CE-102 Water Treatment

7/31/2019 CE-102 Water Treatment

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WATER TREATMENT


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Engineered Systems


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Sources of Drinking Water Groundwater

shallow wells, extract water from unconfined aquifers,generally not used for drinking water source.

deep wells, extract water from confined aquifers, generally

used for drinking water source.

Groundwater is normally potable. Only chlorination ordisinfection is done for water supply.

Surface water

Rivers

Lakes Reservoirs

Surface waters are generally not potable. They need to be

properly treated before supplying for drinking purpose.


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Characteristics Groundwater River Lake/Reservoir

Safety(bacteriological) Generally safe Not safe Better than river

Composition Constant Varying withseason

Relativelyconstant

Hardness Present Variable Variable

Turbidity/SS Nil Present Very Less

Minerals Present Variable(Generallyoptimum)

Variable

(Generallyoptimum)

Iron & Manganese Sometimes Nil VariableColor Low /nil Less/Nil Present

Nitrate Sometimes Less Less

Hydrogen Sulphide Sometimes Nil Nil

Sulfates & Carbonate Present Variable Variable

Taste & Odor Sometimes Less Summer


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Water Treatment objectives

Wholesome water potable as well aspalatable.

Fit for metabolism and health

The availability of reliable supply of

wholesome water is one of the most

important determinants of our health


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Steps Surface Water Hard Groundwater

1 Screening/Microstrainer Aeration

2 Pre-Sedimentation/Pre-

Chlorination

Oxidation/Prechlorination

3 Rapid Mixing (CoagulationTank)

Rapid Mixing (Lime & SodaAddition

4 Slow Mixing (Flocculation

Tank)

Precipitation

5 Sedimentation Sedimentation

6 Filtration Filtration

7 Adsorption Optional fororganics removal for better

taste/color/odor)

Recarbonation

8 Disinfection Disinfection


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Surface Water Treatment

Primary objectives are to

1. Remove suspended material (turbidity)

and color

2. Eliminate pathogenic organisms

Treatment technologies largely based on

coagulation and flocculation


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Type I Settling -- Stokes Law

18

)(2dg

v ss

where

s=settling velocity

s = density of particle (kg/m3)

= density of fluid (kg/m3)

g = gravitational constant (m/s2)

d = particle diameter (m)

= dynamic viscosity (Pas)

Settling ofdiscrete

particles


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Overflow rate

sA

Qv

where

v = overflow

rate (m/s)

Q = water flow

(m3/s)

As= surface

area (m2)


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Surface water

from supplyRapidMix

FlocculationBasin

Sedimentationbasin

Sludge

RapidSand Filter

Disinfection

StorageToDistribution

System

Screen Coagulant


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Surface water

from supplyRapidMix

Clariflocculator

Sludge

RapidSand Filter

Disinfection

StorageToDistribution

System

Screen


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Coagulation and Flocculation

Goal: To alter the surface charge of the

particles that contribute to color and

turbidity so that the particles adhere to one

another to form bigger particles that settleby gravity in reasonable time.


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Colloids

Small particles (0.001 to 1 m)

Usually negatively charged

Particles repel each other; they are calledstable particles.


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Coagulation and Flocculation

Coagulation (process)

+

+

++

+

+

Colloidal particles(0.001 - 1 m)

floc(1 - 100 m)

++

++

+

+ +

++ +

+

+

++

+

++

++

++ +

+

+

++

++ +

++

++

+ + +


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Coagulant

Non-toxic and relatively inexpensive

Alum: Al2(SO4)3.14H2O, Ferric chloride: FeCl3, Ferric

sulfate: FeSO4, Polyelectrolyte

Insoluble in neutral pH range we do not want high

concentrations of metals left in treated water.


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How does alum work?

Al2(SO4)314H2O 2Al3++ 3SO4

2-+ 14H2O

2Al3+ + colloids neutralize surface charge

2Al3+ + 6HCO3- 2Al(OH)3(s) + 6CO2

If insufficient bicarbonate is available:

Al2(SO4)314H2O 2Al(OH)3(s) + 3H2SO4-

+14H2O

Optimum pH: 5.5 to 6.5

Operating pH: 5 to 8


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Aquometallic ions, great

affinity to surfaces

Neutralization of

surface charge on

colloids

Al3+ + H2O -------- Al(OH)2+ + H+

Al3+ + H2O -------- Al(OH)2+ + H+

Al 3+ + H2O -------- Al(OH)3 + H+

Aquometallic ions

Aluminum Hydroxide Flocs


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Rapid Mixing

Used to blend chemicals and water being

treated

Retention time of rapid mixers range from 10 to

30 second. Mechanical mixing using vertical-shaft impeller

in tank with baffles.


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Rapid Mixing


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Flocculation

Paddle units

rotate slowly,

usually


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Flocculation


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Sedimentation/Settling

Following flocculation, the water then flows into

the settling basins

Water in settling basins is nearly quiescent

low flow with little turbulence.

Water resides for at least 2 hours in the tank

and the flocs settle out at the bottom, from

where they are removed. This type of settling of

particle aggregates is called type-II settling.


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Circular Clarifiers


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Filtration

The final step in removing particles isfiltration. It removes those particles thatare too small to be effectively removed

during sedimentation Multiple removal mechanisms depending

on design

Sedimentation effluent: 1 - 10 NTU

Desired effluent level:


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Filtration

Depending the flow rate through filters, they may beclassified as slow sand filter and rapid sand filters.

Sometimes pressure is applied in rapid sand filters to

increase flow rate, then they are called pressure filters.

Depending on the media used, filters are classified as

below.

Single media: sand

Dual media: anthracite coal and sand

Multimedia: anthracite coal, sand and garnet


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Removal Mechanism of filtration

Mechanical Straining of particles. When size of

particle is larger than void space in the media,

particles do not pass through media. (slow sand

filter)

Biological mechanism-Impurities removal my

microorganisms on the top filter layer. (slow

sand filter)

Adsorption to filter media.

Sedimentation on filter media


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Filter Design

whereva=face velocity(m/day) or loadingrate (m3/daym2)

Q = flow rate (m3/day)

As= filter surface area(m2)

Slow sand filters:

va = 2.9 7.6 m3/daym2

Rapid sand filters:

va= 120 m3/daym2

Removal mechanisms

are different

Rapid sand widely used,

s

aA

Qv


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Rapid Sand Filtration

As particles are removed - filter becomes

clogged - headloss increases, turbidity

increases

Must be backwashed which takes about

10-15 min and is done about once per day

Must be designed to handle flow with one

filter out of service


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Rapid Sand Filtration

Backwashing is accomplished by forcing water(and sometimes air) up from the clearwell back

through the filter.

The particles in the filter media become

suspended, releasing the trapped particles.

Backwash water is retreated or disposed of.


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Head Loss


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Groundwater Treatment

Primary objectives are to

1.Remove hardness and other minerals

2.Eliminate pathogenic organisms Treatment technologies largely based on

precipitation


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Groundwater Treatment

Ground waterfrom wells

Sedimentationbasin

Sludge

Recarbo-nation

To Distri-butionSystem

RapidMix

SlowBasin

Disinfection

Storage

CO2

Lime & Soda


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TYPICAL TREATMENT SCHEME FOR RIVER WATER


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ADVANCED WATER TREATMENT FACILITYTYPICAL TREATMENT SCHEME FOR RIVER WATER


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REVERSE OSMOSIS


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REVERSE OSMOSIS

DISINFECTION


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DISINFECTION It is a process of killing pathogens in water.

Some contamination may occur during transporting water to

consumers via pipelines. Some amount of chemicals are left inwater in the disinfection process, which provide residual

protection.

A large number of chemicals are used for disinfection they

include halogen group, i.e. chlorine, bromine and iodine.

Fluorine oxidizes water and therefore it is not used. Chlorine is

frequently used in public water supplies, bromine for pond

waters and iodine often has military applications in remote

areas. Ammonia added with chlorine for disinfection forms

chloramines in water which provide residual protection,because chlorine gas escapes from water very soon after

application. Chlorine forms trihalomethane (THM, e.g.

chloroform) in water which are carcinogens. Good water

supplies therefore use ozone for disinfection, which has the


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DISINFECTION

The factors that affect disinfection are

Characteristics of disinfectant, i.e. its intrinsic

nature

Characteristics of water, i.e. pH, temperature

and presence of extraneous matter

Contact time, i.e. the duration for which the

disinfectant remains in contact with water to be

disinfected


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DISINFECTION

Killing of microorganisms in disinfection process follows firstorder kinetics:

This is called Chicks law.

The amount of chemicals need to be added for achieving a

certain percent kill of microorganism follows the following

relationshipCn t = constant. Where C is concentration of disinfectant, i.e.

chemical that does disinfaction, in mg/L, t is contact time in

min and n is an exponent for a particular species, e.g. 0.88 for

E-Coli.

kteNNor

kNdt

dN

0


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Documents

CE-102 Water Treatment