxidizer than the hypochlorites. As a stronger oxidizer, it is also a stronger disinfectant. Hydrolysis and optimum pH range of chlorination. As previously mentioned, chlorine is supplied in the form of liquefied chlorine. The liquid must then be evapo- rated into a gas. As the gas, Cl 2(g) , is applied into the water or wastewater, it dissolves into aqueous chlorine, Cl 2(aq) , as follows: (17.8) Cl 2(aq) then hydrolyzes, one of the chlorine atoms being oxidized to +1 and the other reduced to −1. This reaction is called disproportionation. The reaction is as follows: (17.9) From Equation (17.9), the hypochlorous acid, HOCl, is formed, which is one of the chlorine disinfectants. If its formula is analyzed, it will be found that the chlorine has an oxidation state of +1, as we mentioned before. Note also that hydrochloric acid is formed. This is a characteristic in the use of the chlorine gas as a disinfectant. The water becomes acidic. Also, as we have mentioned, the chlorine molecule is a much stronger oxidizer than the hypochlorite ion and, hence, a stronger disinfectant. From Equation (17.9), if the water is intentionally made acidic, the reaction will be driven to the left, producing more of the chlorine molecule. This condition will then produce more disinfecting power. As will be shown later, however, this condition, where the chlorine molecule will exist, is at a very low pH hovering around zero. This makes the chlorine molecule useless as a disinfectant. HOCl further reacts to produce the following dissociation reaction: (17.10) Using Equation (17.9), let us calculate the distribution of Cl 2(aq) and HOCl. Expressing in the form of equilibrium equation, (17.11) Taking logarithms, rearranging, and simplifying, (17.12) pK H is the negative logarithm to the base 10 of K H . Table 17.3 shows the ratios of [Cl 2(aq) ]/[HOCl] and [HOCl]/[Cl 2(aq) ] as functions of pH and the chloride concentrations, using Equation (17.12). The concentration of 1.0 gmmole/L of chloride is 35,500 mg/L. This will never be encountered in the normal treatment of water and wastewater. Disregarding this entry in the table, the Cl 2 g()  Cl 2 aq() K Claq 6.2 10 2– ()= Cl 2 aq() H 2 O  HOCl H + Cl 1– +++ K H 4.0 10 4– ()= HOCl  H + OCl − + K a 10 7.5– = K H 4.0 10 4– () HOCl[]H + []Cl 1– [] Cl 2 aq() [] == Cl 2 aq() [] HOCl[] 1 0 { pK H −pH+log Cl 1– []} 10 {3.40−pH+log Cl 1– []} == TX249_frame_C17.fm Page 748 Friday, June 14, 2002 4:49 PM © 2003 by A. P. Sincero and G. A. Sincero [...]... (c) Injector water supply (d) FIGURE 17. 10 Disinfectant mixing devices (From Metcalf & Eddy, Inc (1972) Wastewater Engineering: Treatment, Disposal, Reuse McGraw-Hill, New York, 385 With permission.) Other mixing devices that have not been discussed in that chapter are shown in Figures 17. 10 and 17. 11 The arrangement in Figure 17. 10a is used to inject disinfectant into waters and wastewaters flowing... 4 + 2Na + 2H (17. 66) (17. 67) (17. 68) The effects of the above reactions on the receiving streams are a reduction of dissolved oxygen and an increase in BOD and COD In addition, the right-hand sides of the reactions, except those for sodium sulfite, indicate production of the hydrogen ion Thus, there is the possibility of a decrease in pH of the receiving streams © 2003 by A P Sincero and G A Sincero... of a chlorine disinfectant is measured in terms of available chlorine Available chlorine is defined as the ratio of the mass of chlorine to the mass of the disinfectant that has the same unit of oxidizing power as chlorine The unit of disinfecting power of chlorine may be found as follows in terms of one mole of electrons: − Cl 2 + 2e → 2Cl − (17. 45) From this equation, the unit of oxidizing power of. .. The third method is flow-portioned control Flow-proportioned control proportions dosage according to the flow rate of the water to be disinfected This is the case of the hypochlorite dosing system of Figure 17. 8 The rate of suction of the solution is proportioned to the reading of the meter Flow-proportioned control is also called flow-paced control The fourth method is residual-proportioned control Residualproportioned... substances that could be present in the raw water and raw wastewater and treated water and treated wastewater are ferrous, manganous, nitrites, and hydrogen sulfide Thus, these are the major substances that can interfere with the effectiveness of chlorine as a disinfectant The interfering reactions are written as follows: with ferrous: 2+ 3+ 2Fe → 2Fe + 2e + − (17. 22) − − HOCl + H + 2e → Cl + H 2 O ––––––––––––––––––––––––––––––––––... amount of dosage is proportioned according to the reading of the meter shown in the plan drawing The hypochlorite solution is then injected into the main at the discharge side of the chlorinator through the discharge hose Example 17. 13 Specify the chlorine dosing system to be used for a wastewater 3 treatment plant processing an average of 25,000 m /d of wastewater The peaking factor is 3.0 and the... odorous substances is a major concern of water treatment plant operators Figure 17. 2 shows the threshold odor as a function of pH and the concentration of chlorine dosage Figure 17. 2a uses a concentration of 0.2 mg/L and, at a pH of 9.0, the maximum threshold odor concentration is around 28 µg/L When the pH is reduced to 8.0 this threshold worsens to around 20 µg/L, and when the pH is further reduced... compounds such as fulvic and humic acids and humin produce undesirable by-products These by-products are known as disinfection by-products, DBPs Examples of DBPs are chloroform and bromochloromethane; these DBPs are suspected carcinogens Snoeyink and Jenkins (1980) wrote a series of reactions that demonstrate the basic steps by which chloroform may be formed from an acetyl-group containing organic... HOCl = - = - = 1.35 mg/ L Cl 2 HOCl 1.008 + 16 + 35.5 Therefore, 0.5 0.5 mg/L TRC = - = 0.37 mg/L HOCl 1.35 And, from Equation (17. 50), SO HOCl 2 - ( 0.37 ) - kg/d of sulfur dioxide = ( 25,000 ) 1000 32+32 - ( 0.37 ) 1.008+16+35.5 = - ( 25,000 ) = 11.27 kg/d Ans 1000 3 Example 17. 18 A total flow of 25,000 m /d... and G A Sincero TX249_frame_C17.fm Page 763 Friday, June 14, 2002 4:49 PM 3 Example 17. 11 A flow of 25,000 m /d of treated water is to be disinfected using chlorine in pressurized steel cylinders The raw water comes from a reservoir where the water from the watershed has a very low alkalinity With this low rawwater alkalinity, coupled with the use of alum in the coagulation process, the alkalinity of . methods of disinfection are used: chemical and physical. The chem- ical methods, of course, use chemical agents, and the physical methods use physical agents. Historically, the most widely used chemical. refers to the killing of all organisms. Sterilization is not often practiced in the treatment of water and wastewater. Thus, this chapter will only discuss the unit process of disinfection. This discussion. concentrations of the three species are expressed in terms of the molecular chlorine, Cl 2 . The pertinent reactions are written as follows: (17. 15) (17. 16) (17. 17) TABLE 17. 4 Ratios of and as Functions of