Coagulation and Chemical Applications

Lecture No. 4

1. General Flocculation is the gentle mixing phase that follows the rapid dispersion of coagulant

by the flash mixing unit. The purpose of flocculation is to accelerate the rate of particle collisions, causing the

agglomeration of electrolytically destabilized colloidal particle Flash mixing is an integral part of coagulation

Coagulant 1. destabilizes2. enmeshes

Colloid10-9 to 10-6 m

2. Purpose

The purpose of coagulation is to make the contaminants grow bigger, agglomeration, and begin to settle out because of their increased size. The purpose of flash mixing is to quickly and uniformly disperse water treatment chemicals throughout the water that is being processed.

Metal coagulants such as alum and ferric chloride are absorbed almost immediately and the dispersion should therefore be completed within 1 or 2 seconds.

The ratio of liquid alum to raw water volume is typically 1:50,000.

3. Considerations

Type of Coagulant. The most common coagulants are metal salt coagulants, Meq(OH)p

Z+ : aluminum sulfate (alum), ferric chloride and ferric sulfate. Synthetic coagulants include PDADMA and cationic polymers such as chitosan. Metal coagulants undergo hydrolysis when combined with water whereas all polymers do

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not. The hydrolytic compounds such as Al(H2)63+ and Fe(H2O)3

3+ are formed in less than a second are readily absorbed to colloidal particles thus destabilizing their charge. The solubility of the various chemical is pH dependent. See F3.2.3-1. p.78. The most reasonable chemical application sequence is to first lower the pH of the raw water through the addition alum, thereby allowing divalent and trivalent aluminum hydrocomplexes to form and then adjust the pH of the water to the range of minimum aluminum solubility pH 6.3 in order to allow aluminum hydroxide (floc) formation.

Al(SO4)3 (alum) + H2O Al6(OH)15+3 + Al7(OH)17

+4 + Al8(OH)20+4 + Al13(OH)34

+5

Type of Chemical Diffuser. Most chemicals are converted into solution prior to being fed to the raw water. The chemical is fed as a solution or slurry. The chemical diffuser for solutions is most often a pipe with multiple orifices. For slurries, for pipe applications, the open end of the diffuser pipe acts as the diffuser; for channel flow, a trough with notches or holes at the bottom acts as the diffuser.

Head Loss for Flash Mixer. Head loss of 2’ is required if a non-mechanical type of mixing is used such as a hydraulic jump or in-line static mixer. Studies have shown that mechanical flash mixing may be ineffective due to short circuiting and frequent repairs.

Variations in Flow Rate. GT should be from 300-1600 1/seconds.

4. Type and Selection Guide for Flash Mixing

See F.3.2.3-5 p.86. Alternatives in order of preference include:- Diffusion mixing by pressured water jets- In-line static mixing- In-line mechanical mixer- Hydraulic mixer- Mechanical mixer- Miscellaneous including pipe grid

Pressured Water Jets. No headloss, effective, flexible, less power consumption. Source of the water is plant water. Minimum pressure is 10 psi. Clogging a potential problem.

In-line static mixing. Advantages include: no moving parts, no external energy, fewer clogging problems. Disadvantages include: Minimal flexibility on the degree of mixing which is a function of flowrate and the units are proprietary.

Hydraulic Mixer. Units include: Parshall flumes, Venture meters and weirs. The turbulence is a function of flowrate and hence there is no positive control over the degree of mixing.

Mechanical mixer. Most frequently used in the industry. Common design parameters: G=300s-1, DT=10-30 s and the power requirement is .25-1.0 horsepower per million gallons of water per day. This type of mixing is NOT preferred because: the lack of instantaneous mixing characteristics, short-circuiting, mixing period is too long for metal coagulants, backmixing.

Design Criteria:

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For flash mixing:G = (P/V).5 units p.88GT=300-1600 s-1

Example:Given: Basin 2’x2’x3’, P=150ft.lb/sFind: GG = (P/V).5 V=lxwxd=2’x2’x3’V=12ft3

= 2.73x10-5lb.s/ft2 (assume 50F)G= [].5 = [457,875].5

G = 676 s-1

Given: The above system but G=800Find: What is the power requirementG = (P/V).5 800 = [ ] 1/2

P = 209.6 ft.lb/s

For pump diffusion:GT = 400-1600 (1000 average)Mixing jet velocity 20-25fps at the orificelow pHFor in-line static mixers:GT=350-1700 (1000 average)DT=1-5 s

Examples:Given: Mechanical Flash Mixing System.Q=12.53MGD = 19.39cfs, G=900s-1, Mixing time=1s, = 2.73x10-5lb.s/ft2 (assume 50F)Find: 1.)tank size 2.)mixing time 3.) Horsepower (75% efficiency)1.) tankAn area that will accommodate a mixing device and a smaller actual mixing zone is required. See F3.2.3-8, p92. Say a 8’x8’ square box that is 10’deep. An intermediate slab containing a 3.0’ diameter hole at midheight would be the effective mixing zone. Attach a 3.5’ length of pipe at the opening to act as a mixing reactor tube.2.)Mixing timeG = (P/V).5 units p.79v = Q/A , A= D2 v = 12.53MGD x 1.547cfs/MGD / 32 = 19.39/7.065v = 2.74 fpsDT = length of reactor tube / v = 3.5’ / 2.74fpsDT = 1.28 s

3.)hpThe volume is the size of the whole times the mixing length of the pipe.V = D2 x h = 32 x 3.5 V = 24.73 ft3

= 2.73x10-5lb.s/ft2 (assume 50F)

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G = (P/V).5 900 = (P/2.73x10-5lb.s/ft2 x 24.73 ft3).5 P = 546.85 ft.lb/sAssuming a 75% efficiencyP = 546.85 ft.lb/s / .75P = 729.14 ft.lb/s

Given: Same problem but Pump Diffusion as the Flash MixerQ=12.53MGD=19.39cfs, Pipe diameter=36”, Gt=1000, G=750s-1, minimum jet velocity = 20-25 fps, alum dosage 10-50mg/l, Length of the mixing zone=1.5D,=2.73x10-5lb.s/ft2 (assume 50F)Find: 1.)mixing time 2.)hp 3.) pump capacity(rule of thump, 2-5% plant flow 4.)jet velocity and diameter of the orifice

1.) mixing timeD=36”=3’L=1.5D=1.5(3’)L=4.5’V = D2 x h = 32 x 4.5V = 31.79ft3

DT= V/Q = 31.79ft3 / 19.39cfsDT= 1.64 s

2.)hpG = (P/V).5 750 = (P/2.73x10-5lb.s/ft2 x 31.79 ft3).5 P = 488.18 ft.lb/s

3.)pump capacity2-5% plant flow, say 3.5%pump capacity = 3.5% (plant flow) = .035(12.53MGD)pump capacity = .439MGD x 694.4gpm/MGDpump capacity = 304.5 gpm

4.) jet velocity and diameter p.95Use the basic pump formulahp = Qh/e

hp=488.18 ft.lb/s, from step 2=62.4 lb/ft3 for waterQ=304.5 gpm x .002228cfs/gpm = .678cfse=1=100% since the theoretical is used to determine orifice size488.18 ft.lb/s = 62.4 lb/ft3 x .678cfs x h / 1h = 11.54fth = v2/2g11.54 = v2/(2x32.2)v=27.26fpsQ=vA,A=Q/vA = D2 = .678cfs /27.26fps = .02487ft2

D=.158ft = 1.89inches

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Given: Static Mixer as the Flash Mixer. P. 98. Q=10MGD. Assume dt=2s and the plant influent is 6-8fps, diameter =30”. According to the manufacturer, the pressure drop is P = (.007Q/D4.4))/N.Find: 1.)type 2.)number of mixing elements, rule of thumb, 1 element=1.5-2.5 times the pipe diameter 3.) G

1.)typeSelection of mixer is a matter of obtaining the catalogues of the propriety purveyors who sell static mixers, speaking to their representatives, perhaps obtaining a list of customers, contacting the customers, and weighing disparate items such as cost, reliability, each of installation, durability and so forth in making a preliminary selection.

2.) number of mixing elementsThe practical mixing time is limited to 2s or 3s for pipes larger than 5’, use 2s.distance = rate x time, thus the length of the mixing unit is 2 times the rate, velocity, L=2v. total mixer length =2(6-8fps)total mixer length =12-16’ length of 1 element = 1.5-2.5 times the pipe diameter = (1.5-2.5)30”length of 1 element = 60”= 5’number of elements = total mixer length / length of 1 element = 12-16’ / 5’number of elements = 2 or 3Use 3 elements at 5’ each for a total length of 15’ 3.)GtG = (P/V).5 , P=Qh, units p.88V of mixing unit, volume of pipe or cylinder where mixing occurs = D2 x L = (30/12)2 x1 5’V = 73.59ft3 P = (.007Q/D4.4)NQ=10MGD x 694.4gpm/MGD = 6944 gpm = 15.47cfsP = x 3P = 2.59 psihead loss = 2.59psi x 2.31’/psihead loss = 5.98ftP=Qh=15.47cfs x 62.4lb/ft3 x 5.98ftP= 5773 ft.lb/sG = (P/V).5 = (5773ft.lb/s / 2.73x10-5 lb.s/ft2x 73.59ft3 ).5 G = 1695 s-1

5. Operation and Maintenance

Jar tests have been used successfully to optimize coagulant dosages. The jar test has 4 basic objectives:

- optimization of the coagulant- optimization of the chemical application sequence- optimization of the mixing energy and time, Gt- evaluation of clarifier and filtration performance- evaluation of the corrosive characteristics of the settled water.

Zeta potential has been tried with minimal success

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The primary object of flash mixing is to effectively coagulate colloidal matter present in raw water by applying the proper amount of coagulant through good flash mixing prior to flocculation.

HOMEWORK No. 4, Coagulation and Chemical ApplicationsRead Chapter 1, 2 and part of 3 pp. 1-104Problems: 4A.Given: A rapid-mix basin is to be designed for a water coagulation plant. Q=4MGD, DT=30s, L=W, Depth=1.25L, G=900s-1 @ 50F. = (assume 50F)Find: 1.)Basin dimensions 2.)hp, find P, 1hp=550ft.lb/s

4B.Given: Mechanical Flash Mixing System.Q=5MGD, GT=850s-1, Mixing time=1s, = 2.73x10-5lb.s/ft2 (assume 50F), App.6 p.604, 1kW=1000 J/s, .746hp=1kW.Find: 1.)volume of the tank 2.) Horsepower (75% efficiency)

4C.Given: Pump Diffusion as the Flash MixerQ=50MGD, Pipe diameter=48”, Gt=1000, G=800s-1, minimum jet velocity = 20-25 fps, alum dosage 10-50mg/l, Length of the mixing zone=1.5D.=2.73x10-5lb.s/ft2 (assume 50F)Find: 1.)mixing time 2.)hp 3.) pump capacity in gpm(rule of thump, 2-5% plant flow, use 2.5%) 4.)jet velocity and diameter of the orifice.

4D.Given: Static Mixer as the Flash Mixer. Q=50MGD. Assume dt=2s and the plant influent is 6-8fps, diameter =48”. According to the manufacturer, the pressure drop is P = (.007Q/D4.4) N units p.89, =2.73x10-5lb.s/ft2

Find: 1.) type 2.) number of mixing elements, rule of thumb, 1 element=1.5-2.5 times the pipe diameter, use 2 3.) the head loss (psi and ft)4.) G

4E. Given: Design the rapid mix units for your project.