CEE 371 Water and Wastewater Systems

Transcription

1 Updated: 1 November 009 CEE 371 Water and Wastewater Systems Print version Lecture #16 Drinking Water Treatment: Coagulation, mixing & flocculation Reading: Chapter 7, pp.5-9, David Reckhow CEE 371 L#16 1 Coagulation: Purpose Initiate the chemical reactions that render conventional treatment effective When combined with subsequent physical removal, it achieves: Removal of turbidity historically the reason for coagulation Removal of natural organic matter more recently of importance Some removal of pathogens Giardia, Cryptosporidium David Reckhow CEE 371 L#16 Lecture #16 Dave Reckhow 1

2 Chemical Addition & RM so what are we adding? Coagulants Alum (aluminum hydroxide) and polyaluminum chloride ferric (sulfate, chloride) Oxidants permanganate, chlorine Other PAC (powdered activated carbon) David Reckhow CEE 371 L#14 3 Colloid Stability DLVO theory Repulsive Electrostatic Repulsive Force Net Force En nergy Primary Minimum Van der Waals Attractive Force Distance bt between centers Attractive David Reckhow CEE 371 L#16 4 Lecture #16 Dave Reckhow

3 Charge neutralization David Reckhow CEE 371 L#14 5 Colloid Stability Impact of Charge Repulsive neutralization Electrostatic Repulsive Force En nergy Net Force Van der Waals Attractive Force Distance bt between centers Attractive David Reckhow CEE 371 L#16 6 Lecture #16 Dave Reckhow 3

4 Destabilization with Polymers Natural polymers Alginates Synthetic polymers Cationic, anionic, non-ionic No need to reach primary minimum distance Also used to strengthen floc David Reckhow CEE 371 L#14 7 Coagulant chemistry Ferric Sulfate (also ferric chloride) - Fe ( SO 4 ) 3 + 6OH Fe(OH ) 3 + 3SO4 - Alum (the most common coagulant) + Al ( SO ) 18H O Al( OH) + 3SO + 6H + 1H O GFW= 666 AW= 7 Alum is ~8.4% Al by wt. Mechanisms Charge Neutralization Sweep Floc (enmeshment) Adsorption / complexation for Dissolved substances David Reckhow CEE 371 L#14 8 Lecture #16 Dave Reckhow 4

5 Chemistry of Aluminum LOG (Al) (m mol/l) Precipitation & adsorption zone L CHARGE NEUTRALIZATION TO ZERO ZETA POTENTIAL WITH n+ Al (OH) /Al(OH) (s) X Y Al(OH) + Al (OH) Al Al TOTAL C B A RESTABILIZATION ZONE OPTIMUM SWEEP SWEEP COAGULATION CHARGE NEUTRALIZATION CORONA TO ZERO ZETA POTENTIAL WITH Al(OH) 3(s) Al(OH) 4 - Al(OH) 3(s) ALUM AS Al (SO ) x 14.3 H O-mg/l 4 3 ZETA POTENTIA 0 UNCOATED COLLOID IEP D E IEP (IOSOELECTRIC PAINT) COLLOID COATED n+ WITH ( Al(OH) (s) ) David Reckhow CEE 371 L#14 9 ph OF MIXED SOLUTION Coagulation: Empirical Tests Jar Testing Laboratory experiments with varying coagulant doses at varying phs David Reckhow CEE 371 L#14 10 Lecture #16 Dave Reckhow 5

6 9 Impact of ph on alum coagulation Control (no alum) DOC (mg/l) mg/l dose 96 mg/l dose Rennes IV Raw Water (France) 11/19/84 Reckhow & Bourbigot (unpublished data) 48 mg/l dose Manganese Mn precipitation David Reckhow CEE 371 L#14 11 ph uble Manganese (μg/l) Sol Role of NOM in Coagulation Stoichiometry: Dempsey et al., 1994 J. AWWA More DOC requires more coagulant Depends on ph: affects charges Some typical values 1 mg-al/mg-doc at ph mg-al/mg-doc at ph 5.5 Role of ph Al less soluble and precipitates faster at neutral ph; floc grow and settles faster More NOM may be removed at slightly lower ph David Reckhow CEE 371 L#16 1 Lecture #16 Dave Reckhow 6

7 NOM controls coagulant Particle Charge versus NOM Charge and Coagulant Dosing Particles: For clays; good estimate for particles in general: µeq/mg of clay Use say 0.1 µeq/mg of clay NOM: Aquatic humic matter: good estimate is about 10 µeq/mg DOC Example: Reservoir Supply Turbidity: 5-10 NTU (say clay or similar particles at 3 mg/l) TOC ~ DOC: 3 mg/l (half of the NOM is composed of aquatic humics) Particle Negative Charge : 0.1 μeq x mg mg μeq 3 = 0.3 L L μeq mg μeq NOM Negative Charge :10 x 0.5 x 3 = 15 mg DOC L L Courtesy of Dr. JK Edzwald David Reckhow CEE 371 L#16 13 Coagulation: Empirical Model Moomaw et al., 199 DOC trt ( DOC ) ( ph ) ( Dose ) = raw coag Al David Reckhow CEE 371 L#14 14 Lecture #16 Dave Reckhow 7

8 Question If you have a water that contains 5 mg/l DOC and 50 ntu turbidity, about what will the optimal alum dose be at ph 7.5 in mg-al/l? A. 100 mg/l B. 50 mg/l C. 10 mg/l D. 5 mg/l E..5 mg/l David Reckhow CEE 371 L#16 15 Coagulation: Empirical Tests Jar Testing Laboratory experiments with varying coagulant doses at varying phs David Reckhow CEE 371 L#14 16 Lecture #16 Dave Reckhow 8

9 Jar Test Demo & Question Tonawanda River water has the following characteristics 5 mg/l DOC; entirely humics 50 mg/l alkalinity ph ~7 What will the optimal alum dose be? 1. 0 mg/l. 5 mg/l mg/l mg/l mg/l mg/l David Reckhow CEE 371 L#16 17 Conventional Treatment rapid mix, flocculation, sedimentation in one long tank with baffles H&H, Fig 7-4, pg. 1 David Reckhow CEE 371 L#16 18 Lecture #16 Dave Reckhow 9

10 Coagulant Addition: Rapid Mix Purpose to provide rapid and complete mixing of chemicals at the head of a plant Two types: tank mixer or in-line Tank Mixer Tank 3 to 10 ft diameter flow through, top to bottom 10 to 60 second detention time vertical shaft turbine impeller G= s -1 David Reckhow CEE 371 L#16 19 Rapid mix Tank Impeller Iron deposits Reading, MA David Reckhow CEE 371 L#16 0 Lecture #16 Dave Reckhow 10

11 Rapid Mix Design Detention Time seconds is most common Mixing Energy differences in fluid velocity: velocity gradient change in velocity as you move up or down vertically in a reactor since velocity is [L/T] and vertical distance is [L], the G value is in units of reciprocal time [T -1 ] Camp: related it to power input (P), tank G volume (V) and viscosity (µ) 1 P G = P = μvg μv dv dy David Reckhow CEE 371 L#16 1 RM example design Given: square basin area depth = 1.5 x width Q = 7570 m 3 /d t R = 40 sec temp = 10 C design tank for G = 790 sec -1 David Reckhow CEE 371 L#16 Lecture #16 Dave Reckhow 11

12 example solution calculate tank volume and dimensions m d V = Qt = 40sec 3. 5m ( ) 3 R d 4x60x60sec = ( 3.5m ) = 1. m w = determine power C, µ=1.307 x 10-3 N-s/m P = μvg = 1.307x10 = 850 N m s 3 N s m 3 ( width) depth 1.5w V = = 3 1 ( 3.5m )( 790s ) =.85kW d =1.5w = 1. 76m David Reckhow CEE 371 L#16 3 Rapid mix In-line injector Bath, ME David Reckhow CEE 371 L#16 4 Lecture #16 Dave Reckhow 1

13 Other mixing in-line static mixers injection just upstream of a pump David Reckhow CEE 371 L#16 5 Raw Water Influent Regional WFF Permanganate Feed Not currently used Fitchburg MA David Reckhow 18 Oct 001 CEE 371 L#16 6 Lecture #16 Dave Reckhow 13

14 Flocculation: Purpose Provides slow mixing to allow destabilized particles and precipitates to grow in size Larger size helps with subsequent physical removal Gravity settling Flotation Filtration David Reckhow CEE 371 L#14 7 Flocculation: Purpose Promote agglomeration of particles into larger floc Units often designed on the basis of mixing intensity as described by the velocity gradient, G some mixing is needed to keep particles in contact with other particles too much mixing can cause floc break-up David Reckhow CEE 371 L#14 8 Lecture #16 Dave Reckhow 14

15 Flocculators Drive shaft Usually 4 arms with 3-4 slats per arm David Reckhow CEE 371 L#14 9 Flocculation Horizontal Shaft MWDSC Weymouth Plant 1 Dec 05 David Reckhow CEE 371 L#14 30 Lecture #16 Dave Reckhow 15

16 Flocculation 4 Wooden paddles Chicago David Reckhow CEE 371 L#16 31 Flocculation parallel shafts New Orleans David Reckhow CEE 371 L#16 3 Lecture #16 Dave Reckhow 16

17 Vertical Shaft Flocculator Motor and gear box Andover, MA David Reckhow CEE 371 L#16 33 Flocculation: Design Flow through velocity: 0.5 to 1.5 ft/min For low energy variable speed paddle flocculators peripheral velocities of ft/sec horizontal shaft: slower, best for conventional vertical shaft: faster, best for direct filtration typical dimensions 1 ft deep length/width = 4 30 min detention time; at least 0 min at max daily flow David Reckhow CEE 371 L#14 34 Lecture #16 Dave Reckhow 17

18 Flocculator Mixing Design I Use the same velocity ygradient concept from rapid mix where G is in s -1 ; P is in ft-lb/s or N-m/s, V in ft 3 or m 3 G = absolute viscosity, µ, is in lb-s/ft or N-s/m typical values are s -1, more common s -1 Relate to power dissipated by a paddle P μv 1 Recall that: 1N = 1kg-m/s Cd Aρv P = where: 3 C d = coefficient of drag (1.8 for flat plates) A = area of paddles in ft or m ρ = density of water lb/ft 3 or kg/m 3 v = velocity of paddles relative to water in ft/s or m/s David Reckhow CEE 371 L#14 35 Flocculator Mixing Design II first relate relative water:paddle velocity to an absolute rotational velocity where: ( k) v = v p ( 1 k) = πrn 1 v p = absolute velocity of paddle blade in ft/s or m/s r = radius from rotational axis to center of paddle in ft or m N = rotational speed in rev/s k = ratio of water velocity to paddle velocity (range: ; most ttypical) then combine with the power dissipation equation n P = Cd Aρ π 1 + where: ( 1 k) 3 ( N ) ( r r ) David Reckhow CEE 371 L#14 36 r 1 and r are radii to the first and second paddle, respectively n = # of paddle arms Lecture #16 Dave Reckhow 18

19 Flocculator Mixing Design III Consider Tapered Flocculation Decrease G in successive stages Reduce mixing speed; fewer paddles, etc. Gt Concept G=50s -1 G=0s -1 G=10s -1 Product of G and retention time: 50, ,000 Good practice: Thomas Camp David Reckhow CEE 371 L#16 37 Jar Test Demo Raw Water concentrate Expected optimal alum dose To liters add: X mg Humic acid X mg Tannic acid X mg NaHCO 3 X mg kaolinite Alum solution 5g/L Dilute by 0 x Add 50 ml to 1 L 5 mg/l DOC 50 mg/l alkalinity ph ~7 1 mg-al/mgdoc 5 mg/l Al 60 mg/l alum More because of high SUVA Add the following Alum doses 0 5 ml = 5 mg/l 10 ml = 50 mg/l 15 ml = 75 mg/l 0 ml = 100 mg/l 30 ml = 150 mg/l Procedure 1 min RM, 0 rpm for 0 min David Reckhow CEE 371 L#16 38 Lecture #16 Dave Reckhow 19

20 End To next lecture David Reckhow CEE 371 L#16 39 Lecture #16 Dave Reckhow 0