Water purification is the process of removing undesirable chemicals,
biological contaminants, suspended solids and gases from water. The
goal is to produce water fit for a specific purpose. Most water is
disinfected for human consumption (drinking water), but water
purification may also be designed for a variety of other purposes,
including fulfilling the requirements of medical, pharmacological,
chemical and industrial applications. The methods used include
physical processes such as filtration, sedimentation, and
distillation; biological processes such as slow sand filters or
biologically active carbon; chemical processes such as flocculation
and chlorination and the use of electromagnetic radiation such as
Purifying water may reduce the concentration of particulate matter
including suspended particles, parasites, bacteria, algae, viruses,
fungi, as well as reducing the concentration of a range of dissolved
and particulate matter.
The standards for drinking water quality are typically set by
governments or by international standards. These standards usually
include minimum and maximum concentrations of contaminants, depending
on the intended purpose of water use.
Visual inspection cannot determine if water is of appropriate quality.
Simple procedures such as boiling or the use of a household activated
carbon filter are not sufficient for treating all the possible
contaminants that may be present in water from an unknown source. Even
natural spring water – considered safe for all practical purposes in
the 19th century – must now be tested before determining what kind
of treatment, if any, is needed. Chemical and microbiological
analysis, while expensive, are the only way to obtain the information
necessary for deciding on the appropriate method of purification.
According to a 2007
World Health Organization
World Health Organization (WHO) report, 1.1
billion people lack access to an improved drinking water supply, 88%
of the 4 billion annual cases of diarrheal disease are attributed to
unsafe water and inadequate sanitation and hygiene, while 1.8 million
people die from diarrheal disease each year. The WHO estimates that
94% of these diarrheal disease cases are preventable through
modifications to the environment, including access to safe water.
Simple techniques for treating water at home, such as chlorination,
filters, and solar disinfection, and storing it in safe containers
could save a huge number of lives each year. Reducing deaths from
waterborne diseases is a major public health goal in developing
1 Sources of water
2.2.1 pH adjustment
2.3 Coagulation and flocculation
Sludge storage and removal
2.4.2 Floc blanket clarifiers
2.5 Dissolved air flotation
2.6.1 Rapid sand filters
2.6.2 Slow sand filters
2.7 Membrane filtration
2.8 Removal of ions and other dissolved substances
Chlorine dioxide disinfection
2.9.5 Ultraviolet disinfection
2.10 Potable water purification
2.11 Additional treatment options
3 Other water purification techniques
4 Safety and controversies
4.1 Demineralized water
5.1 Sand filter
5.2 Water chlorination
6 See also
8 Further reading
9 External links
Sources of water
Further information: Water supply
Groundwater: The water emerging from some deep ground water may have
fallen as rain many tens, hundreds, or thousands of years ago. Soil
and rock layers naturally filter the ground water to a high degree of
clarity and often, it does not require additional treatment besides
adding chlorine or chloramines as secondary disinfectants. Such water
may emerge as springs, artesian springs, or may be extracted from
boreholes or wells. Deep ground water is generally of very high
bacteriological quality (i.e., pathogenic bacteria or the pathogenic
protozoa are typically absent), but the water may be rich in dissolved
solids, especially carbonates and sulfates of calcium and magnesium.
Depending on the strata through which the water has flowed, other ions
may also be present including chloride, and bicarbonate. There may be
a requirement to reduce the iron or manganese content of this water to
make it acceptable for drinking, cooking, and laundry use. Primary
disinfection may also be required. Where groundwater recharge is
practiced (a process in which river water is injected into an aquifer
to store the water in times of plenty so that it is available in times
of drought), the groundwater may require additional treatment
depending on applicable state and federal regulations.
Upland lakes and reservoirs: Typically located in the headwaters of
river systems, upland reservoirs are usually sited above any human
habitation and may be surrounded by a protective zone to restrict the
opportunities for contamination.
Bacteria and pathogen levels are
usually low, but some bacteria, protozoa or algae will be present.
Where uplands are forested or peaty, humic acids can colour the water.
Many upland sources have low pH which require adjustment.
Rivers, canals and low land reservoirs: Low land surface waters will
have a significant bacterial load and may also contain algae,
suspended solids and a variety of dissolved constituents.
Atmospheric water generation is a new technology that can provide high
quality drinking water by extracting water from the air by cooling the
air and thus condensing water vapor.
Rainwater harvesting or fog collection which collect water from the
atmosphere can be used especially in areas with significant dry
seasons and in areas which experience fog even when there is little
Desalination of seawater by distillation or reverse osmosis.
Surface Water: Freshwater bodies that are open to the atmosphere and
are not designated as groundwater are termed surface waters.
The goals of the treatment are to remove unwanted constituents in the
water and to make it safe to drink or fit for a specific purpose in
industry or medical applications. Widely varied techniques are
available to remove contaminants like fine solids, micro-organisms and
some dissolved inorganic and organic materials, or environmental
persistent pharmaceutical pollutants. The choice of method will depend
on the quality of the water being treated, the cost of the treatment
process and the quality standards expected of the processed water.
The processes below are the ones commonly used in water purification
plants. Some or most may not be used depending on the scale of the
plant and quality of the raw (source) water.
Pumping and containment – The majority of water must be pumped from
its source or directed into pipes or holding tanks. To avoid adding
contaminants to the water, this physical infrastructure must be made
from appropriate materials and constructed so that accidental
contamination does not occur.
Screening (see also screen filter) – The first step in purifying
surface water is to remove large debris such as sticks, leaves,
rubbish and other large particles which may interfere with subsequent
purification steps. Most deep groundwater does not need screening
before other purification steps.
Storage – Water from rivers may also be stored in bankside
reservoirs for periods between a few days and many months to allow
natural biological purification to take place. This is especially
important if treatment is by slow sand filters. Storage reservoirs
also provide a buffer against short periods of drought or to allow
water supply to be maintained during transitory pollution incidents in
the source river.
Pre-chlorination – In many plants the incoming water was chlorinated
to minimize the growth of fouling organisms on the pipe-work and
tanks. Because of the potential adverse quality effects (see chlorine
below), this has largely been discontinued.
Pure water has a pH close to 7 (neither alkaline nor acidic). Sea
water can have pH values that range from 7.5 to 8.4 (moderately
alkaline). Fresh water can have widely ranging pH values depending on
the geology of the drainage basin or aquifer and the influence of
contaminant inputs (acid rain). If the water is acidic (lower than 7),
lime, soda ash, or sodium hydroxide can be added to raise the pH
during water purification processes. Lime addition increases the
calcium ion concentration, thus raising the water hardness. For highly
acidic waters, forced draft degasifiers can be an effective way to
raise the pH, by stripping dissolved carbon dioxide from the water.
Making the water alkaline helps coagulation and flocculation processes
work effectively and also helps to minimize the risk of lead being
dissolved from lead pipes and from lead solder in pipe fittings.
Sufficient alkalinity also reduces the corrosiveness of water to iron
pipes. Acid (carbonic acid, hydrochloric acid or sulfuric acid) may be
added to alkaline waters in some circumstances to lower the pH.
Alkaline water (above pH 7.0) does not necessarily mean that lead or
copper from the plumbing system will not be dissolved into the water.
The ability of water to precipitate calcium carbonate to protect metal
surfaces and reduce the likelihood of toxic metals being dissolved in
water is a function of pH, mineral content, temperature, alkalinity
and calcium concentration.
Coagulation and flocculation
See also: particle aggregation
One of the first steps in most conventional water purification
processes is the addition of chemicals to assist in the removal of
particles suspended in water. Particles can be inorganic such as clay
and silt or organic such as algae, bacteria, viruses, protozoa and
natural organic matter. Inorganic and organic particles contribute to
the turbidity and color of water.
The addition of inorganic coagulants such as aluminum sulfate (or
alum) or iron (III) salts such as iron(III) chloride cause several
simultaneous chemical and physical interactions on and among the
particles. Within seconds, negative charges on the particles are
neutralized by inorganic coagulants. Also within seconds, metal
hydroxide precipitates of the iron and aluminium ions begin to form.
These precipitates combine into larger particles under natural
processes such as
Brownian motion and through induced mixing which is
sometimes referred to as flocculation. The term most often used for
the amorphous metal hydroxides is "floc." Large, amorphous aluminum
and iron (III) hydroxides adsorb and enmesh particles in suspension
and facilitate the removal of particles by subsequent processes of
sedimentation and filtration.:8.2–8.3
Aluminum hydroxides are formed within a fairly narrow pH range,
typically: 5.5 to about 7.7.
Iron (III) hydroxides can form over a
larger pH range including pH levels lower than are effective for alum,
typically: 5.0 to 8.5.:679
In the literature, there is much debate and confusion over the usage
of the terms coagulation and flocculation—where does coagulation end
and flocculation begin? In water purification plants, there is usually
a high energy, rapid mix unit process (detention time in seconds)
where the coagulant chemicals are added followed by flocculation
basins (detention times range from 15 to 45 minutes) where low energy
inputs turn large paddles or other gentle mixing devices to enhance
the formation of floc. In fact, coagulation and flocculation processes
are ongoing once the metal salt coagulants are added.:74–5
Organic polymers were developed in the 1960s as aids to coagulants
and, in some cases, as replacements for the inorganic metal salt
coagulants. Synthetic organic polymers are high molecular weight
compounds that carry negative, positive or neutral charges. When
organic polymers are added to water with particulates, the high
molecular weight compounds adsorb onto particle surfaces and through
interparticle bridging coalesce with other particles to form floc.
PolyDADMAC is a popular cationic (positively charged) organic polymer
used in water purification plants.:667–8
Waters exiting the flocculation basin may enter the sedimentation
basin, also called a clarifier or settling basin. It is a large tank
with low water velocities, allowing floc to settle to the bottom. The
sedimentation basin is best located close to the flocculation basin so
the transit between the two processes does not permit settlement or
floc break up. Sedimentation basins may be rectangular, where water
flows from end to end, or circular where flow is from the centre
outward. Sedimentation basin outflow is typically over a weir so only
a thin top layer of water—that furthest from the sludge—exits.
Allen Hazen showed that the efficiency of a sedimentation
process was a function of the particle settling velocity, the flow
through the tank and the surface area of tank. Sedimentation tanks are
typically designed within a range of overflow rates of 0.5 to 1.0
gallons per minute per square foot (or 1.25 to 2.5 meters per hour).
In general, sedimentation basin efficiency is not a function of
detention time or depth of the basin. Although, basin depth must be
sufficient so that water currents do not disturb the sludge and
settled particle interactions are promoted. As particle concentrations
in the settled water increase near the sludge surface on the bottom of
the tank, settling velocities can increase due to collisions and
agglomeration of particles. Typical detention times for sedimentation
vary from 1.5 to 4 hours and basin depths vary from 10 to 15 feet (3
to 4.5 meters).:9.39–9.40:790–1:140–2, 171
Inclined flat plates or tubes can be added to traditional
sedimentation basins to improve particle removal performance. Inclined
plates and tubes drastically increase the surface area available for
particles to be removed in concert with Hazen's original theory. The
amount of ground surface area occupied by a sedimentation basin with
inclined plates or tubes can be far smaller than a conventional
Sludge storage and removal
As particles settle to the bottom of a sedimentation basin, a layer of
sludge is formed on the floor of the tank which must be removed and
treated. The amount of sludge generated is significant, often 3 to 5
percent of the total volume of water to be treated. The cost of
treating and disposing of the sludge can impact the operating cost of
a water treatment plant. The sedimentation basin may be equipped with
mechanical cleaning devices that continually clean its bottom, or the
basin can be periodically taken out of service and cleaned manually.
Floc blanket clarifiers
A subcategory of sedimentation is the removal of particulates by
entrapment in a layer of suspended floc as the water is forced upward.
The major advantage of floc blanket clarifiers is that they occupy a
smaller footprint than conventional sedimentation. Disadvantages are
that particle removal efficiency can be highly variable depending on
changes in influent water quality and influent water flow
Dissolved air flotation
When particles to be removed do not settle out of solution easily,
dissolved air flotation (DAF) is often used. After coagulation and
flocculation processes, water flows to DAF tanks where air diffusers
on the tank bottom create fine bubbles that attach to floc resulting
in a floating mass of concentrated floc. The floating floc blanket is
removed from the surface and clarified water is withdrawn from the
bottom of the DAF tank. Water supplies that are particularly
vulnerable to unicellular algae blooms and supplies with low turbidity
and high colour often employ DAF. :9.46
After separating most floc, the water is filtered as the final step to
remove remaining suspended particles and unsettled floc.
Rapid sand filters
Cutaway view of a typical rapid sand filter
The most common type of filter is a rapid sand filter. Water moves
vertically through sand which often has a layer of activated carbon or
anthracite coal above the sand. The top layer removes organic
compounds, which contribute to taste and odour. The space between sand
particles is larger than the smallest suspended particles, so simple
filtration is not enough. Most particles pass through surface layers
but are trapped in pore spaces or adhere to sand particles. Effective
filtration extends into the depth of the filter. This property of the
filter is key to its operation: if the top layer of sand were to block
all the particles, the filter would quickly clog.
To clean the filter, water is passed quickly upward through the
filter, opposite the normal direction (called backflushing or
backwashing) to remove embedded or unwanted particles. Prior to this
step, compressed air may be blown up through the bottom of the filter
to break up the compacted filter media to aid the backwashing process;
this is known as air scouring. This contaminated water can be disposed
of, along with the sludge from the sedimentation basin, or it can be
recycled by mixing with the raw water entering the plant although this
is often considered poor practice since it re-introduces an elevated
concentration of bacteria into the raw water.
Some water treatment plants employ pressure filters. These work on the
same principle as rapid gravity filters, differing in that the filter
medium is enclosed in a steel vessel and the water is forced through
it under pressure.
Filters out much smaller particles than paper and sand filters can.
Filters out virtually all particles larger than their specified pore
They are quite thin and so liquids flow through them fairly rapidly.
They are reasonably strong and so can withstand pressure differences
across them of typically 2–5 atmospheres.
They can be cleaned (back flushed) and reused.
Slow sand filters
Slow "artificial" filtration (a variation of bank filtration) to the
Water purification plant Káraný, Czech Republic
A profile of layers of gravel, sand and fine sand used in a slow sand
Slow sand filters
Slow sand filters may be used where there is sufficient land and
space, as the water must be passed very slowly through the filters.
These filters rely on biological treatment processes for their action
rather than physical filtration. The filters are carefully constructed
using graded layers of sand, with the coarsest sand, along with some
gravel, at the bottom and finest sand at the top. Drains at the base
convey treated water away for disinfection.
Filtration depends on the
development of a thin biological layer, called the zoogleal layer or
Schmutzdecke, on the surface of the filter. An effective slow sand
filter may remain in service for many weeks or even months if the
pretreatment is well designed and produces water with a very low
available nutrient level which physical methods of treatment rarely
achieve. Very low nutrient levels allow water to be safely sent
through distribution systems with very low disinfectant levels,
thereby reducing consumer irritation over offensive levels of chlorine
and chlorine by-products.
Slow sand filters
Slow sand filters are not backwashed; they
are maintained by having the top layer of sand scraped off when flow
is eventually obstructed by biological growth.
A specific "large-scale" form of slow sand filter is the process of
bank filtration, in which natural sediments in a riverbank are used to
provide a first stage of contaminant filtration. While typically not
clean enough to be used directly for drinking water, the water gained
from the associated extraction wells is much less problematic than
river water taken directly from the major streams where bank
filtration is often used.
Membrane filters are widely used for filtering both drinking water and
sewage. For drinking water, membrane filters can remove virtually all
particles larger than 0.2 μm—including giardia and
cryptosporidium. Membrane filters are an effective form of tertiary
treatment when it is desired to reuse the water for industry, for
limited domestic purposes, or before discharging the water into a
river that is used by towns further downstream. They are widely used
in industry, particularly for beverage preparation (including bottled
water). However no filtration can remove substances that are actually
dissolved in the water such as phosphates, nitrates and heavy metal
Removal of ions and other dissolved substances
Ultrafiltration membranes use polymer membranes with chemically formed
microscopic pores that can be used to filter out dissolved substances
avoiding the use of coagulants. The type of membrane media determines
how much pressure is needed to drive the water through and what sizes
of micro-organisms can be filtered out.
Ion exchange systems use ion exchange resin- or
zeolite-packed columns to replace unwanted ions. The most common case
is water softening consisting of removal of Ca2+ and Mg2+ ions
replacing them with benign (soap friendly) Na+ or K+ ions. Ion
exchange resins are also used to remove toxic ions such as nitrite,
lead, mercury, arsenic and many others.
Precipitative softening::13.12–13.58 Water rich in hardness
(calcium and magnesium ions) is treated with lime (calcium oxide)
and/or soda-ash (sodium carbonate) to precipitate calcium carbonate
out of solution utilizing the common-ion effect.
Electrodeionization: Water is passed between a positive electrode
and a negative electrode.
Ion exchange membranes allow only positive
ions to migrate from the treated water toward the negative electrode
and only negative ions toward the positive electrode. High purity
deionized water is produced continuously, similar to ion exchange
treatment. Complete removal of ions from water is possible if the
right conditions are met. The water is normally pre-treated with a
reverse osmosis unit to remove non-ionic organic contaminants, and
with gas transfer membranes to remove carbon dioxide. A water recovery
of 99% is possible if the concentrate stream is fed to the RO inlet.
Pumps used to add required amount of chemicals to the clear water at
the water purification plant before the distribution. From left to
right: sodium hypochlorite for disinfection, zinc orthophosphate as a
corrosion inhibitor, sodium hydroxide for pH adjustment, and fluoride
for tooth decay prevention.
Disinfection is accomplished both by filtering out harmful
micro-organisms and also by adding disinfectant chemicals. Water is
disinfected to kill any pathogens which pass through the filters and
to provide a residual dose of disinfectant to kill or inactivate
potentially harmful micro-organisms in the storage and distribution
systems. Possible pathogens include viruses, bacteria, including
Campylobacter and Shigella, and protozoa,
Giardia lamblia and other cryptosporidia. Following the
introduction of any chemical disinfecting agent, the water is usually
held in temporary storage – often called a contact tank or clear
well to allow the disinfecting action to complete.
Main article: Water chlorination
The most common disinfection method involves some form of chlorine or
its compounds such as chloramine or chlorine dioxide.
Chlorine is a
strong oxidant that rapidly kills many harmful micro-organisms.
Because chlorine is a toxic gas, there is a danger of a release
associated with its use. This problem is avoided by the use of sodium
hypochlorite, which is a relatively inexpensive solution used in
household bleach that releases free chlorine when dissolved in water.
Chlorine solutions can be generated on site by electrolyzing common
salt solutions. A solid form, calcium hypochlorite, releases chlorine
on contact with water. Handling the solid, however, requires greater
routine human contact through opening bags and pouring than the use of
gas cylinders or bleach which are more easily automated. The
generation of liquid sodium hypochlorite is both inexpensive and safer
than the use of gas or solid chlorine.
Chlorine levels up to 4
milligrams per liter (4 parts per million) are considered safe in
drinking water. 
All forms of chlorine are widely used, despite their respective
drawbacks. One drawback is that chlorine from any source reacts with
natural organic compounds in the water to form potentially harmful
chemical by-products. These by-products, trihalomethanes (THMs) and
haloacetic acids (HAAs), are both carcinogenic in large quantities and
are regulated by the
United States Environmental Protection Agency
(EPA) and the
Drinking Water Inspectorate in the UK. The formation of
THMs and haloacetic acids may be minimized by effective removal of as
many organics from the water as possible prior to chlorine addition.
Although chlorine is effective in killing bacteria, it has limited
effectiveness against protozoa that form cysts in water (Giardia
lamblia and Cryptosporidium, both of which are pathogenic).
Chlorine dioxide disinfection
Chlorine dioxide is a faster-acting disinfectant than elemental
chlorine. It is relatively rarely used, because in some circumstances
it may create excessive amounts of chlorite, which is a by-product
regulated to low allowable levels in the United States. Chlorine
dioxide can be supplied as an aqueous solution and added to water to
avoid gas handling problems; chlorine dioxide gas accumulations may
The use of chloramine is becoming more common as a disinfectant.
Although chloramine is not as strong an oxidant, it does provide a
longer-lasting residual than free chlorine and it will not readily
form THMs or haloacetic acids. It is possible to convert chlorine to
chloramine by adding ammonia to the water after addition of chlorine.
The chlorine and ammonia react to form chloramine. Water distribution
systems disinfected with chloramines may experience nitrification, as
ammonia is a nutrient for bacterial growth, with nitrates being
generated as a by-product.
Ozone is an unstable molecule which readily gives up one atom of
oxygen providing a powerful oxidizing agent which is toxic to most
waterborne organisms. It is a very strong, broad spectrum disinfectant
that is widely used in Europe. It is an effective method to inactivate
harmful protozoa that form cysts. It also works well against almost
all other pathogens.
Ozone is made by passing oxygen through
ultraviolet light or a "cold" electrical discharge. To use ozone as a
disinfectant, it must be created on-site and added to the water by
bubble contact. Some of the advantages of ozone include the production
of fewer dangerous by-products and the absence of taste and odour
problems (in comparison to chlorination). Another advantage of ozone
is that it leaves no residual disinfectant in the water.
been used in drinking water plants since 1906 where the first
industrial ozonation plant was built in Nice, France. The U.S. Food
and Drug Administration has accepted ozone as being safe; and it is
applied as an anti-microbiological agent for the treatment, storage,
and processing of foods. However, although fewer by-products are
formed by ozonation, it has been discovered that ozone reacts with
bromide ions in water to produce concentrations of the suspected
carcinogen bromate. Bromide can be found in fresh water supplies in
sufficient concentrations to produce (after ozonation) more than 10
parts per billion (ppb) of bromate — the maximum contaminant level
established by the USEPA.
Main article: Ultraviolet germicidal irradiation
Ultraviolet light (UV) is very effective at inactivating cysts, in low
turbidity water. UV light's disinfection effectiveness decreases as
turbidity increases, a result of the absorption, scattering, and
shadowing caused by the suspended solids. The main disadvantage to the
use of UV radiation is that, like ozone treatment, it leaves no
residual disinfectant in the water; therefore, it is sometimes
necessary to add a residual disinfectant after the primary
disinfection process. This is often done through the addition of
chloramines, discussed above as a primary disinfectant. When used in
this manner, chloramines provide an effective residual disinfectant
with very few of the negative effects of chlorination.
Over 2 million people in 28 developing countries use Solar
Disinfection for daily drinking water treatment. 
Potable water purification
Main article: Portable water purification
Potable water purification devices and methods are available for
disinfection and treatment in emergencies or in remote locations.
Disinfection is the primary goal, since aesthetic considerations such
as taste, odor, appearance, and trace chemical contamination do not
affect the short-term safety of drinking water.
Additional treatment options
Water fluoridation: in many areas fluoride is added to water with the
goal of preventing tooth decay.
Fluoride is usually added after
the disinfection process. In the U.S., fluoridation is usually
accomplished by the addition of hexafluorosilicic acid, which
decomposes in water, yielding fluoride ions.
Water conditioning: This is a method of reducing the effects of hard
water. In water systems subject to heating hardness salts can be
deposited as the decomposition of bicarbonate ions creates carbonate
ions that precipitate out of solution. Water with high concentrations
of hardness salts can be treated with soda ash (sodium carbonate)
which precipitates out the excess salts, through the common-ion
effect, producing calcium carbonate of very high purity. The
precipitated calcium carbonate is traditionally sold to the
manufacturers of toothpaste. Several other methods of industrial and
residential water treatment are claimed (without general scientific
acceptance) to include the use of magnetic and/or electrical fields
reducing the effects of hard water.
Plumbosolvency reduction: In areas with naturally acidic waters of low
conductivity (i.e. surface rainfall in upland mountains of igneous
rocks), the water may be capable of dissolving lead from any lead
pipes that it is carried in. The addition of small quantities of
phosphate ion and increasing the pH slightly both assist in greatly
reducing plumbo-solvency by creating insoluble lead salts on the inner
surfaces of the pipes.
Radium Removal: Some groundwater sources contain radium, a radioactive
chemical element. Typical sources include many groundwater sources
north of the
River in Illinois.
Radium can be removed by ion
exchange, or by water conditioning. The back flush or sludge that is
produced is, however, a low-level radioactive waste.
Fluoride Removal: Although fluoride is added to water in many areas,
some areas of the world have excessive levels of natural fluoride in
the source water. Excessive levels can be toxic or cause undesirable
cosmetic effects such as staining of teeth. Methods of reducing
fluoride levels is through treatment with activated alumina and bone
char filter media.
Other water purification techniques
Other popular methods for purifying water, especially for local
private supplies are listed below. In some countries some of these
methods are also used for large scale municipal supplies. Particularly
important are distillation (de-salination of seawater) and reverse
Boiling: Bringing water to its boiling point ( about 100 °C or 212 F
at sea level), is the oldest and most effective way since it
eliminates most microbes causing intestine related diseases, but
it cannot remove chemical toxins or impurities. For human health,
complete sterilization of water is not required, since the heat
resistant microbes are not intestine affecting. The traditional
advice of boiling water for ten minutes is mainly for additional
safety, since microbes start getting eliminated at temperatures
greater than 60 °C (140 °F). Though the boiling point
decreases with increasing altitude, it is not enough to affect the
disinfecting process. In areas where the water is "hard" (that
is, containing significant dissolved calcium salts), boiling
decomposes the bicarbonate ions, resulting in partial precipitation as
calcium carbonate. This is the "fur" that builds up on kettle
elements, etc., in hard water areas. With the exception of calcium,
boiling does not remove solutes of higher boiling point than water and
in fact increases their concentration (due to some water being lost as
Boiling does not leave a residual disinfectant in the water.
Therefore, water that is boiled and then stored for any length of time
may acquire new pathogens.
Granular Activated Carbon adsorption: a form of activated carbon with
a high surface area, adsorbs many compounds including many toxic
compounds. Water passing through activated carbon is commonly used in
municipal regions with organic contamination, taste or odors. Many
household water filters and fish tanks use activated carbon filters to
further purify the water. Household filters for drinking water
sometimes contain silver as metallic silver nanoparticle. If water is
held in the carbon block for longer periods, microorganisms can grow
inside which results in fouling and contamination. Silver
nanoparticles are excellent anti-bacterial material and they can
decompose toxic halo-organic compounds such as pesticides into
non-toxic organic products.
Distillation involves boiling the water to produce water vapour. The
vapour contacts a cool surface where it condenses as a liquid. Because
the solutes are not normally vaporised, they remain in the boiling
solution. Even distillation does not completely purify water, because
of contaminants with similar boiling points and droplets of
unvapourised liquid carried with the steam. However, 99.9% pure water
can be obtained by distillation.
Reverse osmosis: Mechanical pressure is applied to an impure solution
to force pure water through a semi-permeable membrane. Reverse osmosis
is theoretically the most thorough method of large scale water
purification available, although perfect semi-permeable membranes are
difficult to create. Unless membranes are well-maintained, algae and
other life forms can colonize the membranes.
The use of iron in removing arsenic from water. See Arsenic
contamination of groundwater.
Direct contact membrane distillation (DCMD). Applicable to
desalination. Heated seawater is passed along the surface of a
hydrophobic polymer membrane. Evaporated water passes from the hot
side through pores in the membrane into a stream of cold pure water on
the other side. The difference in vapour pressure between the hot and
cold side helps to push water molecules through.
Desalination – is a process by which saline water (generally sea
water) is converted to fresh water. The most common desalination
processes are distillation and reverse osmosis.
currently expensive compared to most alternative sources of water, and
only a very small fraction of total human use is satisfied by
desalination. It is only economically practical for high-valued uses
(such as household and industrial uses) in arid areas.
Gas hydrate crystals centrifuge method. If carbon dioxide or other low
molecular weight gas is mixed with contaminated water at high pressure
and low temperature, gas hydrate crystals will form exothermically.
Separation of the crystalline hydrate may be performed by centrifuge
or sedimentation and decanting. Water can be released from the hydrate
crystals by heating
In Situ Chemical Oxidation, a form of advanced oxidation processes and
advanced oxidation technology, is an environmental remediation
technique used for soil and/or groundwater remediation to reduce the
concentrations of targeted environmental contaminants to acceptable
levels. ISCO is accomplished by injecting or otherwise introducing
strong chemical oxidizers directly into the contaminated medium (soil
or groundwater) to destroy chemical contaminants in place. It can be
used to remediate a variety of organic compounds, including some that
are resistant to natural degradation
Bioremediation is a technique that uses microorganisms in order to
remove or extract certain waste products from a contaminated area.
Since 1991 bioremediation has been a suggested tactic to remove
impurities from water such as alkanes, perchlorates, and metals.
The treatment of ground and surface water, through bioremediation,
with respect to perchlorate and chloride compounds, has seen success
as perchlorate compounds are highly soluble making it difficult to
remove. Such success by use of Dechloromonas agitata strain CKB
include field studies conducted in Maryland and the Southwest region
of the United States. Although a bioremediation technique
may be successful, implementation is not feasible as there is still
much to be studied regarding rates and after effects of microbial
activity as well as producing a large scale implementation method.
Safety and controversies
Further information: Distilled water § Health concerns
Drinking water pollution detector
Rainbow trout (Oncorhynchus mykiss)
are being used in water purification plants to detect acute water
The examples and perspective in this article deal primarily with the
United States and do not represent a worldwide view of the subject.
You may improve this article, discuss the issue on the talk page, or
create a new article, as appropriate. (April 2011) (Learn how and when
to remove this template message)
In April, 2007, the water supply of
Spencer, Massachusetts became
contaminated with excess sodium hydroxide (lye) when its treatment
Many municipalities have moved from free chlorine to chloramine as a
disinfection agent. However, chloramine appears to be a corrosive
agent in some water systems.
Chloramine can dissolve the "protective"
film inside older service lines, leading to the leaching of lead into
residential spigots. This can result in harmful exposure, including
elevated blood lead levels.
Lead is a known neurotoxin.
Distillation removes all minerals from water, and the membrane methods
of reverse osmosis and nanofiltration remove most to all minerals.
This results in demineralized water which is not considered ideal
drinking water. The
World Health Organization
World Health Organization has investigated the
health effects of demineralized water since 1980. Experiments in
humans found that demineralized water increased diuresis and the
elimination of electrolytes, with decreased blood serum potassium
concentration. Magnesium, calcium, and other minerals in water can
help to protect against nutritional deficiency. Demineralized water
may also increase the risk from toxic metals because it more readily
leaches materials from piping like lead and cadmium, which is
prevented by dissolved minerals such as calcium and magnesium.
Low-mineral water has been implicated in specific cases of lead
poisoning in infants, when lead from pipes leached at especially high
rates into the water. Recommendations for magnesium have been put at a
minimum of 10 mg/L with 20–30 mg/L optimum; for calcium a
20 mg/L minimum and a 40–80 mg/L optimum, and a total
water hardness (adding magnesium and calcium) of 2 to 4 mmol/L.
At water hardness above 5 mmol/L, higher incidence of gallstones,
kidney stones, urinary stones, arthrosis, and arthropathies have been
observed. Additionally, desalination processes can increase the
risk of bacterial contamination.
Manufacturers of home water distillers claim the opposite—that
minerals in water are the cause of many diseases, and that most
beneficial minerals come from food, not water. They quote the
American Medical Association as saying "The body's need for minerals
is largely met through foods, not drinking water." The WHO report
agrees that "drinking water, with some rare exceptions, is not the
major source of essential elements for humans" and is "not the major
source of our calcium and magnesium intake", yet states that
demineralized water is harmful anyway. "Additional evidence comes from
animal experiments and clinical observations in several countries.
Animals given zinc or magnesium dosed in their drinking water had a
significantly higher concentration of these elements in the serum than
animals given the same elements in much higher amounts with food and
provided with low-mineral water to drink."
Demineralized water is also the type of water injection used to
increase thrust in some turbojet engines such as the
J57 engine used
on KC-135A/Q and B-52A/B/C/D/E/F/G aircraft, as well as the civilian
version of that engine, the JT-3C used on early build
Boeing 707 and
Further information: History of water supply and sanitation
Drawing of an apparatus for studying the chemical analysis of mineral
waters in a book from 1799.
The first experiments into water filtration were made in the 17th
Francis Bacon attempted to desalinate sea water by
passing the flow through a sand filter. Although his experiment did
not succeed, it marked the beginning of a new interest in the field.
The fathers of microscopy,
Antonie van Leeuwenhoek
Antonie van Leeuwenhoek and Robert Hooke,
used the newly invented microscope to observe for the first time small
material particles that lay suspended in the water, laying the
groundwork for the future understanding of waterborne pathogens.
Original map by John Snow showing the clusters of cholera cases in the
London epidemic of 1854.
The first documented use of sand filters to purify the water supply
dates to 1804, when the owner of a bleachery in Paisley, Scotland,
John Gibb, installed an experimental filter, selling his unwanted
surplus to the public. This method was refined in the following
two decades by engineers working for private water companies, and it
culminated in the first treated public water supply in the world,
installed by engineer James Simpson for the Chelsea Waterworks Company
London in 1829. This installation provided filtered water for
every resident of the area, and the network design was widely copied
United Kingdom in the ensuing decades.
The practice of water treatment soon became mainstream and common, and
the virtues of the system were made starkly apparent after the
investigations of the physician John Snow during the 1854 Broad Street
cholera outbreak. Snow was sceptical of the then-dominant miasma
theory that stated that diseases were caused by noxious "bad airs".
Although the germ theory of disease had not yet been developed, Snow's
observations led him to discount the prevailing theory. His 1855 essay
On the Mode of Communication of
Cholera conclusively demonstrated the
role of the water supply in spreading the cholera epidemic in
Soho, with the use of a dot distribution map and statistical
proof to illustrate the connection between the quality of the water
source and cholera cases. His data convinced the local council to
disable the water pump, which promptly ended the outbreak.
The Metropolis Water Act introduced the regulation of the water supply
companies in London, including minimum standards of water quality for
the first time. The Act "made provision for securing the supply to the
Metropolis of pure and wholesome water", and required that all water
be "effectually filtered" from 31 December 1855. This was followed
up with legislation for the mandatory inspection of water quality,
including comprehensive chemical analyses, in 1858. This legislation
set a worldwide precedent for similar state public health
interventions across Europe. The
Metropolitan Commission of Sewers
Metropolitan Commission of Sewers was
formed at the same time, water filtration was adopted throughout the
country, and new water intakes on the
Thames were established above
Teddington Lock. Automatic pressure filters, where the water is forced
under pressure through the filtration system, were innovated in 1899
John Snow was the first to successfully use chlorine to disinfect the
water supply in
Soho that had helped spread the cholera outbreak.
William Soper also used chlorinated lime to treat the sewage produced
by typhoid patients in 1879.
In a paper published in 1894,
Moritz Traube formally proposed the
addition of chloride of lime (calcium hypochlorite) to water to render
it "germ-free." Two other investigators confirmed Traube's findings
and published their papers in 1895. Early attempts at implementing
water chlorination at a water treatment plant were made in 1893 in
Germany and in 1897 the city of
England was the
first to have its entire water supply treated with chlorine.
Permanent water chlorination began in 1905, when a faulty slow sand
filter and a contaminated water supply led to a serious typhoid fever
epidemic in Lincoln, England. Dr. Alexander Cruickshank Houston
used chlorination of the water to stem the epidemic. His installation
fed a concentrated solution of chloride of lime to the water being
treated. The chlorination of the water supply helped stop the epidemic
and as a precaution, the chlorination was continued until 1911 when a
new water supply was instituted.
Manual-control chlorinator for the liquefaction of chlorine for water
purification, early 20th century. From Chlorination of Water by Joseph
The first continuous use of chlorine in the
United States for
disinfection took place in 1908 at Boonton Reservoir (on the Rockaway
River), which served as the supply for Jersey City, New Jersey.
Chlorination was achieved by controlled additions of dilute solutions
of chloride of lime (calcium hypochlorite) at doses of 0.2 to 0.35
ppm. The treatment process was conceived by Dr. John L. Leal and the
chlorination plant was designed by George Warren Fuller. Over the
next few years, chlorine disinfection using chloride of lime were
rapidly installed in drinking water systems around the world.
The technique of purification of drinking water by use of compressed
liquefied chlorine gas was developed by a British officer in the
Indian Medical Service, Vincent B. Nesfield, in 1903. According to his
It occurred to me that chlorine gas might be found satisfactory ... if
suitable means could be found for using it.... The next important
question was how to render the gas portable. This might be
accomplished in two ways: By liquefying it, and storing it in
lead-lined iron vessels, having a jet with a very fine capillary
canal, and fitted with a tap or a screw cap. The tap is turned on, and
the cylinder placed in the amount of water required. The chlorine
bubbles out, and in ten to fifteen minutes the water is absolutely
safe. This method would be of use on a large scale, as for service
U.S. Army Major Carl Rogers Darnall, Professor of Chemistry at the
Army Medical School, gave the first practical demonstration of this in
1910. Shortly thereafter, Major William J. L. Lyster of the Army
Medical Department used a solution of calcium hypochlorite in a linen
bag to treat water. For many decades, Lyster's method remained the
standard for U.S. ground forces in the field and in camps, implemented
in the form of the familiar Lyster Bag (also spelled Lister Bag). This
work became the basis for present day systems of municipal water
Sustainable development portal
List of water supply and sanitation by country
Organisms involved in water purification
Portable water purification
^ Combating Waterborne Diseases at the Household Level (PDF). World
Health Organization. 2007. Part 1. ISBN 978-92-4-159522-3.
^ Water for Life: Making it Happen (PDF). World Health Organization
and UNICEF. 2005. ISBN 92-4-156293-5.
^ McGuire, Michael J.; McLain, Jennifer Lara; Obolensky, Alexa (2002).
Information Collection Rule Data Analysis. Denver: AWWA Research
Foundation and American Water Works Association. pp. 376–378.
ISBN 9781583212738. Retrieved 28 April 2017.
^ "Aeration and gas stripping" (PDF). Archived from the original (PDF)
on July 12, 2014. Retrieved 29 June 2017.
^ "Water Knowledge". American Water Works Association. Retrieved 29
^ a b c d Edzwald, James K., ed. (2011). Water Quality and Treatment.
6th Edition. New York:McGraw-Hill. ISBN 978-0-07-163011-5
^ a b c d Crittenden, John C., et al., eds. (2005). Water Treatment:
Principles and Design. 2nd Edition. Hoboken, NJ:Wiley.
^ a b Kawamura, Susumu. Integrated Design and Operation of Water
Treatment Facilities. John Wiley & Sons. pp. 74–75.
ISBN 9780471350934. Retrieved 28 April 2017.
United States Environmental Protection Agency
United States Environmental Protection Agency (EPA)(1990).
Cincinnati, OH. "Technologies for Upgrading Existing or Designing New
Drinking Water Treatment Facilities." Document no. EPA/625/4-89/023.
^ Nair, Abhilash T.; Ahammed, M. Mansoor; Davra, Komal (2014-08-01).
"Influence of operating parameters on the performance of a household
slow sand filter". Water Science and Technology: Water Supply. 14 (4):
643–649. doi:10.2166/ws.2014.021. ISSN 1606-9749.
^ a b Zagorodni, Andrei A. (2007).
Ion exchange materials: properties
and applications. Elsevier. ISBN 978-0-08-044552-6.
^ . CDC
Retrieved 11 February 2018. Missing or empty title= (help)
^ Neemann, Jeff; Hulsey, Robert; Rexing, David; Wert, Eric (2004).
Bromate Formation During Ozonation with
Ammonia". Journal American Water Works Association. 96 (2):
^ Center for Disease Control. CDC
https://www.cdc.gov/safewater/solardisinfection.html. Retrieved 11
February 2018. Missing or empty title= (help)
^ Centers for Disease Control and Prevention (2001). "Recommendations
for using fluoride to prevent and control dental decay caries in the
United States". MMWR Recomm Rep. 50 (RR-14): 1–42.
PMID 11521913. Lay summary – CDC (2007-08-09).
^ Division of Oral Health, National Center for Prevention Services,
CDC (1993). "Fluoridation census 1992" (PDF). Retrieved
2008-12-29. CS1 maint: Multiple names: authors list (link)
^ Reeves TG (1986). "Water fluoridation: a manual for engineers and
technicians" (PDF). Centers for Disease Control. Archived from the
original (PDF) on 2008-10-07. Retrieved 2008-12-10.
^ Penn State Extension "Magnetic Water Treatment Devices" Accessed
^ a b c Backer, Howard (2002). "Water
Disinfection for International
and Wilderness Travelers". Clin Infect Dis. 34 (3): 355–364.
doi:10.1086/324747. PMID 11774083.
^ Curtis, Rick (1998) OA Guide to Water Purification, The Backpacker's
Field Manual, Random House.
^ "Is it true that you can't make a decent cup of tea up a mountain?".
physics.org. Retrieved 2 November 2012.
^ Savage, Nora; Mamadou S. Diallo (May 2005). "Nanomaterials and Water
Purification: Opportunities and Challenges" (PDF). J. Nanopart. Res. 7
(4–5): 331–342. doi:10.1007/s11051-005-7523-5. Retrieved 24 May
^ Osegovic, John P. et al. (2009) Hydrates for Gypsum Stack Water
Purification. AIChE Annual Convention
^ Jr, John T. Wilson; Wilson, Barbara H. (Dec 15, 1987),
Biodegradation of halogenated aliphatic hydrocarbons, retrieved
^ a b Van Trump, James Ian; Coates, John D. (2008-12-18).
"Thermodynamic targeting of microbial perchlorate reduction by
selective electron donors". The ISME Journal. 3 (4): 466–476.
doi:10.1038/ismej.2008.119. ISSN 1751-7362.
^ Hatzinger, P. B.; Diebold, J.; Yates, C. A.; Cramer, R. J.
(2006-01-01). Gu, Baohua; Coates, John D., eds. Perchlorate. Springer
US. pp. 311–341. doi:10.1007/0-387-31113-0_14.
^ Coates, John D.; Achenbach, Laurie A. (2004-07-01). "Microbial
perchlorate reduction: rocket-fuelled metabolism". Nature Reviews
Microbiology. 2 (7): 569–580. doi:10.1038/nrmicro926.
ISSN 1740-1526. PMID 15197392.
^ Poulsen, Kevin (26 April 2007). "Mysterious Glitch Poisons Town
Water Supply". Wired.
^ Miranda, M. L.; Kim, D.; Hull, A. P.; Paul, C. J.; Galeano, M. A. O.
(2006). "Changes in Blood
Lead Levels Associated with Use of
Chloramines in Water Treatment Systems". Environmental Health
Perspectives. 115 (2): 221–225. doi:10.1289/ehp.9432.
PMC 1817676 . PMID 17384768.
^ Health risks from drinking demineralised water. (PDF) . Rolling
revision of the WHO Guidelines for drinking-water quality. World
Health Organization, Geneva, 2004
^ a b Kozisek F. (2004). Health risks from drinking demineralised
^ Water Distillers – Water
Distillation – Myths, Facts, etc.
Naturalsolutions1.com. Retrieved on 2011-02-18.
^ Minerals in Drinking Water. Aquatechnology.net. Retrieved on
^ "The Use of the
Microscope in Water Filter History". History of
^ a b
Filtration of water supplies (PDF), World Health
^ History of the Chelsea Waterworks. ucla.edu
^ Gunn, S. William A. & Masellis, Michele (2007). Concepts and
Practice of Humanitarian Medicine. Springer. p. 87.
^ Bazin, Hervé (2008). L'histoire des vaccinations. John Libbey
Eurotext. p. 290.
^ An Act to make better Provision respecting the Supply of Water to
the Metropolis, (15 & 16 Vict. C.84)
^ Turneaure, F.E. & H.L. Russell (1901). Public Water-Supplies:
Requirements, Resources, and the Construction of Works (1st ed.). New
York: John Wiley & Sons. p. 493.
Typhoid Epidemic at Maidstone". Journal of the Sanitary Institute.
18: 388. October 1897.
^ "A miracle for public health?". Retrieved 2012-12-17.
^ Reece, R.J. (1907). "Report on the Epidemic of Enteric Fever in the
City of Lincoln, 1904-5." In Thirty-Fifth Annual Report of the Local
Government Board, 1905-6: Supplement Containing the Report of the
Medical Officer for 1905-6. London:Local Government Board.
^ Leal, John L. (1909). "The Sterilization Plant of the Jersey City
Water Supply Company at Boonton, N.J." Proceedings American Water
Works Association. pp. 100–9.
^ Fuller, George W. (1909). "Description of the Process and Plant of
the Jersey City Water Supply Company for the Sterilization of the
Water of the Boonton Reservoir." Proceedings AWWA. pp. 110–34.
^ Hazen, Allen. (1916). Clean Water and How to Get It. New York:Wiley.
^ Nesfield, V. B. (1902). "A Chemical Method of Sterilizing Water
Without Affecting its Potability". Public Health: 601–3.
Standard Methods for the Examination of Water & Wastewater.
American Public Health Association. ISBN 0-87553-047-8.
Masters, Gilbert M. Introduction to Environmental Engineering. 2nd ed.
Upper Saddle River, NJ: Prentice Hall, 1998.
US EPA. "Ground Water and Drinking Water." Overview of drinking water
topics and detailed information on US regulatory program. (Updated
Wikimedia Commons has media related to Water_purification.
American Water Works Association
"Water On Tap: What You Need To Know." – Consumer Guide to Drinking
Water in the US (EPA)
Disinfection of Drinking Water – Camping, Hiking and
Code of Federal Regulations, Title 40, Part 141 – U.S. National
Primary Drinking Water Regulations
Environmental impact assessment
Air pollution (control
Solid waste treatment
Water (agricultural wastewater treatment
industrial wastewater treatment
waste-water treatment technologies
Efficient energy use
Fuel (alternative fuel
carbon negative fuel
List of energy storage projects
Renewable energy (commercialization)
Transportation (electric vehicle