August 24, 2017 Arun Kumar (arunku@civil.iitd.ac.in)

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by Dr. Arun Kumar (arunku@civil.iitd.ac.in)

Wk5:Water Treatment Unit Processes

Objective: To introduce concepts of water treatment using sedimentation, coagulation, ion-exchange, adsorption and disinfection processes

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Step 1. List water quality characteristics which need to be removed

• Parameters:

– Ions (arsenate; chromate ions;nitrate)

– Organic compounds (pesticides, etc.)

– Pathogens (viruses, if we have human fecal pollution)

– Solids (depends if there is a fracturing in subsurface)

– Gases(methane, etc.)

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Step 2. Identify unit(s) which can remove at least one type of contamination

• Ions (removal by: adsorption; coagulation-

flocculation; chemical precipitation; bio-adsorption;

membrane process)

• Organic compounds (degradation/removal by:

oxidation; adsorption; biodegradation; reduction-

oxidation; irradiation; membrane process)

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Step 2 contd.

• Pathogens (killed by : adsorption; coagulation-

flocculation; chemical precipitation; disinfection;

boiling; irradiation; membrane process)

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Step 2 contd.

• Solids (removal by : settling; filtration; membrane

process)

• Gases (removal by) : aeration; adsorption;

solubilization)

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Example 1: Groundwater �produce drinking water)

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Treatment schematic (GW� Potable drinking

water)

• Raw Ground water � aeration chamber �

Softening unit � Filtration with chlorination

�Disinfection � Storage

• See sequence of units used

• Chemical is required to be added

• Water is treated

• Chemical sludge is produced

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Units used for (GW� Potable drinking water)

• Aeration chamber (to remove gases; using air)

• Softening unit (to remove cations; using softener and/or cation exchangers)

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Treatment schematic (GW� Potable drinking

water)

• Filtration with chlorination (to remove solids; to kill microbial growth on filter unit surface)

• Disinfection (to kill microorganisms before water is supplied for public consumption)

• Storage

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Example 2: Yamuna River Water �produce drinking water)

• Think for 5 minutes for two steps.

• Step 1: water quality characteristics determination

• Step 2: selection of units, their order

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Example 2: Yamuna River Water �produce drinking water)

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Example 2 schematic contd.

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Treatment schematic (SW� Potable drinking

water)

• Sequence of unit processes: pre-sedimentation

�mixing, flocculation,

settling�filtration�adsorption �disinfection

• Function of unit processes: solids removal �

removal of ions, and solids using chemical

addition�removal of smaller particles �removal of

organic compounds and ions�oxidation of oxygen-

demanding wastes and chemical killing of

pathogens in water

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(SW� Potable drinking water) contd..

• Chlorine is added in sedimentation and filtration

tank to avoid microbial growth

• Chorine is used as a oxidizing agent and

disinfection solution also.

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So far..• Introduction of water treatment schematic concepts

• Discussion on production of water of different final usages

• Discussion on checks– Remove solids before removing bacteria

– Remove solids using sedimentation process before using filtration

– Remove organic compounds and ammonia-based compounds before disinfection

• Discussion on need for calculating solids waste produced from every unit processes and solid waste management

Question 1: GDW to Gardening water

• Which parameters do we need to remove?

• Which unit processes do we need for making

gardening water from groundwater? Is the order

of unit process relevant here?

• How does the schematic of treatment of GDW to

Gardening water differ with that of GDW-

Drinking water?

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Question 2: Injection of gardening

water runoff in soil:Requirments?

• We produced gardening water from GDW.

• Now, if we inject garden water runoff to soil,

would this be ok?

• Which parameters do we need to worry?

• Do we need to do treatment before we can inject

the water?

• Will it depend on soil type, depth to water table,

etc.?

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Question 3: Comparison

• Why schematic for GDW� DW and SW� DW

differ?

• Can one treatment plant treat GDW as well as

SW for producing DW?

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Example 3. Domestic wastewater �Discharge water to river

• Sequence of unit processes: stabilization

tank�sedimentation �biological

process�secondry settling � nitrifier

unit�denitrifier unit �disinfection�discharge water

suitable for river

• Function of unit processes: making incoming flow

rate uniform �solid removal�breakdown of organic

compound �settling of microbial biomass

�removal of ammonium ions �removal of nitrate

ions �killing of microorganisms

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Surface Water Treatment

Conventional Surface Water Treatment

• Screening (remove relatively large floating andsuspended debris)

• Rapid-mix (mixing water with chemicals thatencourage suspended solids to coagulate intolarger particles that will settle easily)

• Flocculation (gently mixing water andcoagulant allowing the formation of largeparticles of floc)

Conventional Surface Water Treatment

• Sedimentation (flow is slowed enough so thatgravity will cause flocs to settle)

• Sludge processing (mixture of solids andliquids collected from settling tank aredewatered and disposed of)

• Disinfection (ensure that water is free ofharmful pathogens)

• Distribution system protection (residualdisinfection)

Solids RemovalSizes of Particles in Water

Solids RemovalMethod 1.Sedimentation

Method1. Sedimentation

• Solids settle based on their gravitational force (with and

without externally added chemicals).

• Settling depend on solid physical characteristics

(diameter, density) and medium temperature, viscosity,

density, etc.

• Some solids do not interact with each other during

settling (i.e., discrete particles) (no change in their size

and shape). The settling is called discrete settling (Type

1 settling). Ex: settling of sand.

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Method1. Sedimentation

• Some solids interact during their settling and change

their size and shape (i.e., flocculent particles) (Type 2

settling). Ex: settling of clay; bacteria.

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Vp/Ap=((4/3*3.14*r3)/(3.14*r2))=4/3*r=2/3*dp

Sedimentation

• Time for settling = column depth/settling velocity at

steady state

• Some particle take less time and some particles take

longer time to settling.

• if t_design>t_settling, particles remove 100%. All patcies

now constitute to solid waste.

• if t_design<t_settling, particles do not remove 100%.

Remaining particles go to next unit in treatment plant

scheme.

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Some coagulants:

aluminum sulfate,ferric sulfate

ferric chloride

Some coagulant aids:

activated silicaclay

polymers

Solids RemovalMethod 2.Coagulation-flocculation-Sedimentation

Coagulation - Flocculation

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++ +Colloidal particles

(0.001 - 10 µm) floc(1 - 100 µm)

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Method2. Coagulation-flocculation (sedimentation

after chemical addition)

• Some solids take very long time to settle (size in

submicron range or in nanometer range).

• Chemicals (ex: alum; ferric chloride) are added in

solution to (1) increase size of particles, (2) capture them

in hydroxide flocs and then precipitate them.

• Coagulation methods: (i) ionic layer compressions, (2)

charge neutralization and surface complexation, (3)

sweep coagulation(iv) polymeric bridging

• Ex: ferric chloride gives ferric ions (acidic pH) and ferric

hydroxide (basic pH). These species work in 2 different

ways to improve particle settling.

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Coagulation-Flocculation

• Double Layer

Compression

• Adsorption of

Aluminum to

Produce Charge

Neutralization

• Interparticle Bridging

• Enmeshment in

Al(OH)3 Precipitate

(sweep floc)

Picture Source: Malvern Instruments, Zeta-Meter Inc.

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Coagulation-Flocculation-Sedimentation

Full-scale

Pilot-scale

Bench-scale