Goals 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.
Pretreatment ;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: 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. This may involve a
screen filter. 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 minimise 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.
pH adjustment 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 caustic soda (
sodium hydroxide) can be added to raise the pH during water purification processes and is especially common practice for
water softening. The choice of chemical used for raising the pH often depends on the carbonate alkalinity in the water. Addition of such chemicals increases the carbonate ion concentration, forcing precipitation of
calcium carbonate, and
magnesium carbonate at even higher pH. Ultimately, the water hardness is reduced. 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 minimise 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 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 colour of water. The addition of inorganic coagulants such as
aluminium 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 neutralised 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. Amorphous metal hydroxides are known as "floc". Large, amorphous aluminium and iron (III) hydroxides adsorb and enmesh particles in suspension and facilitate the removal of particles by subsequent processes of
sedimentation and
filtration. 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. 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) whereby 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. 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. 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. Advantages: • Filters out much smaller particles than paper and sand filters can. • Filters out virtually all particles larger than their specified pore sizes. • 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 ) into the ground at the Water purification plant Káraný, Czech Republic
Slow sand filters may be used where there is sufficient land and space, as the water flows very slowly through the filters. These filters rely on biological treatment processes for their action rather than physical filtration. They are carefully constructed using graded layers of sand, with the coarsest sand, along with some gravel, at the bottom and the 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 are not backwashed; they are maintained by having the top layer of sand scraped off when the flow is eventually obstructed by biological growth.
Bank filtration In
bank filtration, natural sediments in a riverbank are used to provide the 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 river.
Membrane filtration 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 ions.
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: 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: 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 minimised by the 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 pathogenic protozoa that form cysts in water such as
Giardia lamblia and
Cryptosporidium.
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 spontaneously detonate.
Chloramination The use of
chloramine is becoming more common as a disinfectant. Although chloramine is not as strong an oxidant, it provides a longer-lasting residual than free chlorine because of its lower redox potential compared to free chlorine. It also does not readily form THMs or haloacetic acids (
disinfection byproducts). It is possible to convert chlorine to chloramine by adding
ammonia to the water after adding 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 disinfection Ozone is an unstable molecule which readily gives up one atom of oxygen providing a powerful oxidising agent which is toxic to most waterborne organisms. It is a very strong, broad spectrum disinfectant that is widely used in Europe and in a few municipalities in the United States and Canada. Ozone disinfection, or ozonation, 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). No residual ozone is left in the water. In the absence of a residual disinfectant in the water, chlorine or chloramine may be added throughout a distribution system to remove any potential pathogens in the distribution piping. Ozone has 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 bromatethe maximum contaminant level established by the USEPA. Ozone disinfection is also energy intensive.
Ultraviolet disinfection 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.
Solar disinfection Over 2 million people in 28 developing countries use
solar disinfection for daily drinking water treatment.
Ionizing radiation Like UV,
ionizing radiation (X-rays, gamma rays, and electron beams) has been used to sterilise water.
Bromination and iodinisation Bromine and
iodine can also be used as disinfectants. However, chlorine in water is over three times more effective as a disinfectant against
Escherichia coli than an equivalent concentration of
bromine, and over six times more effective than an equivalent concentration of
iodine. Iodine is commonly used for
portable water purification, and bromine is common as a
swimming pool disinfectant.
Portable water purification Portable 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, odour, 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
Illinois River in
Illinois, United States of America. 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==