Water purification

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Water purification is the removal of contaminants from raw water to produce drinking water that is pure enough for human consumption. Substances that are removed during the process of drinking water treatment include bacteria, algae, viruses, fungi, minerals, and man-made chemical pollutants. Many contaminants can be dangerous—but depending on the quality standards, others are removed to improve the water's smell, taste, and appearance.

It is not possible to tell whether water is safe to drink just by looking at it. Simple procedures such as boiling or the use of a household charcoal filter are not sufficient for treating water from an unknown source. Even natural spring water—considered safe for all practical purposes in the 1800's—must now be tested before determining what kind of treatment is needed.

Sources of drinking water

Deep groundwater: The water emerging from some deep groundwaters may have fallen as rain many decades or even hundreds of years ago. Soil and rock layers naturally filter the groundwater to a high degree of clarity before it is pumped to the treatment plant. Such water may emerge as springs, artesian springs, or may be extracted from boreholes or wells. Deep groundwater is generally of very high bacteriological quality (i.e., a low concentration of pathogenic bacteria) but may be rich in dissolved solids, especially carbonates and sulphates of calcium and magnesium. Depending on the strata through which the water has flowed, other ions may also be present including chloride, and bi-carbonate. There may be a requirement to reduce the iron or manganese content of this water to make it pleasant for drinking, cooking, and laundry use. Disinfection is also required. Where groundwater recharge is practised, it is equivalent to lowland surface waters for treatment purposes.
Shallow groundwaters: Water emerging from shallow groundwaters is usually abstracted from wells or boreholes. The bacteriological quality can be variable depending on the nature of the catchment. A variety of soluble materials may be present including potentially toxic metals such as zinc and copper. In parts of Bangladesh, many shallow groundwater sources are contaminated with unacceptably high levels of arsenic.
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.

Water treatment methods

Screening

The first step in purifying surface water is to remove large debris like sticks, leaves, trash and other large particles which may interfere with subsequent purification steps. The smaller the holes in the screen, the smaller the debris must be to pass through. Thus a screen with small holes will filter out more debris but will become clogged quicker and require cleaning more frequently. 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. The filtered water is then treated to remove or inactivate remaining potentially harmful microscopic organisms including viruses, protozoa and bacteria. This removal step comprises part of a multistep process of disinfection which is completed by chemical and/or ultraviolet light treatment which damages and makes non-infectious any remaining viable harmful microbes. For waters that are particularly difficult to treat such as from catchments with intensive agriculture, extra physical, chemical and biological treatment steps may be necessary.

Coagulation and flocculation

Together coagulation and flocculation is a traditional purification method which works by using active chemicals called coagulants that effectively "glue" small suspended particles together so that they settle out of the water or stick to sand or other granules in a granular media filter. In a relatively new and economically attractive development polymer film with chemically formed microscopic pores called micro or ultrafiltration membranes can be used in place of granular media to filter water effectively without coagulants. The type of membrane media determines how much pressure is needed to drive the water through and what sizes of microbes can pass.

Coagulation normally works by eliminating the natural electrical charge of the suspended particles so they attract and stick to each other. The joining of the particles so that they will form larger settleable particles is called flocculation. The larger formed particles are called floc.

Coagulation

Many of the suspended water particles have a negative electrical charge. The charge keeps particles suspended because they repel similar particles. Coagulation processing reduces the surface charge to encourage attraction which forms floc which can settle. The coagulation chemicals are added in a tank (often called a rapid mix tank), which typically has rotating paddles. In most treatment plants, the mixture remains in the rapid mix tank for 10 to 30 seconds to ensure full mixing. The amount of coagulant that is added to the water varies widely due to the different source water quality. It is often easiest to mix varying amounts of coagulants with samples of the source water to see which dosage creates the best floc. The chemicals also act as additional particles which the suspended solids can bond to form floc.

The most common coagulant used in the United States is aluminum sulfate, sometimes called filter alum. Aluminum sulfate reacts in water to form aluminium hydroxide, which attracts smaller suspended particles, forming floc. The water being purified must be alkaline for the aluminium hydroxide reaction to occur. If the water is not buffred to resist acids, lime or soda ash is added to raise the pH. Lime is the more common of the two additives because it is cheaper, but it also adds to the resulting water hardness. Aluminum sulfate is an inexpensive coagulant, but it produces sulfuric acid that may cause rapid corrosion of water mains if soda ash or lime is not used in enough quantity to counteract the acid.

Iron(III) sulfate or chloride are other common coagulants. They also needs a buffered water, so lime or soda ash is often added to the water. Iron(III) coagulants work over a larger pH range than aluminum sulfate but are not effective with many source waters. Other benefits of iron(III) are lower costs and in some cases slightly better removal of natural organic contaminants from some source waters.

Cationic and other polymers can also be used as coagulants in water treatment. They are often called coagulant aids used in conjunction with other regular coagulants. The long chains of positively charged polymers can help to strengthen floc making it larger, faster settling and easier to filter out. The main advantages of polymer coagulants and aids is that they do not need the water to be alkaline to work and that they produce less settled waste than other coagulants which can reduces operating costs. The drawbacks of polymers are that they are expensive, can plug up sand in filters and that they often have a very narrow range of effective doses.

Flocculation

Flocculation is the clumping together of small particles to form larger particles, called floc, which is more readily settled out of the water. Flocculation is the main method to decrease turbidity. After charge neutralization of suspended particles, they will stick to each other and to the coagulant chemical particles. To aid in the flocculation, water is slowly mixed in a large tank called a flocculation basin. Unlike a rapid mix tank, the flocculation paddles turn very slowly to minimize turbulence. The idea is to gently mix the water so particles contact as many others as possible becoming as large as possible without breaking up. Generally, the retention time of a flocculation basin is at least 30 minutes with speeds between 0.5 feet and 1.5 feet per minute (15 to 45 cm / minute). Flow rates less than 0.5 ft/min cause undesirable floc settlement within the basin.

Sedimentation

Water exiting the flocculation basin enters the sedimentation basin, also called a clarifier. It is a large tank with slow flow, allowing floc to settle to the bottom. The sedimentation basin is best located close to the flocculation basin so the transit between does not permit settlement or floc break up. Sedimentation basins can be in the shape of a rectangle, where water flows from end to end, or circular where flow is from the center outward. Sedimentation basin outflow is typically over a weir so only a thin top layer—furthest from the sediment—exits.

The amount of floc that settles out of the water is dependent on the time the water spends in the basin and the depth of the basin. If water spends more time in the basin then the amount of floc that is settled out increases. In order to keep the water in the basin longer, while treating the same amount of water, the basin volume must be increased. The retention time of the water must therefore be balanced with the cost of a larger basin. The minimum clarifier retention time is normally 4 hours. A deep basin will allow more floc to settle out than a shallow basin. This is because large particles settle faster than smaller ones, so large particles bump into and integrate smaller particles as they settle. In effect, large particles sweep vertically though the basin and clean out smaller particles on their way to the bottom.

As particles settle to the bottom of the basin a layer of sludge is formed on the floor of the tank. This layer of sludge must be removed and treated. The amount of sludge that is generated is significant, often 3%-5% of the total volume of water that is treated. The cost of treating and disposing of the sludge can be a significant part of the operating cost of a water treatment plant. The tank may be equipped with mechanical cleaning devices that continually clean the bottom of the tank or the tank can be taken out of service when the bottom needs to be cleaned.

Filtration

After separating most floc, the water is filtered as the final step to remove remaining suspended particles and unsettled floc. 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 could include dangerous disinfection by-products as well as those with taste and odor. 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. So not just the top layer of the filter cleans the water, but 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 blocked all particles the filter would quickly clog. To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called backflushing) to remove embedded particles. 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.

Where sufficient land and space are available, water may be treated in slow sand filters. These rely on biological treatment processes for their action rather than physical filtration. Slow sand filters are carefully constructed using graded layers of sand with the coarsest at the base and the finest at the top. Drains at the base convey treated water away for disinfection. When bringing a new slow sand filter bed into use, raw water is carefully decanted onto the filter material to a water depth of one to three metres, depending on the size of the filter bed. The water passing through the filter for the first few hours is recirculated and not put into supply. Within a few hours, a film of bacteria, protozoa, fungi, and algae builds on the surface of the sand. This is the Schmutzdecke layer that removes all the impurities. An effective slow sand filter may remain in service for many weeks or even months if the pre-treatment is well designed and produces an excellent quality of water which physical methods of treatment rarely achieve.

Disinfection

Disinfection with aggressive chemicals like chlorine or ozone is normally the last step in purifying drinking water. Water is disinfected to destroy any pathogens which passed through the filters. Possible pathogens include viruses, bacteria including Escherichia coli and Shigella, and protozoans including Giardia lamblia and Cryptosporidium. Many water systems intentionally leave residual disinfection agents in the water after exiting the plant so it travels throughout the distribution system. The most common disinfection method is some form of chlorine such as chlorine gas, sodium hypochlorite, chloramine or chlorine dioxide. The water and chemical mix are allowed to sit in a large tank, called a clear well. The water must sit in the clear well to ensure that the water is in contact with the disinfectant for a minimum amount of time because it takes time to inactivate the harmful microbes. Chlorine is a strong oxidant that kills many microorganisms and remains in the water to provide continuing disinfection. Other disinfection methods include using ozone which acts very rapidly or Ultra Violet light that is almost instantaneous also inactivate pathogens.

Chlorine gas and sodium hypochlorite are the most commonly used disinfectants, because they are inexpensive and easy to manage. They are effective in killing bacteria, but have limited effectiveness against protozoans that form cysts in water (Giardia lamblia and Cryptosporidium, both of which are pathogenic). Chlorine gas and sodium hypochlorite both have strong residuals in the water once it enters the distribution system.

The main drawback in using chlorine gas or sodium hypochlorite is that these react with organic compounds in the water to form potentially harmful levels of the chemical by-products trihalomethanes (THMs) and haloacetic acids, both of which are carcinogenic and regulated by the U.S. EPA. The formation of THMs and haloacetic acids is minimized by effective removal of as many organics from the water as possible before disinfection and/or by adding ammonia immediately after chemical disinfection is completed. Formerly, it was common practice to chlorinate the water at the beginning of the purification process, but this practice has mostly been abandoned to minimize the production of THMs.

Chloramines are not as effective disinfectants compared to chlorine gas or sodium hypochlorite, but do not form THMs or haloacetic acids. They are typically used only in stored and distributed treated water. An example of this sort is proceeses using ozone for primary disinfection which is very quickly accomplished then using monochloramine to create a residual level of disinfectant in the water. Chlorine dioxide is another rapid acting disinfectant against bacteria but unlike ozone it leaves a long lasting residual in the water. Despite these beneficial characteristics, it is rarely used because it may creates excessive amounts of chlorate and chlorite, both of which are regulated to low allowable levels.

Ozone is a very strong, broad spectrum disinfectant and is widely used in Europe to disinfect water. It is a most effective method to inactivate harmful protozoans that form cysts and works well against almost all other pathogens. To use ozone as a disinfectant, it must be created on site and added to the water by bubble contact. Other benefits of ozone are that it does not form any dangerous by-products and does not add any taste or odor to the water. One of the main problems with ozone is that it leaves no disinfectant residual in the water.

UV radiation can be used to disinfect water as well. UV radiation is very effective at inactiavitng cysts, as long as the water has a low level of colour so the UV can pass through without being absorbed. The main drawback to UV radiation is that is like ozone also leaves no disinfectant residual in the water.

Many environmental and cost considerations affect the location and design of water purification plants. Groundwater is cheaper to treat, but aquifers usually have limited output and can take thousands of years to recharge. Surface water sources should be carefully monitored for the presence of unusual types or levels of microbial/disease causing contaminants. The treatment plant itself must be kept secure from vandalism and terrorism. The large quantities of dangerous chemicals suggests special training for workers and emergency personnel. Facilities typically reponsibly dispose of waste and prevent them from contaminating the treatment components and the source water. All facilities disinfect finished water, but the exact method of disinfection can be controversial, and the costs and benefits of different methods weighed.

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 osmosis.

Boiling: Water is heated hot enough and long enough to inactivate or kill microorganisms that normally live in water at room temperature. Near sea level, a vigorous rolling boil for at least one minute is sufficient. At high altitudes (greater than two kilometers or 5000 feet) three minutes is recommended. US EPA emergency disinfection recomendations 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 vapour).
Carbon filtering: Charcoal, a form of carbon with a high surface area, absorbs many compounds including some toxic compounds. Water passing through activated charcoal is common in household water filters and fish tanks. Household filters for drinking water sometimes contain silver to releases silver ions which have a bactericidal effect. There are two types of carbon filters. One is granular charcoal which is not very effective for removing many contaminants such has mercury, volatile organic chemicals (this is the most prevalent contaminant found in drinking water and is also not removed by reverse osmosis or distillation), asbestos, pesticides, disinfection byproducts (trihalomethanes), mtbe, pcbs etc. A more effective carbon filter is the sub-micrometre solid block carbon filter which removes all of the contaminants listed above. To see if a particular product removes contaminants or to compare such products, see National Sanitation Foundation, or check the California Health Certificate that comes with most filters. Carbon filters are not true filters such as membrane filters; harmful microbes can easily pass right through them so they are often called contacters rather than filters.
Distilling: 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 vaporized, they remain in the boiling solution. Even distillation does not completely purify water, because of contaminants with similar boiling points and droplets of unvaporized 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. Tight membrane filters like RO or nanofilter membranes will remove salt and colour compounds from water but thorough pretreatment, high pressures and careful cleaning is required leading to high costs per gallon.
Ion exchange: Most common ion exchange systems use a zeolite resin bed to replace unwanted Ca²⁺ and Mg²⁺ ions with benign (soap friendly) Na⁺ or K⁺ ions. This is the common water softener. A more rigorous type of ion exchange swaps H⁺ ions for unwanted cations and hydroxide (OH^-) ions for unwanted anions. The result is H⁺ + OH^- → H₂O. This system is recharged with hydrochloric acid and sodium hydroxide, respectively. The result is essentially deionized water.
Electrodeionization: Water is passed between a positive electrode and a negative electrode. Ion selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity deionized water results. The water is usually passed through a reverse osmosis unit first to remove nonionic organic contaminants.
Water conditioning: This is a method of reducing the effects of hard water. Hardness salts are deposited in water systems subject to heating because the decomposition of bicarbonate ions creates carbonate ions which crystalise out of the saturated solution of calcium or magnesium carbonate. 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, as calcium carbonate of very high purity. The preciptated 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 or/and electrical fields reducing the effects of hard water.
Plumbo-solvency reduction: In areas with naturally acidic waters of low conductivity (i.e surface rainfall in upland mountains of igneous rocks), the water is 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.

Portable water purification

Portable drinking water systems or chemical additives are available for hiking, camping, and travel in remote areas. Portable pump filters are commercially available with ceramic filters that filter 5000 to 50,000 liters per cartridge. Some also utilize activated charcoal filtering. Filters of this kind do not remove any harmful viruses and chemical or UV disinfection is required after filtration for safety. Effective chemical additives include chlorine, chlorine dioxide or iodine.

Iodine is added to water as a solution, crystallized, or in tablets. The iodine kills many—but not all—of the most common pathogens present in natural fresh water sources. Carrying iodine for water purification is a light weight but imperfect solution for those in need of field purification of drinking water. There are kits available in camping stores that include an iodine pill and a second pill that will remove the iodine taste from the water after it has been disinfected.

Bleach may be used for emergency disinfection at the rate of 2 drops of 5% bleach per liter or quart of clear water, and then it is covered for 30 minutes or 1 hour. After this it may be left open to reduce the chlorine smell and taste. Guidelines are available online for safe and effective use of bleach. EPA emergency FAQ, British Columbia Ministry of Health

Neither chlorine (e.g. bleach) nor iodine alone is considered effective against Cryptosporidium, and they are limited in effectiveness against Giardia. Chlorine is slightly better than iodine against Giardia.

SODIS (Solar Water Disinfection): Microbes are destroyed through temperature and UVA radiation provided by the Sun. Water is placed in a transparent plastic bottle, which is oxygenated by shaking. It is placed for six hours in full sun, which raises the temperature and gives an extended dose of solar radiation, killing any microbes that may be present. The combination of the two provides a simple method of disinfection for tropical developing countries.