Methods of Water Treatment

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Methods of water

treatment

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SAND FILTRATION

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Sand filtration

Filtration material: silicate sand ofdifferent granulation

Applications for removal of: Aggregates originally present in water

Flocs after flocculation, iron removal of limesoftening

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Types of sand filters

Slow

Rapid gravity

Rapid pressure

Self cleaning

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Slow filter

Open to gravity Filtration velocity 0.1 0.2 m/h Easy to operate Maintenance and cleaning: Automated or

manual scraping the impurities from the

upper layer of the filter

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Slow filter

Not economically feasible due to highinvestment

Better filtration efficiency Good in removing parasites and bacteria

Protozoan ParasiteGiardia

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Slow filter

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Maintenance of slowfilter

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Rapid gravity filter

Usually for bigger utilities Filtration velocity around 5 m/h

Sand bed layer height around 2 m

Water depth above sand 0.5 m Sand mesh size 0.8-1.5 mm

Bottom layer sand mesh size 4-6 mm

Cleaning in counter-current backwashwith water and compressed air (airscourging reduce consumption ofwater for backwashing)

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Rapid gravity

sand filter

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Pressure rapid sand filter

Sand layer depth up to 3 m

Filtration velocity up to 18 m/h

Mesh size:

Filtering layer 1-2 mm

Bottom layer 4-6 mm

Maximum operating pressure 0.5 0.6

bara

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Sand filters overview

Slow

Best filtration efficiency

Low feasibility

Rapid gravity

Backwash: water + air

Rapid pressure

High filtration velocity Backwash: water + air

Self-cleaning

No control needed

No valves or pumps

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Disinfection

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Disinfection of water Removal of pathogens in water

Prevention of epidemics

Fulfilment of law requirement for drinking water

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Methods of disinfection

Chemica

l

Chlorine

Cl2

Ozone

O3

Silverand

copper

Ag, Cu

Physical

TermicoC

UVFiltration

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Chlorine (Cl2)

Yellow-green gas

Toxic

Oxidizing agent

Disinfection time 30 min

Cl2

+ H2O HCl + HClO

HClO HCl + O

Purchased in steelcontainers

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Chlorine based disinfectants

Chlorine Cl2

Chlorine dioxideClO2

Sodium hypochloriteNaClO

Calcium hypochlorite

Ca(ClO)2

Trichloramines NCl3

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Disinfection chlorine basedchemicals

1. Liquid chlorine (Cl2), usually available in pressurized containers

inexpensive

highly toxic and should be used only by appropriately trained individuals withthe proper chlorinators and ejectors.

2. Sodium hypochlorite (NaOCl)

liquid

more expensive and bulky than liquid chlorine

safer to handle.

5-15 % chlorine

finite shelf life.

3. Calcium hypochlorite (Ca(OCl)2)

solid

approximately 65 percent chlorine

easy to handle in either tablets or granular form

relatively expensive

must be kept dry to prevent degradation

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Disinfection with silver

Inactivates bacteria above 0,015mg/L

Used for cooling water in hospitals

Pools Fisheries (Fish is sensitive to

chlorine)

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Copper and silver disinfection

Positively charged copper ions (Cu2+

) in the water bindto the particles of opposite polarity, such as bacteria,viruses and fungi and disturb cell wall permeability andcause nutrient uptake to fail. Copper ions penetrate thecell wall and as a result they will create an entrance forsilver ions (Ag+). Silver ions bond to various parts ofthe cell, such as the DNA and RNA, cellular proteins and

respiratory enzymes.

Applications of copper-silver disinfection:

Swimming pools

Cooling towers Legionella in hospitals and nursing homes

Drinking water

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Disinfection with UV

irradiation

UV mercury lamps are immersed in water or

placed above the surface of water No chemical addition

It doesnt change chemical composition ofwater,

Simple operation and maintenance Disadvantage: no residual to prevent bacterial

regrowth

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Disinfectionwith UV

irradiation

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Coagulation and flocculation

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Aggregates Colloidal matterMolecules

100 nm 1 nm

Minerals

Turbidity GasesAluminosilicates

OrganicsClay

Size of particles in water

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Floc formation andsedimentation

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Floc formation and flotation

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Flocs separation from water

SedimentationLonger time necessary

Big reactor

Flotation

Usually enhanced by introduction of micro bubbles ofair

Fast (smaller reactor and shorter time necessary)

More expensive due to energy for aeration

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