건축사회환경공학과홍승관교수. potency Cost rank b Harmful. (ph < 7) Chloramines High Yes Fair 2 Maybe not. Ozone Limited No Best 3 Yes, but limited

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1 6.D CHEMICAL AND PHYSICOCHEMICAL TREATMENT METHODS 6.D.1 Disinfection - The central aim of disinfection is to limit the risk of disease transmission associated with potable water and wastewater. - Two common physical methods are heating water (boiling) and irradiating water with ultraviolet light. - In water and wastewater treatment, chemical disinfection, primarily by chlorine, is widely used. Summary of Disinfectant Properties for Drinking Water Disinfectant Solubility Residual a Germicidal potency Cost rank b Harmful by-products Chlorine High Yes Very good 1 Yes (ph < 7) Chloramines High Yes Fair Maybe not Chlorine dioxide High Yes Good 4 Yes Ozone Limited No Best 3 Yes, but limited Ultraviolet light n/a No Fair? None known a Does the use of this disinfectant leave a residual product that can product the water from microbial contamination in the distribution system? b Lowest cost = 1, highest cost = 4. Source : Selleck, Ozonation - widely used in drinking water treatment practice in Europe - very powerful oxidant - no byproduct ( no THM ) - high cost and high energy requirement, no residual UV irradiation - disinfection for drinking water supplies aboard ships - most viable alternative for wastewater disinfection - recommended using UV with chlorine : no residual problem 1

2 Disinfection with chlorine - The active agent in chlorination is the chlorine in hypochlorous acid (HOCl). - An alternative approach, somewhat more expansive but much safer, involves the use of bleach (sodium hypochlorite, NaOCl), which is added to water as a liquid. - The third method is to add calcium hypochlorite, Ca(OCl), a solid, to the water. - byproduct : trihalomethanes ( 소독부산물 ) CHCl, CHCl Br CHClBr.) - reaction with humic substances (e.g., 3, - limited in drinking water having tumorigenic properties Chemistry of Chlorine in pure Water - Chlorine gas (Henry s law) [ 1 Cl( aq)] Cl( g) Cl( aq) KH 98K p (6. D. 1) Cl - Aqueous Cl Cl (aq) + H O HOCl + H + + Cl - [ HOCl][ H ][ Cl ] 4 K K [ Cl ( aq)] (6. D. ) - Hypochlorite ion (OCl - ) [ H ][ OCl ] 8 HOCl H OCl K K (6. D.3) [ HOCl] NaOCl Na (6. D. 4) OCl Ca ( OCl ) Ca ( OCl ) (6. D. 5)

$the fraction of hypochlorous species (HOCl+OCl - ) that is present as undissociated hypochlorous acid (HOCl), versus ph Reactions of Hypochlorous Acid with Chemical Impurities in Water - Reduced$

3 the fraction of hypochlorous species (HOCl+OCl - ) that is present as undissociated hypochlorous acid (HOCl), versus ph Reactions of Hypochlorous Acid with Chemical Impurities in Water - Reduced species that tend to be oxidized by chlorination include the sulfur in hydrogen sulfide and metal ions such as Fe + H S + HOCl S + H O + H+ + Cl - (6. D. 6) Fe + + HOCl + 5H O Fe(OH) 3 (s) + 5H + + CL - (6. D. 7) - If the water contains bromine, hypobromous acid, a compound that is more reactive than hypochlorous acid, may be formed. Br - + HOCl Cl - + HOBr (6. D. 8) HOBr can produce halogenated organics by reacting with natural organic matter in a manner similar to hypochlorous acid (sawyer al.,1994) - Hypochlorous acid can react with ammonia to produce chloramines. - When ammonia and hypochlorous acid are combined, three chloramines are produced : monochloramine (NH Cl), dichloramine (NHCl ), and, usually to a much smaller extant, trichloramine(ncl 3 ) NH 3 + HOCl H O + NH Cl (6. D. 9) NH Cl + HOCl H O + NHCl (6. D. 10) NHCl + HOCl H O + NCl 3 (6. D. 11) 3

4 - Combined chlorine (g/l) = 70.9[NH Cl] + 14[NHCl + 13[NCl 3 ] (6. D. 1) - The coefficients represent the product of the molecular weight of Cl (70.9g/mol) and the relative number of reducible Cl atoms per chloramine molecule compared with the single reducible atom per Cl. - The chlorine residual refers to the sum of the combined and free chlorine, with both expressed on the same mass concentration basis. - The below figure, known as a breakpoint curve, shows how the chlorine residual varies as chlorine is added to water. Breakpoint chlorination curve, for an initial ammonia nitrogen concentration of 1 mg/l. Disinfection Kinetics (for Drinking Water) In a disinfection process, the desired reaction might be written in this form: Live microorganism + disinfectant dead microorganism (6. D. 13) Usually, the rate law is written as dn ) dt ( (6. D. 14) disin fection kn Where N is the number concentration of viable (live) organisms and k is the reaction rate constant. This relationship is known as Chick s law (chick, 1908). Chick s law is useful 4

5 for predicting changes in inactivation percentage with changing contact time. For a fixed contact time, t c, we have N(t c ) = N(0)exp(-kt c ) (6. D. 15) The rate constant, k, in chick s law depend on the specific disinfectant (including its chemical form), its concentration, and the microorganism to be killed. A second relationship was proposed to amount for the effect of disinfectant concentration (Watson, 1908). : C n t c = α (Watson s law) (6. D. 16) Here C is the disinfectant concentration, and n is empirical constant. For a fixed level of inactivation (i.e., a fixed value of the ratio N((tc)/N(0)), α is a constant. Mass balance Conservation of all species involving acid-base reaction CT (total acid added) = [HA] + [A-] Equilibrium relationship k a [ H ][ A ] [ HA] Kw = [H+][OH-] = EXAMPLE 6.D.1 Effect of ph Disinfection Water is to be disinfected by the addition of chlorine gas (Cl ). From experiments, it is determined that at ph = 7, percent kill (4-log removal) is achieved with Ct c =0 min g m -3, where C is the free available chlorine concentration. Predict the product Ct c that is required to attain 4-log removal at ph =8. Assume that the only effect of changing ph is to alter the balance between HOCl and OCl -. Also assume that OCl - is completely ineffective as a disinfectant. Assume that n = 1 in Watson s law. SOLUTION The easiest way to approach this problem is to determine the relative amounts of [HOCl] in water at ph=7 and ph=8 assuming constant concentration of free available chlorine. The ratio [HOCl]/C is determined as follows: [ HOCl] [ OCl ] C MW Cl 5

6 Where MW Cl =70.9g/mol is the molecular weight of chlorine gas. Equation 6.D.3 yields K [ HOCl] OCl ] [ H ] [ Combining the two equations leads to this results : [ HOCl] C MW [ H ] ( [ H ] Cl K At ph=7, the term in brackets equals 0.79, whereas at ph=8 it has a value of 0.8. Thus, for a fixed free available chlorine concentration, C, the concentration of effective disinfection (HOCl) is reduced by a factor.8 (0.79/0.8) by a change of ph from 7 to 8. Since Ct c must be increased by the same factor, we predict Ct c =.8*0=56min g m -3 at ph=8. ) EXAMPLE 6.D. Disinfection Kinetics A series of experiments was conducted in a batch reactor to test the disinfection by chlorine of the protozoan Giardia lamblia. In each experimental run, the concentration of free available chlorine was varied and the time required to kill 99 percent of the organisms was determined. The results showed that a Ct c value of 100min mg L -1 was sufficient to achieve -log removal. (a) In Chick s law, what value of k corresponds to a chlorine concentration of 1 mg/l? (b) At a chlorine dose of 1 mg/l and a contact time of 10 minutes, what fraction of the G. lamblia organisms would be killed? (c) At a chlorine dose of 10 mg/l and a contact time of 5 minutes, estimate the fraction of the G. lamblia organisms that would be killed. SOLUTION (a) Since Ct c =100min mg L -1 for -log removal, and given that C=1 mg/l, we must have t c =100 min. We can use Chick s law (equation 6.D.15) to write the following expression: N ( 100min) 1 N(0) exp( k 100min) 0.01 k 0.046min (b) Again we apply Chick s law, now using the rate constant determined on part (a). We find that 63 percent of the G. lamblia organisms remain viable, or 37 percent are killed with 10min of exposure to 1mg/L of free available chlorine. N(10 min) N ( 10 min) N(0) exp( 0.46) 0.63 N(0) 6

7 (c) If we assume that n=1 in Watson s law, we can assume that Ct c is constant for a given disinfection goal. Then, using the result from (b) of a 37 percent inactivation for a C of 1 mg/l, we can set Ct c for the two doses equal as follows: Ct c =1 mg/l(10 min)=10 mg/l(x min) The time required to achieve 37 percent inactivation at a C of 10mg/L is X = 1min. We can now apply Chick s law to evaluate k for this dose: N N(0) ( 1min) 1 exp( k 1min) 0.63 k 0.46 min Comparing this result with part (a), we see that a 10X increase in disinfectant concentration is estimated to produce a 10X increase in the disinfection rate constant. Finally, this rate constant can be used in Chick s law to evaluate the G. lamblia inactivation at a chlorine dose of 10mg/L and a contact time of 5 min: N(5min) N ( 5min) N(0) exp( 0.465min) 0.10 N(0) We see that 90 percent of the G. lamblia organisms are inactivated with a Ct c value of 50 min mg L -1. In this case each unit of log removal requires a Ct c increment of 50 min mg L -1. The constant α in equation 6.D.16 has a value of 50R min mg L -1, and k=0.046c(min - 1 ), where C has units of mg L -1. This example illustrates an important point: G. lamblia is fairly resistant to disinfection with chlorine (Hoff and Akin, 1986). 7

8 6.D. Coagulation and Flocculation Colloidal Characteristics - colloidal dispersed in water : non-settle by the force of gravity - Hydrophilic colloids : Affinitive for water Shear surface is at the outer boundary of the bound water layer Usually organic (proteins or their end products) Water layer (bound water) surrounds the colloid Charges are due to ionization on the colloidal surface - Hydrophobic colloids : Non affinitive for water Shear surface is at the outer boundary of the fixed layer Usually inorganic (clays) Does not have a sufficient water layer Charges are due to the nonmetallic materials within the colloids 8

9 - Fixed layer : A compact layer of counter ions - Diffused layer : The layer outside of the fixed layer - Shear plane (Shear layer) : Encloses the volume of water that moves with the particles - Zeta potential : Electrical potential at the shear surface Dependant on the distance through which the charge is effective = zeta potential 4q d q = charge per unit area D d = Thickness of the layer surrounding the shear surface through which the charge is effective, as shown in Figure 8.3 D = dielectric constant of the liquid 9

10 Coagulation of Colloids (Destabilization) - The interactions that occur when the colloids are added ⅰ. Reduction of the zeta potential ⅱ. Aggregation of particles ⅲ. Enmeshment of particles in the precipitate floc that is formed - Repulsive forces : zeta potential (Electrical repulsion) Attractive forces : van der Waals forces Net resultant forces : zeta potential + van der Waals forces - The coagulation process is mainly conducted through three processes ⅰ. Reduction of the zeta potential : Coagulant salts are added to create highly positive charged hydroxometallic complexes. These are absorbed to the surface of the colloids to reduce the zeta potential ⅱ. Interparticulate bridging : Organic polymers have ionizable groups which bind reactive sites or groups on the surface of the colloids. In this manner several colloids may be bound to a single polymer. ⅲ. Agitation : The slow stir of the water causes interparticulate contacts which benefit the processes above. 10

11 - Coagulation and flocculation are typically combined in a two-stage process applied in drinking water treatment to assist in the removal of small, suspended particles. - The goal is to cause small particles to combine into larger aggregates that can be more readily separated from water by sedimentation and filtration. - In the coagulation stage of the process, a chemical reagent is added and rapidly mixed into the water to destabilize the colloidal particles. In the flocculation stage, the water is slowly stirred to promote collisions between particles. The aggregates formed in this process are known as flocs. - To understand how coagulation functions, we need to consider two major aspects of coagulation: collision-inducing mechanisms and particle interactions. - For particles to collide, they must move relative to one another. Such movement may originate from one or more of four main processes, as illustrated in the below figure. - Major mechanisms that cause collisions between particles suspended in water. The isomorphic substitution of a cation with a lower oxidation state (in this case, Al(III) replaces Si(IV)) produces a negative surface charge in clay particles. The same process can occur when a divalent cation (e.g., Ca + or Fe + ) substitutes for Al(III). 11

Schematic representation of negatively charged particles surrounded by cation clouds. In (a) the ionic strength is low and the cation clouds are diffuse.

In (b) the ionic strength is high allowing compression of the cationic clouds to the immediate vicinity of the particle surfaces, thereby permitting particles to collide.

12 Schematic representation of negatively charged particles surrounded by cation clouds. In (a) the ionic strength is low and the cation clouds are diffuse. Electrostatic repulsion between the cationic clouds prevents particles from colliding. In (b) the ionic strength is high allowing compression of the cationic clouds to the immediate vicinity of the particle surfaces, thereby permitting particles to collide. - The ionic cloud plus the charged particle surface is called an electrical double layer. The reduction in cloud size that occurs when the ionic strength increase is called double-layer compression. Stable particle suspensions in freshwater become unstable when mixed with seawater because of double-layer compression. - Coagulants applied in water treatment operate through four mechanisms: (1) Double-layer compression () Charge neutralization through adsorption of oppositely charged ions (3) Interparticle bridging (4) Precipitate enmeshment - The most widely used coagulant in drinking water treatment is alum that is, 1

13 hydrated aluminum sulfate, Al (SO 4 ) 3 14H O which is added to water either in dry or liquid form. Al (SO 4 ) 3 14H O + 6HCO 3- Al(OH) 3 (s) + 6CO + 14H O + 3SO 4 - (6. D. 17) - Instead of alum, coagulants based on inorganic iron ions, such as ferric chloride, - COAGULANTS FeCl 3, or ferric sulfate, Fe (SO 4 ) 3, are sometimes used. - varieties of coagulants : aluminum sulfate, ferrous sulfate, ferric sulfate, ferric chloride, lime (fig 8.5~6 참고 ) COAGULANT AIDS - to produce a quick-forming,dense, rapid settling floc, optimum coagulation - alkalinity addition, polyelectrolytes etc - Coagulant types and does are generally estimated using a procedure known as a jar test. Schematic representation of the jar test used to manage the coagulation process in water treatment. 13

14 - The basic procedure involves executing a series of batch experiments that mimics the full-scale process. RAPID MIXING AND FLOCCULATION - Rapid mixing : to disperse the chemicals uniformly throughout the basin to provide the adequate contact between the coagulant and particles - design of rapid mixing basin : * design parameter : velocity gradient, by T.R. Camp (1955) 식 8.9~8.10 G W P V (8.9) 1 G = velocity gradient, fps/ft or sec 1 (mps/m or sec ) W = power imparted to the water per unit volume of the basin, ft-lb/sec- ft 3 (Nm/s- m 3 ) P = power imparted to the water, ft-lb/sec (N-m/s) 3 3 V = basin volume, m ) ft ( = absolute viscosity of the water, lb-force-sec/ ft (at 10 C, = N-s/ m ) The velocity gradient for baffle basins is given by G hl T = specific weight of water (8.10) h L = head loss due to friction, turbulence, and so on T = detention time Velocity gradient too high prevent desired flocculation GT value Flocculation GT main parameter : value related with required power, preventing floc break - design : tapered flocculation basin, ~3 stages basin efficiency for power supply and stable floc P G V - power : (Camp) 1 G = velocity gradient, fps/ft or sec 1 (mps/m or sec ) P = power imparted to the water, ft-lb/sec (N-m/s) 3 3 V = basin volume, ft ( m ) = absolute viscosity of the water, lb-force-sec/ 14 ft (at 10 C, = N-s/ m

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CE 370. Disinfection. Location in the Treatment Plant. After the water has been filtered, it is disinfected. Disinfection follows filtration.

CE 370. Disinfection. Location in the Treatment Plant. After the water has been filtered, it is disinfected. Disinfection follows filtration. CE 70 Disinfection 1 Location in the Treatment Plant After the water has been filtered, it is disinfected. Disinfection follows filtration. 1 Overview of the Process The purpose of disinfecting drinking