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1 23
Applied Water Science
ISSN 2190-5487
Appl Water Sci
DOI 10.1007/s13201-016-0455-7
Drinking water contamination and
treatment techniques
S.Sharma & A.Bhattacharya
1 23
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REVIEW ARTICLE
Drinking water contamination and treatment techniques
S. Sharma
1
•A. Bhattacharya
1
Received: 26 February 2014 / Accepted: 29 July 2016
The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract Water is of fundamental importance for life on
earth. The synthesis and structure of cell constituents and
transport of nutrients into the cells as well as body meta-
bolism depend on water. The contaminations present in
water disturb the spontaneity of the mechanism and result
in long/short-term diseases. The probable contaminations
and their possible routes are discussed in the present
review. Continued research efforts result in some pro-
cesses/technologies to remove the contaminations from
water. The review includes concepts and potentialities of
the technologies in a comprehensible form. It also includes
some meaningful hybrid technologies and promising
awaited technologies in coming years.
Keywords Water Contaminants Purification Hybrid
technology
Introduction
Availability of fresh water, the nature’s gift controls the
major part of the world economy. The adequate supplies of
water are necessary for agriculture, human consumption,
industry as well as recreation. Ironically, sometimes, nat-
ural or added contaminations rob us of the gift and making
us confront a lot more challenging world. It is a well-
known fact that fresh water is an important necessity for
our health. With the advancement of technology and
industrial growth, fresh water resources all over the world
are threatened. One-sixth of the world population suffers
from the freshwater unavailability situation (Elimelech
2006). It is seen that developed countries suffer most from
chemical discharge problems, whereas developing coun-
tries from agricultural sources. Contaminated water causes
problems to health and leads to waterborne diseases which
can be prevented by taking measures even at the household
level. Providing safe water for all is a challenging task.
Continued research efforts in this area for more than few
decades result in many processes/technologies (Shannon
et al. 2008).
Water contamination is a common problem to all over
the world. These may be geological or anthropogenic
(man-made) (Fawell and Nieuwenhuijsen 2003). Higher
levels of contaminants in drinking water are seldom to
cause acute health effects. Of course it depends on indi-
vidual susceptibility and mode of contact with the body.
The types and concentrations of natural contaminates
depend on the nature of the geological materials through
which the groundwater flows and quality of the recharge
water. Groundwater moving through sedimentary rocks
and soils may pick up a wide range of compounds, such as
magnesium, calcium, and chloride, arsenate, fluoride,
nitrate, and iron; thus, the effect of these natural contam-
inations depends on their types and concentrations. The
natural occurring elements present at unacceptable levels
can contaminate water as well (Liu et al.2005; Charles
et al. 2005; Rukah and Alsokhny 2004; Mulligan et al.
2001; Ghrefat et al. 2014; Meenakshi and Maheshwari
2006).
Other contaminants are man-made by-products of
industry, and agriculture, including heavy metals like
mercury, copper, chromium, lead, and hazardous chemi-
cals, dyes and compounds like insecticides and fertilizers.
&A. Bhattacharya
bhattacharyaamit1@rediffmail.com
1
Reverse Osmosis Division, Central Salt and Marine
Chemicals Research Institute, G. B. Marg, Bhavnagar
364002, Gujarat, India
123
Appl Water Sci
DOI 10.1007/s13201-016-0455-7
Improper storing or disposing of household chemicals such
as paints, synthetic detergents, solvents, oils, medicines,
disinfectants, pool chemicals, pesticides, batteries, gasoline
and diesel fuel can lead to ground water contamination
(Kass et al. 2005; Anwar 2003) According to UN report
2003 (UN WWAP 2003) every day 2 million tons of
sewage, industrial and agricultural waste are discharged
into the world’s water.
The microbial contaminants include pathogens like
bacteria, viruses, and parasites such as microscopic proto-
zoa and worms. These living organisms can be spread by
human and animal wastes knowing or unknowingly.
Some contaminants can be easily identified by assessing
color, odor, turbidity and the taste of the water. However,
most cannot be easily detected and require testing to reveal
whether water is contaminated or not. Thus, the contami-
nants may result in unappealing taste or odor and staining
as well as health effects.
Color of the drinking water is a physical characteristic
that cannot be noticed unless it is one of high concentra-
tion. For example, if ground water containing a high iron
concentration, it gives a reddish appearance; similarly, high
tannin concentration makes the water look brown. Gener-
ally, it is measured by comparing a water sample to a color
standard. One color unit has no effect on the water and
usually not detectable while 100 color units could be
compared to the color of light tea (Ligor and Buszewski
2006). Odor is also an indicator for the presence of some
contamination though odor-free water is not necessarily
safe for drinking purpose. Also, some contaminant odors
are noticeable even when present in extremely small
amounts.
On the other hand, the presence of clays, silts or sand, or
organic, algae, and leaf particles results in turbidity. The
turbidity may shield bacteria, preventing disinfection
chemicals from attacking and destroying the cells. The
presence of organic materials in conjunction with chlorine
can form trihalomethanes and other potentially harmful
chemicals. Generally, surface water sources have higher
turbidity compared to groundwater sources. The turbidity
of a surface water source can vary greatly from 1 to 200
NTU (NTU: nephelometric turbidity unit). The immunity
in turbidity level is different from children to adult people.
Types of contaminants
Basically, the contaminants are four types associated with
water pollution-
Inorganic contaminants,
Organic contaminants,
Biological contaminants,
Radiological contaminants.
Inorganic contaminants
The presence of contaminants can also be measured by its
chemical parameters. Hardness of the drinking water is a
naturally occurring contaminate, which basically depends
on the geographical status. It is caused by significant
amounts of calcium or magnesium components; the hard-
ness is classified into carbonate or non-carbonate hardness
depending on what molecules are combined with calcium
or magnesium. If they are combined with carbonate ions
(CO
3
-2
), the hardness is termed as ‘carbonate hardness’; if
combined with other ions, it is non-carbonate hardness.
Generally, 300–400 mg/L hardness is suitable for drinking
purpose. Prolonged exposure to water containing salts
(TDS [500 mg/L) can cause kidney stone, etc.
Apart from carbonate/noncarbonated hardness, there are
several inorganic substances (viz. fluoride, arsenic, lead,
copper, chromium, mercury, antimony, cyanide) that con-
taminate water resource. They can get into drinking water
from natural sources, industrial processes, as well as from
plumbing systems (EPA US 2006; Nriagu 1988).
Sources of fluoride may be geological or anthropogenic.
Weathering of fluoride-bearing minerals (fluorite, fluor-
spar, cryolite, fluorapatite, ralstonite and others) on the
earth’s crust can lead to higher fluoride levels in ground-
water. The over exploitation of ground water also aggra-
vates the problem of fluoride concentration in the water
even more. Further, the anthropogenic sources of fluorides
are certain pharmaceutical products (for treating hyper-
thyroidism), medicines, tooth pastes, insecticides, disin-
fectants, preservatives, super phosphate fertilizer, vitamin
supplements and others. Their effects are especially
harmful to develop children and the elderly people. Fluo-
ride is known to cause dental and skeletal fluorosis. It is
also associated with Alzheimer’s disease and other forms
of dementia (Susheela 1999; Fawell et al. 2006; WHO
2008). Fluoride enters the brain and enables aluminum to
cross the blood–brain barrier, resulting in increased risk for
these diseases (Ram Gopal and Ghosh 1985). Excessive
fluoride (4.0 mg/L by EPA) concentrations have been
reported in ground waters of more than 20 developed and
developing countries including India where 19 states are
facing acute fluorosis problems (Eswar and Devaraj 2011).
Arsenic (MCL 0.01 mg/L) (EPA US 2006) enters in
drinking water supplies from natural deposits in the earth
or from agricultural and industrial practices (Smith et al.
2000). Arsenic contamination is by far the biggest mass
poisoning case in the world, especially in India and
Appl Water Sci
123
Bangladesh (Chatterjee et al. 1995; Khan et al. 2003).
Arsenic contamination of drinking water causes a disease
called arsenicosis (Chen et al. 1988). Non-cancer effects
can include thickening and discoloration of the skin,
stomach pain, nausea, vomiting, diarrhea, numbness in
hands and feet, partial paralysis, and blindness. Arsenic has
been linked to cancer of the skin, bladder, lungs, kidney,
nasal passages, liver, and prostate (Yoshida et al. 2004).
The toxicity and excretion of arsenic compounds and their
metabolites highly depend on the oxidation states [viz.
Arsenite (As III) and Arsenate (As V)] and degree of
methylation of arsenicals. It is seen that As(III) is ten times
more toxic than As(V) (Pontius et al. 1994).
Mercury (MCL 0.002 mg/L) (EPA US 2006)getsinto
drinking water from agricultural runoff as well as seepage
from landfills and some factories. The presence of mercury
in water causes impairment of brain functions, neurological
disorders, and retardation of growth in children, abortion and
disruption of the endocrine system (Clarkson 1992;Counter
and Buchanan 2004). Copper (MCL 1.3 mg/L) (EPA US
2006) can enter into the water through natural deposits in
rock and soil, but more often as a result of corrosion in
household plumbing. In short term, exposure leads to mild
gastrointestinal distress, but long-term exposure can lead to
permanent liver or kidney damage (Semple et al. 1960;
Manuel et al. 1998). Chromium (MCL 0.1 mg/L) (EPA US
2006) occurs naturally in the ground and is often used in
electroplating of metals and leather industries. Generally, it
gets into water from runoff from old mining operations and
improper waste disposal from these industries. A high level
of exposure of chromium causes liver and kidney damage,
dermatitis and respiratory problems (Zhang et al. 1997;Ray
(Arora) and Ray 2009). Lead (MCL 0.015 mg/L) (EPA US
2006) is an increasing problem in cities with older water
systems. Water slowly corrodes the lead in municipal water
systems which can cause a wide range of developmental
difficulties for children and high blood pressure and kidney
ailments in older and adults (Needleman et al. 1990). Anti-
mony (MCL 0.006 mg/L) (EPA US 2006) occurs naturally
in the ground and originates from flame retardant industry. It
is also used in ceramics, glass, batteries, fireworks and
explosives. It may get into drinking water through natural
weathering of rock as well as through industrial and
municipal waste disposal or from manufacturing processes.
It affects cholesterol, glucose in blood levels (Cooper and
Harrison 2009a,b; Public Health Service, US 1992). Nitrate
(MCL 10 mg/L as Nitrogen) (EPA US 2006) contamination
comes through fertilizers. It is found in sewage and wastes
from human and/or farm animals and generally gets into
drinking water from these activities. Excessive levels of
nitrate in drinking water have caused serious illness because
of nitrate conversion to nitrite in the body and interferes
oxygen transport in the blood. The symptoms include
shortness of breath and blueness of the skin (Gupta et al.
2000). Asbestos (MCL 7 million fibers/lit) (EPA US 2006)is
a mineral that forms minute fibers in the environment.
Asbestos fibers in water have been linked to an increase in
the risk of certain cancers and regulated by EPA because of
asbestos exposure from water (EPA US 2009a,b; Bull
2007). Selenium (MCL 0.05 mg/L) (EPA US 2006)con-
tamination comes through mainly food and soils. It is used
in electronics, photocopy operations, manufacture of glass,
chemicals, drugs, and as a fungicide and feed additive.
Exposure to high levels of selenium over a long period of
time has resulted in a number of adverse health effects,
including a loss of feeling and control in the arms and legs
(Olson 1986; Fan and Kizer 1990). Barium (MCL 2 mg/L)
(EPA US 2006) occurs naturally in some aquifers that serve
as sources of ground water. It generally gets into drinking
water after dissolving from naturally occurring minerals in
the ground. It may damage heart and cardiovascular system,
and is associated with high blood pressure in laboratory
animals such as rats exposed to high levels during their
lifetimes (Brenniman et al. 1979; Wones et al. 1990).
Drinking water that meets the EPA standard is associated
with little to none of this risk and is considered safe with
respect to Barium. Beryllium (MCL 0.004 mg/L) (EPA US
2006) generally gets into water from runoff from mining
operations, discharge from processing plants and improper
waste disposal. Beryllium compounds have been associated
with damage to the bones and lungs and also may increase
the risk of cancer in humans who are exposed over long
periods of time (Cooper and Harrison 2009a,b). Cyanide
(MCL 0.2 mg/L) (EPA US 2006) usually gets into water as
a result of improper waste disposal. It has been shown to
damage the spleen, brain and liver of humans fatally poi-
soned with cyanide (Ronald 1991).
Organic contaminants
The major anthropogenic sources of organic contamination
are pesticides, domestic waste, and industrial wastes, etc.
Contamination through organic materials can cause serious
health problems like cancers, hormonal disruptions, and
nervous system disorder (Ram et al. 1990; Harvey et al.
1984). Trihalomethanes (THMs) are formed when chlorine
in the treated drinking water combines with naturally
occurring organic matter.
•Pesticides contaminate through agricultural as well as
public hygienic sources (Damalas and Eleftherohorinos
2011; Younes and Galal-Gorchev 2000). The adverse
environmental effects of pesticides used in agriculture
and public health are due to an improper handling and
application procedure (WHO 2010). Pesticides are
designed to interact with various chemical processes
Appl Water Sci
123
in the pest’s living body chemistry. Unfortunately,
doing this, all pesticides may interact with the
metabolism of non-targeted living organism. Mostly,
pesticides damage the liver and nervous system. Tumor
formation in the liver has also been reported (Bolognesi
2003). Environmental agencies have fixed their MCL’s
(EPA US 2009a,b). Some of the pesticides with their
MCLs are in the ensemble (Table 1).
•Volatile organic chemicals (VOCs) include solvents
and organic chemicals like toluene benzene, styrene,
trichloroethylene (TCE) and vinyl chloride, etc.,
degreasers, adhesives, gasoline additives, and fuel
additives (Wehrmann et al. 1996). These VOCs cause
chronic health effects like cancer, central nervous
system disorders, liver and kidney damage, reproduc-
tive disorders, and birth defects (Brown et al. 1984).
•Dyes constitute one of the largest groups of organic
compounds that represent an increasing environmental
concern. The release of this contaminated water into the
environment is a considerable source of non-esthetic
pollution and eutrophication, which can originate
dangerous byproducts through oxidation, hydrolysis,
or other chemical reactions taking place in the
wastewater phase (Pagga and Bruan 1986; Prevot
et al. 2001).
•Apart from the above, compounds present in the water
have the potential to cause known or suspected adverse
ecological or human health effects. These compounds
are termed ‘Emerging Organic Contaminants’ (Pal
et al. 2010,2014; Stuart et al. 2012; Lapworth et al.
2012). It includes pharmaceuticals (viz. ciprofloxacin,
erythromycin, tetracycline, codeine, salbutamol, carba-
mazepine, paracetamol, ibuprofen, salicylic acid, Tam-
iflu, chemotherapy drugs such as 5-flurourcil,
ifosfamide) industrial compounds (viz. chlorinated
solvents, petroleum hydrocarbons, including the pol-
yaromatic hydrocarbons, the fuel oxygenate methyl
tertiary butyl ether, plasticizers/resins bisphenols, adi-
pates and phthalates), personal care products (viz.
N,Ndiethyl meta toluamide, alkyl esters of p-hydroxy
benzoic acid, triclosan), fragrances (viz. tonalide,
galaxolide), water treatment by products (viz. tri-
halomethanes, haloacetic acids, N-nitroso dimethyl
amine), plasticizers, flame retardants as well as surfac-
tants. Mostly, they are endocrine disruptors,
carcinogenic.
•These are commonly derived from a variety of
municipal, agriculture and industrial sources and path-
ways. The pharmaceuticals (viz. antibiotics, analgesic
and anti-inflammatory) come from hospital effluents
and/or chemical manufactures. There are reports of
lowest predicted no-effect concentration (PNEC) values
of emerging organic contaminants. Some of them are
listed in Table 2(Pal et al. 2010,2014).
Biological contaminants
Biological contamination of water is caused by the pres-
ence of living organisms, such as algae, bacteria, protozoan
or viruses. Each of these can cause distinctive problems in
water (Daschner et al. 1996; Ashbolt 2004). Algae are in
general single celled and microscopic. These are quite
abundant and depend on nutrients (viz. Phosphorus) in
water. The nutrients are generally from domestic run-off or
industrial pollution. The excess algae growth is not only
imparted taste and odor problems in water; it clogs filters,
and produces unwanted slime growths on the carriers.
Sometimes, they [viz. blue-green algae (Anabaena, Apha-
nizomenon and Microcystis)] are capable of liberating
toxins and they damage the liver (hepatotoxins), nervous
system (neurotoxins) and skin (Hitzfeld et al. 2000; Rao
et al. 2002).
Bacteria are also microscopic single celled. There are
numerous pathogenic bacteria and can be contaminated
with water (Inamori and Fujimoto 2009). They can result in
typhoid, dysentery, cholera and gastroenteritis. Some non-
pathogenic bacteria (viz. sulfur, crenothrix iron bacteria),
although not harmful, may cause taste and odor problems
(Nwachcuku and Gerba 2004; Rusin et al. 1997). Similarly,
Protozoans are also single-celled and microscopic organ-
isms. Some protozoans (like Giardia and Cryptosporidium)
are commonly found in rivers, lakes, and streams
Table 1 Some of the pesticides with their maximum contamination
level (MCL) (Adapted from EPA, US Protection agency)
Pesticides Nature Maximum
contamination
level (MCL), lg/L
Carbofuran Nematicide 40
Dalapon Herbicide 200
Dibromochloropropane Nematocide 0.2
Dinoseb Insecticide/miticide 7
Dioxin Herbicide 0.0003
Diquat Herbicide 20
Endothall Algicide 100
Ethylene dibromide Insecticide 0.05
Glyphosate herbicide 700
Methoxychlor Insecticide 40
Oxamyl Insecticide 200
Pentachlorophenol Fungicide 1
Picloram Herbicide 500
Simazine Herbicide 4
Toxaphene Insecticide 3
Appl Water Sci
123
contaminated with animal feces or which receive wastew-
ater from sewage treatment plants. These may cause diar-
rhea, stomach cramps, nausea, fatigue, dehydration and
headaches. Viruses are the smallest living organisms cap-
able of producing infection and causing diseases. Hepatitis
and polio viruses are commonly reported in the contami-
nated water.
Radiological contaminants
Radiological contaminants are caused by radioactive ele-
ments. Sources of radioactive material could be soils or
rocks the water moves through or some industrial waste.
Erosion of natural deposits of certain minerals (radioactive)
may emit radiations (like a,b). Radiological elements (viz.
U
226
,Ra
226
,Ra
228
and Rn
228
) tend to be a greater problem
in groundwater than in surface water. All types of radio-
logical contamination increase the risk of cancer (Alireza
et al. 2010; Haki et al. 1995). Some of the radioactive
contaminants with their MCLs are listed in Table 3.
Solving approaches
The famous saying of Minora Shirota’s statement is ‘‘Pre-
vent disease rather than treat disease: a healthy intestine
leads to a long life, and deliver health benefits to as many
people as possible at an affordable price’’ (Heasman and
Mellentin 2001). This philosophy, elaborated almost half a
century ago, is becoming more valid now than ever before.
The need of science-based solutions for uncontaminated
water provisioning results in several water treatment meth-
ods to counter the problem. Of course, the suitable technol-
ogy is based on raw water characteristics (i.e., the nature and
extent of contamination), infrastructure (i.e., power, man-
power, availability of chemicals), affordability/cost as well
as acceptability. Some of the common water purification
methods are sedimentation or settling, boiling/distillation,
chemical treatment (precipitation/coagulation/adsorbents),
disinfection and filtration. The processes and techniques in
mitigating the contaminations are as follows.
Precipitation and coagulation
Precipitation is a technique of removing one or more sub-
stances from a solution by adding reagents so that insoluble
Table 2 Lowest predicted no effect concentration (PNEC) values for some of the emerging organic contaminants (Pal et al. 2010,2014)
Compounds Lowest PNEC (ng/l) Compounds Lowest PNEC (ng/l)
Antibiotics
Trimethoprim 1000 Bisphenol A (making plastics) 60–150
Ciprofloxacin 20
Sulfamethaoxazole 20,000
Analgesic and anti-inflammatory
Naproxen 37,000 PPOS (protective coatings, surfactants) 1100
Ibuprofen 5000
Ketoprofen 15.6 910
6
Diclonofenac 10,000
Beta blockers
Propranolol 500 Fipronil (termiticide) 250
Atenolol 10 910
6
Blood lipid regulators
Clofibric acid 12000 NP1EO (surfactant) 330
Gemfibrozil 100,000
Benzafibrate 100,000
Hormones
Estriol 0.8 4MBC (sun screen) 560
Estrone 18
Sucralose (sugar substitute) 93 910
4
DEET (mosquito repellent) 5–24 910
6
Table 3 Radioactive contaminants and their MCLs (adapted by EPA,
US)
Contaminants MCL
Alpha particles 15 (pCi/L)
Beta particles and photon emitters 4 mrems/year
Radium 226 and Radium 228 (combined) 5 pCi/L
Uranium 30 ug/L
Appl Water Sci
123
solids appear. The ‘solubility’ rules the technique, i.e., when
the product of ion concentrations (in simple) in the solution
over the solubility product of the respective solid, the pre-
cipitation occurs. It is one of the simple methods to purify
water. The chemicals are added to form particles which
settle and remove contaminants from water. The treated
water is reused whereas the settled portion is dewatered and
disposed of. The technique is used in softening of water as
well as to remove impurities like phosphorus, fluoride,
arsenic, ferrocyanide and heavy metals, etc. (EPA US 2000;
Matlock et al. 2002; Eikebrokk et al. 2006).
Softening of water
The presence of Ca/Mg in terms of carbonate, bicarbonate,
chloride and sulfate results in hardness of water. Addition
of proper chemical forms precipitation and makes it soft.
Addition of Ca(OH)
2
forms precipitation with bicar-
bonate and sulfate in water.
Ca HCO3
ðÞ
2þCa OHðÞ
2!2CaCO3#þ2H2O,
MgSO4þCa OHðÞ
2!Mg OHðÞ
2#þ CaSO4:
Addition of Na-aluminate forms precipitation of
hydroxide with sulfate and chloride in water. Actually,
Na-aluminate forms sodium hydroxide with water, and
with sulfate/chloride it forms hydroxide.
MgSO4=Cl2þNa2Al2O4þ4H
2O
!Mg OHðÞ
2#þNa2SO4=NaCl þ2Al OHðÞ
3#:
Formation of aluminum hydroxide aids in floc formation,
sludge blanket conditioning and silica reduction.
Softening of water is also feasible by simple boiling
Ca HCO3
ðÞ
2þheat !CaCO3#þH2OþCO2:
Removal of heavy metals
Heavy metals (e.g., Ba, Cd, Pb, Hg, Ni, Cu) typically
precipitated from waste water as sulfates, sulfides,
hydroxides, and carbonates (Matlock et al. 2002). Metal
co-precipitation during flocculation with iron and alu-
minum salts is also possible for some metals (e.g., As, Cd,
Hg, Cr). The following reaction represents as chromium
co-precipitation in terms of hydroxides or sulfates
H2Cr2O7þ6FeSO4þ6H2SO4!Cr2SO4
ðÞ
3
#þ3Fe
2SO4
ðÞ
3þ7H2O,
Cr2SO4
ðÞ
3þ3Ca OHðÞ
2!2Cr OHðÞ
3#þ3CaSO4:
Removal of arsenic
Arsenic removal with coagulants, viz. Alum [Al
2
(SO
4
)
3-
18H
2
O] ferric chloride (FeCl
3
) and ferric sulfate
[Fe
2
(SO
4
)
3
7H
2
O] is effective (Harper and Kingham 1992;
Fields et al. 2000). In these cases, arsenic (V) can be more
effectively removed than arsenic (III). The microparticles
and negatively charged arsenic ions are attached to the
flocs by electrostatic attachment during the process. The
possible steps of coagulation and co-precipitation are as
follows:
Alum dissolution:
Al2SO4
ðÞ
318H2O!2Alþ3þ3SO2
4þ18H2O:
Aluminum precipitation (acidic):
2Alþ3þ6H2O!2Al OHðÞ
3#þ6Hþ:
Co-precipitation (non-stoichiometric, non-defined
product):
H2AsO
4þAl OHðÞ
3!Al As complexðÞ
þOther Products:
Similar reactions take place in case of ferric chloride
and sulfate with the formation of Fe–As complex as an end
product which is removed by the process of sedimentation
and filtration (Mok and Wai 1994; Hering et al.1997). The
efficient removal depends on pH range.
Removal of phosphorus
The removal of phosphates is generally done by coagulant,
i.e., by mixing coagulant into waste water (Xie et al. 2005).
The most commonly used multivalent metal ions are Ca,
Al, and Fe.
10 Ca2þþ6PO
3
4þ2OH!Ca10 PO4
ðÞ6OHðÞ
2#;
Al3þþHnPO3n
4!AlPO4þnHþ;
Fe3þþHnPO3n
4!FePO4þnHþ:
Removal of fluoride
Precipitation of fluoride species into chemically
stable form is the most effective option for the removal of
fluoride (in terms of Ca, Mg, Al) from effluent streams
(Dahi 1997; Lawrence et al. 2005). Among all metal flu-
orides, CaF
2
is less soluble in water. Consequently,
removal of fluoride from the effluents by converting it into
CaF
2
has become the most widely used method of treat-
ment. CaCl
2
, limes, may be used for this purpose, but
CaCl
2
is preferred with respect to lime due to its higher
solubility and the lower ratio of additive to effluent.
2HFþCa OHðÞ
2!CaF2þ2H2O,
CaCl2þ2HF !CaF2#þ2HCl:
The reaction of hydrofluoric acid and ammonium
fluoride with the aluminum treatment agent is as follows:
Appl Water Sci
123
3HFþAlO2!AlF3#þH2OþOH;
3NH4FþAlO2!AlF3#þ3NHþ
4þO2;
6NH4HF2þ4 AlO2!4 AlF3#
þ6NHþ
4þ2O2þ2H2Oþ2OH:
Inorganic flocculants have the potential in different
separations (Gray et al. 1995; Jiang and Graham 1998), but
they are used in very large quantities. These leaves large
amounts of sludge and strongly affected by pH changes,
whereas polymeric flocculants cause the formation of large
cohesive aggregates (flocs) and inert to pH changes. Both
natural and synthetic polymers are useful for this purpose.
Generally, synthetic polymers (viz. polyacrylamide,
polyethylene oxide, poly (diallyl dimethyl ammonium
chloride), poly (styrene sulfonic acid) are highly effective
flocculants at small dosages and have high tailor ability but
poor shear stability, whereas though natural polymers (viz.
starch, guar gum, alginate, glycogen, dextran) are
biodegradable and effectively shear stable (Brostow et al.
2009).
Removal of dyes
Dyes are non-biodegradable, and precipitation with CaCO
3
can be one of the approaches to remove them from the
water (Hoffmann et al. 1995; Reife and Freeman 1996).
As a whole, the precipitation technique has the features
Benefits:
•Simple process,
•Effective for the removal of As, Cd, Ba, Cd, Cr, Pb, Hg,
Se, Ag, etc.,
•It is also applicable to remove natural organic matter
(NOM) or dissolved organic carbon (DOC).
Limitations:
•Requires continuous supply of huge chemicals,
•Handling of by-products,
•Disposal of coagulation/precipitation sludge is a
concern.
Distillation
It is the most common separation technique (http://www.
msue.msu.edu; Veil. 2008). In this separation technique,
the mixed components in water are separated by the
application of heat. It is based on the differences in boiling
points of the individual components. The boiling point
characteristics depend on the concentrations of the com-
ponents present. Thus, the distillation process depends on
the vapor pressure characteristics of liquid mixtures. The
basic principle described as the input of heat energy raises
vapor pressure. When the vapor pressure reaches its sur-
rounding pressure, the liquid mixture boils and distillation
occurs because of the differences of volatility in the
mixture.
This process results in a separation between water and
inorganic substances, such as lead, calcium, magnesium,
etc. are also destroying bacteria. However, organics with
boiling points lower than 100 C cannot be removed effi-
ciently and can actually become concentrated in the pro-
duct water. Distilled water purification technology was
originally developed for industrial purpose. However, it
came eventually for home use. Since, this process is not
very effective in removing organic chemicals so the carbon
filter system must be added to make the water really safe to
drink. The carbon filters require regular changing because
they can quickly become breeding grounds for bacterial
growth.
Although distilled water is safe, it is not healthy as this
contains no nutrient minerals, which are essential for the
drinking purpose. This type of water purification technol-
ogy is also very slow. Adding to that, the cost of a carbon
filter and the result is an unwieldy system of water
purification.
Benefits:
•Removes a broad range of contaminants (toxic chem-
icals, heavy metals, bacteria, viruses, parasites),
•Continuous,
•Does not rely on physical barriers (filters),
•Does not require additional disinfecting process.
Limitations:
•It consumes an enormous amount of energy both in
terms of cooling and heating requirements,
•Some contaminants can be carried into the condensate,
•Requires careful maintenance to ensure purity,
•The process is not very effective which are of lower
volatility (viz. organics) compared to water.
Adsorption
In this physical process, dissolved contaminants adhere to
the porous surface of the solid particles (Jiuhui 2008). It is
the surface phenomena and the outcome of surface energy.
With the material, all the bonding requirements of the
constituent atoms of the material are filled with other
atoms. However, atoms on the surface of the adsorbent are
not wholly surrounded by other adsorbent atoms and
physical attractive force results. It can be physisorption
(originates from vanderwaals forces) and chemisorption
(originates from co-valent forces).
The adsorbent systems are added directly to the water
supply or via mixing basin. Adsorbents combine chemical
Appl Water Sci
123
and physical processes to remove the compounds that
impart color, taste, and odor to water. In principle, all
microporous materials can be used as adsorbents. However,
those with well controlled and highly microporous are the
most preferred (Yang 1997). The porous solids, e.g., acti-
vated carbon, silica gels, aluminas, zeolites, etc. contain
many cavities or pores with diameters as small as a fraction
of a nanometer is useful (Ali and Gupta 2007;Qu2008).
The isotherms are the quantitative interrelation between
the adsorbate and adsorbent. The three most well-known
isotherms are Freundlich, Langmuir and Linear. The most
commonly used for the water contaminants is Freundlich
and it is expressed as:
x
m¼KC1=n
e:
where xis the mass of solute adsorbed, Mis the mass of
adsorbent, C
e
is the equilibrium concentration of solute,
and Kand nare experimental constants.
Activated carbon
The most commonly used adsorbent is activated car-
bon—a substance which is quite similar to common
charcoal. Actually, the active carbon is much more
efficient because of its high porous character. The high
porous character is generated by treating carbon to steam
and high temperature (1300 C) with or without oxygen
in the presence of inorganic salts (physical method). The
carbon may be of petroleum coke, bituminous coal,
lignite, wood products, and coconut/peanut shells. At
high temperature, parts of carbon are oxidized in CO
2
and steam. The gases are evacuated and micro fractures
and pores are generated in the carbon structure. It dra-
matically increases the carbon surface area, making a
useful material for the removal of contaminants (Baudu
et al. 1991; Yang and Benton 2003). In some cases, the
carbonaceous matter may be treated with a chemical
activating agent such as phosphoric acid, zinc chloride
andthemixturecarbonizedatanelevatedtemperature,
followed by the removal of activating agent by water
washing (chemical method).
Active carbon uses the physical adsorption process,
whereby Vanderwaals attractive forces pull the solute
contamination out of the solution and onto its surface. The
efficiency of the adsorption depends on the nature of the
carbon particle and pore size, surface area, density and
hardness as well as the nature of the contaminants (con-
centration, hydrophobicity, polarity and solubility of the
contaminant and contaminant attraction to the carbon
surface).
There are two different forms of activated carbon in
common use, granular activated carbon (GAC) and pow-
dered activated carbon (PAC). Physically, the two differ as
their names suggested by particle size and diameter. The
reusability of the carbon is done primarily with the GAC as
PAC particles are too small to be reactivated.
Benefits:
•Activated charcoal is effective for trapping carbon-
based impurities (volatile organic chemicals), chlorine
(including cancer-causing by-product trihalomethanes)
as well as colors and odors,
•Very cost effective,
•Long life (high capacity).
Limitations:
•In GAC scheduled filter replacements, it is important to
eliminate the possibility of ‘channeling’ which reduces
the contact between the contaminant and the carbon.
Therefore, it reduces efficiency, and the accumulation
of bacteria in the filter,
•Frequent filter changes often required,
•Can generate carbon fines.
Activated alumina
Activated alumina consists mainly of aluminum oxide
(Al
2
O
3
) spherical beads, highly porous and exhibits tremen-
dous surface area. The surface area of activated alumina is in
the range 345–415 m
2
/g. It does not shrink, swell, soften or
disintegrate when immersed in water. It can exist in three
forms, viz. activated alumina sorbent, activated alumina
desiccant and activated alumina catalyst carrier. The granu-
lated alumina has the internal active surface of the alumina.
In this process, contaminated water is passed through a
cartridge or canister of activated alumina. The contaminant
adsorbs on the alumina (Chen et al. 1987). As the physical
adsorption has a particular limit, the cartridge of activated
alumina must be replaced periodically.
Benefits:
•Tailoring of activated alumina is possible by varying
the activation process and dopant variation,
•Effective in removing As
5?
,PO
43-
,Cl
-
, and F
-
from
water,
•Removal of Se, Sb, Pb and Bi from the water is also
possible.
Limitations:
•The method is not very much capable of reducing levels
of other contaminants of health concern. It needs
another support.
Appl Water Sci
123
Zeolite
Zeolites are aluminosilicates with an Si/Al ratio between 1
and infinity. It has a tetrahedral network of silicon and
oxygen atoms, and some of the silicon atoms are replaced
by aluminum to form alumino-silicates. The adsorptive
property of zeolite is considered due to the crystalline
nature of the materials. The channels in it are of extended
honeycomb and cavities. Zeolites have the surface area
1–20 m
2
/g. Synthetic Zeolites are manufactured by
hydrothermal processes in a temperature range of
90–100 C, an autoclave followed by ion exchange with
certain cations (Na
?
,Li
?
,Ca
2?
,K
?
,NH
4
?
) (Rahman et al.
2012). The high cation exchange and molecular sieve
properties, such as zeolites, have been widely used as
adsorbents. The water softening process is by exchanging
Na
?
with the Ca
2?
/Mg
2?
in water, as follows:
Na Zeolite þCa2þ=Mg2þ!Ca=Mg Zeolite þNaþ:
Natural zeolites in the waste-water treatment are very
useful (Margeta et al.2013; Kallo
´2001). Many natural
zeolites (e.g., Clinoptilolite, mordenite, phillipsite, chaba-
zite) show selective separation towards NH
4
?
and also for
transition metals (e.g., Cu
2?
,Ag
?
,Zn
2?
,Cd
2?
, and Hg
2?
)
(Jafarpour et al. 2010; Karapınar 2009).
Benefits:
•Recharging of zeolite is feasible by exchanging the
cations with the initial one and thus reuse is also feasible,
•Removes NH
4
?
and heavy metal removal of inorganic
anions (nitrates, phosphates, arsenates, chromates and
fluorides) as well as radionuclides (e.g., 137Cs, 90Sr,
60Co, 45Ca, 51Cr, 111mCd, 110mAg) is also possible,
•Removal of organics and other humic substances (including
humic, fulvic acid, and humin) and odor is also possible,
•Microorganisms capturing (the large surface area of the
zeolites is accessible for adhering microorganisms. This
makes selecting a suitable material for biofilter for
removal of pathogenic microorganisms),
•By the zeolites, the permeable reactive barriers (PRB)
can be prepared in the waste disposal site, so that
contaminations could not spread in the ground water.
Limitations:
•As zeolites are used as softener in detergent formulations
and insoluble, they lead to increase in sewage sludge mass.
Silica gel
Silica gel is an amorphous hard glass-like granules or
beaded material made of silicon dioxide (SiO
2
). Basically,
it is a naturally occurring mineral which is purified and
processed. Silica gel is a high capacity adsorbent with fine
pores on the surface and can be used especially as desic-
cant, moisture-proof, rust inhibitor as well as catalysis
(Heckel and Seebach 2000).
Generally, it is formed by two routes: (1) polymerizing
silicic acid, and (2) aggregation of particles of colloidal
silica. Silicic acid, Si(OH)
4
, has a strong tendency to
polymerize and form a network of siloxane (Si–O–Si),
leaving a minimum number of uncondensed Si–O–H
groups. The aggregation is by Van der Waals forces or by
cations bridging as coagulants. Commercial silica is pre-
pared through the first route by mixing a sodium silicate
solution with a mineral acid, such as sulfuric or
hydrochloric acid. The reaction produces a concentrated
dispersion of finely divided particles of hydrated SiO
2
,
known as silica hydrosol or silicic acid:
Na2SiO3þ2HCl þnH2O!2NaCl þSiO2nH2O
þH2O:
The hydrosol, on standing, polymerizes into a white jelly-
like precipitate, which is silica gel. The resulting gel is
washed, dried, and activated. Various silica gels with a wide
range of properties, such as surface area, pore volume, and
strength, can be made by varying the silica concentration,
temperature, pH, and activation temperature (Iler 1979).
Two common types of silica gel are known as regular-
density and low-density silica gels, although they have the
same densities (true and bulk). The regular-density gel has a
surface area of 750–850 m
2
/g and an average pore diameter
of 22–26 A
, whereas the respective values for the low
density gel are 300–350 m
2
/g and 100–150 A
).
Because of its large pore volume and mesoporosity, silica
gel is used as desiccant. The modified silica gel (modified by
the impregnation) with a high-molecular weight quaternary
amine (triethyl octadecyl ammonium iodide) has been used
for the concentration of heavy metals (Cs, Ag, Hg, Cu, Cd,
etc.) for water purification (Tzvetkova and Nickolov 2012;
Bowe et al. 2003; Bowe and Martin. 2004).
Benefits:
•Silica gel is non-toxic, non-corrosive material,
•It has high adsorption capacity because of very high
surface area and porosity.
Limitations:
•Preparative aspects needed very precise control,
•Modification is needed to remove the contaminants.
Ion exchange
The coulombic attractive force between ions and charged
functional groups is more commonly classified as ion
exchange. It is a typical reversible chemical reaction where
Appl Water Sci
123
an ion from a solution is exchanged for a similarly charged
ion attached to an immobile solid particle.
The selectivity coefficient controls the preference for
ions of particular resins and is expressed as follows:
KAþ
Bþ¼f
AgfBþg
fAþgf
Bg
for the exchange of
Ain solution for Bþon the resin:
Aþþ
B$Bþþ
A
The barred terms indicate location on the resin (solid
phase) as opposed to solution phase. The superscript and
subscript on the selectivity coefficient indicate the direction
of the reaction.
Ion exchange materials are insoluble substances con-
taining loosely held ions, capable of exchanging particular
ions within them with ions in a solution that is passed through
them. Many natural substances like proteins, cellulose, liv-
ing cells and soil particles exhibit ion exchange properties,
which play an important role in the way the function in
nature. Synthetic ion-exchange polymers can be made in two
forms, viz. beaded polymer matrix (resins) and membranes.
Ion exchange resins
Ion exchange resins are very small polymer matrix (beads),
with a diameter of about 0.6–1.0 mm. The ion exchange
resins can be manufactured in one of the two physical
structures, gel and porous. The gel resins are crosslinked
polymers having no porous structure, while porous resins
have considerable external and pore surfaces (microporous,
mesoporous and macroporous) where ions can attach. The
porous polymer matrices contain invisible water inside the
pores of the beads, measured as ‘‘humidity’’ or ‘‘moisture
content’’. The functional groups (ions) can be attached on
the polymer matrix which cannot be removed or displaced.
Based on their functional groups attached on polymer
matrix, the ion exchange resins are two types: cation and
anion exchange resins, which further subdivided into four
categories-
•Strongly acidic (typically, sulfonic acid groups, e.g.,
sodium polystyrene, sulfonate, etc.,
•Strongly basic (quaternary amino groups, for example,
trimethylamonium group,
•Weakly acidic (mostly, carboxylic acid groups),
•Weakly basic (primary, secondary, and/or ternary
amino groups, e.g., polyethylene amine).
Cation exchange resins (Fig. 1) exchange cations like cal-
cium, magnesium, radium, and anion resins, used to remove
anions like nitrate, arsenate, arsenite, or chromate from waste
solution/water (Alexandratos 2009;Calmon1986). Regenera-
tion can be possible using sodium chloride. In case of cation
resins, sodium ion displaces the cation from the exchange site;
whereas in case of anion resins, the chloride ion displaces the
anion from the exchange site. Resins can be designed to show a
preference for specific ions, so that the process can be easily
adapted to a wide range of different contaminants.
The mode of preparation of ion exchange resins is
through suspension polymerization technique containing the
monomers, cross-linkers and initiators. Various types of
polymeric beads like styrene, MMA, MAA, DVB, etc. can
be prepared by this technique, varying the ratio of mono-
mers, diluents, the stabilizer, concentration and the agitation
rate is dispersed by agitation in a liquid phase, usually water,
in which the monomer droplets are polymerized while they
are dispersed by continuous agitation also known as pearl
polymerization technique (Penlidis et al. 1997).
The most important issue in the practical operation of
suspension polymerization is the control of the final par-
ticle size distribution. The size of the particles depends on
monomer type, monomer purity, interfacial tension, stabi-
lizer concentration, agitation condition in the reactor (de-
gree of agitation) design of reactor/stirrer.
Benefits:
•Simple and low running cost technique,
•Technique is very useful in separating components/con-
taminations (cations and anions) from dilute solutions/
wastes and in water purification, etc.,
•Useful for the recovery of expensive materials from
industrial waste (e.g., precious metals),
•Recycling components present in the solutions and/or
regenerating chemicals,
Fig. 1 Schematic diagram regarding the behavior cation exchange
and anion exchange resin
Appl Water Sci
123
•Capability to handle hazardous wastes,
•Simple regeneration process and well-maintained resins
last for many years.
Limitations:
•Limitation on the concentration in the effluent to be
treated,
•Ion exchange resin-treated water contains sodium,
which cannot be recommended for the diet requiring
low sodium intake,
•Generation of waste (sodium wastewater) as a result of
ion exchange regeneration,
•Ion exchange resins do not remove organic compounds
or biological contaminants,
•If resin is not sanitized or regenerated regularly,
bacterial colonies proliferate on resin surfaces and
can contaminate drinking water.
The ion exchange membranes are discussed in the fol-
lowing part.
Apart from the above, interests are growing to develop
different low cost adsorbents. For this purpose, numerous
agro-waste biomaterials are found suitable, viz. rice-husk,
soyabean hulls, coconut shells, rice straw, sugarcane
bagasse, tea leaves, petiolar felt-sheath of palm trees, etc.
(Ahluwalia and Goyal 2007; Tee and Khan 1988; Low
et al. 1993; Mustafiz et al. 2002). These are useful for
removal of heavy metal ions (Pb
2?
,Ni
2?
,Cd
2?
,Zn
2?
,
etc.) in low concentrations. Biosorption is a rapid phe-
nomenon of passive metal uptake sequestration of non-
growing biomass (Beveridge and Doyle 1989). Biomass of
Aspergillusniger, Penicillum Chrysogenoum, Rhizopusni-
gricans, Ascophyllumnodosum, Sargassumnatans, Chlor-
ella fusca, Oscillatoriaanguistissima, Bacillus firmus and
Streptomyces sp. has also the potential to sequester metal
ions by forming metal complexes from solution and obvi-
ates the necessity to maintain special growth-supporting
conditions.
Membrane water treatment
Membrane technology is one of the innovative ideas of
water treatment. Over here, a semipermeable membrane is
used for the removal of water impurities. There are two
types of membrane water treatment technologies, namely
pressure-driven (e.g., reverse osmosis) and electrically
driven (electro-membrane) (Charcosset 2009).
Reverse osmosis
The two processes (viz. osmosis and reverse osmosis) are
the regulator of life. Though they are termed as concen-
tration and pressure driven simultaneously, both are con-
trolled by thermodynamic function, i.e., ‘chemical
potential’ of the systems. It is essentially a driving force
expressed as a change in the free energy of the system as a
result of the change in the composition of the system.
Though literally the two signify just the opposite process,
thermodynamically they are similar. Under isothermal
operating condition, the tendency for material transport is
always in the direction of lower chemical potential for both
the processes. In osmosis, the flow is occurring solvent to
solution side through a semipermeable membrane, whereas
in reverse osmosis the flow is a solution for solvent. In both
cases, only solvent molecules migrate from one side to
another. The schematic diagram of osmosis and reverse
osmosis is presented in Fig. 2.
The main two characteristics of a membrane process are
flux and rejection. If an RO membrane is considered as
permeating water only, the water and solute flux can be
written as:
Jw¼ADPDPðÞ;
Js¼BDcs;
where Ais the permeability coefficient, and DPand DPare
the hydraulic pressure and osmotic pressure difference
Fig. 2 Schematic diagram of
osmosis and reverse osmosis
Appl Water Sci
123
across the membrane and Bis the solute permeability
coefficient and DCsis the solute concentration difference
across the membrane.
The microfiltration and ultrafiltration membranes have a
pore size in the range of [10 and 1–100 nm, respectively,
whereas in the case of nanofiltration and reverse osmosis
membranes are in the range of *1and\1 nm. Size selective
separation operates in case of micro and ultrafiltration, whereas
the size and charge selective separation operate in the latter two.
The membranes are generally based on natural and
synthetic polymers (cellulose acetate, cellulose triacetate,
polysulfone, polyamide, etc.). The most popular RO
membrane is thin film composite membranes (i.e., poly-
amide layer on asymmetric polysulfone) (Cadotte and
Peterson 1981). The polyamide layer is formed by inter-
facial polymerization of diamine and acyl halide and shows
the charge holding capacity in it (Fig. 3).
Reverse osmosis (RO) is one of the most effective types
of water treatment and widely used water purification
processes in the world. It is usually used for home water
treatment to remove salts (Bhattacharya and Ghosh 2004),
chemical toxins (Pawlak et al. 2005), organic contaminants
(Bhattacharya et al. 2008), dyes (Nataraj et al. 2009),
pesticides (Bhattacharya 2006) and microbes (Park and Hu
2010). In reverse osmosis, the raw water is forced (with
pressure) through a dense membrane filter that prevents
passing of impurities.
Benefits:
•No phase changes and thus requirement of low energy,
•Eco-friendly as they do not produce or use any harmful
chemicals; compactness and space requirements are
less compared to distillation, and can be designed
according to the requirement,
•Ability to remove almost all kinds of contaminates like
Cl
-
,NO
3
-
,F
-
,SO
4
=
,Pb
2?
,Na
?
,K
?
,Mg
2?
, organics
as well as microorganisms,
•No alteration in the taste and smell of water and
effective removal of microbes and toxins.
Limitations:
•The purified water obtained after reverse osmosis
treatment is devoid of useful minerals,
•Membrane may become clogged after prolonged use
and, hence, requires periodical replacement of the
membrane.
Electrodialysis membrane treatment
Electrodialysis (ED) is electric potential-driven membrane-
based separation process. The basic principle of the
membrane separation is similar to ion exchange reactions
(Xu 2005; Strathmann 2010a). The charged groups are
attached to the polymer backbone of the membrane mate-
rial and it is obvious that the fixed charge groups partially
or completely exclude ions of the same charge from the
membrane, i.e., an anionic membrane with fixed positive
groups excludes positive ions, but is freely permeable to
negatively charged ions whereas cationic membrane with
fixed negative groups excludes negative ions but is freely
permeable to positively charged ions.
Since the membrane is of ion selective, it separates or
rejects opposite charge ions, useful in removal, or separa-
tion of electrolytes (Koter and Warszawski 2000; Strath-
mann 2010b). The schematic diagram is presented in
Fig. 4.
The ion transportation depends on the current efficiency
in the particular system. Generally, the current efficiencies
[80 % are required in commercial stacks to minimize
energy operating costs. The low current efficiencies result
in water splitting in the dilute or concentrate streams, shunt
currents between the electrodes, or back-diffusion of ions
from the concentrate to the dilute. The current efficiency is
NHCO
NH CO
CO
n
NH
2
NH
2
COCl
+
ClOC COCl
C
CH
3
CH
3
S
O
O
n
O
O
O
(i) Interfacial
Polymerization
Polyamide Ultrathin barrier layer
Microporous polysulfone
(< 1 µm)
(30-40 µm)
Reinforcing polyester non-woven fabric (100 µm)
(100 µm)
(ii) Thermal
Curing
m-phenylene
diamine
Trimesoyl
chloride
Fig. 3 Schematic diagram of
polyamide thin film composite
membrane
Appl Water Sci
123
calculated according to the following equation (Shaffer and
Mintz 1980).
n¼zFQfCinlet Coutlet
ðÞ=NI;
where nis the current utilization efficiency, zis the charge
of the ion, Fis the Faraday constant 96485 Amp-s/mol,
Q
f
is the dilute flow, L/s C
inlet
is the dilute ED cell inlet
concentration, mol/L, C
outlet
is the ED cell outlet concen-
tration, mol/L, Nis the number of cell pairs, and Iis the
current, Amps.
Apart from their chemical structure (cation and anion),
the commercial ion-exchange membranes can be divided,
according to their structure and preparation procedure, into
two major categories, homogeneous and heterogeneous and
depending on the degree of heterogeneity of the ion-ex-
change membranes, these can be further classified into
different types: mono polar (cation/anion) ion-exchange,
amphoteric ion-exchange, bipolar ion-exchange, inter-
polymer membranes.
The ion-exchange membranes are very similar to normal
ion-exchange resins in terms of chemical structure as well
as of high selectivity and low resistivity. The difference
between membranes and resins arises largely from the
mechanical requirement of the membrane process. Thus, it
is generally not possible to use sheets of material that have
been prepared in the same way as a bead resin. However,
the most common solution to this problem is the prepara-
tion of membrane with a backing of a stable reinforcing
material that gives the necessary strength and dimensional
stability. The preparation method of ion-exchange mem-
branes can be summarized in three different steps, viz.
Polymerization or polycondensation of monomers; at least
one of them must contain a moiety that either is or can be
made anionic or cationic groups, respectively, introduction
of anionic or cationic moieties into a preformed solid film
such as styrene-DVB-based membrane, and introduction of
anionic or cationic moieties into a polymer, such as Poly-
sulfone, followed by dissolving the polymer and casting it
onto a film.
Benefits:
•Non-pollution, safety and reliability,
•Completely eliminated the chemicals for regeneration,
•Effective for complete removal of dissolved ionic
particles (cation and anions), heavy metals, etc.,
•Ability to treat feed water with higher SDI, TOC and
silica concentrations.
Limitations:
•Removal of low-molecular weight ionic
contaminations,
•Non-charged, higher molecular weight, and less mobile
ionic species cannot be significantly removed by the
process,
•Large membrane areas are required to satisfy capacity
requirements for low concentration (and sparingly
conductive) feed solutions.
Catalytic processes
Catalytic processes are typically achieved by the following
three methods: hydrogenation of nitrate, photocatalytic and
electrocatalytic.
Hydrogenation of nitrate
The hydrogenation via catalytic method is one of the
promising techniques for removal of nitrate from water. It
needs very active catalysts because the reaction is per-
formed preferably at an ambient/low temperature. The
reaction scheme shows that nitrate is reduced to the desired
products involving NO
2
-
, NO, N
2
O and N
2
. The undesired
byproduct NH
4
?
is also formed by a side reaction due to
over hydrogenation (Soares et al. 2010; Mikami et al. 2006;
Berndt et al. 2001). Supported bimetallic catalyst (viz. Pd/
Cu, Pd/In, and Pd/Sn) has emerged as efficient catalysts for
nitrate hydrogenation (Gao et al. 2003; Mikami et al. 2003;
Deganello et al. 2000). Apart from Pd, the other metals
(e.g., Cu, In, Sn, Co) serve as the role of promoter for the
first reduction step to convert NO
3
-
into NO
2
-
(Soares
et al. 2008; Pintar et al. 2004; Arino et al. 2004; Qi et al.
2006). It is seen in the schematic reaction, below that N
2
and ammonium (NH
4
?
) are the stable end products of the
catalytic reduction process. N
2
is not harmful, but the
second one is considered a hazardous aquatic pollutant.
Fig. 4 Schematic diagram of electrodialysis. DC diluted chamber,
EW electrode wash, CC concentrated chamber, CEM cation exchange
membrane, AEM anion exchange membrane
Appl Water Sci
123
That is why target is to convert NO
3
-
into N
2
as an end
product.
Benefits:
•The method can be of single operation mode,
•Selectivity of catalyst can counter the formation of
ammonia ions,
•Addition of other chemicals can be avoided.
Limitations:
•Increase in pH in the reaction medium forms ammonia
in dissolved condition, which is more harmful than
nitrate.
Photocatalytic method
The method is based on the acceleration of photodegradation
of organic pollutants, pathogens, green algae, and substances
in the presence of catalyst (Esplugas et al. 2002; Pera-Titus
et al. 2004; Akira et al. 2000;Egginsetal.1997;Bekbolet
et al. 1998; Ibhadon and Fitzpatrick 2013; Gaya and Abdullah
2008;Chongetal.2010). In response to UV light, when they
excited charge separation followed by scavenging e–s and
holes by surface adsorbed species. The heterogeneous pho-
tocatalysts employing semiconductor catalysts (TiO
2
,ZnO,
Fe
2
O
3
,) have shown their efficiency in degrading a wide
range of pollutants in water. Metal oxides are more suitable,
since they are more resistant to poisoning and deactivation.
Upon UV-irradiation, photocatalytic reactions are initi-
ated by the absorption of illumination with photo-energy
equal to or greater than the band gap of the semiconductor.
It results in electron–hole (e–/h
?
) pairs as shown in Fig. 5.
Thus, it participates in the redox reaction with the adsorbed
pollutant species in water. Apart from the reaction, the
semiconductor also oxidizes water to produce OH•,a
powerful oxidant, which rapidly reacts with the pollutants
in the water (Teoh et al. 2012).
To improve the catalytic activity using visible light,
various approaches are also developed, viz. addition of
dopants, stoichiometry of catalytic metal oxides and mixed
metal oxides, particle size and shape. TiO
2
doped with
nitrogen showed excellent photo catalytic activities com-
pared to unmodified TiO
2
nanoparticles in both degradation
of chemicals and bactericidal reaction (Daneshvar et al.
2007).
Benefits:
•Reusability of the catalyst as it is unchanged during the
process,
•Reactions can occur in ambient condition as well as no
consumable chemicals are required,
•Operational process is simple
•It is good enough to treat low concentration of
pollutants.
Limitations:
•Post-separation of the semiconductor catalysts after
water treatment is important and failing results in
catalyst poisoning.
•The catalysts with their fine particle size and large
surface area to volume ratio create a strong catalyst
agglomeration tendency during the operation.
Electrocatalytic oxidation
In the electrocatalysis, the oxidation occurs through surface
mediator on the anodic surface. The rate of oxidation
depends on temperature, pH and diffusion rate of gener-
ating oxidants in indirect electrolysis (Mohana and Bala-
subramanian 2006). This is somewhat different from
electrolysis where direct oxidation of pollutants takes place
and rate of oxidation depends on electrode activity, pol-
lutants diffusion rate and current density.
The electrocatalysis through metal oxide (MO
x
) elec-
trode can be shown (Comninellis 1994) as follows:
MOxþH2O!MOxðOHÞ:
Fig. 5 Schematic diagram of the photocatalytic arrangement
Appl Water Sci
123
In the presence of organics (R) present in waste water,
the physiosorbed active oxygen (
OH) involves in complete
combustion of organics (1) and chemisorbed active oxygen
in the form of MO
x?1
(2) does the selective oxidation
RþMOxðOHÞ!CO2þHþþeþMOxþ1;ð1Þ
RþMOxþ1!RO þMOx:ð2Þ
The key role in the electrocatalytic process is electrocat-
alytic material. Ru/Pb/Sn oxide and Pb/PbO
2
coated with
Ti is used in the dye oxidation (Mohana and Balasubra-
manian 2006; Morsia et al. 2011). Pt, TiO
2
, IrO
2
, PbO
2
,
several Ti-based alloys and boron-doped diamond (BDD)
electrodes are employed for the removal of effluents con-
taining various organics, viz. phenols, pharmaceuticals,
alcohols, carboxylic acids, anionic surfactants and pesti-
cides (Comninellis and Nerini 1995; Radovici et al. 2009;
Klavarioti et al. 2009; Canizares et al. 2008; Louhichi et al.
2008; Ventura et al. 2002). Pt(acac)
2
onto ruthenium
nanoparticles is used for the removal of formic acid (Chen
et al. 2009).
Electrocatalytic reduction is largely used for NO
3–
removal. In this regard, the development of electrodes (viz.
Ti–Rh, Ti/IrO
2
–Pt, PPy–Graphite, Carbon cloth–Rh, Pd–Sn
activated carbon fiber (Tucker et al. 2004; Li et al. 2010;
Zhang et al. 2005;Peeletal.2003; Wang et al. 2006)is
interesting direction.
Benefits:
•High pollutant degradation, easy control and low cost,
•It can be easily controlled by putting on/off the power,
•Environmentally compatible since there is little or no
need for additional chemicals,
•It has the potential to eliminate different types of
pollutants as well as bulk volume,
•It operates at low temperature and pressure compared to
nonelectrochemical methods; thus, the volatilization
and discharge of un-reacted wastes can also be avoided.
Limitations:
•High operating cost due to the high energy consump-
tion during operation,
•Electrode fouling may also occur on the surface of the
electrodes,
•It needs, conducting nature of the effluent. Sometimes
the addition of an electrolyte may be necessary,
•Theuseofmetalionsresultedinaneffluentwitha
higher toxicity than that of the initial effluent. Thus,
this approach requires a separation step to recover
the metallic species (Martinez-Huitle and Ferro
2006).
Bioremediation
Phytoremediation
It signifies the removal of pollutants from the environment
by the use of plants. The technology involves different
mechanisms, viz. phytoextraction, rhizofiltration, phy-
tostabilization, phytotransformation/phytodegradation (Rai
2009). Phytoextraction involves metal accumulation into
the harvestable parts of the roots and the above ground
shoot. Rhizofiltration indicates the absorption, precipitation
and concentration of toxic metals from polluted effluents.
Phytostabilization is a process in which mobility of heavy
metals is reduced through the use of tolerant plants,
whereas phytotransformation/phytodegradation is the pro-
cess in which contaminants can be eliminated via phy-
todegradation or phytotransformation by plant enzymes or
enzyme co-factors.
The history of the particular study, including the uptake
of toxic metals Hg, As, and other metals, begins in the 70’s
(Dolar et al. 1971) and other metals (Stanley 1974). In this
regard, macrophytes water hyacinth (Eichhorniacrassipes)
(Zhu et al. 1999); pennywort (Hydrocotyle umbellate L)
(Dierberg et al. 1987); duckweeds (Lemna minor L.) (Rai
2007a) and water velvet (Azollapinnata) (Rai 2007a,b) are
considered the biological filters and play the important role
in the maintenance of the aquatic ecosystem. The floating
plants Lemna minor (Zayed et al. 1998), Eichhorniacras-
sipes (Zhu et al. 1999) and Pistiastratiotesand Salvinia-
herzogii (Maine et al. 2001,2004) show good potential in
accumulating the metals directly from industrial effluents.
Benefits:
•Cost effective,
•Eco-friendly.
Limitations:
•Seasonal growth of the plants,
•Biomass disposal.
Vegetated filter strips
The filter strips are meant as land areas of either planted or
indigenous vegetation, situated between a potential, pol-
lutant-source area and a surface-water body that receives
runoff. Vegetated filter strips (viz. grassed filter strips, filter
strips, and grassed filters) are vegetated surfaces that are
designed to treat sheet flow from adjacent surfaces (Dillaha
et al.1989; Delgado et al. 1995). The run-off usually carries
sediment, organics, plant nutrients and pesticides.
Appl Water Sci
123
The trapped plant nutrients and pesticides may be easily
degraded or transformed by biological and chemical pro-
cesses. Cole et al. (1997) report the removal of chloropyrifos
(62–99 %), dicamba (90–100 %), 2,4D (89–98 %), and
mecoprop (89–95 %) using Bermuda grass buffer. On the
other hand, atrazine (98 %) and pyrethroid (100 %) removal
is possible using vegetated drainage ditch (Moore et al. 2001)
Benefits:
•Trap sediments,
•Capture nutrients both through plant uptake and
adsorption of soil particles,
•Promote transformation and degradation of pollutants
into less toxic forms,
•Removal of pathogens is possible.
Limitations:
•The design is important,
•Proper vegetation is necessary,
Biologically active carbon filtration
Biologically active carbon is another prospective process
with this bioremediation technique. The process utilizes
granulated activate carbon (GAC) as its water filtration.
The microbial (bacterial) colonization is possible over the
GAC media particles form ‘biofilm’ (Scholz and Martin
1997). Actually, it is described as a ‘porous tangled mass of
slime matrix (Weber et al. 1978). It consists of microbial
cells, either immobilized on the surface of the GAC (sub-
stratum) or embedded in an extracellular microbial organic
polymer matrix (Ghosh et al. 1999; Lawrence and Tong
2005). Bacteria and fungi cells in the biofilms secrete
extracellular polymeric substances to form a cohesive,
stable matrix in which cells are held in dense agglomera-
tion (Branda et al. 2005; Lazarova and Manem 1995). The
extracellular matrix is composed of polysaccharides, pro-
teins, nucleic acids and lipids (Goodwin and Forster 1985).
The activity of the biofilm relates to the physiological
modifications associated with the promotion of certain
genes (Dagostino et al. 1991), or changes the bacteria cell
surrounding to increase the local concentration of nutrients,
oxygen and enzymes (Ghosh et al. 1999) or limit the
invasion of toxic or inhibiting substances (Blenkinsopp and
Costerton 1991).
Most of the dissolved organic chemical removal occurs
through physical adsorption in the GAC media. Apart from
the adsorption, biodegradation can also operate.
Benefits:
•It can avoid chemical disinfection water treatment
processes,
•Because of the microbial biodegradation of organic
substrates on the GAC media, the service life can be
extended,
•Bacterial regrowth is less possible,
•Eliminates the need for coagulant in source filtration
processes (Hillis 2000).
Limitations:
•The control of the growth of the process is necessary.
Magnetic separation
In the magnetic separation process, the high-gradient
magnetic separation (HGMS) is a commonly used process
(Hoffmann and Franzreb 2004a,b; Ditsch et al. 2005;
Okada et al. 2005). In this case, device comprising bed of
magnetically susceptible wires placed inside an electro-
magnet is used. There are various influencing factors, viz.
nature of impurities, concentration, size, magnetic sus-
ceptibility, spacing design, and intensity of magnetic field
and its orientation, magnetic field strength.
Generally, there are three categories of separators based
on magnet type, viz. permanent magnet, electromagnet and
superconducting magnet. The permanent magnet (ferro-
magnets of iron-based, nickel, cobalt or rare earth element)
is having magnetic fields of less than 1 T, though trend is
to improve the magnetic field strength by the development
of materials and shape design parameters (Ormerod and
Constantinides 1997; Zhu and Halbach 2001; Iwashita
et al. 2008). The electromagnetic-based device consists of a
solenoid of electrically conducting wires which can gen-
erate a magnetic field of 2–4 T within their cavity on
passage of electric current (Li et al. 2007) (Timoshenko
and Ugarov 1994). The third category of magnetic sepa-
rators generates the highest intensity magnetic field from 2
to 10 T (Selvaggi et al. 1998; Yan et al. 1996).
The magnetically assisted water purification can be
primarily classified into the following type depending on
the difference in adoption of physical processes, viz. direct
purification, seeding and separation by magnetic flocculant,
and magnetic sorbent application in organic and inorganic
contaminants including radionuclides. In the direct purifi-
cation technique, there is no carrier magnetic component
utilized for the separation. The basic properties of ions or
solid response to the magnetic field are utilized for purifi-
cation. In this method, the anti-scaling technique is most
commonly practiced. In the area of anti-scale magnetic
treatment, the most common constituents of scale are
CaCO
3
, CaSO
4
2H
2
O and silica, BaSO
4
, SrSO
4
,Ca
3
(PO
4
)
2
and ferric and aluminum hydroxides (Busch and Busch
1997; Gabrielli et al. 2001; Fathia et al. 2006; Jianxin et al.
Appl Water Sci
123
2007). In the magnetic flocculant separation, coagulant
cation [viz. Fe(III)] forms an insoluble precipitate under
applied magnetic field. It is an effective means of lowering
significantly both the oil and suspended solids of water
effluent streams (Kakihara et al. 2004; Nishijima and
Takeda 2007). Ions (polymerise as polyhydroxycomplexes,
or nitroso-hydroxy, or hydroxy-carbonato or halogenohy-
droxo-carbonato complexes), which are difficult to coag-
ulate magnetic sorbents, are utilized for waste water
purification.
Benefits:
•Useful for the separation of pollutants,
•Magnetic pre-treatment improvises purification RO
membrane filters,
•Calcium carbonate scale formation in heat exchanger
can be reduced,
•Promotes the homogeneous precipitation of calcium
carbonate scales.
Limitations:
•Not fully sufficient.
Disinfection
The disinfection methods are classified as physical and
chemical methods. In physical treatment UV, solar radia-
tion, and ultrasound are included, whereas chlorine, iodine,
ozone are included in chemical treatment (Kerwick et al.
2005).
The features of the treatments are described in the
following:
By ultraviolet radiation:
In the ultraviolet treatment, the water to be treated passes
through germicidal ultraviolet (UV) light configured inside
a low-pressure lamp. As the water passes the ultraviolet
purifier, the biological contaminants are exposed to UV
light, which damages the genetic components of the
microbes. The microbes are killed this way using ultravi-
olet water treatment (Hijnen et al. 2006; Bergmann et al.
2002). They are the pioneers of using UV in water puri-
fiers. A major drawback of this water treatment type is that
it is ineffective in removal of dissolved chemicals and other
particulate matter.
Benefits:
•Ability to destroy or make inactive many pathogenic
microorganisms,
•It has no effect on minerals in water,
•Ability to degrade some organic contaminants,
•No additional toxic and nontoxic chemicals are
introduced.
Limitations:
•Not suitable for water with high levels of suspended
solids, turbidity, color or soluble organic matter,
•Without electricity, it could not operate.
It is employed by solar radiation also. It is very useful to
inactivate pathogens, especially diarrhea. The contaminated
water is to fill into transparent plastic bottles and expose to
the full sunlight for 6 h. The UV-A radiation (wavelength
320–400 nm) of the sunlight destroys the pathogen.
Benefits:
•Easy to use as well as inexpensive,
•Good bacterial and viral disinfection,
•No toxic chemicals except plastic bottles,
•Does not require constant attention to use,
•No effect for minerals in water.
Limitations:
•Dependent on climatic condition,
•Toxicity can come from poor quality of plastic bottles,
•Need turbidity of 30NTU or less,
•Less effective against bacterial spores and cysts stage
of some parasites.
By ultrasound
Ultrasound is the cyclic sound pressure with a frequency
greater than the upper limit of human hearing. The ultra-
sound is used in many different fields by penetrating the
medium, measuring the reflection signature or supplying
focussed energy. The mechanical vibration of the waves
can be caused to damage cellular structures of bacteria.
Thus, it can be useful to disinfect water. However, the
regrowth of the microorganisms is also possible. Thus,
combination of this and chemical disinfectant is the best
technique.
Benefits:
•Easy to use,
•Does not require constant attention.
Limitations:
•Regrowth of microorganism is also feasible,
•Not fully self-sufficient.
By ozone
Ozone, O
3
is an unstable form of oxygen and protective
layer of UV-radiation. But in drinking water, it makes an
Appl Water Sci
123
effective disinfectant (VonGunten 2003a,b). It readily
gives up oxygen and thus a powerful oxidizing agent.
Ozone is made by passing oxygen through UV-light or a
‘cold’ electrical discharge. The very high oxidation
potential of ozone is easy enough to insert oxygen into the
bonds of organic compounds to form aldehydes and
ketones. It is effective for killing the biological contami-
nants (viz. pathogens) than that of chemical disinfection
method like chlorination. Actually, the ozone oxidizes the
organics in bacterial membrane, which weakens the cell
wall and leads to cellular rupture. This exposes the
organism to the external environment, which causes almost
immediate death of the cell. Ozone also improves the
clarity (clarifying iron, sulfur and manganese). The soluble
Fe(II) and Mn(II) which are not filtered in the normal
condition transformed to insoluble Fe(III) and Mn(VII)
with ozone treatment and, thus, filtration is possible. It also
reduces odor problems and concentrations of sulfur and
other dissolved chemicals. The main advantage of ozone is
that it leaves no disinfectant residual in the water. To use
ozone as disinfectant, it is generated and immediately
applied on site. The limitations of using ozone as disin-
fectant are a significant air pollutant, explosive, and an
irritant to skin, eyes, respiratory tract and mucous mem-
brane. It can produce carcinogens if little bromine is there
in the water.
By chlorine
The most common strong oxidant in the form of chlorine
and its compounds, viz. chloramine or chlorine oxide are
used in disinfection technique. Chlorine is well to do against
bacteria and protozoa that form cysts (viz. Giardia lamblia
and Cryptospordium) (Gala-Gorchev 1996); Melvin et al.
1967). Handling of chlorine gas is dangerous, thus the use of
sodium and calcium hypochlorite is the trend. It releases free
chlorine in water. Electrolytic method is another mode to get
chlorine solution. The free chlorine is released when dis-
solved in water. The limitation of using chlorine that reacts
with natural organic compounds in the water forms poten-
tially harmful chemical by-products, such as tri-
halomethanes and haloacetic acids. They are shown to cause
cancer (Univ. Florida Report 1998). The maximum allow-
able annual average level of trihalomethane and haloacetic
acids is 80 and 60lg/L, respectively. They tend to increase
with pH, temperature, time and the level of organics in
water. One way to decrease the level of trihalomethane and
haloacetic acids is to reduce the organics (EPA, US 2012).
Thus, it is preferable to use after the removal of organic
compounds from water. EPA has suggested ‘enhanced
coagulation’ (i.e., the process by increasing the feed rate of
coagulants, adding better coagulants, possibly ferric coagu-
lants) process to remove the organics for controlling
trihalomethane and trihaloacetic acids. The advantage of
using chloramine is that it will not form THMs or haloacetic
acids, but it results in nitrification, as ammonia is a nutrient
for bacterial growth, with nitrates being generated as by-
products
Cl2þH2O!HOCl þHCl,
HOCl !HCl þO½:
Benefits:
•Simple method,
•Availability of inexpensive chemicals/cheaply
available,
Limitations:
•Excess of chlorine produces characteristic unpleasant
taste and odor and irritating effect on mucous
membrane.
However, the usual practice is bleaching powder in
place of chlorine. The mechanistic process is as follows:
Ca OClðÞCl þH2O!Ca OHðÞ
2þCl2;
Cl2þH2O!HOCl þHCl,
HOCl !HCl þO½:
Bleaching powder should be used only in calculating
amount because excess of it will give a bad taste and dis-
agreeable odor, while the lesser amount of it will not
sterilize the water completely (Snoeyink and Jenkins
1980).
Benefits:
•Simple method,
•Availability of inexpensive chemicals.
Limitations:
•It requires continuous supply of the chemicals and
trained personal so that the chlorine is at effective
levels.
By iodine
Similar to chlorine, it is also a good oxidizing agent. It is
effective against many varieties of pathogenic organisms
including spores, cysts, viruses, etc. in a short time (Pu-
nayani et al. 2006). Compounds-based formulations (or-
ganic iodide compounds: bisglycine hydroiodide,
potassium tetraglycine triiodide, etc., iodophores: combi-
nation of iodine and solubilizing compounds, i.e., non-
ionic surfactants, iodine incorporated resins, cross-linked
copolymer of styrene and divinylbenzene with I
3
-
, iodine
with polyvinyl pyrrolidone) are there to regulate the release
of iodine.
Appl Water Sci
123
The mechanistic way of inhibition of protein function is
forming N-iodo compounds, i.e., reacting with basic –NH
functions of amino acids and nucleotides. Thus, important
positions for H-bonds are blocked, resulting in a lethal
change in protein structure. The –SH groups in the cyto-
plasm are oxidised. Thus, the ability to make disulphide
bonds in protein formation is lost. The addition of olefinic
double bonds of unsaturated fatty acids may also be the
reason to decrease the fluidity of cell membranes.
Benefits:
•Effective against many varieties of pathogenic organ-
isms including spores, cysts, viruses, etc. in a short time,
•Eliminates the chances of disease caused due to
deficiency of iodine,
•Ammonia and other nitrogeneous substance have no
pronounced effect on the efficiency.
Limitations:
•Higher concentrations are required compared to chlorine,
•Costly than chlorine per unit of germicidal
effectiveness,
•Taste and slight color produced can affect palatability
and esthetic quality.
By hydrogen peroxide
Though it is known for high oxidative and biocidal effi-
ciency, the use in drinking water disinfection is not avail-
able, but coupled with ozone, UV-radiation it can be used
(Andreozzi et al. 1999). The disinfection mechanism is
based on the decomposition of peroxide, i.e., the release of
free oxygen radicals. The free radicals have both oxidizing
and disinfecting abilities.
Contrary to other chemical substances, it does not pro-
duce residues or gases. The limitations of peroxide are: it
can irritate the eyes, skin, and lung. Skin exposure causes
painful blisters, burns and skin whitening.
Apart from the above, recent trend is there to employ
Ag, Au, Cu, Zn, titanium nanoparticles supported in solid
matrix. Due to the bactericidal effect, the water passed in
the matrix will be free from bacterial contamination (Li
et al. 2008; Savage and Diallo 2005).
Hybrid technologies
In true sense, no technologies independently counter all the
problems. The development of technology is a dynamic
process that moves forward slowly and recommendations
are made based on the best science available at that time.
However, with new research and new results, the flaws of
existing technologies may be removed. That is why the
concept of the combinations of technologies or in other
sense hybrid technologies has come. Scientists and tech-
nologists have orchestrated according to the requirement.
Let us discuss with the synergistic RO technology first. In
the RO technology, feed pretreatment is vital for RO to
avoid problems, i.e., fouling, damaging the membranes.
Conventional pretreatment steps include chemicals addition,
i.e., acid, coagulant/flocculant, disinfection. Coagulation and
flocculation (coagulants–flocculants) are dealt in water
treatment process. Chlorine treatment is treated as disin-
fection process and commonly employed. But chlorination
shortens the stability of the membrane and, thus, dechlori-
nation treatment (viz. sodium bi sulfite) is required. In media
filtration, water is treated by passing through granular media
like pumice, anthracite, gravels, etc. that can be used in
combination. Cartridge filter (made up of papers, woven
wire, cloth) is used as the last pretreatment step to retain
particles in the size range 1–10lm. To check the quality,
‘Silt Intensity Index’ or SDI parameter is important. Actu-
ally, SDI considers the ratio of two flow measurements, one
at the beginning, and the other at the end by passing feed
water through a 0.45 lm filter paper in dead end mode at
constant pressure (Saha and Bhattacharya 2010).
Similarly, the pretreatment step coagulation is coupled
with ion exchange treatment of water. The coupled elec-
trodeionization technology based on electrodialysis and ion
exchange results in a process which effectively deionizes
water, while the ion exchange membranes are continuously
regenerated by the electric current in the unit. This electro-
chemical regeneration replaces the chemical regeneration of
conventional ion exchange systems. Recently, hybrid tech-
nologies like ED-RO or ED-RO and distillation have been
developed for the water purification and these processes offer
many advantages over the traditional technologies (Saha and
Bhattacharya 2010; Makwana et al. 2010). The ED-RO
technologies desalinate the brackish water with high recov-
eries along with zero discharge and reduced energy con-
sumption. ED-RO is a high recovery system since RO
concentrates can be recycled through the ED system to reduce
the feed flow rate, pre-treatment cost and the reduced amount
of effluent. Thus, coupling of the technologies/processes
offers a solution to an increasing important issue in water
treatment as well as for water conservation. To get better
results, UV is typically used as a final purification stage in
terms of removing contaminants bacteria and viruses.
Awaited/coming technologies
The torch of the scientific quest along with the traditional
technologies has now been handed over to the nanotech-
nologists of the twenty-first century, to whom a major
challenge is to transform this into the field. Nanotechnology
Appl Water Sci
123
refers to technologies involving particles on the approximate
size scale of a few to hundreds of nanometers in diameter.
The elevated surface area to mass ratio, a common charac-
teristic of nanoparticles, makes it promising. In terms of
applicability, three approaches are there, viz. individual
nanoparticles (Watlungton 2005), binding the nanoparticles
to a powder/granule form and nanoparticles onto mem-
branes/polymers. As individual zero valent iron particles
(Can-Bao and Zhang 1997), palladium-coated iron particles
and palladium-coated gold nanoparticles (Michael et al.
2005) are very promising in terms of permeable reactive
barriers and photocatalysts. The nanoscopic materials such
as carbon nanotubes and alumina fibers embedded in zeolite
filtration membranes (Valli et al. 2010), TiO
2
/Al
2
O
3
mem-
branes (Zhang et al. 2006), carbon nanotubes, wrapped
around a carbon block filter structure (Cooper et al. 2007)
have the capacity to remove the impurities from water.
Nanoreactive membranes are able to decompose pollutants
such as 4-nitrophenol (Dotzauer et al. 2006) and bind metal
ions (Hollman and Bhattacharya 2004) in water solution.
Polysulfone membranes impregnated with silver nanoparti-
cles are found to be effective in bacteria and virus removal
(Zodrow et al. 2009). Superchlorination is another advanced
technique to get clean and disinfected water. It signifies that
extra dose of chlorine oxidize organics kill and remove algae
and pathogens from the water within the short-contact time.
HOCl is the active chemical that provides sanitation as well
as shows reactivity towards organic pollutants. When there
is sufficient HOCl, the pollutants are easily oxidized. But in
case of low level of HOCl compared to organic pollutants
combined chlorine is formed. These combined chlorine
compounds can be oxidized by increasing the level of HOCl
level in water. The point at which all the organic impurities
are oxidized is called the break point (Bahadori et al. 2013).
To avoid the flaws (viz. corrosion, bleaching of hair and
skin, foul smelling), sometimes superchlorination followed
by dechlorination is necessary before the use of water.
Superchlorination is practiced after the sunset as there are no
possibilities to react with UV-rays from the sun.
Conclusions
The world is facing turbulent water future. With the growing
economy and rising population, the theme of all nations is
‘Save water’. Quantity and quality of water should be given
equal importance. Awareness related to ‘water conservation’
and ‘safe drinking water’ is extremely important, and should
be given a good thought to the people.
The technological solution depends on raw water char-
acteristics, affordability and acceptability and level of
application. Of course, sustainability depends on an
awareness of the related issues. Since there are limitations
in every individual treatment technologies and, thus, hybrid
technologies are always beneficial; however, availability,
selection, optimization, etc. are important for the best
performances of the system. Lastly, it must be mentioned
through the gambling of research that the future of the
water treatment technology is highly prosperous and hope
1 day we will fulfill the demand ‘fresh water for everyone’.
Acknowledgments The authors are grateful to SERB, Department of
Science and Technology, India for research funding and Council of
Scientific Industrial Research, New Delhi for the support. The authors
also wish to thank Mr. Chirag Sharma and Mr. Mayank Saxena,
CSMCRI, Bhavnagar for their help.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://
creativecommons.org/licenses/by/4.0/), which permits unrestricted
use, distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
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