water treatment calculations updated

8/13/2019 Water Treatment Calculations Updated

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Water Treatment


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Water tretment process

Basic steps

Raw Water

Storage

Mixing Flocculation

Sedimentation

Filtration

Clear Well

Distribution

AerationCoagulant, pH Adjustment

Disinfectant (Cl2, NaOCl)

Screening

Raw water


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Coagulation

Find the requirement of alum and lime to

treat water (107 L/day) at alum dosage (30mg/L) when original alkalinity present is 8.5

mg/L.


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Alum required =7

= 300 /

4.5 mg/L alkalinity (CaCO3) is required for 10mg/L dosage of alum.

Alkalinity required = (4.5/10)* 30 8.5= 5mg/ L

56 mg of CaO is required for obtaining 100 mg/L

of CaCO3.Lime required = 5*(100/56)*(107 / 106)

= 90 kg/day


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Sedimentation

Factors affect for size of settling basin

Detention time

Overflow rate Settling velocity of particle

Horizontal velocity (for rectangular tanks)


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Detention time =

Detention time (days)

Basin volume(m3)

Volumetric flow rate (m3/day)

Horizontal velocity =

Flow area


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Settling velocity of particle

=

Total surface area of the basin

Overflow rate surface loading) =

Over flow rate (m3

/m2

day)

Length of the tank =


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Strokes law

=

18

2

=

18 1 2

, Density of particle and water respectively

Particle diameter

Viscosity of water

Specific gravity of particle

Acceleration due to gravity


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Filtration

The required filtration rate is calculated using the formulabelow

( 2) = ( )

2


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Filter backwash

The amount of water required for backwash depends on,

Design of the filter

Quality of the water being filtered

( 2) = ( ) 2


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Chlorination

Chlorine usage in the treatment of 18.9

million litres of water is 7.71 kg/day. The

residual after 10 min contact is 0.2 mg/L.

Compute the dosage in milligrams per liter

and chlorine demand of the water.


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Dosage= 7.71 *1000/ 18.9*106

= 0.407 mg/L

Chlorine demand = DosageResidual

= 0.407- 0.2

= 0.207 mg/L


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Activated Carbon Filters

Activated carbon filtration can effectively

reduce,

certain organic compounds such as volatile

organic compounds, pesticides and benzene

and chlorine in drinking water.

the quantity of lead and harmless taste- and

odor-causing compounds


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Treatment Principles

An adsorptive process in which thecontaminant is attracted to and held

(adsorbed) onto the surface of the carbon

particles.


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Medium for an activated carbon filter

petroleum coke

bituminous coal

lignite wood products

coconut shell or peanut shells


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Preparation of activated carbon

Subject carbon medium to steam and hightemperature (2300F) without oxygen to activatethe product

the carbon can process by an acid wash or coat

with a compound to enhance the removal ofspecific contaminants

activation produces carbon with many smallpores and, therefore, a very high surface area

Activated carbon is then crushed to produce agranular or pulverized carbon product

This creates small particles with more outsidesurface area available for adsorption


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The efficiency of the adsorption process is

influenced by carbon characteristics (particle and pore size,

surface area, density and hardness)

the contaminant characteristics (concentration,

tendency of chemical to leave the water, solubility

of the contaminant, and contaminant attraction to

the carbon surface)

contact time between the water and the carbon(the rate of water flow)


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Breakthrough point

When the activated carbon becomes

saturated (all adsorption sites filled),

contaminants can flow from the carbon back

into solution. This is called breakthrough.

In order to prevent breakthrough, some AC

filtration units will shut off the water supply

after a specified number of gallons have beentreated


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Advanced Water

Treatment


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Ion exchange

In the ion exchange process an insoluble resin

removes ions of either positive charge or

negative charge from solution and releases

other ions of equivalent charge into solutionwith no structural changes in the resin


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Purpose of using ion exchanger in

water treatment

Remove

Anions- nitrate, fluoride, arsenic and other

contaminants

CationsCalcium, Magnesium


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Types of ion exchangers

Natural: Proteins, Soils, Lignin, Coal, Metal oxides,

Aluminosilicates (zeolites) (NaOAl2O3.4SiO2).

Synthetic zeolite gels and most common -

polymeric resins (macroreticular, large pores).


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Ion exchange resin


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Plastic beads made of cross linked polystyrene

with functional groups (sulphonates) that actas ion exchange sites.

The sulphonate group has a negative charge

allowing it to attract and hold (exchange)

positive ions or cations such as H+, Ca+2, Mg+2,

Fe+2, Na+.

Those ions remain on the bead until the bead

encounters other ions for which it has a

greater affinity


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Classification of ion exchange resins


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Resin classification

Resins are classified based on the type of

functional group they contain and their % of

cross-linkages

Cationic Exchangers:- Strongly acidicfunctional groups derived from strong

acids e.g., R-SO3H (sulfonic).

- Weakly acidicfunctional groups derived from weak

acids, e.g., R-COOH (carboxylic).


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Cation exchange Softening

Use to reduce hardness

Cation exchange reaction

++

++ 2

+

represent the anionic component of the resin(Ca and Mg cations are absorbed and an equivalentamoun of Na ions is released to the solution)

Reaction during regeneration

2

++

++


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Regeneration cycle consist of three stages Fill the resin bed with brine

Slow rinse

Back wash


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The sodium concentration after regeneration

should not exceed recommended maximum

value

The concentration of sodium in the softened

water increases in proportion to the hardness

ions removed

Ex: Hardness of 43 mg/l produces water

containing 20 mg/l of sodium


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Anionic Exchangers

Strongly basicfunctional groups derived

from quaternary ammonia compounds, R-N-

OH.

Weakly basic - functional groups derived from

primary and secondary amines, R-NH3OH or R-

R-NH2OH.


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Disadvantages

High operating cost

The problem of brine water disposal


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Factors influencing resin life

Type of resin

Chemical characteristics of the water beingtreated

Operating temperature Regeneration temperature

Regeneration level ( salt applied per unit bedvolume)

Water feed rate

Bed depth


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Membrane Processes

Microfiltrtion


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A membrane is a selective barrier that permits

the separation of certain species in a fluid bycombination of sieving and diffusion

mechanisms

Membranes can separate particles andmolecules and over a wide particle size range

and molecular weights


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Membranes commonly consist of a porous

support layer with a thin dense layer on top

that forms the actual membrane

Active layer

Porous support layer


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Pressure-Driven Membrane Processes

Separate by size and chemistry

Concentration, Porosity Effects


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MF

10-300 kPa

RO

0.5-1.5 MPa

NF

0.5-1.5 MPa

UF

50-500 kPaP=

Bacteria, parasites, particles

High molecular substances, viruses

Mid-size organic substances,multiple charged ions

Low molecular substances, single charge

ions


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Membrane filtration configuration


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The growing use of MF

1. More attention paid to environmental

problems linked to drinking and non-drinking

water

2. Increased demand for water (using

currently available sources more effectively)

3. Market power


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Pore size of MF membranes


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Pores and pore geometries

Porous MF membranes consist of polymeric matrix in which pores

are present.

The existence of different pore geometries implies that different

mathematical models have been developed to describe transport

phenomena.


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MF membranes preparation

Stretched PTFE memb rane

Stretching

Semycristalline polymers (PTFE, PE, PP)

if stretched perpendicular to the axis of

crystallite orientation, may fracture in

such a way as to make reproduciblemicrochannels.

The porosity of these membranes is very

high, and values up to 90% can be

obtained.


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Phase inversion (PI)

A polymer is transformed in a controlled

manner from liquid to solid phase. The

process of solidification is initiated by the

transition from one liquid state into two

liquids (liquid-liquid demixing) at a certain

stage during demixing. The high polymer

concentration phase will solidify and a

solid matrix is formed.By contrtolling the initial stage of phase

transition the membrane morphology can

be controlled and porous as well as

nonporeous membranes can be prepered.

Chemical phase inversion

0.45m PVDF membrane


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4. Sintering

This method involves compressing a powder consisting of

particles of a given size and sintering at high temperatures.

The required temperature depends on the material used.

HEAT

pore


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Materials used in MF

Synthetic polymeric membranes:

a) Hydrophobic

b) Hydrophilic

Ceramic membranes

PTFE, teflon

PVDF

PP

PE

Cellulose esters

PC

PSf/PES

PI/PEI

PAPEEK

Alumina, Al2O3

Zirconia, ZrO2

Titania, TiO2

Silicium Carbide, SiC


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1. Polymeric MF membranes

Phase inversion Stretching

Track-etching


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2. Ceramic MF membranes

Anodec, anodic oxidation (surface) US Filter, sintering (cross section, upper part)


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Retentate: how will it be used?

1.Sent to a treatment plant

2.Discharged into a body of water

3.Sent to a storage facility

4.For ground applications5.Recycled back to water source


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Some other industrial applications

1. Waste-water treatment2. Clarification of fruit juice, wine and beer

3. Ultrapure water in the semiconductor industry

4. Metal recovery as colloidal oxides or hydroxides5. Cold sterilization of beverages andpharmaceuticals

6. Medical applications: transfusion filter set,

purification of surgical water7. Purification of condensed water at nuclear plants

8. Separation of oil-water emulsions


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Membrane Processes

Reverse Osmosis


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Pore size in reverse osmosis


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Reverse Osmosis

This process will allow the removal of particles as smallas ions from a solution.

Reverse osmosis is used to purify water and removesalts and other impurities in order to improve the color,taste or properties of the fluid.

The most common use for reverse osmosis is inpurifying water. It is used to produce water that meetsthe most demanding specifications that are currently inplace.

It can be used to purify fluids such as ethanol and

glycol, which will pass through the reverse osmosismembrane, while rejecting other ions andcontaminants from passing.


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Contd.

Reverse osmosis uses a membrane that is semi-permeable, allowing the fluid that is being purified topass through it, while rejecting the contaminants thatremain.

The process of reverse osmosis requires a driving force

to push the fluid through the membrane, and the mostcommon force is pressure from a pump. As the concentration of the fluid being rejected

increases, the driving force required to continueconcentrating the fluid increases.

Reverse osmosis is capable of rejecting bacteria, salts,sugars, proteins, particles, dyes, and other constituentsthat have a molecular weight of greater than 150-250daltons.


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What is osmosis?

If two solutions of differentconcentration are separated bya semi-permeable membranewhich is permeable to thesmaller solvent molecules butnot to the larger solutemolecules, then the solventwill tend to diffuse across themembrane from the lessconcentrated to the more

concentrated solution. Thisprocess is called osmosis.

The energy which drives theprocess is usually discussed in

terms of osmotic pressure.


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Pressure and flux range

Membrane processPressure range

(bar)Flux range(l/m2hbar)

Microfiltration 0,1 - 2,0 >50

Ultrafiltration 1,0 - 5,0 10 50

Nanofiltration 5,0 20 1,4 - 12

Reverse Osmosis 20 - 100 0,05 - 1,4

The pressures used in reverse osmosis range from 20 to 100 bar and the

flux from 0,05 to 1,4 l / m2h

Energy Requirements


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Energy Requirements

Pressure driven processes

Power devices applied in pressure driven

membrane process


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Energy requirements

The energy

consumption to

pressurize a liquid is

given by:

A turbine may be

utilized to recover the

energy:

pump

pump

PqE

PqE turbineturbine

fluxq


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Membrane selection

o Membrane accounts for 15 to 40 percent of the price in

reverse osmosis.

o Membranes must be replaced periodically

o

CAREFUL MEMBRANE SELECTION ISESSENTIAL

SELECTION CRITERIA:

Chemical tolerance

Mechanical suitability

Price

Cleanability

Separation performance

GOOD DESIGN:

Consistent performance

Needs less frequent membranecleaning

Reasonable consum of power

Little operational attention


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Membrane configurations in RO

Spiral-wound configuration

Next logical step from a flat

membrane but with higher

packing density300 1000 m2/m3

Permeate is collected in the

central tube


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Tubular conf iguration

Not self supporting in

contrast to hol low fiber

modulesPermeate crosses the

membrane layer to the

outside

Low sur face-volume ratio

Usual ly the active layer is

inside


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Capil lary/hollow fiber conf igurationFibers diameter:


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Applications

Production of drinking water

Treatment of urban waste water

Production of water for industrial uses

Treatment of different wastes

Concentration of fruit juices, white of an egg,

whey...

Fermentation

water treatment calculations updated

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