CE 6301: Theory of Water Treatment

Prof. Dr. Md. Abdul Jalil

 Syllabus
• Water and its impurities
• Criteria of water quality
• Physical, chemical and biological treatment
processes
• Controls of aquatic growth
• Control of taste and odor
References:
1. Process Chemistry for Water and Wastewater Treatment by Benefield,
Judkin & Weand.
2. Water Treatment Principles and Design by James M. Montgomery.
3. Water Works Engineering: Planning, Design & Operation by Qasim,
Motlay & Zhu.
4. Water and Wastewater Engineering Vol. 2: Water Purification and
Wastewater Treatment and Disposal by Fair, Geyer & Okun
5. Surface Water Treatment for Communities in Developing Countries by
Schulz & Okun.
6. Environmental Engineering by Peavy, Rowe & Tchobanoglous.
7. Introduction to Water Treatment Principles and Practices of Water
Supply Operations by AWWA.
8. Water Treatment Plant Design by AWWA.
9. Water treatment principles and design by Crittenden, Trussel, Hand,
Howe & Tchobanoglous.
 INTRODUCTION
• For the existence of all living beings (human,
animals and plants), water is very crucial. Almost
all human activities—domestic, agricultural and
industrial, demand use of water.
• Although water is nature’s most wonderful and
abundant compound but only less than 1% of the
world’s water resources are available for ready
use.
• Hence it is required to use it carefully and
economically.
 SOURCE OF WATER
The main sources of water are:
(i) Surface water: It includes flowing water
(streams and rivers) and still water (lakes,
ponds and reservoirs).
(ii) Underground water: It includes water from
wells and springs.
(iii) Rain water
(iv) Sea water.
 IMPURITIES OF WATER
The impurities present in water may be categorized
as follows:
(A) Dissolved Impurities
(a) Dissolved gases: O2 , CO2 , H2S etc.
(b) Inorganic salts:
i) Cations: Ca2+, Mg2+, Na+ , K+ , Fe2+, Al3+ etc.
ii) Anions: HCO3– , Cl– , SO4– , NO3– etc.
(c) Organic salts.
 IMPURITIES OF WATER
(B) Suspended Impurities
(a) Inorganic: Clay and sand.
(b) Organic: Oil globules, vegetables, and animal
material.
(C) Colloidal Impurities
Finely divided clay and silica, Al(OH)3 , Fe(OH)3 ,
organic waste products, coloring matter, amino
acids etc.
(D) Microscopic Matters
Bacteria, algae, fungi etc.
 SOURCES OF IMPURITIES IN WATER
Following are the sources of impurities in water:

i. Gases (O2 , CO2 etc.) are picked up from the atmosphere

by rainwater.
ii. Algal photosynthesis adds O2 to water.
iii. Decomposition of plants and animals remains introduce
organic impurities and gases (CO2, H2S etc.) in water.
iv. Water dissolves impurities when it comes in contact with
ground, soil or rocks.
v. Impurities are also introduced in water when it comes in
contact with sewage or industrial waste.
 Impurities in various sources of water

• River water contains dissolved minerals like chlorides,

sulfates, bicarbonates of sodium, magnesium, calcium
and iron. It also contains suspended impurities of sand,
rocks and organic matter. The composition of river
water is not constant. The amount of dissolved
impurities in it depends on its contacts with the soil.
Greater the duration of contact, more soluble is the
minerals of soil in it.
• Lake water has high quantity of organic matter present
in it but lesser amount of dissolved minerals. Its
chemical composition is also more or less constant.
• Rain water is obtained mainly as a result of evaporation
from the surface water. Probably it is the purest form of
natural water. But during its downward journey through
the atmosphere it dissolves organic and inorganic
suspended particles and considerable amount of
industrial gases like (CO2 , NO2 , SO2 etc.). Rain water is
expensive to collect and is irregular in supply.
• Groundwater is free from organic impurities and is
clearer in appearance due to the filtering action of the
soil. But it contains large amount of dissolved salts.
• Sea water is very impure due to two reasons:
i. Continuous evaporation increases the dissolved impurities
content, which is further increased by the impurities thrown
by rivers as they join sea.
ii. It is too saline for most industrial uses except cooling.
 Table 1: Common impurities found in water.

Constituent Chemical Formula Difficulties Caused Means of Treatment

imparts unsightly
appearance to water;
non-expressed in deposits in water lines, coagulation, settling,
Turbidity
analysis as units process equipment, and filtration
etc.; interferes with
most process uses
chief source of scale in
softening;
heat exchange
calcium and demineralization;
equipment, boilers, pipe
Hardness magnesium salts, internal boiler water
lines, etc.; forms curds
expressed as CaCO3 treatment; surface
with soap, interferes
active agents
with dyeing, etc.
foam and carryover of
lime and lime-soda
solids with steam;
softening; acid
bicarbonate(HCO3-), embrittlement of boiler
treatment; hydrogen
carbonate (CO32-), and steel; bicarbonate and
Alkalinity zeolite softening;
hydroxide(OH-), carbonate produce
demineralization
expressed as CaCO3 CO2 in steam, a source
dealkalization by anion
of corrosion in
exchange
condensate lines
 H2SO4 , HCI. etc.,
Free Mineral Acid corrosion neutralization with alkalies
expressed as CaCO3
corrosion in water lines,
aeration, deaeration,
Carbon Dioxide CO2 particularly steam and
neutralization with alkalies
condensate lines
pH pH = - log [H+] pH varies according to
acidic or alkaline solids pH can be increased by
in water; most natural alkalies and decreased by
waters have a pH of 6.0- acids
8.0
adds to solids content
of water, but in itself is
demineralization, reverse
not usually significant,
Sulfate SO42- osmosis, electrodialysis,
combines with calcium
evaporation
to form calcium sulfate
scale
adds to solids content demineralization, reverse
Chloride Cl - and increases corrosive osmosis, electrodialysis,
character of water evaporation
adds to solids content,
but is not usually
significant industrially:
high concentrations demineralization, reverse
Nitrate NO3- cause osmosis, electrodialysis,
methemoglobinemia in evaporation
infants; useful for
control of boiler metal
embrittlement
 cause of mottled
enamel in teeth; also adsorption with magnesium
used for control of hydroxide, calcium
Fluoride F-
dental decay: not phosphate, or bone black;
usually significant alum coagulation
industrially
adds to solids content
of water: when
demineralization, reverse
combined with OH-,
Sodium Na+ osmosis, electrodialysis,
causes corrosion in
evaporation
boilers under certain
conditions
hot and warm process
scale in boilers and removal by magnesium
cooling water systems; salts; adsorption by highly
Silica SiO2 insoluble turbine blade basic anion exchange
deposits due to silica resins, in conjunction with
vaporization demineralization, reverse
osmosis, evaporation
discolors water on
aeration; coagulation and
precipitation; source of
filtration; lime softening;
Fe2+ (ferrous) deposits in water lines,
Iron cation exchange; contact
Fe3+ (ferric) boilers. etc.; interferes
filtration; surface active
with dyeing, tanning,
agents for iron retention
papermaking, etc.
Manganese Mn2+ same as iron same as iron
 Arsenic As3+ Gastrointestinal Co-precipitation,
tract, nervous adsorption, ion
system, respiratory exchange,
tract, and skin membrane
diseases. techniques
usually present as a
result of floc carryover
from clarifier; can causeimproved clarifier and
Aluminum AI3+
deposits in cooling filter operation
systems and contribute
to complex boiler scales
corrosion of water lines,
deaeration; sodium
heat exchange
Oxygen O2 sulfite; corrosion
equipment, boilers,
inhibitors
return lines, etc.
aeration; chlorination;
cause of "rotten egg"
Hydrogen Sulfide H2S highly basic anion
odor; corrosion
exchange
corrosion of copper and cation exchange with
Ammonia NH3 zinc alloys by formation hydrogen zeolite;
of complex soluble ion chlorination; deaeration
Ion exchange,
Nitrate NO3- Algal growth, child disease
denitrification
 refers to the measure of
undissolved matter,
determined subsidence; filtration,
Suspended Solids none gravimetrically; usually preceded by
deposits in heat coagulation and settling
exchange equipment,
boilers, water lines, etc.
refers to the sum of
dissolved and see "Dissolved Solids"
Total Solids none suspended solids, and "Suspended
determined Solids"
gravimetrically
Vegetable dyes -- Color, acidity Coagulation,
adsorption
Bacteria Turbidity at high Filtration,
concentrations, Disinfection
diseases
Viruses Diseases Disinfection
Algae, protozoa, Turbidity, odor, Filtration,
fungi color, diseases, Disinfection
toxicity
Radioactive Diseases Confinement
impurities