In this article we will discuss about the methods and procedure to purify water. Learn about:- 1. Introduction to Water Purification 2. Water Treatment Processes 3. Softening 4. Aeration 5. Coagulation and Flocculation 6. Filtration 7. Ion – Exchange and Carbon Adsorption.
- Introduction to Water Purification
- Water Treatment Processes
- Coagulation and Flocculation
- Ion – Exchange and Carbon Adsorption
1. Introduction to Water Purification:
An adequate supply of pure water is absolutely essential to human existence. The consequences of a contaminated water supply can be illustrated by conditions prevalent during the industrial revolution in Europe when large numbers of peasants were attracted to the cities where they crowded together with little or no sanitary facilities.
Human waste or ‘night soil’ as it was called, was tossed into the streets or emptied into pits in common courtyards, often near the shallow wells that served as the neighbourhood water supply. Seepage into these wells and runoff into nearby streams provided a direct link in the infection cycle and once an outbreak of disease occurred it usually spread rapidly through the community.
The development of effective water-treatment methods has virtually eliminated major waterborne epidemics in developed countries. This is not to suggest, however, that the problem of waterborne diseases has been eliminated.
Developing nations, where treated water is not available to all the population, still experience occasional epidemics of cholera and typhoid, as well as many outbreaks of less severe disease. Even highly developed countries, including the United States, where public water supplies are almost universally treated, are not totally immune from an occasional outbreak of gastrointestinal illnesses traceable to biologically contaminated water supplies.
Chemical contamination of water supplies has become a concern in more recent times. Industrial facilities in developed countries produce and use literally thousands of chemical compounds. Along with an abundant array of household and agricultural chemicals, these materials often find their way into water supplies.
While some of these chemical compounds are known toxicants, mutagents or carcinogens, the health effects of many others are not presently known. It is ironic that the high standard of living that allows industrialised nations to provide biologically pure water to the majority of the population also results in the discharge of chemical waste that may eventually have more deleterious effects on human health than the domestic waste that helped spread the plagues of past centuries.
The treatment of water intended for human consumption is a very old practice. Baker reports references in Sanskrit literature dating back to 2000 BC to such practices as the boiling and filtering of drinking water. Wick siphons that transferred water from one vessel to another, filtering out the suspended impurities in the process, were pictured in Egyptian drawings of the thirteenth century BC and were referred to in early Greek and Roman literature.
The fact that these practices were recorded in the medical documents of the times indicates that the connection between water and health had been observed. In fact, Hippocrates (460-354 BC), considered to be the father of modern medicine, wrote that ‘whosoever wishes to investigate medicine properly should—consider the water that the inhabitants use—for water contributes much to health.’
These early water-treatment devices were used in individual households; there is no indication of community water supplies being treated until around the first century. Some of the Roman aqueducts had settling basins at the headworks and incorporated ‘pebble catchers’ in the aqueduct channel.
These aqueducts supplied a few private taps and provided fountains or reservoirs for use by the general public. The city of Venice, situated on islands with no freshwater resource, channelled rain water from roofs and courtyards into elaborate cisterns through sand filters surrounding the reservoir. The first of these cisterns was built around the fifth century AD and provided private and public water supplies for about 13 centuries.
Water-treatment practice apparently lagged during the Middle Ages, with a renewed interest emerging in the eighteenth century. Several patents were issued for filtering devices, primarily in France and England. As in ancient times, however, these devices were for use in private households, institutions, ships, etc.
It was not until the beginning of the nineteenth century that the treatment of public water supplies was attempted on a large scale. The city of Paisley, Scotland, is generally credited with being the first city with a treated water supply.
That system consisted of settling operations followed by filtration and was put in service in 1804. This practice slowly spread through Europe and by the end of the century, most major municipal water supplies were filtered. These filters were the ‘slow sand’ type.
The development of water treatment in America lagged behind the European practice. The first attempt at filtration was made at Richmond, Virginia, in 1932. This project was a failure and several years intervened before another significant effort was made.
After the Civil War, other attempts were made to follow the sand filtration practice of Europe, few of which were successful. Apparently the nature of the suspended solids in American streams was significantly different from that of the solids in European streams and the slow sand process was not as effective.
The development of the hydraulically cleaned rapid sand filter during the latter part of the nineteenth century provided a more workable process and by the end of the century its use was widespread.
During the first two-thirds of the nineteenth century, filtration was practiced to improve the aesthetic quality of the drinking water. An unknown benefit was the removal of micro-organisms, including pathogens, which made the water more wholesome as well.
The acceptance of this fact in the last quarter of the century spurred the construction of the filter plants throughout Europe and America. At the turn of the century, filtration was the primary defense against waterborne disease.
Acceptance of the germ theory of disease transmittal led to the disinfection of public water supplies. First used on a temporary basis, disinfection with bleach powders and hypochlorites was used in isolated cases in the eighteen-nineties. The first permanent installation for chlorinating water was made in Belgium in 1902.
The production of liquid chlorine began in 1909 and was first used for water disinfection in Philadelphia in 1913. Other means of disinfection, notably ozonation, were developed simultaneously but did not find widespread use. The drastic reduction in deaths due to waterborne diseases as a result of disinfection led to the widespread chlorination of public water supplies.
Other water-treatment processes developed more slowly and less dramatically. Coagulation as an adjunct to settling was developed along with the rapid sand filter in America. Softening of hard waters was demonstrated in Europe during the nineteenth century but did not find widespread use in public water supplies until well into the twentieth century.
The capacity of charcoal to remove dissolved organics was observed by early experimenters in filtration but did not find application in public water supplies. The improvement of this material into activated carbon and its use in water-treatment plants is a recent occurrence, as is the use of synthetic membranes for hyper filtration to remove dissolved inorganic material.
More progress has been made in water purification in the last century than in all of the previously recorded history. With few exceptions, treatment processes developed in the absence of scientific knowledge concerning the basic principles upon which they operate and often with little means to quantitatively assess their effectiveness.
Only within the last 30 to 40 years has the body of scientific knowledge caught up with the practice of water purification. It is interesting to note that the development of a theory base has resulted in few changes in the basic processes of water purification.
Understanding of scientific principles has, however, led to refinements of processes, development of better equipment and an overall increase in operating efficiencies in water treatment.
Past practices in US and other developing countries have often been to obtain the purest possible source, even at the expense of transporting water over long distances and to deliver it to the consumer with little or no treatment. Some cities still own large tracts of land near the headwaters of stream and restrict activities on these watersheds to minimise contamination.
Although the benefits of source protection are recognised as a ‘first line of defense’ in preserving water quality, all natural waters will require some degree of treatment in order to meet modern drinking-water standards. The nature and extent of treatment will, of course, depend upon the nature and extent of impurities.
The processes selected for the treatment of potable water depend on the quality of the raw water supply. Most ground waters are clear and pathogen-free and do not contain significant amounts of organic materials. Such waters may often be used in portable systems with a minimal dose of chlorine to prevent contamination in the distribution system.
Other groundwater may contain large quantities of dissolved solids or gases. When these include excessive amounts of iron, manganese or hardness, chemical and physical treatment processes may be required. Treatment systems commonly used to prepare potable water from groundwater.
Surface waters often contain a wider variety of contaminants than groundwater and treatment processes may be more complex. Most surface waters contain turbidity in excess of drinking-water standards. Although fast-moving streams may carry larger material in suspension, most of the solids will be colloidal in size and will require chemical coagulation for removal.
Depending on the geology of the watershed, hardness may or may not be a problem in surface waters. If low levels of colour and other organic material are present, adsorption onto surface-active material, a process not significant in natural water systems, may be necessary. A wide variety of micro-organisms, some of which may be pathogenic, are also common constituents of surface waters. Treatment systems commonly used in treating surface waters.
It is generally convenient to group human use of water into two broad categories depending upon the location of the use relative to the source. In-place use of water includes navigation, recreation, wildlife propagation and the dilution, assimilation and transportation of waste-water. Although hydroelectric power generation requires brief diversion of water through turbine penstocks, this use is also considered an in-place use.
For irrigation and industrial use and for individual and public domestic supplies, water must be withdrawn from streams, lakes or aquifers in the natural hydrologic cycle. The pollutants most deleterious to crops (inorganic salts and metals) are difficult and expensive to remove.
The vast quantity of irrigation water used and the low margin of profit associated with farming virtually preclude any treatment of this water. Water not suited for irrigation is simply abandoned and available capital is used instead to secure an alternate source of acceptable quality.
Many industries with needs for small amounts of essentially potable water obtain their supplies from public systems. Some industrial water supplies, such as boiler feed water, may require a chemical purity an order of magnitude greater than potable water.
Engineering design for treatment of other types of industrial water supplies may also be necessary. Cooling water, particularly that used only once and discharged back to nature, has few quality constraints.
Individual domestic supplies are usually drawn from wells or springs of acceptable quality and serve individual homes or farmsteads. Such systems are seldom engineered but are installed and operated by the home owners, perhaps with the advice of the well-driller and the distributor of home water-treatment units.
Public water supplies, while only a fraction of the total water use, require by far the largest amount of effort expended by environmental engineers in the water-treatment field.
Softening of water is the removal of bivalent calcium and magnesium ions (Ca+2, Mg+2). These ions come from the dissolved compounds of calcium and magnesium. Their presence is known as hardness of water. Hardness is defined as the concentration of multivalent metallic cations in solution.
At supersaturated conditions, the hardness cations will react with anions in the water to form a solid precipitate. Hardness is classified as carbonate hardness and non-carbonate hardness, depending upon the anion with which it associates. The hardness that is equivalent to the alkalinity is termed carbonate hardness with any remaining hardness being called non-carbonate hardness.
Carbonate hardness is sensitive to heat and precipitates readily at high temperatures.
The multivalent metallic ions most abundant in natural waters are calcium and magnesium. Others may include iron and manganese in their reduced states (Fe2+, Mn2+), strontium (Sr2+) and aluminum (Al3+). The latter are usually found in much smaller quantities than calcium and magnesium and for all practical purposes, hardness may be represented by the sum of the calcium and magnesium ions.
The reduction of hardness or softening, is a process commonly practised in water treatment. Softening may be done by the water utility at the treatment plant or by the consumer at the point of use, depending on the economics of the situation and the public desire for soft water.
Generally, softening of moderately hard water (50 to 150 mg/l hardness) is best left to the consumer, while harder water should be softened at the water-treatment plant. Softening processes commonly used are chemical precipitation and ion exchange, either of which may be employed at the utility-owned treatment plant. Home-use softeners are almost exclusively ion-exchange units.
Aeration occupies a significant place in waste-water quality management and is an important factor in the purification of polluted water. Gas transfer is a physical phenomena in which gas molecules are exchanged between a liquid and a gas at a gas-liquid interface.
This physical phenomenon of gas molecules exchanged between the liquid and gas at the liquid-gas interface may also be accompanied by biological, biochemical, biophysical and chemical action. These results are often the primary purpose of the gas transfer operation and methods of achieving the desired results may vary. Principal objectives of aeration, however, usually add or remove gases or volatile substances to water or carry out both objectives simultaneously.
In the biological process, aerators function to transfer the required oxygen and include sufficient mixing to maintain uniform dispersed oxygen throughout the basin and keep biological solids in suspension in aerobic basins and the activated sludge process. For high-rate organic loadings, the power required may be determined by oxygen transfer requirements rather than mixing.
5. Coagulation and Flocculation:
Coagulation and flocculation convert non-settlable turbidity particles into settlable form for their effective removal by gravity.
After pre-sedimentation, these particles are mostly colloidal type. Colloidal turbidity particles are too small (1-100 nm) to settle by gravity. They stay suspended and cause turbidity. Mostly, they are negatively charged. Their removal is accomplished by using substances that make them clump together to form large and heavy particles known as floe that will settle.
These substances are known as coagulants. A coagulant is an electrolyte that provides cations (positively charged ions) to precipitate out the negatively charged colloidal turbidity particles. As a rule, the higher the charge on the cation, the more effective is the coagulant.
Therefore, commonly used coagulants are aluminium and ferric compounds that provide Al+3 and Fe+3 cations, respectively. Some other substances are often used to facilitate the coagulants; those are known as coagulant aids. This treatment phase is the second barrier to remove turbidity, waterborne pathogens and other contaminants.
It consists of three parts:
Rapid mixing is the fast and thorough mixing—flash mixing—of the various chemicals, such as coagulants and coagulant aids, with water for their proper chemical reactions. It has only 30 ± 15 seconds detention time in a small tank. Rapid mixing disperses the chemicals immediately to reach their targets and start their precipitation.
Precipitation (separation of a solid from a liquid) in water treatment is known as coagulation. It is the start of the removal of colloidal particles. Rapid mixing is followed by coagulation.
Coagulation is the precipitation of the colloidal turbidity particles, coagulants and coagulant aids.
Flocculation is the clumping of microfloc particles to form large particles called floe. It is achieved by the gentle mixing of coagulated water, in tanks known as flocculation basins to allow further clumping of the coagulated matter and turbidity particles, to form large floe particles.
Flocculation basins have slow mixing mechanical paddles, known as flocculators and baffles to provide adequate mixing and low velocity. These basins have a velocity about 1 ± 0.25 ft/sec and detention time of 15 to 45 minutes.
The best floe is pinhead size and visible a few feet below the surface of coagulated water. Floe particles are heavy enough to settle to the bottom of basin by gravity. The flocculated water flows to the primary sedimentation basin for the next phase, the sedimentation.
The hardness removal is also achieved at this phase of treatment by using lime and soda ash. If chlorine dioxide is used for pre-disinfection, then chlorite removal can also be done here by using ferrous ions. For the proper chemical reactions, these chemicals should be applied in the following sequence- ferrous ions, alum, lime and then soda ash.
Filtration is the mechanical removal of turbidity particles by passing the water through a porous medium, which is either a granular bed or a membrane. Filtration’s purpose is to remove all the turbidity particles carried over from the sedimentation phase, thus producing a sparkling clear water with almost zero turbidity.
Thus, filtration is a fundamental unit operation that, separates suspended particle matter from water. Although industrial applications of this operation vary significantly, all filtration equipment operate by passing the solution or suspension through a porous membrane or medium, upon which the solid particles are retained on the medium’s surface or within the pores of the medium, while the fluid, referred to as the filtrate, passes through.
In a very general sense, the operation is performed for one or both of the following reasons. It can be used for the recovery of valuable products (either the suspended solids or the fluid) or it may be applied to purify the liquid stream, thereby improving product quality or both.
Examples of various processes that rely on filtration include adsorption, chromatography, operations involving the flow of suspensions through packed columns, ion exchange and various reactor engineering applications. In petroleum engineering, filtration principles are applied to the displacement of oil with gas (i.e. liquid-liquid separations), in the separation of water and miscible solvents (including solutions of surface-active agents) and in reservoir flow applications.
In hydrology, interest is in the movement of trace pollutants in water systems, the purification of water for drinking and irrigation and to prevent saltwater encroachment into freshwater reservoirs. In soil physics, applications are in the movement of water, nutrients and pollutants into plants.
In biophysics, the subject of flow through a porous media touches upon life processes such as the flow of fluids in the lungs and the kidney. Although there are numerous industry-specific applications of filtration, water treatment has historically and continues to be the largest general application of this unit operation.
The objective is to provide an overview of filtration terminology and basic engineering principles, as well as calculation methods that describe the filtration process in a generalised way. The basis equations describing the generalised process of filtration have been around for nearly 100 years and with few refinements, continue to be applied to modern design practices.
7. Ion – Exchange and Carbon Adsorption:
Ion exchange and carbon adsorption are unrelated technologies and often have different objectives. They are, however, often times used in compliment to achieve high water quality attributes.
Ion exchange is a reversible chemical reaction wherein an ion (an atom or molecule that has lost or gained an electron and thus acquired an electrical charge) from solution is exchanged for a similarly charged ion attached to an immobile solid particle.
These solid ion exchange particles are either naturally occurring inorganic zeolites or synthetically produced organic resins. The synthetic organic resins are the predominant type used today because their characteristics can be tailored to specific applications.
An organic ion exchange resin is composed of high-molecular-weight polyelectrolytes that can exchange their mobile ions for ions of similar charge from the surrounding medium. Each resin has a distinct number of mobile ion sites that set the maximum quantity of exchanges per unit of resin.
The industry application most familiar with, ion exchange technology is metal plating. Most plating process water is used to cleanse the surface of the parts after each process bath. To maintain quality standards, the level of dissolved solids in the rinse water must be regulated.
Freshwater added to the rinse tank accomplishes this purpose and the overflow water is treated to remove pollutants and then discharged. As the metal salts, acids and bases used in metal finishing are primarily inorganic compounds, they are ionised in water and could be removed by contact with ion exchange resins.
In a water deionisation process, the resins exchange hydrogen ions (H+) for the positively charged ions (such as nickel, copper and sodium) and hydroxyl ions (OH–) for negatively charged sulphates, chromates and chlorides.
Because the quantity of H+ and OH ions is balanced, the result of the ion exchange treatment is relatively pure, neutral water. Ion exchange technology is applied in many other industry sectors, including the petroleum and chemical industries, as well as general waste-water treatment applications. The technology is most often compared to reverse osmosis, since both technologies are often aimed at similar objectives.
Activated carbon is a crude form of graphite, the substance used for pencil leads. It differs from graphite by having a random imperfect structure which is highly porous over a broad range of pore sizes from visible cracks and crevices to molecular dimensions.
The graphite structure gives the carbon it’s very large surface area which allows the carbon to adsorb a wide range of compounds. Activated carbon can have a surface of greater than 1000 m2/g. This means 5 grams of activated carbon can have the surface area of a football field.
Adsorption is the process by which liquid or gaseous molecules are concentrated on a solid surface, in this case activated carbon. This is different from absorption, where molecules are taken up by a liquid or gas. Activated carbon can made from many substances, containing a high carbon content such as coal, wood and coconut shells. The raw material has a very large influence on the characteristics and performance activated carbon.
The term activation refers to the development of the adsorption properties of carbon. Raw materials such as coal and charcoal do have some adsorption capacity, but this is greatly enhanced by the activation process.
There are three main forms of activated carbon:
1. Granular Activated Carbon (GAC)—irregular shaped particles with sizes ranging from 0.2 to 5 mm. This type is used in both liquid and gas phase applications.
2. Powder Activated Carbon (PAC)—pulverised carbon with a size predominantly less than 0.18 mm (US Mesh 80). These are mainly used in liquid phase applications and for flue gas treatment.
3. Pelleted Activated Carbon — extruded and cylindrical shaped with diameters from 0.8 to 5 mm. These are mainly used for gas phase applications because of their low pressure drop, high mechanical strength and low dust content.
Activated carbon is also available in special forms such as a cloth and fibers. Activated Charcoal Cloth (ACC) represents a family of activated carbons in cloth form. These products are fundamentally unique in several important ways compared with the traditional forms of activated carbon and with other filtration media that incorporate small particles of activated carbon.
ACC products are similar to the traditional activated carbon products in that they are 100 per cent activated carbon. This gives the products the same high capacity for adsorption of organic compounds and other odorous gases as the more traditional, pelletised, granular and powder forms of activated carbon.
As with the traditional forms of activated carbons, ACC products can be impregnated with a range of chemicals to enhance the chemisorption capacity for selected gases. By being constructed of bundles of activated carbon filaments and fibers in a textile form, several important advantages are imparted to ACC.
The diameter of these fibres is approximately 20 mm, so the kinetics for ACC products are similar to that of a very tine carbon particle. Gases and liquids can flow through the fabric and the accelerated adsorption kinetics mean that the ACC can retain the advantages of mass transfer zones associated with deeper filter beds. Faster adsorption rates mean smaller adsorption equipment and up to twenty times less carbon on line.
Adsorption is the process where molecules are concentrated on the surface of the activated carbon. Adsorption is caused by London Dispersion Forces, a type of Van der Waals Force which exists between molecules. The force act in a similar way to gravitational forces between planets. London Dispersion Forces are extremely short ranged and therefore sensitive to the distance between the carbon surface and the adsorbate molecule.
They are also additive, meaning the adsorption force is the sum of all interactions between all the atoms. The short range and additive nature of these forces results in activated carbon having the strongest physical adsorption forces of any material known to mankind.
All compounds are adsorbable to some extent. In practice, activated carbon is used for the adsorption of mainly organic compounds along with some larger molecular weight inorganic compounds such as iodine and mercury.
In general, the adsorbability of a compound increases with:
(i) Increasing molecular weight,
(ii) A higher number of functional groups such as double bonds or halogen compounds, and
(iii) Increasing polarisability of the molecule.
The most common manufacturing process is high temperature steam activation though activated carbon can also be manufactured with chemicals. Along with the raw material, the activation process has a very large influence on the characteristics and performance of activated carbon.