Various processes for the treatment of impure water are given below. It is not essential that all these processes will have to be employed at all places, but it depends upon the quality of raw water.
Process # 1. Removal of Dissolved Gases:
It has been found that some of the gases if present in water in dissolved form may cause certain difficulties. Dissolved carbon dioxide corrodes the pipes. Similarly oxygen, chlorine, and other gases, if in dissolved form, are present in excess amounts, also cause difficulties.
Many dissolved gases can be removed by boiling, decompression or by means of chemical treatment. Except oxygen and nitrogen all other gases can be reduced by aeration. Aeration process removes carbon dioxide, hydrogen sulphide, and odours very rapidly.
Following are some of the methods of aeration:
(i) By mechanically agitating impure water.
(ii) By diffusing compressed air inside the impure water.
(iii) Mixing air in water under pressure.
(iv) By spraying water into the atmosphere through nozzle; 1 to 2-3 metres.
(v) Flowing impure water through perforated trays and coke beds, so that the water filters through them.
(vi) By flowing impure water over weirs, steps etc., so that water is exposed to sun as much as possible.
Process # 2. Removal of Iron and Manganese:
Manganese and Iron are generally found together, in impure waters. Iron is found in the form of ferrous sulphate and ferrous bicarbonates.
The presence of iron and manganese in excess of 0.3 ppm renders water objectionable due to following reasons:
(i) They cause corrosion to plumbing works.
(ii) They cause taste and odour.
(iii) They cause troubles in various manufacturing processes and make them uneconomical.
(iv) They cause spots on clothes during washing or during their use in textiles.
(v) They may make water reddish due to presence of iron bacteria.
Removal of iron and manganese can also be done by any one of the following methods:
(i) By base-exchange processes.
(ii) By chlorination.
(iii) By aeration of impure water.
Iron alone in the absence of organic matter can usually be removed by aeration of any type, followed by sedimentation and filtration.
Combination of iron and manganese or iron alone loosely bound to organic matter may require aeration in multiple coke trays (Fig. 1) containing coke, gravel or crushed pyrolusite (pyrolusite is a negative manganese dioxide). It has been revealed that meta-phosphates may be used to prevent precipitation of iron or manganese. Their use is generally applicable when the iron concentration is less than 1 ppm.
Process # 3. Removal of Silica:
The following are the methods which may be used for silica removal:
(i) By using magnesium hydroxide with carbon dioxide, calcium bicarbonate or magnesium bicarbonate which produce magnesium carbonate absorbing silica.
(ii) Apply ferric sulphate and lime to develop ferric hydroxide which absorbs silica.
Process # 4. Removal of Manganese Alone:
For the removal of manganese alone, green sand and carbonaceous cation exchangers may be used, with salt for regeneration. During aeration soluble ferrous and manganese compounds get converted into insoluble ferric and manganese compounds which are then removed in settling tanks or filters. The iron is mostly present in water in ferrous bicarbonate form.
During aeration the following reactions take place:
Fe (HCO3)2 + 2H2O → FeO + 2CO2 + 3H2O
4FeO + O2 = 2Fe2O3
Fe2O, + 3H2O = 2Fe(OH)3
Fe(OH)3 is insoluble in water.
Similar action takes place with manganese bicarbonate. When iron and Mn occur in water in combination with organic matter, it becomes difficult to break the bond between them. Once the bond is broken, the treatment is as mentioned above. The bond may be broken either by adding lime and raising pH value of water to about 8.50 to 9.00 or by adding chlorine or potassium permanganate.
Process # 5. Removal of Taste, Odour and Colour:
Coagulation followed by filtration, pre-chlorination, super chlorination followed by de-chlorination and use of chlorine dioxide are the methods which help in the removal of taste, odour and colour.
Process # 6. Conversion of Saline Water:
Sea water contains about 35000 ppm of dissolved solids. No single process of treatment can be suitable for making such water safe. Vapour- compression method, ion exchange method, solar distillation, freezing, osmotic processes and ultrasonic are the methods which may be employed for purification of such waters.
Ion exchange method is more promising when the concentration of dissolved material is below 4000 to 5000 ppm. Several plants for applying this method have been constructed recently in U.S.A.
Process # 7. Removal of Radio-Activity from Water:
Radio-active materials may pollute the sources of water supply which is used for drinking purposes. Dangerous materials may be mixed with water due to nuclear blasts, wastes from atomic energy installations or use in research, industry or medicine.
The wastes from atomic energy installations are so controlled that an appreciable health hazard is unlikely. Radio-active materials may be partly removed from water by ordinary methods of coagulation. Removals up to 80 to 90% can be expected. No other feasible method have been devised so far.
Process # 8. Removal of Dissolved Minerals:
Kenzelite and Zepholite proprietary, base-exchange compounds, have been used successfully in the removal of lead, zinc, copper and tin. According to Mr. Stretcher, waters having dissolved solids from 1000 to 3000 ppm may be demineralized successfully by the application of a direct electric current in specially designed cells with canvas or similar diaphragms.
Process # 9. Removal of Oils:
It may be removed by absorption by passing the impure water through containers of excelsior.
Hardness of Water or Softening of Water:
Hard water has the following bad effects:
1. It develops bad taste.
2. It develops corrosion and incrustations in pipes.
3. It also influences the working of dyeing.
4. It develops scales in the boilers.
5. It consumes more soap.
The hard water has to be made soft by certain methods before it is supplied to the consumers.
Types of hardness:
Temporary hardness is caused due to the presence of bicarbonates of calcium and magnesium. The permanent hardness is caused by the presence of sulphates, chlorides of calcium and magnesium. This is also called non-carbonate hardness.
Removal of temporary hardness:
This hardness of water can be removed by either boiling or by adding lime.
Chemical reaction may be as follows:
Ca(HCO3)2 + Heating → CaCO3 + H2O + CO2
Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 + 2H2O
Mg(HCO3)2 + Ca(OH)2 → CaCO3 + MgCO3 + 2H2O
Process # 10. Removal of Permanent Hardness:
The following three methods may be adopted for this purpose:
1. Zeolite process
2. Demineralization process
3. Lime soda process
1. Lime soda process:
Hydrated lime removes permanent hardness due to magnesium sulphate, magnesium chloride and calcium chloride while washing soda eliminates permanent hardness due to calcium sulphate, calcium chloride and magnesium chloride.
Chemical reactions are given here:
(a) Excess lime treatment:
In this method, impure water is over-treated with lime in order to completely precipitate magnesium. Soda ash is added to neutralize the excess lime, converting all alkalinity to sodium alkalinity. After Alteration if the pH is about 8.0 it, will be good for water with hardness of about 30 ppm.
In this process excess lime is added to impure water. Excess lime is then neutralized by the action of CO2.
Process # 11. Sewage Treatment:
For sewage treatment we have Rotating Biological contactor, anaerobic filter and grass plots. RBC can be operated with raw sewage while the anaerobic filter can function as a composite secondary treatment device in which case the sewage should be free from grit and preferably partly homogenized.
The filter can also be used as a secondary treatment device for treating effluent from septic tanks. Overland grass filtration treatment of filter effluent will be useful adjunct for further ‘polishing’ the effluent from filter or RBC.
Based on the studies so far conducted, the process design criteria have been evolved and are summarized briefly as follows:
Process # 12. RBC for Treating Raw Sewage:
Process # 13. Grass Plot: Hydraulic Loading:
0.8 to 1.5 m3/day/sq. meter of area minimum 2 plots.
Slope: 1 in 80 to 120. BOD removal efficiency: 30 to 60 percent.
Process # 14. Chlorinators for Disinfection of Drinking Water:
Direct feed type chlorinator:
A simple method of disinfection for piped supplies in to feed bleaching power solution directly in the suction line of the centrifugal pump, used for drawing water. The bleaching powder is fed from a storage tank through an intermediate container.
The strength of solution used is 1%. The flow into the intermediate container is regulated such that sufficient solution is always present to avoid air bubbles being sucked in. This arrangement is simple and does not require any special gadgets, See fig. 2.
Process # 15. Differential Pressure Type Chlorinator:
This type of chlorinator is commercially available for use with piped water supplies. A solution containing bleaching powder and soda ash in proportion of 5 : 1 filled in a rubber bag (housed in a metal container) is gradually squeezed out in proportion to the differential pressure across an orifice plate, see fig. 3.
Fig.4 shows a chlorinator used for application of gaseous chlorine. The figure 4 is self explanatory to kill the germs in drinking water.
A = Chlorine supply pipe
B = Water supply to chlorine solution.
C = Chloronome or pulsating meter.
D = Chlorine high pressure gauge.
E = Stop valve.
F = Pressure reducing valve No. 1.
G = Chlorine absorption tower.
H = Chlorine solution outlet pipe connected to main supply.
J = Chlorine low pressure gauge.
K = Regulating valve.
L = No. 2 pressure reducing valve.
M = Connecting tube.
N = Filter.
O = Cylinder stop valve.
P = Connector valve.
Q = Cylinder cap.