Chapter 7. Adsorption & Activated Carbon

Transcription

1 Chapter 7 Adsorption & Activated Carbon

2 Contents 1. Introduction 2. Manufacture of activated carbon 3. Specific methods for evaluating the adsorption capacity of activated carbon 4. Adsorption equilibria 5. Factors affecting adsorption equilibria 6. Equilibrium batch adsorption 7. Kinetics of adsorption (HSDM Model) 8. Fixed bed adsorption columns 9. GAC adsorption systems 10. PAC adsorption systems

3 1. Introduction Adsorption : Accumulation of a substance at the interface between two phases Adsorbent: adsorbing phase (A.C.) Adsorbate: solute, material adsorbed - In water purification, adsorbents are used to remove organic impurities (nonbiodegradable organics associated with i) taste, ii) odor, iii) color and iv) health effects such as EDCs & Pharmaceuticals.) - Classification of Adsorption i) physical adsorption (activated carbon) van der waals forces, reversible ii) chemical adsorption chemical reaction occurs between solids and adsorbed solutes, irreversible

4 1. Nature of Adsorbent - Common industrial adsorbents : activated carbon (most widely used), silica gel, activated alumina, molecular sieves - Classification of adsorbent : Micropores, rp < 1 nm Mesopores, 1nm < rp < 25 nm Macropores, 25 nm < rp or Powdered Activated Carbon (PAC) : less than 200 mesh (74 um) * About 1um of PAC is prepared by NGK company in Japan. Granular Activated Carbon (GAC) : more than 100 um - Surface area of Activated Carbon : 500 ~ 1,000 m 2 /g As the pore surface area greatly exceeds the surface area of particles, most of the adsorption occurs on the pore surfaces except the smaller size of PAC (~ 1um)

5 2. Manufacture of Activated Carbon 1) Dehydration (up to C ) - Carbonaceous Material (coal, wood, coconut shells, peat) is dehydrated to remove excess water. - Sometimes ZnCl 2, H 3 PO 4 may be used in addition to heat as a dehydrating agent. 2) Carbonization (pyrolysis at 400~600 0 C ) - Heating in the absence of air makes decomposition of the material and drives off impurities such as tars and methanol. - The escape of these volatile substances causes pores to form within the material and leaves a product usually referred to as char. - Although surface area is opened up during carbonization, chars possess relatively little adsorptive power due to the presence of amorphous residues (tars), which block their pores.

6 2. Manufacture of Activated Carbon 3) Activation (750~950 0 C ) - To blow CO 2, air, and steam at temperature of 750~950 0 C so as to burn off the amorphous residues. Tars are removed and the pores are cleaned and enlarged by activation process. 4) Reactivation (~ C) - Adsorbed organics in spent carbon are burned off in a steam-air atmosphere in a multiple hearth furnaces at ~ C. - Carbon loss during reactivation is 5~10 %.

7 2. Source materials for Activated Carbon ( Natural carbon containing materials) Bagasse Blood/Flesh Residues Bones Carbon Black Cereals Coal * (Bitminous; 역청탄 ) Coconut Shells * Coffee Beans Corn Cobs Cottonseed Hulls Distillery Waste Fish Fruit Pits Kelp Lampblack Lignin Lignite Molasses Municipal Refuse Nutshells Peat * Petroleum Acid Sludge Petroleum Coke * Pulp-Mill Chars Sawdust Woods * * The most common materials used in the production of activated carbons suited for water treatment

8 2. Surface functional group

9 2. Surface functional group

10 2. Surface functional group

11 2. Surface functional group

12 2. Surface functional group

13 3. Specific Methods for evaluating the adsorption capacity of AC a) Surface Area (SA) - Typically 600 ~ 1,200 m 2 /g up to 2,500 m 2 /g - Compare : a cube of 1cm SA = o.ooo6 m 2 If it is divided into 10 9 cubes ( each cube of 10-5 m), then SA = 0.6 m 2. For AC, SA = X f (particle size ) b) Molasses Test - Especially relevant for carbons to be used for decolorization purposes such as those used in sugar processing - Molasses number: the weight of carbon required to decolorize a standard molasses to the same level as a standard dose of a standard carbon. - Relative efficiency: different amounts of carbon are added to kg/m 3 solutions of molasses. The dosage of carbon required to achieve a 90 to 95% color removal is compared to that of a standard carbon and reported as relative efficiency.

14 3. Specific Methods for evaluating the adsorption capacity of AC mgi2 c) Iodine number = at Ce = 0. 02N ga. c. - The test results give a good indication of the microporosity and are often correlated to the BET surface area values :

15 3. Specific Methods for evaluating the adsorption capacity of AC d) Particle porosity - A carbon particle consists of the carbon material and intra-particle pore space - ε p (particle porosity) : ratio of the pore volume to the total volume of a carbon particle ρ M (material density) : ratio of the weight of the dry and unloaded particles to the volume of the material only (true density) the density of graphite ρ p (particle density) : ratio of the weight of the dry and unloaded particles to the total volume of particles including the pore volume. ε F (filter bed porosity) : ratio of void to total volume within a filter layer - ε p ρm ρ p = = 1 ρ M ρ ρ p M

16 3. Specific Methods for evaluating the adsorption capacity of AC e) Ash content : - The adsorption properties of activated carbon are determined not only by the porous structure of the carbon, but also by the chemical properties of the inner surface - Noncarbon elements (metals + surface bound oxygen) = 5~20% of activated carbon by weight ash (inorganic) contents : Al, Si, Fe, Mg, Ca, Na, K, S, P - Ash content can be determined by ignition of sample to a constant weight (ASTM D 2866)

17 4. Adsorption Equilibria Adsorption Equilibria - Nomenclature V m V = volume of liquid m = mass of solid C 0 = initial conc. of solute C = final conc. of solute (C e ) x = V(C 0 -C) = mass of solute adsorbed q e = x/m = solute adsorbed per unit weight of solid adsorbent

18 4. Adsorption Equilibria Isotherms Irreversible Favorable q e linear unfavorable C e

19 4. Langmuir Isotherm Kinetic derivation : rate of adsorption K a (rate constant) C (conc. of solute) (1-θ) rate of desorption max q e = Q 0 (all sites occupied) K d (rate constant) θ θ = q e /Q 0 = fraction of sites occupied Q0 : the surface concentration at monolayer coverage, maximum value of q e that can be achieved as C e is increased. Assumption: 1) fixed number of adsorption sites 2) one molecule per site 3) same ΔH for all sites 4) adsorbed molecules do not interact

20 4. Langmuir Isotherm 1. At equilibrium : K a C (1-θ) = K d θ 2. Solve for θ : θ = 3. Substitute q e /Q 0 for θ : 4. Rearrange : q e q e / Q / Q 0 K 0 d = K a + K d C K K + a a C C K a C ( Ka / Kd ) C = 1+ ( K / K ) C a 5. Let K a /K d = b (b is related to the energy of adsorption and increases as the strength of the adsorption bond increases ) d 6. Substitute 0 Q bc q e = 1 + bc Langmuir equation

21 4. Linearized Langmuir Isotherm Invert equation : or : bc = 0 q e Q bc 1 = + 0 q e Q bc 1 Q 0 1/q e 1/Q 0 b 1/Q 0 1/C

22 4. Freundlich Isotherm q e = K F C 1/n log q e = log K F + 1/n log C log q e 1/n log K F log C

23 4. Freundlich Isotherm; example for wastewater

24 5. Factors affecting adsorption equilibria 1) - nonpolar compounds adsorb more strongly to nonpolar adsorbent (Hydrophobic Bonding) - In water treatment, there is often interest in the adsorption of an organic adsorbate from a polar solvent (water) onto a nonpolar (or slightly polar) adsorbent (Activated carbon) - For a homologous series of organic molecules in water, the extent of adsorption usually increases with an increase in the numbers of carbon atoms because the longer hydrocarbon chain is more nonpolar (Traube s rule) - However, when a size increase causes the molecule to be excluded from some pores, adsorption capacity may decrease as solubility decrease (more -CH 2 -)

25 5. Factors affecting adsorption equilibria - When ph in a range where the molecule is in the neutral form, adsorption capacity is relatively high, because the ionized forms tend to be stabilized by interaction with polar water

26 5. Factors affecting adsorption equilibria 2) Inorganic composition of water - Ca 2+ associates (complexes) with the fulvic acid anion to make it more adsorbable

27 5. Factors affecting adsorption equilibria - Inorganic substances (Fe, Mn, Ca) salts or precipitates may deposit on the adsorbent to interfere with adsorption pretreatment is necessary - Activated carbon usually carries a net surface charge that is slightly negative 3) Pore Size Distribution Micropores (d < 2 mm) Transition pores (2 < d < 50 mm) Macropores (d > 50 mm) - The maximum amount of adsorption is proportional to the amount of surface area within pores that is accessible to the adsorbate - Activated carbons that have a relatively small volume of macropores have a relatively low capacity for the large fulvic acid molecule

28 3) Pore Size Distribution

29 3) Pore Size Distribution

30

31 5. Factors affecting adsorption equilibria 4) Adsorption for heterogeneous mixtures Group parameter TOC DOC COD DOX (Dissolved Organic Halogen) U.V. absorbance Fluorescence TOX - The mixture of compounds is treated as a single compound in isotherm equations (Freundlich or Langmuir) - The shape of isotherm depends on the relative amounts of compounds in the mixture because each compound has different affinity for an adsorbent - Isotherm for a heterogeneous mixture (Initial concentration, fraction of the mixture)

32 4) Adsorption for heterogeneous mixtures - The shape of isotherm depends on the relative amounts of compounds in the mixture because each compound has different affinity for an adsorbent- - Isotherm for a heterogeneous mixture (Initial concentration, fraction of the mixture)

33 5. Factors affecting adsorption equilibria 5) Competitive Adsorption - The adsorbent s surface is shared by the competing substance - The extent of competition depends on i) the strength of adsorption of the competing molecule ii) the concentration of each molecule iii) the type of activated carbon

34 Fig. Breakthrough curves for sequential feed of DMP and DCP to a GAC adsorber. DMP; dimethylphenol, DCP;dichlorophenol

35 Fig. Adsorption of p-nitrophenol as a function of p-bromophenol concentration from a solution containing neutral p-nitrophenol and neutral p-bromophenol

36 5) Competitive Adsorption Fig. Adsorption of Geosmin next page. 1) distilled water, 2) 10mg/humic acid, 3) 40mg/humic acid

37 5) Competitive Adsorption - The presence of 10 mg/l of a humic substance caused about 90% reduction in capacity for the musty-odor compound geosmin at C e =1 mg/l - Geosmin, Methyisoborneol (MIB) - Geosmin and MIB are metabolites of actinomycetes and blue-green algae and responsible for the common earthy-musty odors in water supplies - Odor threshold concentrations : < 10 ng/l

38 6. Equilibrium Batch Adsorption Batch adsorption operation : adsorbent + liquid separation by settling filtration centrifugation regenerated or discarded Time required to approach equilibrium depends upon concentration of solute (Adsorbate) amount of solid (A.C.) particle size of adsorbent powdered form (10~50 μm) to increase the surface area and reduce the diffusional resistance inside the pores Agitation increase contact of particles with liquid and decreases the mass transfer resistance at the surface with powdered carbon, a contact time 10~60 min is often sufficient to approach equilibrium

39 7. Kinetics of Adsorption The rate of adsorption is limited by one of the various mass transport mechanisms involved. 1) Movement of the solute from the bulk solution to the liquid film or boundary layer surrounding the adsorbent solid

40 7. Kinetics of Adsorption 2) Diffusion of the solute through the liquid film (film diffusion or external) 3) Diffusion of the solute inward through the capillaries or pores within the adsorbent solid. (Pore diffusion) 4) Diffusion along the surface of the pores (pore-surface diffusion) 5) Adsorption of the solute onto the capillary walls or surfaces. (adsorption)

41 7. Kinetics of Adsorption (HSDM Model) Homogeneous Surface Diffusion Model (Crittenden & Weber) i) Assumption 1 Each particle is a spherical homogeneous solid. Since it is difficult to quantify the impact of the pore structure on the diffusion rate and adsorbate concentration the HSDM assume that the adsorbent is homogeneous and the surface diffusive flux can be described by Fick s law 2 Surface diffusion flux is much greater than pore diffusion flux as an intraparticle mass transfer mechanism. Therefore, pore diffusion flux is neglected.

42 7. Kinetics of Adsorption (HSDM Model)

43 7. Kinetics of Adsorption (HSDM Model)

44 7. Kinetics of Adsorption (HSDM Model) 3 Liquid diffusion resistance, characterized by the film transfer coefficient, K f, occurs at the external surface of the particle 4 Instantaneous surface-solution equilibrium occurs during transport ii) The suitability of this model for describing the adsorption process in batch and column contactors has been adequately demonstrated for single- and bi-solute systems.

45 7. Kinetics of Adsorption (HSDM Model) iii) Batch Reactor Model (for a single solute) C : liquid-phase concentration (M/L 3 ) C s : liquid-phase concentration adjacent to the particle surface (M/L 3 ) X : dosage of carbon (M) V : Reactor Volume (L 3 ) K f : film transfer coefficient (L/t) R : Adsorbent Radius (L) t : time (t) D s : surface diffusion coefficient (L 2 /t) q : solid-phase concentration (M/M) ρ a : Adsorbent density that includes pore volume (M/L 3 ) q s : surface concentration at external surface of particle (M/M)

46 r s r r s r r q D r r q D r r t q π π π = + = r q r r D s 2 4π = r q r r r D t q s 2 2 1) = R s f qr t C C K R ) ( 4 π ρ π Divide by Δr and letting Δr tend to zero r r r q D r r q D r t q r s r r s r = lim 4 π π π at t=0, 0 r R : q=0 at t 0, r=0 : 0 = r q at t 0, r=r : at r=r : C s = f (q s ) 7. Kinetics of Adsorption (HSDM Model)

47 ) ( t C 3 2 s f a C C K R X R V = π ρ π ε ) ( R V 3 a s f C C K X t C = ρ ε * The rate of mass transfer through the surface film is equal to the rate of change of the average surface concentration inside the particle 2) The mass balance on the liquid-phase is given by the equation ) ( ) / (1 3 D 0 0 s a a f C C R D K t C = ρ ρ ε : porosity of batch adsorber ( = 1 D 0 /ρ a ) V : solution volume D 0 : Dosage of carbon (M/L 3 ) 3) Adsorption Equation (Freundlich) n s F s C K q 1/ = 7. Kinetics of Adsorption (HSDM Model) Linear driving force Total No. of activated carbon Total surface area of activated carbon

48 7. Kinetics of Adsorption (HSDM Model) Remark : i) These equations cannot be directly solved analytically ii) These equations can be put into a nondimensional form, and solutions may be obtained using finite difference method (FDM) or orthogonal collocation techniques.

49 Example.1) Time required to approach equilibrium in simulated adsorption experiments

50 Example.2 Adsorption of Trichlorophenol (TCP) on PAC

51 Example.2 Adsorption of Trichlorophenol on PAC

52 Example.2 Adsorption of Trichlorophenol on PAC

53 = r q r r r D t s 2 2 q iv) Column Model (Fixed Bed) A fixed-bed model must account for both the flow of solution between the particles and the diffusion of the solutes within the surface film and the particles. The model for a single solute : 1) = R s f qr t C C K R ) ( 4 π ρ π at t=0, 0 r R : q=0 at t 0, r=0 : 0 = r q at t 0, r=r : at r=r : C s = f (q s ) 7. Kinetics of Adsorption (HSDM Model)

54 7. Kinetics of Adsorption (HSDM Model) 2) [rate of accumulation of adsorbate] = [rate of flow of adsorbate into longitudinal shell by advection] - [rate of flow of adsorbate out of longitudinal shell by advection] - [rate of removal of adsorbate by adsorption ] C t = V C Z 2 n 4πR 4 3 n πr 3 ε (1 ε ) B B K f ( C C s ) Area available for liquid phase mass transfer / volume of bed Volume of liquid phase

55 7. Kinetics of Adsorption (HSDM Model) C t = V C Z 3(1 ε R ) ε R B K f ( C C s ) ε B : fraction of volumetric space unoccupied by the adsorbent or void fraction L B : Bend Length V : Interstitial velocity Z : longitudinal dimension at t=0, 0 Z L B : C=0 at t 0, Z=0 : C=C 0 q s =K F C s 1/n

56 7. Kinetics of Adsorption (HSDM Model) Remark : i) These equations cannot be directly solved analytically ii) These equations can be put into a nondimensional form, and solutions may be obtained using finite difference method (FDM) or orthogonal collocation techniques.

57 8. Fixed bed adsorption columns - Adsorption zone (Z s ) : The length of the column in which adsorption is occurred (most of the solute is removed) : = sorption zone : = mass transfer zone

58 8. Fixed bed adsorption columns - Above the adsorption zone, solute (C 0 ) in the liquid phase is in equilibrium with that sorbed (q e )on the solid phase. q 0 = K F (C 0 )1/n - The length of Mass Transfer Zone (MTZ or Z S ) depends on 1) solution flow rate 2) rate of adsorption i) smaller carbon particle size ii) higher Temperature iii) large diffusion coefficient of adsorbate iv) greater strength of adsorption of adsorbate (larger K value)

59 8. Fixed bed adsorption columns - Once the sorption zone reaches the bottom of the column, the effluent solute concentration (C) becomes a finite value and breakthrough begins - C B : breakthrough concentration ; Maximum acceptable effluent concentration. Usually C B = 0.05C 0 when C e =C B, GAC must be replaced. - As the sorption zone disappears, the effluent solute concentration ultimately increases to C 0. The column is called exhausted when C E = 0.95C 0 Fig. Typical breakthrough curve

60 - Breakthrough curve C e /C 0 spread out sharpness Effluent volume (V) Time of treatment (t) or Bed volumes (BV) = volume treated volume of GAC The shape of curve, i) Anything that causes the rate of adsorption to increase will increase the sharpness of the curve ii) Increasing the flow rate will cause the curve to spread out over a larger volume of water treated. iii) If L MTZ =0, the curve will be vertical

61 - Various shapes of breakthrough curve

62 Biological Activity; adsorbable biodegradable compounds may be adsorbed first, and then desorbed and degraded.

63 9. Fixed bed adsorption columns - Breakthrough Capacity : The amount of solute adsorbed by the column or Mass of adsorbate removed by the adsorber at breakthrough. = area above the breakthrough curve V = ( C C 0 0 ) dv

64 9. Fixed bed adsorption columns - Degree of column utilization = the mass adsorbed at breakthrough the mass adsorbed at complete saturation at the influent concentration (C * It creases as the rate of adsorption increases. 0 )

65 8. Fixed bed adsorption columns - EBCT (Empty Bed Contact Time) EBCT = = V Q = L Bed (m) Q/A (m/hr) Bulk volume of carbon in contactor (m volumetric flow rate to contactor (m Actual contact time = EBCT Interparticle porosity - L critical (Critical depth of a column) (0.4~0.5) : The depth that leads to the immediate appearance of an effluent concentration equal to C B when the column is started-up 3 3 ) /hr) L critical = MTZ when C B is the minimum detectable concentration EBCT min = L Q/A critical

66 8. Fixed bed adsorption columns - Carbon Usage Rate (CUR) : the mass of activated carbon required per unit volume of water treated. mass CUR = vol. CUR CUR g L g L mass of activated carbon in column volume treated to breakthrough, ρgac (g/l) = Bed volumes to breakthrough (C0 - C1) (mg/l) = q (mg/g) e,c 0 C 0 : the influent concentration C 1 : the effluent concentration that represents an average for the entire column run C 1 = 0 when the length of MTZ is negligible (sharp breakthrough curve) or C 1 = the concentration of nonadsorbable compounds when such substances are present. ρ GAC : apparent density of GAC (= the mass of non-stratified dry activated carbon per unit volume of activated carbon, including the volume of voids between grain) V B

67 8. Fixed bed adsorption columns - Y (Bed life) = volume of water that can be treated unit volume of carbon Y L L GAC q H O e,c0 (C 0 (mg/g - C 1 GAC 2 = ) ) mg/l ρ GAC (g/l) C 0 : the influent concentration C 1 : the effluent concentration that represents an average for the entire column run C 1 = 0 when the length of MTZ is negligible (sharp breakthrough curve) or C 1 = the concentration of nonadsorbable compounds when such substances are present. ρ GAC : apparent density of GAC ( the mass of nonstratified dry activated carbon per unit volume of activated carbon, including the volume of voids between grain)

68 9. GAC Adsorption Systems Influent Influent Influent Single-stage system Treated effluent Series system Treated effluent Parallel system Treated effluent Flow Arrangement single-stage contactor Series Parallel Parallel-series

69 9. GAC Adsorption Systems i) Single-stage contactor - GAC in a single-stage contactor must be removed about the time when the MTZ begins to exit the column. - Activated carbon usage rate (CUR) may be relatively high because only a portion of the GAC is saturated at the influent concentration. Influent ii) Series Column - MTZ is entirely contained within the downstream columns after the lead column has been saturated. - Activated carbon moves countercurrent to the flow of water. - Lower activated carbon usage rates (CUR) are achieved than with single-stage contactors. - The increased costs of plumbing counters the cost benefit of reduced CUR Treated effluent Influent Treated effluent

70 ii) Series Column Breakthrough curves for two columns in series

71 ii) Series Column Breakthrough curves for two columns in series - With only a single organic compound CUR (g/l) = Mass of GAC in one column (g) Total volume of water processed between replacements, V 2 - V 1 (L) = V 3 * Pilot test can be terminated at fig. (a) because V 3 = V 2 - V 1 - With the presence of competing organics CUR (g/l) = Mass of GAC in one column (g) V 3 (L) * Pilot test can be terminated at fig. (a) & (b) because V 3 V 2 - V 1

72 iii) Parallel Column iii) Parallel Column - Decrease the Activated Carbon Usage Rate (CUR) - Because the effluent from each of the units is blended, Only the composite flow must meet the effluent quality goals. Influent - The integral curve is developed, assuming that, i) Each contactor same size Treated effluent ii) Each contactor same θ n iii) At intervals of θ n /n, only one contactor is replaced at one time

73 iii) Parallel - Series - 1 influent Bank A 2 3 Bank B 4 effluent (partially spent carbon cells) (fresh carbon cells) - 5 influent Bank A 2 3 Bank B 6 effluent (partially spent carbon cells) (fresh carbon cells)

74 10. PAC Adsorption Systems 1) Comparison to GAC -commercially available PACs : 65~90%<325mesh(44µm) -ρ(apparent density) = 0.36~0.74g/cm3 -Iodine Number > 500 (AWWA standard) GACs have higher Iodine Numbers - Advantage Low capital investment costs The ability of the change in the PAC dose as the water quality changes (For the systems that do not require an adsorbent for much of the year) - Disadvantage High operating costs (if high doses were required for long periods of time) The inability to regenerate The low TOC removal The difficulty of sludge disposal The difficulty of completely removing the PAC particles from the water

75 10. PAC Adsorption Systems point of addition Point of addition Advantages Disadvantages Intake Long contact time, some substances may adsorb that otherwise would good mixing be removed by coagulation, thus increasing the activated carbon usage rate. Rapid mix Good mixing during rapid Possible reduction in rate of adsorption because of mix and flocculation; rea- interference by coagulants. sonable contact time Contact time may be too short for equilibrium to be reached for some contaminants. Some competition may occur form molecules that otherwise would be removed by coagulation. Filter inlet Efficient use of PAC Possible loss of PAC to the clear well and distribution system. Slurry con- Excellent mixing for the A new basin and mixer may have to be installed. tactor design contact time, no inter- Preceeding ference by coagulants, additional Some competition may occur from molecules that rapid mix contact time possible during otherwise would be removed by coagulants. flocculation and sedimentation

76 10. PAC Adsorption Systems point of addition Important criteria for selecting the point of addition. i) The provision of good mixing or good contact between the PAC and all the water being treated. ii) Sufficient time of contact for adsorption of the contaminant As the molecular size increases, the time to reach equilibrium increases. As PAC size increase, the time to reach equilibrium increases. -When adding PAC at the rapid mix, incorporation of PAC into floc particles is one factor that reduce rate of adsorption. The adsorbate must diffuse through the part of the floc surrounding the PAC particle and then into the particle itself to be adsorbed - PAC added after the alum floc formed adhered to the outer surface of an alum floc rather than being incorporate into the floc, thus avoiding interferences. - However, if insufficient time is allowed for equilibration, an increased PAC dose must be used to compensate.

77 10. PAC Adsorption Systems point of addition iii) Minimal interference of treatment chemicals with adsorption on PAC. - Activated carbon is an efficient chemical reducing agent. Thereby increasing the demand for these substances and the cost of treatment, and reducing the adsorption capacity of the activated carbon for selected compounds. HOCl + C* C*O + H + +Cl - OCl - +C* C*O + Cl - (C* : carbon surface C*O: surface oxide) NH 2 Cl+H 2 O+C* NH 3 (aq) + C*O + H + + Cl - 2NH 2 Cl+C*O N 2 (g)+2h + +2Cl - +H 2 O+C* ClO 2- +C* C*O 2 +Cl - - Adsorption at high ph is often poorer than at low ph because many organic contaminants are weak acids that ionize at high ph. - An increase of ph cause supersaturation of CaCO3 precipitates coat the PAC particle, and decrease in adsorption efficiency.

78 10. PAC Adsorption Systems point of addition iv) No degradation of finished water quality. - Addition of PAC just before the filter (sand filter) is advantageous because the PAC can be remain in the filter and be kept in contact with the water longer - The average PAC residence time is ½ of the time between two successive backwashings, assuming PAC is continuously added to the filter influent. ( PAC added right after backwashing has the longest residence time in the sand filter, but PAC added just before the shortest residence time.) - Careful monitoring of filter is necessary to ensure the following points: i) The ability of the filter to retain the PAC. ii) The rate of head loss build-up in the filter. iii) To avoid penetration of PAC to the distribution system.

79 11. PAC UF/MF hybrid System - PAC removes dissolved organic matters (pesticides, taste-or odor- causing compounds), which UF/MF does not. - Membrane (UF/MF) retains PAC.

80 11. PAC UF/MF hybrid System -Floc blanket reactor (FBR)- PAC- UF system - The backwashed effluent is recycled to the FBR, where PAC is trapped in the floc. - The FBR effluent concentration of organics is much higher than the UF effluent concentration (due to backwash with water), so the amount of adsorption per unit mass of PAC is significantly increased.