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In lime soda process all the soluble hardness causing impurities are chemically converted into insoluble precipitates, which may be removed by settling and filtration In this method calculated quantity of lime [Ca OH 2] and soda Na2CO3 are added into water. The precipitated formed are finely divided, so they do not settle easily and cannot be filtered easily. Therefore we add a small amount of coagulants. Lime is rather cheap and it removes temporary hardness efficiently without introducing any soluble salts in the water.
Magnesium hydroxide produced in the above reactions precipitates as an insoluble sludge. The reaction of soda with the permanent calcium hardness produces insoluble CaCO3.
Addition of a coagulant such as sodium aluminate or alum helps in accelerating the coagulation of the carbonate sludge, which is subsequently removed by filtration. Water softened by this process contains appreciable concentrations of soluble salts, such as sodium sulphate, and cannot be used in high- pressure boiler installations. Each tank is provided with inlets for raw water and chemicals, outlets for softened water and sludge, and a mechanical stirrer Fig.
Raw water and calculated quantities of the chemicals are slowly sent into the tank simultaneously under agitation with the help of the stirrer. Intermittent cold lime-soda softener. Some sludge from a previous operation is also added which forms nucleus for fresh precipitation and thus accelerates the process.
Thus by the time the tank is full, the reaction is more or less complete. Stirring is stopped and the sludge formed is allowed to settle. The clear softened water is collected through a float pipe and sent to the filtering unit.
The sludge formed in the tank is removed through the sludge outlet. By employing a set of tanks planned for alternate cycles of reaction and settling, continuous supply of softened water may be ensured. The raw water Softened water and the chemicals flowing down the chamber come into close contact and the softening reactions take place.
The sludge formed settles Fibre filter down to the bottom of the outer chamber from Paddle stirrer where it is periodically removed through the sludge outlet. The softened water rising up passes through the fibre filter where traces Sludge of sludge are removed and filtered soft water Sludge outlet passes through the outlet provided. Water treated by the cold lime-soda process Fig. Conventional type of lime-soda softner.
The tank used may be either open for gravity operation Softend water or closed for operation under pressure. In both the cases the raw water and the calculated Catalyst quantities of chemicals enter the tank tangentially crushed and graded near the bottom of the cone and spiral upwards Raw calcite or green sand water through the suspended catalyst bed.
The catalyst Chemicals employed is a finely granuled 0. The Fig. Catalyst or spiractor type cold lime- retention time is about 8 to 12 minutes. The soda water softener. The softened water rises to the top from where it is drawn off.
The catalyst or spiractor type of continuous water softener is of interest as it gives a granular sludge which drains and dries rapidly and can be handled easily.
These softeners differ from the conventional type in that the treated water is filtered upwardly through a suspended sludge blanket composed of previously formed precipitates. Thus in a single unit, all the three processes namely mixing, softening and clarification take place In the conventional type of equipment, some of the added lime suspension is carried down in the sludge formed by the precipitates, before it has time to dissolve and react with the hardness causing impurities of the raw water and thus some of the lime is wasted.
In the sludge blanket type, this does not happen because the upward filtration through the suspended sludge blanket ensures complete utilization of the added lime. With the conventional type of equipment, it is generally observed that after precipitates or after deposits form on the granules or filter media employed, and in pipe lines or distribution systems carrying the filtered effluents. This usually necessitates recarbonation with CO2 to obviate the formation of such deposits.
However, in the sludge blanket type of equipment, the intimate contact of the treated water with a large mass of solid phase mostly prevents super-saturation or the formation of after deposits. The retention period required with sludge blanket type equipment is one hour as against four hours with the conventional units.
Further, silica is removed better in sludge blanket units. The sludge blanket type of water softening equipment, owing to its higher efficienc , shorter detention period and smaller space requirements, is rapidly displacing the conventional type.
Parts per million 0. The rate of these precipitation reactions It can be solutions respectively. Thus effect is more pronounced with the precipitation of magnesium compounds. For efficient softening, cold lime-soda softening plants must be of considerable area and water-storage capacity, whereas hot lime-soda softeners are much more rapid in operation and therefore for a given through-put, much more compact.
Elevated temperatures not only accelerate the actual chemical reactions but also reduce the viscosity of the water and increase the rate of aggregation of the particles. Thus, both the settling rates and filtration rates are increased. Thus the softening capacity of the hot lime-soda process will be several times higher than the cold process.
Since the sludge formed settles down rapidly, there is no need of adding any coagulants. A smaller excess of chemicals is needed than with the cold process. Further, dissolved gases are driven out of the solution to some extent at the high temperature. The hot lime-soda process yields softened water having relatively lower residual hardness about 17 to 34 ppm as against the cold process about 50 — 60 ppm.
A typical hot lime- soda water softening unit is shown in Fig. If the water is alkaline, filtration through sand and gravel beds might contaminate the Exhaust or water with dissolved silica, particularly if the Live Chemicals quartz used is of inferior quality. Other filtering steam media used are anthracite coal, calcite and magnetite. If slight excess of chemicals are used over that Sludge Clarified theoretically required, more rapid and more water complete removal of hardness will result.
But if larger excess of chemicals are used, naturally Sludge outlet they will appear in the softened water. Lime- soda plants do not produce water of zero Fig. Hot lime-soda water softner. Tips for the solvening problems on water treatment by Lime-soda process. Accordingly, their respective equivalents must be considered for calculating the lime requirement. Accordingly, these constituents also should be considered while calculating the soda requirement.
Thus for every one equivalent of HCO3— present, the corresponding reduction in the dose of soda has to be made in the calculations for soda requirement. For solving numerical problems on lime-soda requirements for softening of hard water, the following steps may be followed: 1. The units in which the impurities analysed are expressed i. Substances which do not contribute towards hardness e.
This fact should be explicitly stated. All the substances causing hardness should be converted into their respective CaCO3 equivalent, as a matter of convention and convenience. Table 1. The name was first used by cronstedt a Swedis Geologist in to a certain group of natural minerals, which released their water of hydration or combination in the form of steam. They are also known as permutits. They are of two types: a Natural Zeolites: They are non-porous and derived from green sands by washing, heating and treatment with sodium-hydroxide NaOH.
They are more durable. They are prepared from solution of sodium silicate and aluminium hydroxide. They may also be prepared by heating together a china clay, felspar, and soda ash or b solution of sodium silicate, Al2 SO4 3 and NaAlO3 or c solution of sodium silicate and Al2 SO4 3 or d solution of sodium silicate and NaAlO2 and granulating the resultant mass after cooling.
Water softening by zeolites Sodium zeolites are used in water softening process. It may by represented as: Na2O. X Y H2O For simplification they are represented as Na2Z. Since zeolites are capable of exchanging basic radicals, they are generally known as base exchanger.
Coarse Gravel. Zeolite softener. Regeneration: When the zeolite bad is exhausted i. Sodium nitrate, sodium sulphate potassium chloride or potassium nitrate could also be used. Schemes for softening of water by Zeolite process. Advantages of Zeolite Process 1. Hardness is completely removed. Equipment used is compact and occupies less place. It automatically adjusts itself to waters of different hardness. It can work under pressure. Hence the plant can be installed in the water supply line itself, avoiding double pumping.
In this process, the hardness causing ions are simply exchanged with sodium ions. As the process does not involve any precipitation, there is no problem of sludge formation and after precipitation in the softened water at later stages. Limitation of Zeolites Process i Water with turbidity should not be used as pores of Zeolite get dogged.
Zeolite process Lime-soda process 1. This process produces water of almost 1. This process produces water having zero hardness hardness of 15 to 60 ppm depending on 2. The cost of the plant and the Zeolite are whether it is hot process or cold process. Hence the capital cost is higher.
The capital cost is lower. The exhausted Zeolite bed can be 3. The chemicals needed viz. Hence the operating cost is process. Hence the operating cost is lesser higher. The plant is compact and occupies less 4. The plant occupies more space. The size space. The size of the plant depends on of the plant depends on the amount of the hardness of water being treated. Cannot be used for hot water, acidic 5. The process is free from such limitations. This process can operate under pressure 6.
This process cannot be operated under and can be designed for fully automatic pressure. This process does not involve a number of 7. This process involves all the problems operations such as settling, coagulation, associated with settling, coagulation filtration. Water treated with the Zeolite process 8. Treated water contains lesser percentage contains larger amounts of sodium salts of dissolved solids and lesser quantities and greater percentage of dissolved salts of sodium salts.
This process adjusts itself to waters of 9. Reagent doses must be adjusted for waters different hardness. Salts causing temporary hardness are Temporary hardness is completely re- converted to NaHCO3 which will be moved in the form present in the softened water. No problems of after precipitation. There may be problems of after precipi- tation in distribuion systems and even in boilers when used as boiler feed water.
C Ion Exchange Process or Deionization or Deminerabization Process The process of complete removal of all ions present in water is called demineralization. Demineralization of water is carried out by ion-exchange resins. Ion-exchange resins are cross-linked, long chain organic polymers with a microporous structure.
The functional groups attached to the polymeric chain are responsible for ion exchange. They are also called H form cation exchangers. An Cation Exchanger Resin. Resins having SO3H group are known as strongly acidic resins.
While resins having COOH group are known as weakly acidic resins. They are styrene divinyl benzene complexes which are capable of exchanging their basic functional groups with the anions of the solution. An Anion Exchanger Resin. For effective water treatment, ion exchangers should possess the following properties. They should be non-toxic.
They should not decolourise the water being treated. They should possess a high ion-exchange capacity. It depends upon the total number of ion active groups per unit weight of the exchanger and is expressed as mill equivalents per gram of the exchanger. They should be physically durable. They should be resistant to chemical attack. They should be cheap and commonly available. They must be capable of being regenerated and back-washed easily and economically.
They should have a large surface area since ion-exchange is a surface phenomenon. At the same time, their resistance to flow must be compatible with hydraulic requirements Process: The ion exchange unit consists of two tanks.
Cation exchange resins are placed in first tank and anion exchange resins are placed in second tank. At first hard water is passed through a cation exchange resin. It is known as deiomized or demineralized water. Raw water To vacuum pump. Alkali for regeneration Acid for regeneration Deionised water Fig. Demineralization of water. After some time the cation and the anion exchangers get exhausted and stop working. The anion exchanger may be regenerated by treating it with NaOH solution.
This method produces an effluent which is far superior to that produced by the two bed operation. Thus the process is highly efficient When the resins are exhausted the bed is backwashed when the two resins are separated in different layers due to difference in their densities. Then the resins are separately regenerated, washed and mixed again by injecting air and reused for a fresh cycle. Advantages i Highly acidic or alkaline water can be softened. Disadvantage i The process is costly.
This is mainly used as a corrective treatment to remove the slight residual hardness and also sometimes to remove the corrosive tendencies in water. In modern heavy-duty high pressure boilers, water of zero hardness is required, since even an egg-shell thickness of scale may be extremely detrimental. The above principles are used in the carbonate conditioning when sodium carbonate solution is added to boiler water, the concentration of CO32— ion increased and when it becomes greater than the solubility product of SO4— — i.
B Phosphate conditioning: Phosphate conditioning is applicable to high pressure boilers. In this method an excess of soluble phosphate is added to boiler water. For instance, if the feed water tends to produce an acidic condition in the boiler, the alkaline Na3PO4 should be chosen. If the feed water produces almost the right alkalinity desired in the boiler, it is preferable to use Na2HPO4 which is practically neutral.
If the boiler water becomes too alkaline, the acidic NaH2PO4 would be selected. Both sodium pyrophosphate and metaphosphate are rapidly hydrolysed under boiler water temperatures to orthophosphate. Hence the former would be preferred if the use of NaH2PO4 causes feedline corrosion.
However, precaution should be taken to inspect them at least once in six months and remove the scale and sludge accumulations. C Colloidal conditioning : Scale formation can also be minimised by adding some colloidal conditioning agents such as glue, agar agar, tannins, starches and sea-weed extract into the boiler feed water.
These substances act as protective colloids. Thus, the precipitated scale-forming salts are maintained in loose suspended form which can easily be removed by blow- down operation. Thus the scale formation is prevented.
D Calgon conditioning : Another approach for preventing scale formation is to convert the scale forming salts into highly soluble complexes which are not easily precipitated under the boiler conditions.
This substance interacts with the residual calcium ions forming highly soluble calcium hexametaphosphate and thus prevents the precipitation of scale- forming salts. If impure water is made use of as boiler feed water, then following boiler problems may occurs i Sludge and scale formation ii Corrosion of boiler metal iii Caustic embrittlement iv Carry over: priming and foaming i Sludge and scale formation In a boilder, water is continuously evaporated to form steam.
This increases the concentration of dissolved salts. Finally a stage is reached when the ionic product of these salts exceeds their solubility product and hence they are thrown out as precipitates. If the precipitates formed are soft loose and slimy, these are known as sludges; while if the precipitate is hard and adhering on the inner walls, it is called as scale.
Sludge : Sludge is a soft, loose and slimy precipitate formed within the boiler. Sludges are formed by substances which have greater solubilities in hot water than in cold water, e. They are formed at comparatively colder portions of the boiler and get.
If sludges are formed along with scales, then former gets entrapped in the latter and both get deposited as scales. Disadvantage of sludge formation i Sludge are poor conductors of heat, they tend to waste a portion of heat generated and thus decrease boiler efficienc. Sludge sittles in the regions of poor water circulation e. Prevention of sludge formation i By using softened water ii By frequent blow down operation i.
Scale : Scales are hard deposits firmly sticking to the inner walls of the boiler. They are difficult to remove, even with the help of hammer and chisel, and are the main source of boiler troubles. CaSO4 is soluble in cold water, but almost completely insoluble in super-heated water. Consequently, CaSO4 gets precipitated as hard scale on the hotter parts, of the boiler.
This type of scale causes troubles mainly in high-pressure boilers. Calcium sulphate scale is quite adherent and difficult to remove, even with the help of hammer and chisel iii Hydrolysis of magnesium salts Dissolved magnesium salts get hydrolysed at prevailing high temperature inside the boiler forming magnesium hydroxide precipitate, which forms a soft type of scale, e.
These deposite adhere very firmly on the inner side of the boiler surface and are very difficult to remove. One important source of silica in water is the sand filt. Disadvantage of Scale Formation i Wastage of Fuel. Scales are the poor conductor of heat. They reduces the rate of heat transfer from boiler to inside water. Thus scale formation will result in wastage of fuel and reduction in boiler efficienc. Scale formation on the boiler tubes or other heated surfaces insulates the metal so well that it becomes overheated.
The metal becomes soft and weak thus making the boiler unsafe particularly at high pressures. The overheating also causes burning out of the metal plates and tubes and breakdown of the expanded joints. In addition to the loss of strength due to overheating, rapid reaction between water and iron occurs at high temperatures, causing additional thinning of the tube wall. Since the scale acts as heat insulator, the metal of the boiler is overheated.
Under the high pressure of steam existing in the boiler, the metal expands until the scale on it cracks. Sudden entry of the water through these cracks to the very hot metal causes sudden cooling of the boiler metal with the simultaneous conversion of water into steam.
The sudden increase in pressure due to this large quantity of steam thus formed may lead to explosion. Scales deposited in the valves and condensers of the boiler, block them partially, which lowers down the efficiency of the boile.
Prevention of Scale Formation Scale formation can be prevented by the following methods. It involves removal of hardness causing impurities such as calcium and magnesium salts and silica from the water before entering the boiler.
The various methods of external treatment of water are already discussed earlier. Internal treatment consists of adding chemicals directly to the water in the boilers for removing dangerous scale forming salts which were not completely removed in the external treatment for water softening. This treatment is not usually applied to raw waters, except for small boilers, but it is usually practised in larger power stations. Difference Between Sludge And Scales. Sludge Scales 1.
They are soft, loose and slimy precipitate They are hard deposits 2. They are non-adherent deposits and can be They stick very firmly to the inner surface of easily removed boiler and are very difficult to remov 3. It formed at comparatively colder portion of It formed generally at heated portion of the the boiler boiler 5.
They are less dangerous They are more dangerous as compare to sludge. Boiler corrosion is disintegration of boiler body material either due to chemical or electrochemical reaction with its environment. Boiler corrosion takes place due to following factors: a Presence of free acids in water. Some of these factors are discussed below: a Dissolved oxygen.
Dissolved oxygen is the main source of corrosion in boilers. The concentration of oxygen in boiler waters should be below 0. Oxygen enters the boilers through raw make up water and also through infiltration of air into the condensate system. When the water containing dissolved oxygen is heated in the boiler, the free gas is evolved which corrodes the metal parts under the conditions obtaining in the boiler. Dissolved oxygen can be removed by: Mechanical de-areation of water.
These two principles are made use of in the design of mechanical de-areratior. In mechanical dearators, dissolved oxygen is removed by injecting hot feed-water as a fine spray into a vacuum chamber heated externally by steam. Chemical Treatment For complete removal of dissolved oxygen, chemical methods of treatment are adopted. Mechanical de-areration of Sodium sulfite is suitable for boilers operating below water.
The sulphite reacts with the dissolved oxygen to form sulphate. Ferrous sulphate is also used sometimes. The pure compound is an explosive inflammable liquid B. However, rubber gloves must be worn to guard against the danger of dermatitis in some cases. One of the important advantage of hydrazine treatment is that combination with oxygen does not produce any salts. Nitrogen and water are the only reaction products obtained.
The residual hydrazine can be measured easily by a colorimeter. The amount of hydrazine added must be closely controlled. Some inorganic salts like magnesium chloride and calcium chloride are also corrosive agents. Silicic acid catalyzes the reaction so that in water containing silica, appreciable quantities of HCl may be formed at lower temperatures. If the amount of HCl formed is small, it might get neutralized by the alkalinity present in the water, but otherwise it should be neutralized by the addition of alkali.
Water containing bicarbonates release CO2 on heating. If CO2 is released inside the boiler, it will go along with the steam and as the steam condenses, the CO2 is dissolved in water forming carbonic acid. This produces intense local corrosion called pitting. CO2 along with oxygen in water, can be removed by mechanical deaeration. CO2 in water can be removed by lime treatment.
CO2 can be converted into ammonium carbonate by the addition of ammonia. If some O2 is also present in the condensate, the ammonia may attack the condenser tubes made of copper. This may also lead to pitting corrosion. This can be prevented by suspending zinc plates which act as sacrificial anodes iii Caustic embrittlement : Caustic embrittlement is a form of corrosion caused by a high concentration of sodium hydroxide in the boiler water.
It is characterised by the formation of irregular intergranular cracks on the boiler metal, particularly at places of high local stress, such as riveted seams, bends and joints. It is caused by the high concentration of NaOH which is capable of reacting with steels stressed beyond their yield point. The rate and extent, of corrosion by caustic embrittlement increases with the concentration of NaOH and temperature and hence with increasing operating pressure.
The NaOH travels to the minute hair cracks, bends and joints. Water evaporates and the concentration of NaOH at these points keep on increasing with time due to poor circulation of water at these points. This caustic soda attracks the surrounding areas and corroding the boiler material or dissolving the iron of the boiler. This causes caustic embrittlement of the boiler parts. The caustic embrittlement can be explained by the formation of the concentration cell as Iron at Rivets Conc.
The concentrated alkali dissolves the iron as sodium ferroate Na2FeO2. The caustic embrittlement occurs more rapily in following cases. Prevention of Caustic Embrittlement a By using sodium sulphate or sodium phosphate as softening agent in place of sodium carbonate: It is observed that boiler water containing sodium sulphate or sodium phosphate prevent the caustic embrittlement by blocking the capillaries, thereby preventing the infiltration of caustic soda solution to these.
As steam rises from the surface of the boiling water in the boiler, it may be associated with small droplets of water.
Such steam, containing liquid water, is called wet steam. These droplets of water naturally carry with them some suspended and dissolved impurities present in the boiler water.
This is mainly due to priming and foaming. Steam used for power production is usually superheated for achieving greater efficiencies in the turbine or engine in which steam is used. Wet steam causes corrosion in the inlet ends of the superheaters.
If the steam contains high percentage of moisture, the extent of superheating will decrease with the consequent reduction in efficiency of the turbine or engine. Further, the water carried over with the steam contains salts and sludges, these are carried into the superheater where they may deposit as the water evaporates.
This will seriously restrict the flow of steam. Moreover, due to the insulating effect of these deposits, the superheater tubes also may burn out. A part of the dried salts may be carried along with the steam farther and deposit on the high-pressure turbine blades or in engine valves.
Even a small amount of deposit on the turbine blades decreases its efficiency considerably. In order to eliminate the bad effects of moisture, mechanical steam purifiers are often installed in the steam drums of the boiler or between the boiler and the superheater. These devices force the steam to take curved paths whereby due to centrifugal action, the moisture is thrown out of the steam. Foaming is the formation of small but presistent bubbles at the water surface.
These bubbles are carried along with steam leading to excessive priming. According to Bancroft, foams are formed when there is a difference in concentration of solute or suspended matter between the surface film and the bulk of the liquid. Substances which increase the viscosity of the film favour production of foam. Any material which lowers the surface tension of the water will collect at the interface and thus increase the foaming tendency of the liquid. Pure water has no tendency to form foam but almost all impurities causes foaming, specially the dissolved impurities like soap which reduces the surface tension of water.
Foaming can be avoided by : i By the removal of the foaming and stabilizing agents from the water. The foam is fi- nally destroyed by mutual antagenistic effect of the difference in charge Positive and nega- tive on the colloidal partical of the two foams.
When the steam is generated rapidly in the boilers, some droplet of the liquid water are carried along with steam. This process is known as wet steaming or priming. Causes of priming: The main causes of priming are given below: i It is due to the presence of suspended impurities and to some extent to dissolved impurities in the water. Minimization of primining.
It can be minimised by: i By using a proper designed boiler ii By maintaing low water level iii By avoiding rapid changes in the steaming rate iv By minimizing foaming v By using only soft water. Water that is fit for drinking purposes is called potable water. Water used for drinking and other domestic used should be colourless, odourless, free from suspended impurities, free from germs, bacteria and other pathogenic organisms and should be free from harmful dissolved impurities.
Therefore, the raw or impure water obtained by municipalities, from sources such as rivers, likes, wells, tube wells, etc. Porous drain tiles Fig. Set of Water Filtration Units. This removes bad odours, CO2, etc. At this stage, some of the heavier impurities present in water settle down by gravity. Also, the bacteria present are partially eliminated due to the UV radiation from sunlight.
The suspended impurities are trapped by the resulting precipitate of Al OH 3 and settle down at the bottom, thereby bringing about partial clarification of the water. Also, the negatively charged colloidal impurities neutralized by the trivalent aluminium cations, followed by agglomeration and settling down by gravity. These comprise of rectangular tanks which contain : a a top layer about meter thick of fine sand b a middle layer 0.
A series of porous drains are provided at the bottom of the gravel layer through which filtered water is collected. The slimy surface layer comprising of finely divided clay, algae, bacteria etc. The filters are backwashed periodically to remove the precipitated matter from the surface, so as the ensure efficient filtration. Activated carbon may be used for filtration if the water contains undesirable odours v Chlorination : The filtered water is sterilized by chlorination by adding chlorine or bleaching powder to destroy the pathogenic micro-organisms.
The water is now pumped to over-head tanks for domestic distribution. Fresh Waters. Classificatio Best Use A — Drinking water source after disinfection, without any other treatment B — Outdoor bathing C — Drinking water source with conventional treatment followed by disinfection. D — Propagation of wild-life and fisheries E — Irrigation, industrial cooling and controlled waste disposal.
Water supplied by municipalities for domestic purposes must be free from pathogenic bacteria. It should be clear, colourless and pleasant to taste. It should be free from excessive dissolved salts, suspended impurities and harmful microorganisms. Water for domestic purposes should be obtained from such a source which is least contaminated by animal and vegetable matter as well as industrial effluents. Rivers, lakes and wells are the most common sources of water used by municipalities.
Generally the treatment of these waters involves removal of suspended impurities and removal of colloidal impurities if any, followed by sterilization. If the water is very hard, certain amount of softening may be needed, which is very rare. Sedimentation, coagulation, filtration and sterilization are the treatment techniques usually employed depending on the requirements of the situation. Sedimentation is a process of removing relatively large particles suspended solids into large reservoirs of settlement tanks in which it is left for a few days or even weeks, where the suspended impurities partially sink to the bottom.
The principle involved is to slow down the flow of water so that substances held up by the turbulence of fast moving water can fall gravitationally to the bottom of the tank when water flow is stilled. Periodically the accumulations of the debris are to be scraped away. In order to remove floating impurities, screens of various kinds e. These screens also must be continuously cleaned. During sedimentation, solid particles settle by gravity on the bottom of a settling tank in which the water being clarified is at rest or in slow horizontal or upward motion.
The velocity with which a particle in water will fall under the action of gravity depends upon i the horizontal flow velocity of the water, ii the size of the particle, iii the specific gravity of the particles, iv the shape of the particle and v the temperature of the water.
Accordingly, several formulae have been given to calculate the velocity of falling spherical particles in slowly moving water on the basis of which several types of sedimentation tanks have been designed.
The sedimentation tanks commonly used are of horizontal flow rectangular type and circular shaped upward flow typ Sedimentation takes a long time, requires large-capacity settling tanks and cannot ensure complete removal of coarse-dispersed impurities from water.
Finely divided silica, clay and organic matter does not settle down easily and hence cannot be removed by simple sedimentation. Most of these are in colloidal form e.
Such impurities are generally removed by chemically assisted sedimentation, in which certain chemicals are added which produce ions of right electrical charge that neutralize the oppositely charged colloidal particles and brings about their coalescence.
This process is called coagulation. This permits the particles to aggregate together until a denser particle is formed which falls through still water at a reasonable rate and is called flocculation Aluminium sulphate is the most common coagulating agent used for removing clay particles and is generally called filter alum. Other coagulants which also find application in water treatment include ferric sulphate, ferrous sulphate copperas , chlorinated copperas, alum, ammonia alum or potash alum and sodium aluminate.
Thus the smaller particles join together to form denser particles which settle down to the bottom. Some bacteria and colour associated with these particles also get removed simulataneously. In order to render the Al OH 3 filterable and also in order to neutralize the H2SO4 liberated to permit the hydrolysis reaction to completion, some alkali will have to be added if the water is not sufficiently alkaline.
The coagulants are generally added in solution form to the water with the help of mechanical flocculators provided with slow moving rotating baffles or stationary baffles turning the flow of water; thus ensuring a gentle contact with the water and the reagents.
After any sedimentation process, especially after that utilizing a chemical flocculent, there will be a substantial reduction in the bacterial count in the water. A round brilliant cut diamond set in a ring. Main article: Diamond gemstone. Main articles: Diamond cutting and Diamond cut. See also: List of diamond mines and Exploration diamond drilling. Play media. Main articles: Kimberley Process , Blood diamond , and Child labour in the diamond industry. Main article: Synthetic diamond.
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Sheet size in mm: Full Sheet x mm. Technical Datasheet. Material Safety Datasheet. Show less information -. Colour Bottom Layer Black White.
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