In this essay we will discuss about:- 1. Necessity of Sedimentation with Coagulation 2. Process of Sedimentation with Coagulation 3. Chemical used as Coagulants 4. Comparison of Alum and Iron Salts for Use as Coagulants 5. Handling and Storing of Coagulants 6. Dosage of Coagulants 7. Devices used 8. Flocculation and Clarifiers 9. Limitation of the Process.
Essay # 1. Necessity of Sedimentation with Coagulation:
Very fine suspended clay particles are not removed by plain sedimentation. Silt particle of 0.06 mm size requires 10 hours to settle in 3 m deep plain sedimentation tank and 0.02 mm particle will require about 4 days for settling.
This settling time is impracticable, because water cannot be detained for such a long time. In plain sedimentation tanks detention time of about 2 hours for mechanically cleaned basins and about 6 hours for ordinary tanks, can be allowed.
In addition to fine suspended matter, water also contains electrically charged colloidal matter which are continuously in motion and never settle down due to gravitational force.
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When water contains such fine clay particles and colloidal impurities, it becomes necessary to apply such process which can easily remove them from the water. After long experience it has been found that such impurities can be removed by sedimentation with coagulation.
Essay # 2. Process of Sedimentation with Coagulation:
It has been found that when certain chemicals are added to water an insoluble, gelatinous, flocculent precipitation is formed. This gelatinous precipitate during its formation and descent through the water absorb and entangle very fine suspended matter and colloidal impurities.
The gelatinous precipitate therefore has the property of removing fine and colloidal particles quickly and completely than by plain sedimentation. These coagulants further have the advantages of removing colour, odour and taste from the water. These coagulants if properly applied are harmless to the public.
First the coagulants are mixed in the water to produce the required precipitate, then the water is sent in sedimentation basins where sedimentation of fine and colloidal particles takes place through the precipitate.
Essay # 3. Chemical used as Coagulants in Sedimentation with Coagulation:
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The following are the most commonly used coagulants:
(a) Aluminium sulphate [Al2, (SO4)3 18H2O]:
It is also called simply as alum. Alum which is available in market, is dirty grey solid in the form of lumps containing about 17% aluminium sulphate. This is the chemical coagulant which is widely used in water treatment plants. Alum reacts in water in the presence of alkalinity; if natural alkalinity is not present sufficient lime is added.
The following chemical reactions take place with the various types of alkalinity:
The insoluble and colloidal aluminium hydroxide [Al(OH)3] forms the floe which removes the fine suspended and colloidal impurities. For best results the pH value of water should be between 6.5 and 8.5. The dose of alum should be 0.03 to 0.13 gm/litre depending on the turbidity of water.
The amount of alum required for coagulation mainly depends on the turbidity and colour of water. The quantity of optimum dose of alum is determined by practical test in the laboratory and it should be adopted in the practice also. Usually the dose of alum varies from 5 mg/litre for relatively clear water to about 25 mg/litre for very turbid waters. The average dose is about 19 mg/litre.
Filter alum is very effective coagulant and is now-a-days extensively used throughout the world. It is cheap, forms very good stable floe, and also does not require skilled supervision or handling. The water obtained after its treatment is very clear. The main difficulty in using alum was to remove the water from the floe and its disposal.
But now-a- days processes have been discovered to recover the alum from the sludge and reuse it in coagulation. The cost of recovery is also about 1/4th the cost of first cost of alum. The reuse of sludge has also removed the problem of sludge disposal.
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(b) Sodium Aluminate [Na2Al2O3]:
This is an alkaline compound. The best grade it contains Al2O3, 55%; Na2O3, 34%; Na2CO3 4.5%; Na (OH), 6.3%. This can be used for treatment very easily in the water having no alkalinity. It reacts very quickly and forms the precipitate of aluminium hydroxide.
Its chemical equations are as follows:
CaAl2O4 is the required floe, which causes sedimentation. Sodium aluminate does not increase the non-carbonate hardness and it can be easily mixed with lime and soda ash solution. It has the further advantage of removing corrosive qualities of the water. But it is costly due to which it is not widely employed in the water works practice.
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(c) Ferric Coagulants:
Generally ferric chloride (FeCI3), ferric sulphate [Fe2(SO4)3] or the mixture of both is used for coagulation purpose.
The various chemical reactions which take place are as follows:
(d) Chlorinated Copperas:
It is a mixture of ferric chloride and ferric sulphate prepared by adding chlorine to a solution of ferrous sulphate in the ratio of 1 part chlorine to 7.9 parts copperas.
It is very good coagulant and requires less amount of alkalinity in the water for floe formation. The produced floe is tough and easily settles due to which only small residue goes in the filters. This coagulant removes colours very well.
(e) Ferrous Sulphate and Lime:
Fe (OH)3 is the floc, which causes sedimentation. Ferric coagulants are good oxidising agents due to which these also remove hydrogen sulphate, tastes and odours from the water. These coagulants are generally used in the treatment of sewage.
Ferrous sulphate as [FeSO4. 7H2O] is commonly used in coagulation. It is cheaper than alum, and gives good results above pH-value of 8.5. But for coloured raw water it is not used. The quantity of ferrous sulphate as required is more or less the same as that of alum.
Essay # 4. Comparison of Alum and Iron Salts for Use as Coagulants:
The Alum and the iron salts for use as coagulants have their own merits and demerits, which are as under:
(i) Iron salts can be used over a wider range of pH-values.
(ii) Iron salts cause more corrosiveness to the water, than the alum.
(iii) The floe produced by iron salts is more heavy and can remove more % of suspended solids than alum.
(iv) Being good oxidising agents, the iron salts can remove hydrogen sulphide and its corresponding odour and tastes from water.
(v) Iron salts promote the growth of iron bacteria in the distribution system and also cause straining.
(vi) More skilled workers are required for storing and handling the Iron salts than alum.
(vii) Iron salts are corrosive and deliquescent in nature.
Essay # 5. Handling and Storing of Coagulants:
Generally the coagulants are delivered to water-purification plants in paper bags in paper-lined boxes or in paper-lined wooden barrels. At the treatment plant the coagulants should be stored in dry place where deliquescence or caking may not take place.
These chemicals should be stored in the room just above the coagulant preparation room, so that while preparing the solutions or going from one device to another, it may move under gravitational force only.
If the storing of coagulants is required in bulks, it should be done in bunkers or silos with hopper bottom, so that it can be easily taken out. The storage container may be of concrete, steel or wood materials. Lead, rubber, stainless steel and acid resistant bronzes materials may be used for handling the coagulants.
Essay # 6. Dosage of Coagulants:
The dosage of coagulants which should be added in the water, depends on the following factors:
(i) Kind of coagulant
(ii) Turbidity of water
(iii) Colour of water
(iv) pH-value of water
(v) Temperature of water
(vi) Mixing and flocculation time.
In case of more turbid water at lower temperature more quality of coagulants are required. The pH-Value of water should be properly controlled for better floe formation. The floe formed by alum coagulant will disappear if the pH value is lower than the optimum. Similarly if pH value is more, aluminium hydroxide ionizes into aluminate, which is also soluble in water.
Table 12.1 gives the normal dose and pH value required for best floe formation with various coagulant.
Determination of Optimum Coagulant Dose:
The optimum dose of coagulants is determined by Jar-Test Apparatus. Fig. 12.1 shows the jar test apparatus. It essentially consists of four or more large size beakers of 1-2 litres capacity. Stirring paddles of non-corrosive metal are placed in each jar, which can be rotated at any desired speed by gear and spindle system.
For starting the experiment first of all the sample of water in real amounts is taken in every jar. Then coagulant is added in jar in varying amounts. The quantity of coagulant added in each jar is noted. Then with the help of electric motor all the paddles are rotated at a speed of 30-40 R.P.M. for about 10 minutes.
After this the speed is reduced and paddles are rotated for about 20-30 minutes. The rotation of paddles is stopped and the floe formed in each jar is noted and is allowed to settle. The dose of coagulant which gives the best floe is the optimum dose of coagulants.
In water works this test should be done frequently to determine optimum dose and economical use of coagulants. Similarly when the quality and characteristics of water change, the test should be done continuously.
Dose of Chemicals in the Process of Sedimentation with Coagulation:
The amount of coagulants and the auxiliary chemicals which are to be added for best floe formation mainly depends on the turbidity of the raw water, its pH-value and the standard of purity desired. The optimum dose is determined actually by conducting the practical’s on jar test apparatus described above.
Essay # 7. Devices used during Sedimentation with Coagulation:
a. Feeding Devices:
Coagulants can be fed in dry or liquid form. Dry feed devices are desirable because they are simple, require small space for installation, keep neatness, are free from corrosion and are economical. But all the coagulants cannot be fed by dry feed devices, because some coagulants have characteristics of clogging, caking and deliquescence.
The coagulants which have uniformity in grain size, constancy in composition, free from being hygroscopic or efflorescent and remain dry under various conditions of temperature and pressure are suitable for feeding in dry form. Aluminium sulphate can be fed in dry condition. The coagulants which are of corrosive nature and create difficulties in solution feed method should also be fed in dry form.
The water of crystallization of ferrous sulphate changes with temperature therefore it is difficult to be fed in dry form because its powder may change to a solid mass, in the same way hydrated lime cannot be easily fed in dry form, because it may absorb moisture from air and become slaked lime.
The choice of feeding device depends on the type of coagulant and the economy in total cost of plant. In large water works where large quantity of coagulants is required, the chemicals are purchased in the cheapest form and then feeding device is decided.
In the case of small waterworks or where small quantity of water is to be treated the cost of feeding device is the main deciding factor. At such places cheapest type of feeding device is installed and the coagulants which suit it are used.
b. Dry Feed Devices:
These devices are designed on volumetric or gravimetric displacement of dry-chemicals. The chemicals are kept in the hoppers. Fig. 12.2 shows two types of dry feeding devices.
Required amount of coagulant is fed by revolving the helical screw or the toothed wheel fixed at the bottom of the conical hopper. Only required amount of coagulant is drawn off continuously. The speed of rotation of helical screw or toothed wheel is controlled by means of a venturi device installed in the raw water channels or pipes through which raw water flows to the treatment plant. When the quantity of raw water increases automatically the speed of rotation is increased and required amount of coagulant is fed in water at every time.
c. Solution Feed Devices (Wet Feeding):
The chemicals whose solutions can be easily prepared are suitable to be fed by this method. First of all solution is prepared by placing the coagulants in a metal basket, perforated concrete box or perforated wooden box and then spraying warm water over it. The solution so prepared is kept in large tanks to hold sufficient quantity for one operation shift.
Sometimes solution pot is used for this purpose which is most satisfactory method. In the pot coagulants are kept through which the water flows. The rate of flow in the solution pot is directly proportional to the flows in the main channel. Sufficient coagulants are kept in the pot to maintain a saturated solution in the effluent pipe.
Fig. 12.3 shows a solution feed device for adding coagulant solution in the raw water channel. The solution is kept in a constant solution level tank, having a tapered hole in the bottom, which is controlled by a conical plug operated by a rod connected to the pulley.
A small float chamber is constructed and connected to the raw water channel by means of a pipe. A float is kept in float chamber and it operates the pulley by means of a rack and pinion arrangement as shown in Fig. 12.3. When the quantity of raw water increases, the water level in float chamber also increases and lifts and float.
The lifting of float operates the pulley and the conical plug is also lifted thus increases and lifts the flot. The lifting of float operates the pulley and the conical plug is also lifted thus increasing the opening of tapered hole causing more solution to reach the raw water.
The float and conical plugs are so interconnected by means of pulley, shaft, rack and pinion arrangement that only required amount of solution reaches the raw water channel in every case. Thus, this device is an automatic device for feeding solution of coagulants.
Fig. 12.4 shows another type of solution feed device which controls the quantity of solution going in raw water by means of adjustable weir. Its operation can be easily understood from Fig. 12.4.
d. Mixing Devices:
After adding coagulants in water, the next operation is to mix them thoroughly in water so that they fully disperse in the whole water. This mixing is done by mixing devices. In these devices first the coagulants are vigorously and rapidly mixed for about one minute.
Then the mixture is gently agitated for about half an hour so that coagulants may react and start coagulation. The velocity of flow of water in mixing basins is kept between 15-30 cm/sec. The velocity in no case should be less than 10 cm/sec. and more than 75 cm/sec., because in first case the floe will settle down and in second case disintegrate.
Mixing can be done by one of the following devices:
(i) Baffle Type Basins:
In these basins water may flow round about the end baffles or up and down past under and over baffles, the baffle walls are placed 60-100 cm apart and the velocity of water is kept between 15-30 cm/sec. Figs. 10.5 and 10.6 show both the types of mixing basins. The detention period in these basins is kept 20-50 minutes. These are not suitable for small plants because these are costly in construction, have less flexibility of control, and greater loss of head.
(ii) Flash Mixer:
In this device the solution of coagulants is mixed thoroughly in the water by means of a fan operated by electric motor suitable drive. The water enters in through the inlet, the deflecting wall deflects the water towards fan blades where chemicals also reach through chemical pipe.
The rotating fan mixes coagulants with water which finally goes out from outlet.
(iii) Deflector Plate Mixer:
Fig. 12.8 shows this device. In this device the mixing is done by diffusing the water through a deflector plate. Water enters from inlet pipe, then it comes out from the holes provided below the deflector plate where it is agitated rapidly. Chemical pipe brings the coagulants near deflector plate, where they are thoroughly mixed with water.
Essay # 8. Flocculation and Clarifiers used during Sedimentation with Coagulation:
After thoroughly mixing of coagulants in the water the next operation is flocculation. Flocculators are slow stirring mechanisms, which forms floe. Flocculators mostly consist of paddles which are revoling at very slow speed about 2-3 r.p.m. The paddles may revolve on a vertical or horizontal shaft.
The flocculators provide number of gentle contacts between the flocculating particles which are necessary for the successful formation of floe. In one type there are number of compartments fitted with rotating paddles. The water enters from the inlet and leaves through outlet. The detention time for best results should be between 30-60 minutes. Fig. 12.9 shows mechanical flocculator which is most commonly used in waterworks practice.
Clarifiers:
In this operation the floe which has been formed above is allowed to settle and is separated from the water. This is done by keeping the water in sedimentation tanks which are also known as coagulation basins. The design of clarifiers is similar to that of plain sedimentation tanks. Some clarifiers are filled with a moving arm known as raking arm. These devices are also fitted with continuous sludge removing arrangements. Figs. 11.7 to 11.9 show some of the modern clarifiers.
Now a days some firms manufacture combined unit comprising of feeding, mixing, flocculator and clarifier devices. Fig. 12.10 shows one such unit of M/s. Dor. Oliver and Co., U.S.A
Following are the advantages of mechanical flocculators over horizontal flow rectangular baffle wall tanks:
(a) Requirement of chemicals is reduced by 10-40%.
(b) Better floe formation.
(c) Less capacity of tank is required therefore it is cheaper in construction.
(d) More flexibility in operation, because it can be easily controlled.
(e) Very small loss in head of water.
(f) Can be easily installed in the existing plants.
Mechanical flocculators have the following disadvantages:
(a) Dead spaces in corners.
(b) Low velocity near the shaft of paddles.
(c) Bad short-circuiting.
(d) Require careful supervision and maintenance.
Essay # 9. Limitations of the Process of Sedimentation with Coagulation:
The coagulation process removes the suspended impurities of water and considerably reduces the load on the filtration process. The turbidity of water can be removed less than 20 ppm and if the process is properly controlled it can go upto 5-10 ppm. The floe formed in this process also removes bacteria upto 65%. 5-coli index is removed by 70%. The efficiency of the coagulation depends on the proper control of various processes.
Example 1:
Waterworks of a town treat 35 × 106 litres/day. The water is treated by coagulation-sedimentation tanks. The quantity of filter alum is consumed at 20 mg/litres of water. If the alkalinity of the raw water is equivalent to 4.5 mg/litre ofCaCO3 determine the quantity of filter alum and the quick lime (containing 80% of CaO) required per month by the water works. Molecular weights are given as [Ca = 40, C = 12, S = 32, 0= 16, Al = 27 and H =1].
Solution:
Example 2:
Determine the quantity of copperas and the time required per year to treat 4 x106 litre/day, if 11 mg of copperas is consumed with lime at a coagulation basin.
Molecular wright of Fe = 55.85, S = 32, O = 16,H = 1, Ca = 40.
Solution:
Example 3:
The requirements of a city is 40 × 106 litres/day. The detention period is one hour in the tank, and the flow velocity is 20 cm/sec. Design baffle-wall sedimentation tank. Any data not given may be suitably assumed.
Solution:
Fig. 12.11 shows the plane and section of the designed baffle wall sedimentation tank.
Assuming the sedimentation tank to have three channels of 10.0 m clear width.
The effective length of each channel
Example 4:
The population of a town is 1,00,000 and the average per capita demand is 135 litre/day/capita. Design the coagulation cum-Sedimentation tank for the water works, supplying water to the town. The maximum demand may be taken as 1.5 times the average demand.
Assume the detention periods of 5 hours and 30 minutes for setting the average demand. Assume the detention periods of 5 hours and 30 minutes for setting tank and floe chamber respectively. Also assume the flow rate as 900 litres/hour/m2 of plan area.
Solution:
