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Water Purification

Water purification is the process of removing contaminants from a raw water source. The goal is to produce water for a specific purpose with a treatment profile designed to limit the inclusion of specific materials; most water is purified for human consumption (drinking water). Water purification may also be designed for a variety of other purposes and water purified to meet the requirements of medical, pharmacology, chemical and industrial applications. Methods include, but are not limited to: ultra violet light, filtration, water softening, reverse osmosis, ultrafiltration, molecular stripping, deionization, and carbon treatment.

Water purification may remove, particulate sand; suspended particles of organic materal; Parasites, Giardia; Cryptosporidium; bacteria; algae; virus; fungi; etc. Minerals calcium, silica, magnesium, etc., and Toxic metals lead; copper; chromium; etc. Some purification may be elective in its inclusion in the purification process; examples, smell (hydrogen sulfide remediation), taste (mineral extraction), and appearance (iron incapsulation).

Governments usually dictate the quality standards for drinking water water quality, these standards will require minimum / maximum setpoints for the extraction of contaminants and the inclusion of control elements that produce potable drinking water. Quality standards in the United States require specific amounts of disinfectant (example, residual chlorine content) in the water after it leaves the WTP (Water Treatment Plant), at the end of the treatment process to reduce the risk of re-contamination while the water is in the distribution system.

Ground water (usually supplied as well water) is typically a more economical choice than surface water as a source for drinking water, as it is inherently pre-filtered, by the aquifer from which it is extracted. Over large areas of the world, aquifers are recharged as part of the hydrologic cycle, and their water is a renewable resource. In more arid regions, water from an aquifer will have a limited output and can take thousands of years to recharge. Surface water; (rivers, lakes, streams) is locally more abundant where subsurface formations do not function as aquifers; however, ground water is far more abundant than the more-visible surface water. Surface water is a typical raw water source used to make drinking water where it is abundant, ground water is unavailable or poor quality, however, it is much more exposed to human activity and its byproducts. As a water source it is carefully monitored for the presence of a variety of contaminants by the WTP operators.

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 all the possible contaminants that may be in water from an unknown source. Even natural spring water; considered safe for all practical purposes in the 1800s; and must now be tested before determining what kind of treatment is needed. Laboratory analysis will define the contaminants in the water sample, with both qualitative and quantitative measurements. Lab analysis, while expensive, it is the only way you will be able to obtain the bench mark information necessary for establishment of a purification process, methodology for purification.

Sources of drinking water
1.Deep ground water :

The water emerging from some deep groundwaters may have fallen as rain many decades, hundreds, thousands or in some cases millions of years ago. Soil and rock layers naturally filter the ground water 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 ground water is generally of very high bacteriological quality (i.e., pathogenic bacteria such as Campylobacter or the pathogenic protozoa Cryptosporidium and Giardia) are typically absent, but the water typically is 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 bicarbonate. 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.

2.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 (rarely) potentially toxic metals such as zinc and copper. Arsenic contamination of groundwater is a serious problem in some areas, notably from shallow wells in Bangladesh and West Bengal in the Ganges Delta.

3.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.

4.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 vapour.

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 rain .

Pre-treatment
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, trash and other large particles which may interfere with subsequentpurification 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-conditioning

Many waters rich in hardness salts are treated with soda-ash (Sodium carbonate) to precipitate calcium carbonate out utilising the common ion effect.

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. [citation needed]

Widely varied techniques are available to remove the fine solids, micro-organisms and some dissolved inorganic and organic materials. 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.

pH adjustment

Distilled water has an average pH of 7 (neither alkaline or acidic) and sea water has an average pH of 8.3 (slightly alkaline). If the water is acidic (lower than 7), 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. Making the water slightly alkaline ensures that coagulation and flocculation processes work effectively and also helps to minimise the risk of lead being dissolved from lead pipes and lead solder in pipe fittings.

FLOCUATION

FLOCUATION: is a process in which we first clarify the water. Clarifying means removing any turbidity or colour so that the water is sparklingly clear and colourless. Clarification is done by causing a precipitate to form in the water. Initially the precipitate forms as very small particles but as the water is gently stirred, these particles stick together to form bigger particles. We can say that the small particles coagulate; this process is sometimes called flocculation. Many of the small particles that were originally present in the raw water absorb onto the surface of these small precipitate particles and so get incorporated into the larger particles that coagulation produces. In this way the coagulated precipitate takes most of the suspended matter out of the water and is then filtered of, generally by passing the mixture through a coarse sand filter or sometimes through a mixture of sand and granulated anthracite (high quality coal). Anthracite with its high carbon content is able to absorb much of the organic matter present in solution and this can remove odour and taste from the water. A precipitate that is widely used to clarify water is iron (III) hydroxide. This is formed first by adjusting (if necessary) the pH of the incoming water to above 7 (by adding lime or sodium hydroxide), then by adding a solution of an iron (III) compound such as iron (III) chloride. Iron (III) hydroxide is extremely insoluble and forms even at a pH as low as 7. Aluminium hydroxide is also widely used as the flocculating precipitate.

Sedimentation

Water exiting the flocculation basin may enter the sedimentation basin, also called a clarifier or settling basin. 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. The retention time of the water must therefore be balanced against 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 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 particles. Prior to this, 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.

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.

Membrane filtration: is essentially a thin film of synthetic polymer through which there are pores of fairly uniform size. This filters water as it flows through.

ADVANTAGES: Filter out much smaller particles than paper and sand filters can Filter out virtually all particles larger than their specified pore sizes They are quite thin and so liquids flow through them fairly rapidly. They are reasonalbly strong and so can withstand pressure differences across them of typically 2-5 atmospheres. They can be cleaned (back flushed) and reused.

Membrane filters are widely used for filtering both drinking water and sewage(for reuse). For drinking water membrane filters can remove virtually all particles larger than 0.2 um including Giardia and cryptosporidium. Membrane filters is an effective form of tertiary treatment when it is dissired to reuse the water for industry or for limited domestic purposes or before discharging the water into a river that is used by towns further downstream. Is 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 phospherus and nitrates and heavey metal ions.

Slow sand filters

Membrane filters are widely used for filtering both drinking water and sewage(for reuse). For drinking water membrane filters can remove virtually all particles larger than 0.2 um including Giardia and cryptosporidium. Membrane filters is an effective form of tertiary treatment when it is dissired to reuse the water for industry or for limited domestic purposes or before discharging the water into a river that is used by towns further downstream. Is 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 phospherus and nitrates and heavey metal ions.

Ultrafiltration

Ultrafiltrationmembranes are a relatively new development; they use polymer film with chemically formed microscopic pores that 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 micro-organisms can be filtered out.

Disinfection

is normally the last step in purifying drinking water. Water is disinfected to kill any pathogens which pass through the filters. Possible pathogens include viruses, bacteria, including Escherichia coli, Campylobacter and Shigella, and protozoans, including G. lamblia and other Cryptosporidia. In most developed countries, public water supplies are required to maintain a residual disinfecting agent throughout the distribution system, in which water may remain for days before reaching the consumer. 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. is the sanitising of sterilisation of water. This is done by adding gaseous dissloved chlorine in the water. Chlorine at a concentration of 1 or 2 ppm destroys bacteria and some viruses. Sufficient chlorine is added to the water (with careful monitoring) to ensure that the concentration stays slightly above 1ppm until the water reaches the end user.

1.Chlorination

The most common disinfection method is some form of chlorine or its compounds such as chloramine or chlorine dioxide. Chlorine is a strong oxidant that kills many 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 either a relatively inexpensive solid that releases free chlorine when dissolved in water or a liquid (bleach)that is typically generated on site using common salt and high voltage DC. Handling the solid, however, requires greater routine human contact through opening bags and pouring than the use of gas cylinders which are more easily automated. The generation of liquid sodium hypochlorite is both inexpensive and safer than the use of gas or solid chlorine. Both disinfectants are widely used despite their respective drawbacks. One drawback to using chlorine gas or sodium hypochlorite is that they react with organic compounds in the water to form potentially harmful chemical by-products trihalomethanes (THMs) and haloacetic acids (HAAs), both of which are carcinogenic in large quantities and regulated by the U.S. Environmental Protection Agency (EPA). The formation of THMs and haloacetic acids is 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 protozoans that form cysts in water. (Giardia lamblia and Cryptosporidium, both of which are pathogenic).

2.Chlorine dioxide

Chlorine dioxide is another fast-acting disinfectant. It is, however, rarely used, because it may create excessive amounts of chlorate and chlorite, both of which are regulated to low allowable levels. Chlorine dioxide also poses extreme risks in handling: not only is the gas toxic, but it may spontaneously detonate upon release to the atmosphere in an accident.

3.Chloramines

Chloramines are another chlorine-based disinfectant. Although chloramines are not as strong of an oxidant or provide a reliable residual, as compared to chlorine gas or sodium hypochlorite, they are less prone to form THMs or haloacetic acids. It is possible to convert chlorine to chloramine by adding ammonia to the water along with the chlorine: The chlorine and ammonia react to form chloramine. Water distribution systems disinfected with chloramines may experience nitrification, wherein ammonia is used a nitrogen source for bacterial growth, with nitrates being generated as a byproduct.

4.Ozone

Ozone (O 3) is a relatively unstable molecule "free radical" of oxygen which readily gives up one atom of oxygen providing a powerful oxidising agent which is toxic to most water borne organisms. It is a very strong, broad spectrum disinfectant that is widely used in Europe. It is an effective method to inactivate harmful protozoans 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 relatively fewer dangerous by-products (in comparison to chlorination) and the lack of taste and odor produced by ozonation. Although fewer by-products are formed by ozonation, it has been discovered that the use of ozone produces a small amount of the suspected carcinogen Bromate, although little Bromine should be present in treated water. Another one of the main disadvantages of ozone is that it leaves no disinfectant residual in the water. 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.

5.UV radiation

UV radiation (light) is very effective at inactivating cysts, as long as the water has a low level of colour so the UV can pass through without being absorbed. The main disadvantage to the use of UV radiation is that, like ozone treatment, it leaves no residual disinfectant in the water. Because neither ozone nor UV radiation leaves a residual disinfectant in the water, it is sometimes necessary to add a residual disinfectant after they are used. 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 little of the negative aspects of chlorination.

Additional treatment options
1.Fluoridation

Fluoridation -in many areas fluoride is added to water for the purpose of preventing tooth decay. This process is referred to as water fluoridation. Fluoride is usually added after the disinfection process. In the United States, fluoridation is usually accomplished by the addition of hexafluorosilicic acid, which decomposes in water, yielding fluoride ions.

2.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, 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 or/and electrical fields reducing the effects of hard water. [citation needed]

3.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.

4.Radium Removal :

Some groundwater sources contain radium, a radioactive chemical element. Typical sources include many groundwater sources north of the Illinois 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.

5.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. One method of reducing fluoride levels is through treatment with activated alumina.

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.

1.Boiling :

Water is heated hot enough and long enough to inactivate or kill micro-organisms 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. [1] 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). Boiling does not leave a residual disinfectant in the water. Therefore, water that has been boiled and then stored for any length of time may have acquired new pathogens.

2.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 release silver ions which have an anti-bacterial effect.

3.Distillation

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 unvaporised liquid carried with the steam. However, 99.9% pure water can be obtained by distillation. Distillation does not confer any residual disinfectant and the distillation apparatus may be the ideal place to harbour Legionnaires' disease.

4.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 colonise the membranes.

5.Ion exchange :

Most common ion exchange systems use a zeolite resin bed to replace unwanted Ca 2+ and Mg 2+ions with benign (soap friendly) Na + or K + ions. This is the common water softener.

through a reverse osmosis unit first to remove non-ionic organic contaminants.

6.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 Descaling agent

The use of iron in removing arsenic from water.


Reverse Osmosis

Reverse osmosis is a separation process that uses pressure to force a solvent through a membrane that retains the solute on one side and allows the pure solvent to pass to the otherside. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.

The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2–17 bar (30–250 psi) for fresh and brackish water, and 40–70 bar (600–1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.

This process is best known for its use in desalination (removing the salt from sea water to get fresh water), but has also purified naturally occurring freshwater for medical, industrial process and rinsing applications since the early 1970s

Method

When two solutions with different concentrations of a solute are mixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. This is achieved by diffusion, in which solutes will move from areas of higher concentration to areas of lower concentrations until the concentration in all the different areas of the resulting mixture are the same, a state called equilibrium.

Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration, it is instead achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as "osmotic pressure", since it is analogous to flow caused by a pressure differential.

In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure. In the same way as in conventional osmosis, the solute cannot move from areas of high pressure to areas of low pressure because the membrane is not permeable to it. Only the solvent can pass through the membrane. When the effect of the externally applied pressure is greater than that of the concentration difference, net solvent movement will be from areas of high solute concentration to low solute concentration, and reverse osmosis occurs.

Applications

Drinking water purification

In the United States, household drinking water purification systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking.

Such systems typically include four or five stages:

  • a sediment filter to trap particles including rust and calcium carbonate

  • optionally a second sediment filter with smaller pores

  • an activated carbon filter to trap organic chemicals, and chlorine which will attack and degrade TFC reverse osmosis membranes

  • a reverse osmosis (RO) filter which is a thin film composite membrane ( TFM or TFC)

  • optionally a second carbon filter to capture those chemicals not removed by the RO membrane.

  • optionally an ultra-violet lamp is used for disinfection of any microbes that may escape filtering by the reverse osmosis membrane.

In some systems, the carbon pre-filter is omitted and cellulose triacetate membrane (CTA) is used. The CTA membrane is prone to rotting unless protected by the chlorinated water, while the TFC membrane is prone to breaking down under the influence of chlorine. In CTA systems, a carbon post-filter is needed to remove chlorine from the final product water.

Portable reverse osmosis (RO) water processors are sold for personal water purification in various locations. To work effectively, the water feeding to these units should best be under some pressure (40psi or over is the norm). Portable RO water processors can be used by people who live in rural areas without clean water, far away from the city's water pipes. Rural people filter river or ocean water themselves, as the device is easy to use (Saline water may need special membranes). Some travelers on long boating trips, fishing, island camping, or in countries where the local water supply is polluted or substandard, use RO water processors coupled with one or more UV sterilizers. RO systems are also now extensively used by marine aquarium enthusiasts, as the domestic water supply contains substances that are extremely toxic to most species of saltwater fish. In the production of bottled mineral water, the water passes through a RO water processor to remove pollutants and microorganisms. In European countries, though, such processing of Natural Mineral Water (as defined by a European Directive) is not allowed under European law.(In practice, a fraction of the living bacteria can and do pass through RO membranes through minor imperfections, or bypass the membrane entirely through tiny leaks in surrounding seals. Thus, complete RO systems may include additional water treatment stages that use ultraviolet light or ozone to prevent microbiological contamination.)

In the water treatment industry there is a chart of types of contaminants, their sizes and which ones pass through the various types of membranes. [1] Membrane pore sizes can vary from 1 to 50,000 angstroms depending on filter type. "Particle filtration" removes particles of 10,000 angstroms or larger. Microfiltration removes particles of 500 angstroms or larger. "Ultrafiltration" removes particles of roughly 30 angstroms or larger. "Nanofiltration" removes particles of 10 angstroms or larger. Reverse osmosis is in the final category of membrane filtration, "Hyperfiltration," and removes particles larger than 1 angstrom.

Water and wastewater purification

Rain water collected from storm drains is purified with reverse osmosis water processors and used for landscape irrigation and industrial cooling in Los Angeles and other cities, as a solution to the problem of water shortages.

In industry, reverse osmosis removes minerals from boiler water at power plants. The water is boiled and condensed over and over again and must be as pure as possible so that it does not leave deposits on the machinery or cause corrosion. It is also used to clean effluent and brackish groundwater.

Reverse osmosis product can be used for the production of deionized water.

In July 2002, Singapore announced that a process named NEWater would be a significant part of its future water plans. It involves using reverse osmosis to treat domestic wastewater before discharging the NEWater back into the reservoirs. [1]

Food industry

In addition to desalination, reverse osmosis is a more economical operation for concentrating food liquids (such as fruit juices) than conventional heat-treatment processes. Research has been done on concentration of orange juice and tomato juice. Its advantages include a low operating cost and the ability to avoid heat treatment processes, which makes it suitable for heat-sensitive substances like the protein and enzymes found in most food products.

Reverse osmosis is extensively used in the dairy industry for the production of whey protein powders and for the concentration of milk to reduce shipping costs. In whey applications, the whey (liquid remaining after cheese manufacture) is pre-concentrated with RO from 6% total solids to 10-20% total solids before UF (ultrafiltration) processing. The UF retentate can then be used to make various whey powders including WPI (whey protein isolate) used in bodybuilding formulations. Additionally, the UF permeate, which contains lactose, is concentrated by RO from 5% total solids to 18-22% total solids to reduce crystallization and drying costs of the lactose powder.

Although use of the process was once frowned upon in the wine industry, it is now widely understood and used. An estimated 60 reverse osmosis machines were in use in Bordeaux, France in 2002. Known users include many of the elite classed growths (Kramer) such as Château Léoville-Las Cases in Bordeaux.

Reverse osmosis is used globally throughout the wine industry for many practices including wine and juice concentration, taint removal; such as acetic acid, smoke taint and brettanomyces taint; and alcohol removal. The patent holder for these processes, Vinovation, Inc., claims to have served over 1000 wineries worldwide, either directly or through one if its licensed partners, in the last 15 years. Its use has become so widely accepted that patent infringers have sprung up on several continents.

Car washing

Because of its lower mineral content, RO water is often used in car washes during the final vehicle rinse to prevent water spotting on the vehicle. RO water also enables the car wash operators to reduce the demands on the vehicle drying equipment.

Maple syrup production

Starting in the 1970s, some maple syrup producers started using reverse osmosis to remove water from sap before being further boiled down to syrup. The use of reverse osmosis allows approximately 75–80% of the water to be removed from the sap, reducing energy consumption and exposure of the syrup to high temperatures. Microbial contamination and degradation of the membranes has to be monitored.

Hydrogen production

For small scale production of hydrogen, reverse osmosis is sometimes used to prevent formation of minerals on the surface of the electrodes and to remove organics and chlorine from drinking water.

Reef Aquarium Keeping

Many Reef Aquarium Keepers use Reverse Osmosis for their artificial mixture of seawater, In fact pure fresh water is the only substitute for evaporated water in reef aquariums. Therefore, each and every aquarist should use only purest from of water. Water from water tap often is contaminated & contains excessive chlorine, chloramines, copper nitrates, phosphates, silicates besides, many other harmful chemical that are also found in tap water making it more vulnerable to many life forms in reef aquarium. An effective combination of both Reverse Osmosis & De-ionization (RO/DI) is considered to be the most popular amongst Reef Aquarium Keepers above other water purification processes as its convenient method with low cost of ownership and minimal running costs

Desalination

Desalination refers to any of several processes that remove excess salt and other minerals from water in order to obtain fresh water suitable for animal consumption or irrigation, or, if almost all of the salt is removed, for human consumption. Sometimes the process produces table salt as a by-product. Desalination of ocean water is practiced in many regions that have scarce natural freshwater supplies; it is widespread in the Middle East and the Caribbean, and is increasingly used in parts of the United States, North Africa, Singapore, Spain, Australia, China and now in INDIA. It is also used on many ships and submarines.

Desalination typically requires large amounts of energy as well as specialized, expensive infrastructure, making it very costly compared to the use of fresh water from rivers or wells (bores). The large energy reserves of many Middle Eastern countries have allowed for desalination to be employed relatively cheaply. Saudi Arabia's desalination plants account for about 24% of total world capacity. The world's largest desalination plant is the Jebel Ali Desalination Plant (Phase 2) in the United Arab Emirates. It is a dual-purpose facility that uses multi-stage flash distillation and is capable of producing 300 million cubic meters of water per year

Methods

  • Distillation
    1. Multi-stage flash distillation
    2. Multiple-effect evaporator
    3. Vapor-compression evaporation
    4. Evaporation/condensation
  • Membrane processes
    1. Electrodialysis reversal
    2. Reverse osmosis
    3. Nanofiltration
    4. Forward osmosis
    5. Membrane distillation
  • Freezing
  • Geothermal desalination
  • Solar humidification
  • Methane hydrate crystallisation
  • High grade water recycling

The two leading methods were reverse osmosis (47.2% of installed capacity world-wide) and multi-stage flash (36.5%).The traditional process used in these operations is vacuum distillation—essentially the boiling of water at less than atmospheric pressure and thus a much lower temperature than normal. Due to the reduced temperature, energy is saved.

In the last decade, membrane processes have grown very fast, and reverse osmosis has taken nearly half the world's installed capacity. Membrane processes use semi-permeable membranes to filter out dissolved material or fine solids. The systems are usually driven by high-pressure pumps, but the growth of more efficient energy-recovery devices has reduced the power consumption of these plants and made them much more viable; however, they remain energy intensive and, as energy costs rise, so will the cost of reverse osmosis water.

Forward osmosis employs a passive membrane filter that is hydrophilic and slowly permeable to water, and blocks a portion of the solutes. Water is driven across the membrane by osmotic pressure created by food grade concentrate on the clean side of the membrane. Forward osmosis systems are passive in that they require no energy input. They are used for emergency desalination purposes in seawater and floodwater settings


RO Membrane

Thin film composite membrane

Thin film composite membranes (TFC or TFM) are semipermeable membranes manufactured principally for use in water purification or desalination systems. They also have use in chemical applications such as batteries and fuel cells.

Essentially, a TFC material is a molecular sieve constructed in the form of a film from two or more layered materials.

Membranes used in reverse osmosis are typically made out of polyimide, chosen primarily for its permeability to water and relative impermeability to various dissolved impurities including salt ions and other small, unfilterable molecules.

History

The first viable reverse osmosis membrane was made from cellulose acetate as an integrally skinned asymmetric semi-permeable membrane. This membrane was made by Loeb and Sourirajan at UCLA in 1959 and patented in 1960. The current generation of reverse osmosis (RO) membrane materials are based on a composite material patented by FilmTec Corporation in 1970. FilmTec's FT30 membrane is known as a polyamide thin film composite membrane.

Structure and Materials

As is suggested by the name, TFC membranes are composed of multiple layers. Membranes designed for desalination use an active thin-film layer of polyimide layered with polysulfone as a porous support layer.

Other materials, usually zeolites, are also used in the manufacture of TFC membranes.

Element construction

membranes are thin film composite membranes packed in a spiral wound configuration. Spiral wound designs offer many advantages compared to other module designs, such as tubular, plate and frame and hollow fiber module design for most of the reverse osmosis applications in water treatment. Typically, a spiral wound configuration offers significantly lower replacement costs, simpler plumbing systems, easier maintenance and greater design freedom than other configurations, making it the industry standard for reverse osmosis and nanofiltration membranes in water treatment.

The construction of a spiral wound membrane element as well as its installation in a pressure vessel is schematically element contains from one, to more than 30 membrane leafs, depending on the element diameter and element type. Using unique automated manufacturing process, each leaf is made of two membrane sheets glued together back-to-back with a permeate spacer in-between them. Automated process produces consistent glue lines about 1.5 in (4 cm) wide that seal the inner (permeate) side of the leaf against the outer (feed/concentrate) side. There is a side glue line at the feed end and at the concentrate end of the element, and a closing glue line at the outer diameter of the element. The open side of the leaf is connected to and sealed against the perforated central part of the product water tube, which collects the permeate from all leaves. The leaves are rolled up with a sheet of feed spacer between each of them, which provides the channel for the feed and concentrate flow. In operation, the feed water enters the face of the element through the feed spacer channels and exits on the opposite end as concentrate. A part of the feed water – typically 10-20 % – permeates through the membrane into the leaves and exits the permeate water tube.

When elements are used for high permeate production rates, the pressure drop of the permeate flow inside the leaves reduces the efficiency of the element. Therefore elements have been optimized with a higher number of shorter membrane leaves and thin and consistent glue lines. The element construction also optimizes the actual active membrane area (the area inside the glue lines) and the thickness of the feed spacer. Element productivity is enhanced by high active area while a thick feed spacer reduces fouling and increases cleaning success. Such precision in element manufacture can only be achieved by using advanced automated precision manufacturing equipment

In membrane systems the elements are placed in series inside of a pressure vessel. The concentrate of the first element becomes the feed to the second element and so on. The permeate tubes are connected with interconnectors (also called couplers), and the combined total permeate exits the pressure vessel at one side (sometimes at both sides) of the vessel.

Semipermeable Membranes are at the Heart of RO Systems

The process of reverse osmosis (RO) represents the finest level of liquid filtration available today. While ordinary liquid filters use a screen to separate particles from water streams, an RO system employs a semipermeable membrane that separates an extremely high percentage of un wanted molecules.

For example, the membrane may be permeable to water molecules, but not to molecules of dissolved salt. If this membrane is placed between two compartments in a container as shown in Figure 1, and a salt solution is placed in one half of the container and pure water in the other, water passes through the membrane while the salt cannot.

Pressure is Applied to Reverse Natural Osmotic Flow

Now a fundamental scientific principle comes into play. That is, dissimilar liquid systems will try to reach the same concentration of materials on both sides of the membrane. The only way for this to happen in our example is for pure water to pass through the membrane to the salt water side in an attempt to dilute the salt solution. This attempt to reach equilibrium is called osmosis.

But if the goal in our example water purification system is to remove the salt from water, it is necessary to reverse the natural osmotic flow by forcing the salt water through the membrane in the reverse direction. This can be accomplished by applying pressure to the salt water as it’s fed into the system, creating a condition know as “reverse osmosis

Cross-flow Filtration Permits Long-term Performance

While the principals of reverse osmosis are simple, in practical terms, the RO process cannot go on indefinitely unless steps are taken to ensure that the membrane doesn’t become clogged by precipitated salts and other impurities forced against it by the pressurized stream of feed water. To significantly reduce the rate of membrane fouling, RO

systems employ cross-flow filtration which allows water to pass through the membrane while the separate flow of concentrate sweeps rejected salts away from the membrane surface

Elements Maximize the Performance of RO Water Purification Systems

The membrane element is the heart of any RO water purification system. To make sure you’re getting the most effective, efficient system available, make sure it’s built around a element..

  • Home drinking water membrane elements are constructed using advanced automated manufacturing technology. This means elements are built to optimum physical tolerances, and element-to-element differences are minimized. What’s more, critical fastening points are sonic-welded for maximum strength and durability, and extensive quality tests are performed to ensure that high standards for fabrication of elements are met. Some other manufacturers handroll and assemble their elements on primitive manufacturing lines.

  • Elements have been installed as part of more systems...in more applications...than any other thin-film composite membrane. Besides offering superior performance, their consistency and reliability in service are well documented. elements have been the market leader in both consumer and commercial RO systems for more than a decade. Hundreds of thousands of drinking water systems based on elements are in successful operation today.

  • The membrane inside screens out a higher percentage of dissolved solids than cellulosic membranes. Salt passage, for instance, is often 50% lower than that of cellulose acetate membranes operated at the same water recovery rate. The membrane has also been found to reject a higher percentage of undesirable dissolved solids- such as chloride, lead and nitrates- than other thin-film composite membranes.

  • Elements produce more high-quality water per day- two to three times more- than elements containing cellulosic membranes. They also last longer than cellulose based membranes under typical operating conditions due to superior resistance to compaction, chemical degradation and microbiological attack.

Applications

Thim film composite membranes are used in

  • Water purification;

  • as a chemical reaction buffer (batteries and fuel cells);

  • and in industrial gas separations.

Limitations

Thin film composites membranes typically suffer from compaction effects under pressure. As the water pressure increases, the polymers are slightly reorganized into a tighter fitting structure that results in a lower porosity, ultimately limiting the efficiency of the system designed to use them. In general, the higher the pressure, the greater the compaction.

Surface fouling: Colloidal particulates, bacteria infestation (biofouling).

Chemical decomposition and oxidation.

Performance

A filtration membrane's performance is rated by selectivity, chemical resistance, operational pressure differential and the pure water flow rate per unit area.

Due to the emphasis on flow rate, a membrane is manufactured as thinly as possible. These thin layers introduce defects that may affect selectivity, so system design usually trades off the desired flow rate against both selectivity and operational pressure.

In applications other than filtration, parameters such as mechanical strength, temperature stability, and electrical conductivity may dominate.


RO Membrane

Factors Affecting RO Membrane Performance

Reverse osmosis (RO) technology can be a complicated subject, particularly without an understanding of the specific terminology that describes various aspects of RO system operation and the relationships between these operating variables.

This bulletin defines some of these key terms and provides a brief overview of the factors that affect the performance of RO membranes, including pressure, temperature, feedwater salt concentration, permeate recovery, and system pH.

Definitions

Recovery

The percentage of membrane system feedwater that emerges from the system as product water or Òpermeate.Ó Membrane system design is based on expected feedwater quality and recovery is fixed through initial adjustment of valves on the concentrate stream. Recovery is often fixed at the highest level that maximizes permeate flow while preventing precipitation of super-saturated salts within the membrane system.

Passage

The opposite of "rejection," passage is the percentage of dissolved constituents(contaminants) in the feedwaterallowed to pass through the membrane.

Permeate

The purified productwater produced by a membrane system.

Flow

Feed flow is the rate of feedwater introduced to the membrane element, usually measured in gallons per minute (gpm). Concentrate flow is the rate of flow of non-permeated feedwater that exits the membrane element. This concentrate contains most of the dissolved constituents originally carried into the element from the feed source. It is usually measured in gallons per minute (gpm).

Flux

The rate of permeate transported per unit of membrane area, usually measured in gallons per square foot per day (gfd). Dilute solution Ð purified water solution, RO system product water.

Concentrated solution

Ð brackish water solution such as RO system feedwater.

Effect of pressure

Feedwater pressure affects both the water flux and salt rejection of RO membranes. Osmosis is the flow of water across a membrane from the dilute side toward the concentrated solution side. Reverse osmosis technology involves application of pressure to the feedwater stream to overcome the natural osmotic pressure. Pressure in excess of the osmotic pressure is applied to the concentrated solution and the flow of water is reversed. A portion of the feedwater (concentrated solution) is forced through the membrane to emerge as purified product water of the dilute solution side (please see Figure 1).

As shown in Figure 2, water flux across the membrane increases in direct relationship to increases in feedwater pressure. Increased feedwater pressure also results in increased salt rejection but, as Figure 2 demonstrates, the relationship is less direct than for water flux.

Because RO membranes are imperfect barriers to dissolved salts in feedwater, there is always some salt passage through the membrane. As feedwater pressure is increased, this salt passage is increasingly overcome as water is pushed through the membrane at a faster rate than salt can be transported.

However, there is an upper limit to the amount of salt that can be excluded via increasing feedwater pressure. As the plateau in the salt rejection curve (Figure 2) indicates, above a certain pressure level, salt rejection no longer increases and some salt flow remains coupled with water flowing through the membrane.

Effect of temperature

As Figure 3 demonstrates, membrane productivity is very sensitive to changes in feedwater temperature. As water temperature increases, water flux increases almost linearly, due primarily to the higher diffusion rate of water through the membrane.

Increased feedwater temperature also results in lower salt rejection or higher salt passage. This is due to a higher diffusion rate for salt through the membrane.

The ability of a membrane to tolerate elevated temperatures increases operating latitude and is also important during cleaning operations because it permits use of stronger, faster cleaning processes. This is illustrated by the comparison of the pH and temperature ranges of thin-film composite membrane and a cellulose acetate (CA) membrane in Figure 4.

Effect of salt concentration

Osmotic pressure is a function of the type and concentration of salts or organics contained in feedwater. As salt concentration increases, so does osmotic pressure. The amount of feedwater driving pressure necessary to reverse the natural direction of osmotic flow is, therefore, largely determined by the level of salts in the feedwater.

Figure 5 demonstrates that, if feed pressure remains constant, higher salt concentration results in lower membrane water flux. The increasing osmotic pressure offsets the feedwater driving pressure. Also illustrated in Figure 5 is the increase in salt passage through the membrane (decrease in rejection) as the water flux declines.

Effect of recovery

As shown in Figure 1, reverse osmosis occurs when the natural osmotic flow between a dilute solution and a concentrated solution is reversed through application of feedwater pressure. If percentage recovery is increased (and feedwater pressure remains constant), the salts in the residual feed become more concentrated and the natural osmotic pressure will increase until it is as high as the applied feed pressure. This can negate the driving effect of feed pressure, slowing or halting the reverse osmosis process and causing permeate flux and salt rejection to decrease and even stop (please see Figure 6).

The maximum percent recovery possible in any RO system usually depends not on a limiting osmotic pressure, but on the concentration of salts present in the feedwater and their tendency to precipitate on the membrane surface as mineral scale. The most common sparingly soluble salts are calcium carbonate (limestone), calcium sulfate (gypsum), and silica. Chemical treatment of feedwater can be used to inhibit mineral scaling.

Effect of pH

The pH tolerance of various types of RO membranes can vary widely. Thin-film composite membranes s are typically stable over a broader pH range than cellulose acetate (CA) membranes and, therefore, offer greater operating latitude (please see Figure 4).

Membrane salt rejection performance depends on pH. Water flux may also be affected. Figure 7 shows that water flux and salt rejection for membranes are essentially stable over a broad pH range.

As illustrated in Figure 4, the stability of membrane over a broad pH range permits stronger, faster, and more effective cleaning procedures to be used compared to CA membranes.


RO Membrane

Reverse Osmosis Membrane Specifications

Features

Reverse osmosis membrane gives excellent performance for a wide variety of applications including low-pressure tapwater purification, single-pass seawater desalination, chemical processing, and waste treatment. This membrane exhibits high rejection at low pressures with very stable long-term operation.

Solute MW Rejection (%)
Sodium fluoride NaF1 42 99
Sodium chloride NaCl 58 99
Silica SiO2 (50 ppm) 60 60
Sodium bicarbonate NaHCO3 84 99
Sodium nitrate NaNO3 85 97
Magnesium chloride MgCl2 95 99
Calcium chloride CaCl2 111 99
Magnesium sulfate MgSO4 120 >99
Nickel sulfate NiSO4 155 >99
Copper sulfate CuSO4 160 >90
Formaldehyde 30 35
Methanol 32 25
Ethanol 46 70
Isopropanol 60 90
Urea 60 97
Lactic acid (pH 2) 90 94
Lactic acid (pH 5) 90 99
Lactic acid (pH 5) 90 99
Glucose 180 98
Sucrose 342 99
Chlorinated pesticides (traces) >99
  1. Solute rejection (approximate) 2,000 ppm solute, 225 psi (1.6 MPa), 77°F (25°C), pH 7 (unless otherwise noted).
  2. Fluoride rejection is strongly pH dependent (about 75% at pH 5, 50% at pH 4, 30% at pH 3.5 and 0% below pH 3).
  3. FT30 membrane is available in a wide variety of spiral-wound configurations.

Operating Limits

Membrane typeThin-film composite polyamide
Maximum operating pressure 1,000 psi (6.9 MPa)
Maximum operating temperature113°F (45°C)
Free chlorine tolerance < 0.1 ppm
pH range, continuous operation2 - 11
pH range, short-term cleaning (30 min.)1 - 13

RO Membrane

Reverse Osmosis Membrane Cleaning

Introduction

The sanitization of RO/NF membrane systems as described in this chapter is the application of biocidally effective solutions or hot water to the membranes while the system is offline, i.e. not in production mode. The online dosage of biocidal chemicals while the system is in production mode is dealt with in Biological Fouling Prevention

Membrane systems are sanitized in order to keep the number of living microorganisms at an acceptably low level. There are two main reasons why sanitization is required:

  • Smooth operation. Microorganisms may grow into a biofilm at the membrane and feed spacer surface and cause biofouling. Biofouling is a major threat to system operation, and regular sanitization is part of a strategy to control biofouling. Regular sanitization helps to keep the level of biological growth low enough to avoid operational problems. In RO systems operating with biologically active feed water, a biofilm can appear within 3–5 days after inoculation with viable organisms. Consequently, the most common frequency of sanitization is every 3–5 days during peak biological activity (summer) and about every 7 days during low biological activity (winter). The optimal frequency for sanitization will be site-specific and must be determined by the operating characteristics of the RO system.

  • Smooth operation. Microorganisms may grow into a biofilm at the membrane and feed spacer surface and cause biofouling. Biofouling is a major threat to system operation, and regular sanitization is part of a strategy to control biofouling. Regular sanitization helps to keep the level of biological growth low enough to avoid operational problems. In RO systems operating with biologically active feed water, a biofilm can appear within 3–5 days after inoculation with viable organisms. Consequently, the most common frequency of sanitization is every 3–5 days during peak biological activity (summer) and about every 7 days during low biological activity (winter). The optimal frequency for sanitization will be site-specific and must be determined by the operating characteristics of the RO system.


Softener

Sources of Hardness Minerals in Drinking Water

Water is a good solvent and picks up impurities easily. Pure water -- tasteless, colorless, and odorless -- is often called the universal solvent. When water is combined with carbon dioxide to form very weak carbonic acid, an even better solvent results. As water moves through soil and rock, it dissolves very small amounts of minerals and holds them in solution. Calcium and magnesium dissolved in water are the two most common minerals that make water "hard." The degree of hardness becomes greater as the calcium and magnesium content increases and is related to the concentration of multivalent cations dissolved in the water..

Every household and every factory uses water, and none of it is pure.

One class of impurity that is of special interest is "hardness". This refers to the presence of dissolved ions, mainly of calcium Ca 2+ and magnesium Mg 2+ which are acquired through contact with rocks and sediments in the environment. The positive electrical charges of these ions are balanced by the presence of anions (negative ions), of which bicarbonate HCO 3 – and carbonate CO 3 2– are most important. These ions have their origins in limestone sediments and also from carbon dioxide which is present in all waters exposed to the atmosphere and especially in groundwaters.

Indications of Hard Water

Hard water interferes with almost every cleaning task from laundering and dishwashing to bathing and personal grooming. Clothes laundered in hard water may look dingy and feel harsh and scratchy. Dishes and glasses may be spotted when dry. Hard water may cause a film on glass shower doors, shower walls, bathtubs, sinks, faucets, etc. Hair washed in hard water may feel sticky and look dull. Water flow may be reduced by deposits in pipes.

Dealing with hard water problems in the home can be a nuisance. The amount of hardness minerals in water affects the amount of soap and detergent necessary for cleaning. Soap used in hard water combines with the minerals to form a sticky soap curd. Some synthetic detergents are less effective in hard water because the active ingredient is partially inactivated by hardness, even though it stays dissolved. Bathing with soap in hard water leaves a film of sticky soap curd on the skin. The film may prevent removal of soil and bacteria. Soap curd interferes with the return of skin to its normal, slightly acid condition, and may lead to irritation. Soap curd on hair may make it dull, lifeless and difficult to manage.

When doing laundry in hard water, soap curds lodge in fabric during washing to make fabric stiff and rough. Incomplete soil removal from laundry causes graying of white fabric and the loss of brightness in colors. A sour odor can develop in clothes. Continuous laundering in hard water can shorten the life of clothes. In addition, soap curds can deposit on dishes, bathtubs and showers, and all water fixtures.

Hard water also contributes to inefficient and costly operation of water-using appliances. Heated hard water forms a scale of calcium and magnesium minerals that can contribute to the inefficient operation or failure of water-using appliances. Pipes can become clogged with scale that reduces water flow and ultimately requires pipe replacement.

Potential Health Effects

Hard water is not a health hazard. In fact, the National Research Council (National Academy of Sciences) states that hard drinking water generally contributes a small amount toward total calcium and magnesium human dietary needs. They further state that in some instances, where dissolved calcium and magnesium are very high, water could be a major contributor of calcium and magnesium to the diet.

Researchers have studied water hardness and cardiovascular disease mortality. Such studies have been "epidemiological studies," which are statistical relationship studies.

While some studies suggest a correlation between hard water and lower cardiovascular disease mortality, other studies do not suggest a correlation. The National Research Council states that results at this time are inconclusive and recommends that further studies should be conducted.

Testing

If you are on a municipal water system, the water supplier can tell you the hardness level of the water they deliver. If you have a private water supply, you can have the water tested for hardness. Most water testing laboratories offer hardness tests for a fee, including the Environmental Quality Center. Also many companies that sell water treatment equipment offer hardness tests. When using these water tests, be certain you understand the nature of the test, the water condition being measured, and the significance of the test results. An approximate estimate of water hardness can be obtained without the aid of outside testing facilities. Water hardness testing kits are available for purchase through water testing supply companies. If more accurate measurements are needed, contact a testing laboratory.

Interpreting Test Results

The hardness of your water will be reported in grains per gallon, milligrams per liter (mg/l) or parts per million (ppm). One grain of hardness equals 17.1 mg/l or ppm of hardness.

The Environmental Protection Agency (EPA) establishes standards for drinking water which fall into two categories -- Primary Standards and Secondary Standards.

Primary Standards are based on health considerations and Secondary Standards are based on taste, odor, color, corrosivity, foaming, and staining properties of water. There is no Primary or Secondary standard for water hardness. Water hardness is classified by the U.S. Department of Interior and the Water Quality Association as follows:

Classification mg/l or ppm grains/gal
Soft 0 - 17.1 0 - 1
Slightly hard 17.1 - 60 1 - 3.5
Hard 120 - 180 7.0 - 10.5
Very Hard 180 & over 10.5 & over

NOTE:

Other organizations may use slightly different classifications.

Options

There are two ways to help control water hardness: use a packaged water softener or use a mechanical water softening unit.

Packaged water softeners are chemicals that help control water hardness. They fall into two categories: precipitating and non-precipitating.

Precipitating water softeners

include washing soda and borax. These products form an insoluble precipitate with calcium and magnesium ions. The mineral ions then cannot interfere with cleaning efficiency, but the precipitate makes water cloudy and can build up on surfaces. Precipitating water softeners increase alkalinity of the cleaning solution and this may damage skin and other materials being cleaned.

Non-precipitating water softeners

use complex phosphates to sequester calcium and magnesium ions. There is no precipitate to form deposits and alkalinity is not increased. If used in enough quantity, non-precipitating water softeners will help dissolve soap curd for a period of time.

Mechanical water softening units can be permanently installed into the plumbing system to continuously remove calcium and magnesium. Water softeners operate on the ion exchange process. In this process, water passes through a media bed, usually sulfonated polystyrene beads. The beads are supersaturated with sodium. The ion exchange process takes place as hard water passes through the softening material. The hardness minerals attach themselves to the resin beads while sodium on the resin beads is released simultaneously into the water. When the resin becomes saturated with calcium and magnesium, it must be recharged. The recharging is done by passing a salt (brine) solution through the resin. The sodium replaces the calcium and magnesium which are discharged in the waste water. Hard water treated with an ion exchange water softener has sodium added. According to the Water Quality Association (WQA), the ion exchange softening process adds sodium at the rate of about 8 mg/liter for each grain of hardness removed per gallon of water.

For example

if the water has a hardness of 10 grains per gallon, it will contain about 80 mg/liter of sodium after being softened in an ion exchange water softener if all hardness minerals are removed.

Because of the sodium content of softened water, some individuals may be advised by their physician, not to install water softeners, to soften only hot water or to bypass the water softener with a cold water line to provide unsoftened water for drinking and cooking; usually to a separate faucet at the kitchen sink.

Softened water is not recommended for watering plants, lawns, and gardens due to its sodium content.

Although not commonly used, potassium chloride can be used to create the salt brine. In that case potassium rather than sodium is exchanged with calcium and magnesium.

Before selecting a mechanical water softener, test water for hardness and iron content. When selecting a water softener, the regeneration control system, the hardness removal capacity and the iron limitations are three important elements to consider.

There are three common regeneration control systems. These include a time-clock control (you program the clock to regenerate on a fixed schedule); water meter control (regenerates after a fixed amount of water has passed through the softener); and hardness sensor control (sensor detects hardness of the water leaving the unit, and signals softener when regeneration is needed).

Hardness removal capacity, between regenerations, will vary with units. Softeners with small capacities must regenerate more often. Your daily softening need depends on the amount of water used daily in your household and the hardness of your water. To determine your daily hardness removal need, multiply daily household water use (measured in gallons) by the hardness of the water (measured in grains per gallon).

Example:

400 gallons used per day X 15 grains per gallon hardness = 6,000 grains of hardness must be removed daily.

Iron removal limitations will vary with water softener units. If the iron level in your water exceeds the maximum iron removal capacity recommended by the manufacturer of the unit you are considering, iron may foul the softener, eventually causing it to become plugged.

How Softeners Work

Below is a diagram of our Softener. It shows how our softener, and everyone else’s, works. A water softener is an ion exchanger.

Hard water—water with a high calcium/magnesium content—enters the softener through the “In” port indicated by the green arrow. It passes through the control valve and into the tank, where it goes from top to bottom through a specially prepared resin that “softens” it.

The resin consists of specially manufactured beads that have been saturated with sodium ions. “Softening” occurs as the hardness minerals in the water attach themselves to the resin and are “exchanged” for sodium.

The softened water then enters the long center tube, called a riser, via the strainer basket in the bottom of the tank and passes upward through the riser. The water exits the softener via the control valve (blue arrow) and is sent to the home.

When the resin becomes saturated by hardness minerals, the softener automatically goes into regeneration. (The regeneration process is initiated by a timer or a meter, depending on the type of softener you purchase.) By this process the hardness minerals are washed down the drain (via a drain tube not shown in the diagram), and the resin bed is rinsed, resettled, and recharged with sodium. It is now again ready to soften your water.

The regeneration process is accomplished by passing very salty water from the brine tank through the resin.

The brine tank must remain filled with softener salt at all times so that it can regenerate the softening resin again and again

Water Softeners Make Water Work Better

Water softeners combat this nuisance by eliminating the minerals that cause hard water. The most common kind of water softener is a mechanical appliance plumbed directly into the home’s water supply intake. (See figure 1) The water softener exchanges calcium and magnesium with sodium in a process called ion exchange.

The water softening system consists of a mineral tank and a brine tank. The water supply pipe is connected to the mineral tank so that water coming into the house must pass through the tank before it can be used.

The mineral tank holds small beads (also known as resin) that carry a negative electrical charge. The positively charged calcium and magnesium (called ions) are attracted to the negatively charged beads. This attraction makes the minerals stick to the beads as the hard water passes through the mineral tank. (See figure 3.)

Eventually the surfaces of the beads in the mineral tank become coated with the calcium and magnesium minerals. To clean the beads, a strong sodium (salt) solution held in the brine tank is flushed through the mineral tank. Sodium ions also have a positive electrical charge, just not quite as strong as that of calcium and magnesium. This large volume of sodium ions overpowers the calcium and magnesium ions and drives them off of the beads and into the solution. The sodium solution carrying the minerals is then drained out of the unit. Some sodium ions remain in the tank attached to the surfaces of the beads

Conventional water softening

Most conventional water-softening devices depend on a process known as ion-exchange in which "hardness" ions trade places with sodium and chloride ions that are loosely bound to an ion-exchange resin or a zeolite (many zeolite minerals occur in nature, but specialized ones are often made artificially.)

The illustration depicts a negatively-charged zeolite to which [positive] sodium ions are attached. Calcium or magnesium ions in the water displace sodium ions, which are released into the water. In a similar way, positively-charged zeolites bind negatively-charged chloride ions (Cl –), which get displaced by bicarbonate ions in the water. As the zeolites become converted to their Ca 2+ and HCO 3 – forms they gradually lose their effectiveness and must be regenerated. This is accomplished by passing a concentrated brine solution though them, causing the above reaction to be reversed. Herein lies one of the drawbacks of this process: most of the salt employed in the regeneration process gets flushed out of the system and and is usually released into the soil or drainage system— something that can have damaging consequences to the environment, especially in arid regions. For this reason, many jurisdications prohibit such release, and require users to dispose of the spent brine at an approved site or to use a commercial service company.

The Softening Process

The normal water softening cycle operates like this: Hard water enters the mineral tank. Inside the tank, the calcium and magnesium ions carried in the water attach themselves to the beads. The surfaces of the beads eventually hold their limit of calcium and magnesium and can’t remove any more from the water. At this point the water softener must be "regenerated". (See figure 2.) The three-step regeneration cycle can be scheduled according to a timer or by a flow detection meter.

The first step, called the backwash phase, reverses the water’s flow and flushes any accumulated dirt particles out of the tank and down the drain. Next, in the regeneration or recharge phase, the sodium rich brine solution flows from the brine tank into and through the mineral tank. The brine washes the calcium and magnesium off the beads. In the final phase, the mineral tank is flushed of the excess brine, which now also holds the calcium and magnesium, and the solution is disposed of down the drain.

Sodium ions from the previous regeneration cycle cling to the beads. Now when the hard water flows into the mineral tank, the calcium and magnesium ions change places with the sodium ions on the resin. The displaced sodium ions remain dissolved in the water.


Iron Removal

Sources of Hardness Minerals in Drinking Water

Iron is an objectionable constituent of a drinking water. Appreciale amounts of Iron in water impart a bitter characteristic, metallic taste and cause oxdized precipitate.Coloration of water which may be yellowish brown to reddish brown and renders the water objectionable or unsuitable for domestic purpose. In addition Iron stain everything with which it come in contact. In hotels,hospitals, clubs, Institutions, office buildings and homes, Iron-bearing water stain wash basins, toilets, urinals, bath tubs, showers, tiled floors and walls.Iron tolerances for municipal or house hold use should not exceed 0.3 ppm. Concentration of Iron in excess of 0.2 to 0.3 mg/l may cause nuisance even though its presence does not affect the hygienic quality of water.

Source and Nature

Iron exists in water in two levels.One as the bi-valent , Ferrous Iron ( Fe ++) and the second one as the tri-valent, Ferric Iron (Fe+++). The Ferric Iron generally occuring in the precipitated form. Iron forms complexes of hydroxides and other in-organic complexes in solution with substantial amounts of bi-carbonate, sulphate, Phosphate, Cyanide or Halides. Presence of organic substances induces the formation of organic complexes which increase the solubility of Iron. The waters of high alkalinity have lower iron than waters of low alkalinity.

The commonest form in which Iron is found in water supplies is as Ferrous bi-carbonate which is a soluble, colourless salt and exists only in solution. Its solubility is increased by increasing the free Carbon-di-oxide content of the water. The unaerated water is clear and colourless. It develops a slight whitish haze, which on longer standing turns yellowish and then forms yellowish brown to reddish brown deposits of hydrated Ferric oxide after aeration..

Removal of Iron

Oxidation by aeration or use of chemicals like chlorine, chlorine di-oxide or potassium permanganate followed by filtration alone or by settling and filtration can bring about the precipitation of iron and its removal. Use of zeolites as well as catalytic oxidation also serve the purpose.

Precipitation

Iron in water in the reduced form is converted to ferric compound by oxidation and these are removed by filtration alone or by sedimentation and filtration. The reaction period is about 5 minutes or less at a pH of 7.0 to 7.5 and 0.14 mg of oxygen is needed to convert 1 mg ferrous iron to ferric hydroxide as indicated below:

4 Fe 2+ + O 2 + 10 H 2O 4 Fe ( OH) 3 + 8 H +
                              4x56 mg Fe 2 = 2x 16 mg O 2

The rate of oxidation of ferrous iron by aeration is slow under conditions of low pH and is fast under high pH conditions. Rate of precipitation and filtration are accelerated in practice by contact and catalysis. Water is allowed to trickle over coke or crushed stone. The deposition of hydrated oxides of iron and bacteria on the contact media is believed to act as catalysis which accelerate the oxidation of Iron.

Simple Technique for Iron removal

A simple and inexpensive treatment unit for the removal of iron is suggested so that the difficulties of operation and maintenance can also be minimized.

When the source is a well or a sump and the water consumption rate is in the order of 40 lpcd and when hand pump is used, a tray type aerator with two trays operated at an aeration rate of 1.26 m 3/m 2/hr are employed and the water aerated. Then the water is settled in a sedimentation basin having a detention period of 3 hours and the clarified water passed through a sand filter having a depth of 0.3 m supported by gravel 3-6 mm in size 0.1 m deep. The effective size of the sand is 0.30-0.45 mm and its uniformity coefficient 2-3.Sand is cleaned by manual scraping.

Iron Removal: A World Without Rules

Iron can often be detected visibly in water or by staining on plumbing fixtures.

There is one rule to keep in mind when selecting a method for iron removal — and that is there is no rule. You will find — as with all problem water applications — the solution is 50 percent science and 50 percent experience.

The following information describing the different types of iron removal process applications are the basics. Before using any of these applications, it’s good to have an understanding of the type of iron present; the equipment and its limitations; and the product and processes involved with method.

Equipment

Care must be taken when considering iron removal advice from different regions of the country as water temperature, pH, alkalinity, dissolved oxygen content and other factors will affect the actual results.

Most application failures are caused simply by not selecting the right equipment for the water conditions present. It is important to follow manufacturer’s guidelines regarding flow rates, backwash rates, pH levels, maximum iron input levels, water temperatures and any other application limitations that the manufacturer has noted in order for the equipment and media to deliver their best result as designed.

Water filter

Most iron filtration systems operate on the principal of oxidizing the iron (oxidation) to convert it from a ferrous (dissolved or soluble) to a ferric or undissolved state. Once in the ferric state, iron can be filtered.

Water filters are the most widely used equipment in removing iron. Its popularity comes from its versatility due to the various media products available and the process involved with each media.

The most common reasons for filter failure are a lack of flow in backwash or a lack of frequency of regenerations. Low pH levels when using filters are another reason for unsatisfactory results.

Water softener

Water softeners exchange ions by design. When used in iron removal, the softener uses a cation resin to exchange iron for sodium, in addition to the calcium and magnesium exchanged for sodium in the softening process.

Softeners are commonly used in removing low levels of ferrous iron (1-3 ppm), though it is not uncommon to remove 10 or more ppm depending on water conditions and control settings.

The last thing a water softener needs is for the ferrous iron to oxidize and convert to a ferric state. Since pH plays a big part in how quickly this conversion takes place, it is important to note that softeners perform better on low pH, which will also prolong bed life.

In the ferric state, iron will coat the resin, plugging the exchange sites and fouling the resin. Iron fouling will eventually happen in any iron application and requires replacement of the media.

High saltings, longer backwashes, frequent regenerations and the use of iron cleaners are keys to longer bed life. However, even after taking these steps to prevent the bed from fouling, the resin will eventually succumb to the iron and require replacement.

Media selection

Each type of treatment has its own strengths and weaknesses. As in the selection of equipment, it is important to follow manufacturers’ recommendations and note any application limitations such as water temperature, pH alkalinity and dissolved oxygen content to get the best result.

To do this, water treatment professionals need a clear understanding of all limitations of the product and equipment selected.

Filtration using various means of oxidation is the most common method of iron removal. Depending on the media selected, other common processes such as ozone, aeration, chlorine or peroxide injection may be used to boost the oxidizing properties of the water being treated.

Greensand

Greensand is one of the oldest but proven oxidation technologies. Potassium permanganate, itself an oxidizer, is used to regenerate the greensand.

In this application, potassium permanganate produces manganese dioxide on the surface of the mineral and — once the water comes in contact with it — any iron is immediately oxidized. The iron can be filtered and then cleaned away in the backwash cycle. Greensand is also effective with low levels of H 2S (hydrogen sulfide) and manganese.

Synthetic greensand is a granular mineral with a manganese dioxide coating having the same ability as regular greensand. It is much lighter and requires less of a backwash rate than standard greensand.

Manganese dioxide

Manganese dioxide is a naturally mined ore with the ability to remove iron, manganese and hydrogen sulfide. The hydrogen sulfide capability exceeds that of either greensand or synthetic greensand and requires no chemicals to regenerate.

It does, however, require adequate amounts of dissolved oxygen in the water as a catalyst and may require some type of pre-oxidation to achieve its maximum ability.

Birm

Birm has the ability to remove iron and manganese and has no effect on hydrogen sulfide. Like manganese dioxide, birm also uses dissolved oxygen as a catalyst and may require some type of pre-oxidation in cases where the dissolved oxygen content is too low to affect a maximum iron removal result.

Redox

Redox media, which requires adequate dissolved oxygen to be effective, consists of two metals - 85 percent copper and 15 percent zinc. These two dissimilar metals create a small electrical field in the bed that will not allow bacterial growth in the media.

This property earns redox the unique distinction of being effective on bacterial iron without the use of chlorine injection and being rated as bacterial static.

Effective on removal of iron and hydrogen sulfide, able to reduce chlorine and heavy metals such as lead and mercury, redox is not effective with manganese.

The biggest drawback for this media is its weight. Being almost twice as heavy as other minerals, it requires more than twice the backwash rate of other minerals. Sizing mineral tanks is crucial.

Catalysts & Considerations

Once you have identified the enemy and selected the equipment with compatible backwash and flow rates for the media selected, the water itself must be scrutinized.

Check for dissolved oxygen and pH levels and determine what, if any, pre-treatment is necessary for the selected application to deliver maximum iron removal efficiency.

What is the role of pH?

The pH of a given water source plays an important role in how quickly ferrous (dissolved) iron converts to a ferric (solid) state. The higher the pH, the faster iron will convert to the ferric state that can then be filtered.

This is good in all equipment selections with the exception of a water softener where the ferric iron plugs the exchange sites and fouls the resin.

When using an iron filter a pH above 6.5 is necessary for iron to properly convert and is the recommendation of most manufacturers. However, most experienced water treatment professionals agree that a pH above 7.0 is a must and an 8.0 to 8.5 pH greatly enhances the chance of a successful application.

If it is necessary to increase the pH level, chemical feed of either sodium carbonate (soda ash) or sodium hydroxide (caustic soda) is preferred over a filter filled with calcium carbonate or magnesium oxide, as the filter method may foul quickly

Pre-oxidation

Most chemical-free iron filters and several chemical filter media require some dissolved oxygen in the water to act as a catalyst. Pre-oxidation is required in cases where the dissolved oxygen content is too low.

Pre-oxidation can come from aeration, chlorine or peroxide injection, ozone and other methods.

Chemical feed

There are several types of chemical feed applications. Using sodium carbonate or sodium hydroxide to raise pH is common. Using 5 percent to 10 percent chlorine or 7 percent hydrogen peroxide as oxidizers to the water before a filter is also widely used.

Different rules apply to each of these methods, from retention or contact tanks to using static mixers. When using different chemicals together, it’s important to understand the compatibility of the chemicals and the safety considerations.

For greater success, follow the manufacturers’ recommendations closely regarding proper feed rates and installation when injecting chemicals.

Aeration

When aeration is used as a pre-oxidizer it is generally done with either an air inductor or an air pump.

An air inductor is a venturi installed inline. The water flowing through the inductor creates a vacuum and sucks air into the water line. The faster the water flows, the more air induced into the water.

Watch for pressure drop and perform routine maintenance of the inductor, as they will clog with iron over time.

The air pump method allows more air induced into the water, as a mechanical pump is used to force air into the water. A contact tank is often used.

This method has proven effective with the only cautions being maintenance to the pump and injection fittings.


Water Quality

Sources of Hardness Minerals in Drinking Water

Water quality is the physical, chemical and biological characteristics of water, characterized through the methods of hydrometry. The primary bases for such characterization are parameters which relate to drinking water, safety of human contact and for health of ecosystems. The vast majority of surface water on the planet is neither potable nor toxic. This remains true even if sea water in the oceans (which is too salty to drink) isn't counted. Another general perception of water quality is that of a simple property that tells whether water is water pollution or not. In fact, water quality is a very complex subject, in part because water is a complex medium intrinsically tied to the ecology of the Earth. Industrial pollution is a major cause of water pollution, as well as runoff from agricultural areas, urban stormwater runoff and discharge of untreated sewage (especially in developing countries).

Overview

Contaminants that may be in untreated water include microorganisms such as viruses and bacteria; inorganic contaminants such as salts and metals; pesticides and herbicides; organic chemical contaminants from industrial processes and petroleum use; and radioactive contaminants. Water quality depends on the local geology and ecosystem, as well as human uses such as sewage dispersion, industrial pollution, use of water bodies as a heat sink, and overuse (which may lower the level of the water).

The Environmental Protection Agency prescribes regulations that limit the amount of certain contaminants in the water provided by public water systems for tap water. Food and Drug Administration (FDA) regulations establish limits for contaminants in bottled water that must provide the same protection for public health. Drinking water, including bottled water, may reasonably be expected to contain at least small amounts of some contaminants. The presence of these contaminants does not necessarily indicate that the water poses a health risk.

Some people use water purification technology to remove contaminants from the municipal water supply they get in their homes, or from local pumps or bodies of water. For people who get water from a local stream, lake, or aquifer, their drinking water is not filtered by the local government.

Toxic substances and high populations of certain microorganisms can present a health hazard for non-drinking purposes such as irrigation, swimming, fishing, rafting, boating, and industrial uses. These conditions may also impact wildlife which use the water for drinking or as a Habitat.

Interest by individuals and volunteer groups in making local water quality observations is high, and an understanding of the basic chemistry of many water quality parameters is an essential first step to making good measurements. Most citizens harbor great concern over the purity of their drinking water, but there is far more to water quality than water treatment for human consumption.

Statements to the effect that "uses must be preserved" are included within water quality regulations because they provide for broad interpretation of water quality results, while preserving the ultimate goal of the regulations. Technical measures of water quality—that is, the values obtained when making water quality measurements—are always subject to interpretation from multiple perspectives. Is it reasonable to expect a river to be pristine in a landscape that no longer is? If a river has always carried sediment, is it polluted even if the cause is not man induced? Can water quality be maintained when water quantity can not? The questions that arise from consideration of water quality relative to human uses of the water become more complex when consideration must also be given to conditions required to sustain aquatic biota. Yet inherent in the concept of preserving uses is a mandate that waterways must be much more than conduits for a fluid we might want to drink, fill our swimming pool with, or carry our wastes out of town.

Measurement of water quality

The complexity of water quality as a subject is reflected in the many types of measurements of water and Wastewater quality indicators. These measurements include (from simple and basic to more complex):

  • Electrical conductivity|Conductivity (also see salinity)
  • Dissolved Oxygen
  • pH
  • Color of water
  • Taste and odor (geosmin, 2-methylisoborneol (MIB), etc)
  • Turbidity
  • Total suspended solids (TSS)
  • Dissolved metals and salts (sodium, chloride, potassium, calcium, manganese, magnesium)
  • Chemical oxygen demand (COD)
  • Biochemical oxygen demand (BOD)
  • Microorganisms such as fecal coliform bacteria (Escherichia coli), Cryptosporidium, and Giardia lamblia
  • Nutrients, such as nitrogen and phosphorus
  • Dissolved metals and metalloids (lead, Mercury (element),arsenic, etc.)
  • Dissolved organics: Colored Dissolved Organic Matter (CDOM), Dissolved Organic Carbon (DOC)
  • Temperature
  • Pesticides
  • Heavy Metals
  • Pharmaceuticals
  • Hormone analogs

Some of the simple measurements listed above can be made on-site (temperature, pH, dissolved oxygen, conductivity), in direct contact with the water source in question. More complex measurements that must be made in a lab setting require a water sample to be collected, preserved, and analyzed at another location. Making these complex measurements can be expensive.

Because direct measurements of water quality can be expensive, ongoing monitoring programs are typical conducted by government agencies. Individuals interested in monitoring water quality who cannot afford or manage lab scale analysis can also use biological indicators to get a general reading of water quality. Biological monitoring metrics have been developed in many places, and one widely used measure is the presence and abundance of members of the insect orders Ephemeroptera, Plecoptera and Trichoptera (EPT). EPT indexes will naturally vary from region to region, but generally, within a region, the greater the number of taxa from these orders, the better the water quality. A number of websites originating in the United States offer guidance on developing a monitoring program and identifying members of these and other aquatic insect orders.