The sources and the quality of raw water determine the type of treatments to be undertaken. For example, if the water is drawn from deep tube-wells, coagulation and filtration are not necessary. Only softening may be necessary where the water is hard, or aeration where the water contains excessive iron taste or odour. For surface water, flocculation and disinfection are essential whereas pre-chlorination and aeration may be necessary in specific cases, and especially during rainy season.
1. Aeration:
ADVERTISEMENTS:
The principle behind this method is to mix air with water to increase the oxygen content and remove carbon dioxide. Aeration is done to remove iron from groundwater and taste and odour from surface water. It is mostly done in the initial stage of water treatment.
It is a simple process and is accomplished by the following methods:
(i) Cascades – Water is made to fall in a thin sheet down a series of steps like a fountain.
(ii) Spray Nozzles – Water is sprayed under pressure to open air through nozzles.
(iii) Waterfall or Multiples Tray Aerators – Water is discharged through a riser pipe and allowed to fall on a series of trays from which it falls either through small openings or over the edges.
(iv) Mechanical Aerators – These are most applied in waste water treatment, where a large amount of organic matters are to be oxidised, within a short period of time. This method is suitable also for the area where land is a limiting factor.
2. Coagulation and Flocculation:
ADVERTISEMENTS:
Finely divided suspended matters, both organic and inorganic which cause colour and turbidity, do not settle easily through sedimentation in a short period, the object of coagulation is to convert these into easily settleable size, by reducing the zeta potential forces. The latter are repulsive forces which on reduction, bind together the colloidal particles.
Thus, the aggregated molecules of suspended colloidal particles become large enough in size and settle down easily. When the coagulants are added to water, large size flocks are formed on rapid agitation. These flocks absorb colour and during their downward journey entrap suspended matter and bacteria. This process is known as flocculation.
The most common coagulant is aluminium sulphate or Al2 (SO4)3, 18H2O and the next is ferrous sulphate, FeSO4, 7H2O.
Alum is preferred because it requires lesser skill and control.
ADVERTISEMENTS:
The reactions that take place are as such:
In above reaction Ca(HCO3)2 represents the natural alkalinity of waters. Some waters which do not have sufficient alkalinity need the addition of lime. CaO (lime) react with water to form calcium hydroxide, Ca(OH)2 which further reacts with alum as –
In both case the flock aluminium hydroxide is formed as coagulation depends on the pH of water. Different coagulants may be suitable tor different pH range. Alum is most commonly used in public water supply scheme as coagulant. The use of alum in the form of solid in water treatment process is not desirable at all, because as such, the natural buffering capacity of aluminised water becomes locally upset, which in turn, produces excessively nigh turbidity.
Thus, for efficient coagulation and flock formation, optimum pH, adequate does, rapid agitation and gentle transfer of flocculated water to selling tank is necessary.
Quality Criteria for Alum and Lime as Coagulants:
Good quality of alum must contain alumina (Al2O3) in the range of 16 – 17%. Besides there must not be more than 0.7% of insoluble matter and 0.3% iron. The pH value of 10% aqueous solution of alum should vary in the range of 2.7 to 3.2 (TS 299 – 1975) otherwise natural alkalinity of the water to be coagulated would be severely damaged resulting in very high consumption of lime.
Relaxation has however, been granted by Indian Standard Institution, Government of India, by reducing the limit of alumina to 15% for the use as coagulant especially in public water supply purification process (IS 299 – 1980). In case of coal washery wastes the use of alum as coagulant does not achieve the desirable degree of coagulation. For effective coagulation of the suspended coal particulates, recently developed poly-electrolyte in very little dose of 0.3 to 0.7 ppm in combination with alum and lime is of great success. The poly-electrolyte as such works as coagulant aid.
Lime needs to be of equally good quality otherwise effective coagulation is not possible. Good quality of lime must have the minimum 90% content of available lime as CaO (IS-1540- 1978). Too high content of lime as CaO in lime sample is too good to be used in river water treatment rather is wasteful in the case where natural alkalinity of raw water is more than 80 ppm.
Experiments have conclusively proved that lime within 78.6% as CaO is good enough to be used in water purification, especially where natural alkalinity of water ranges from 65 to 110 ppm. Further the content of CaO in lime is shown to be considerably reduced on ageing, especially when the same is stored in the open air, owing to the conversion of CaO into CaCO3.
Approximately, 50% of its total CaO content is lost during one and half month storage in the haphazard manner, Insoluble matter present in the lime as silica is though permitted upto 1.0 per cent (IS 1540 11-1971), but practically of no harm, even if it exceeds a number of times. This is rather helpful in reason where the hardness of water is found usually in the range of 100 to 125 ppm. At the same time, it should always be in mind that the insoluble matters, other than silica, are not desirable, because as such, the treated water becomes even more turbid than the raw water itself.
Therefore, special care is needed in the identification of the types of insoluble matters present as the impurities in both alum as well as in the lime.
For quantitative analysis of alum and lime, well equipped laboratory is needed. Special care in weighing the samples and making volumetric solutions is of vital importance in achieving the accuracy in the result. The details of the procedures for the estimation of alum and lime are described in the IS 299 – 975 and IS 1514-1059 respectively.
3. Filtration:
Filtration aids in the removal of taste, odour, colour, turbidity, iron and manganese and to a large extent of bacterial load. Three types of filters are usually used in water treatment plants.
These are:
(i) Rapid stand filter,
(ii) Slow sand filter and
(iii) Pressure filter.
Relative merits and demerits for each of them are discussed as follows:
Essential characteristic of rapid sand filters arte high rate of filtration and consequently lesser area requirement and washing the filters through reverse flow of filter water through the filter beds. A rapid sand filter consist of a bed supported by gravels of different sizes in layers.
Filter sand for a rapid sand filter should satisfy the following norms:
(a) Sand shall be hard, resistant and free from dirt of every description
(b) Effective size should range between 0.45 to 0.70 mm
(c) Uniformity coefficient must not be more than 1.7 and less than 1.3
(d) Ignition loss not to be exceed 0.7 per cent by weight
(e) Not more than 5 per cent of the sand be soluble in HCI.
(f) Silica content should not be less than 99.8 per cent
(g) Specific gravity to be in the range of 2.55 to 2.65,
The depth of filter sand layer should be between 60 to 75 cm. The standing depth of water above filter sand must be between 1 to 2 meters. The free board above water level should be at least 50 cm.
Besides these, the operation and maintenance of rapid sand filter need the strict implementation of the following knowhow:
The standard rate of Alteration through a rapid sand filter is usually 80 to 100 lpm/m2 (litres per minute/square meter). High rate can be achieved by proper conditioning of water before filtration. For plants of 1000 md (million litres a day) capacity or more, 100 m2 area is needed to be provided in two units of 50 m2 each. Besides the under drain consists of a grid of manifolds, laterals, headers covering the entire filter bottom to collect filtered water and also to distribute wash water. These are perforated and pipes of cost iron, asbestos cement, concrete or other material.
For perforated pipe under-drain system, minimum gravel size should be 2 mm and maximum size 25 mm. Depth of gravel should be 50 cm. Filter gravel shall be spherical, hard, clean and uniform in size. Gravel is placed between the sand and the under-drainage system, the lower size being at the top and higher at the bottom above the under-drains.
It is essential to wash the filter beds at regular intervals to prevent clogging. Washing is done by wash water under pressure, sent through the under-drainage system into the filter beds. Pressure should be such as not to disturb the sand beds. Normal rates of application are 600 lpm/m2, equivalent to a rise of 60 cm/min for a period of 10 minutes. Air wash system is used to precede water wash of the beds. The combined systems of washing reduces the use of wash water, therefore, very useful in case of large filter units. Air is pushed through the under- drains for about 3.5 minutes to agitate the sand beds before introducing wash water.
The disposal of the waste generated during water treatment process by itself a major task for the sanitary engineers and chemists.
The solid wastes, produced in the water treatment plants mainly consist of following composition:
(a) Sludge from flocculation, residual chemical coagulants plankton etc.
(b) Wastes from backwashing of filter beds.
(c) Wastes generated during ion-exchange water softening process.
The erection of this type of treatment plant involves initially very high cost and need a large area than that of rapid sand filter process. The effective size of sand ranges from 0.2 to 0.3 mm. This process is very efficient in removing bacteria, colour and odour, even without the use of any chemical e g. alum, lime and bleaching powder.
However, this process is not at all suitable for the water which has turbidity value greater than 50 ppm and involves a large number of people. This process in fact is meant for the rural area where electricity is a problem and the availability of land is not a limiting factor.
The comparative merits and demerits for the rapid and slow sand filters have, however, been shown in the following table for assessing their feasible at a glance:
(iii) Pressure Filter:
Pressure filter is a type of rapid sand filter in a closed container through which water is passed under pressure. It is not considered very reliable in removing bacteria. Therefore, it is better used to soften and remove iron from groundwater. It has a wide use in clarifying softened water in industrial plants. Nevertheless pressure filter is used in small water works in our country.
A battery of units is installed, as per requirement, all receiving raw or chemically treated water and discharging into a common header pipe or pipes. Vertical pressure, filters are generally 16-96 inch in diameter, while horizontal units are obtainable in 7 to 8 ft. diameter length varies from 8 to 25 ft.
4. Disinfection:
Treatment of water by coagulation, sedimentation and Alteration renders water chemically and aesthetically acceptable.
Although a large part of the bacterial load is reduced during such treatment even then the water does not become absolutely free of them. Water, therefore, requires disinfection which implies the destruction of pathogen, i.e., (disease producing) bacteria.
Disinfection does not completely destroy all micro-organisms, which can be accomplished only by sterilisation. Complete sterilisation however, is not easily obtained, nor is it necessary, for making water potable. The commonly used chemicals and disinfectants are bleaching powder, liquid chlorine iodine, etc. The latter is available in the form of tablets, known as tetra glycine hydro-periodide, galabaline, etc., iodine is used in disinfecting the water in such places, e.g., in the field of war, where no organised water supply facility exists.
Chlorine not only removes bacteria but also eliminates taste, colour, and odour controls algae, protozoa and fungi, improves coagulation and oxidises iron and manganese. It is available as calcium hypochlorite in form of bleaching powder containing 33-35 per cent available chlorine (IS- 1065.1971) and in the form of HTH (High test hypochlorite) powder which is stable carries 65-75 per cent of available chlorine.
These are used in small water works. For medium and large water works, chlorine gas is used because it is more economical and effective. It is compressed into liquid form and supplied in steel cylinders and is injected in gaseous state through chlorinators. Chlorine reacts with water to form hypochlorous acid (HOCl) and hydrochloric acid (HCl).
The reaction is represented as:
Hypochlorous acid being unstable dissociates into hydrogen ions (H=) and hypochlorite ions (OCl) pt.
Both the above equations are reversible, the undissociated HOCl is 80 to 100 times more potent as a disinfectant than the OCl ion. The amount of dissociation is directly dependent upon the pH value of water. The disinfecting action is rapid when the pH value is about 7 or slightly higher. Beyond, it is reduced gradually because more and more HOCl dissociates into OCl ions. Equal amount of HOCl and OCl is present at pH 7.5.
Chlorine present in water as hypochlorous acid, hypochlorite ions and molecular chlorine is defined as free available chlorine. Normal chlorine dose varies from trace to 1 ppm depending on the pollution load. The dose is determined with precision because disinfection will not be complete with insufficient dose, whereas a higher dose will form bad taste and odour, necessitating dechlorination. An effective does of chlorine will leave a residual chlorine charge of 0.1 to 0.2 ppm which indicates that the water has been properly disinfected.
Chlorine, when added to water as free chlorine, oxidises some of the organic and inorganic compounds. While reacting with ammonia and organic amines, it forms chloramines. Chlorine present in water in chemical combination with ammonia or nitrogenous compounds is known as combined available chlorine.
Chloramines are also oxidising agents but are less potent than hypochlorous acid.
The following reactions mark the formation of the three chloramines:
Monochloramine predominates at pH over 7.5 dichloramine at pH between 5 to 6.5 and odorours trichloramine (nitrogen trichloride) is produced at pH below 4.4. It is, however, volatile and disappears within a few hours retention period, but is carcinogenic in nature. The chlorination of water to the extent that all the ammonia be converted into trichloramine (NCl3) or oxidised to free nitrogen is termed as break-point chlorination.
This is estimated by knowing the sudden fall in residual chlorine (RC) value inspite of continued chlorine addition to the same volume of water under test. Similarly, the difference between the amount of chlorine applied to the constant volume of water (say of one litre) and the amount of free, combined or total available residual chlorine remaining at the end of the desired contact period is known as chlorine demand of water.
The latter determines the quality of raw water and thus, bears the positive relationship with the amount of oxidisable organic pollution load present in water. Raw water with a chlorine demand upto a value of 1.1 ppm is considered to be fit to be used as source of public water supply.
To ensure the wholesomeness of the drinking water, accurate estimation of free, residual, combined residual and total residual chlorine is essential. The instrument used for chlorine estimation on spot is of pocket size known as chloroscope. Its manufacture has recently been commercialized in India and it is easily available from any scientific instrument dealer.
(II) Methods for Estimation of Available Chlorine in Water:
Put 0.5 ml of 0.2 per cent orthotoluidine solution + 10 ml water sample + 0.5 ml of 0.5 per cent solution of sodium arsenate (NaASO3).
(a) Read immediately and discard A
(b) Put 0.5 ml NaASO3 + 10 ml water sample + 0.5 ml orthotoluidine solution.
Read immediately B1.
Read after 5 minutes- B2.
(c) 0.5 ml orthotoluidine + 10 ml water sample, Read immediately C
Total residual chlorine = C – B1
Free residual chlorine = A – B1,
Combined residual=(C-B2) – (A-B1)
At 20-25°C and pH 6.8, 0.2 ppm of free residual chlorine (FRC) for 10 minutes contact period is considered as the optimum dose for killing most of the bacteria or rendering them ineffective. The optimum FRC concentration (0.2 ppm) at the furthest points of the distribution network can only be maintained, provided, the following three points are satisfied.
(a) FRC should range from 0.5 to 1.2 ppm at the source.
(b) The distance between the source and the furthest distribution must be preferably within 10 kms, measured along the delivery pipes. For longer distribution system, re-chlorination will have to be done at suitable point to maintain requisite FRC.
(c) The minimum pressure 1.4 kg/cm2 must be maintained throughout the distribution system, otherwise back flows of polluted water may occur.
(III) Estimation of Available Chlorine in Bleaching Powder:
Prepare 1 per cent aqueous solution of bleaching powder. Out of the total prepared solution, 10 ml is taken out and to which 5 ml acetic along with 2 g solid potassium iodine are added. It is then titrated against N/40 sodium thiosulphate (Na2S4O3) solution, using starch as indicator, till the end point comes.
The calculation for the available chlorine is done as indicated below:
% Available chlorine = ml of N/40 Na2S4O3 × 0.89.
(IV) Other Methods of Disinfection:
a. Boiling:
This is the easiest method to destroy all kinds of bacteria, algae etc. It needs vigorous boiling for 15 to 20 minutes. It is applicable for individual use because large volumes of water cannot be treated by this method. During epidemic, water must be consumed after boiling, even if the same is chlorinated, to become doubly sure about its sterility.
b. Ultraviolet Irradiation:
This is the most modern of sterilisation and consists of passing ultra violet rays from mercury vapour arc lamps. The astonishing results of this method is shown here.
Some Special Treatments for Ground Water:
Ground waters are usually harder as compared to the surface waters. Besides, the groundwater in some regions contains large proportion of dissolved elements viz. Iron, manganese, fluoride, chloride, iodine, etc., which necessitate removal.
The groundwater in the extreme southern parts along the plateau regions across the border of Orissa, Madhya Pradesh and Bihar is acidic and as such need to be treated.
1. Hardness:
There are two types of hardness, temporary and permanent. Temporary hardness is caused due to the presence of carbonate and bi-carbonate of calcium, magnesium and other bivalent metals. It can be removed by boiling. This is known as carbonate hardness. Permanent hardness is caused due to the presence of sulphates and chlorides of hardness constituting cations. This is also known as non-carbonate hardness and cannot be removed by easy means like boiling. These salts are very corrosive and deposit scales in steam boilers and in distribution system.
Waters having hardness values of less than 45 ppm is known to be very soft water and with more than 230 ppm as excessively hard water. The consumption of very soft water, especially in old age is desirable, because the toxic elements are removed from the body. The use of very hard water causes the loss of the tenderness of vegetables and meat cooked. Possible inflammation of intestines have been reported to be lessened or removed by consuming moderately soft water.
Removal of Temporary Hardness:
(i) Boiling:
Soluble Ca and Mg bio-carbonates are converted into their insoluble carbonates.
The mechanism of the reactions is as shown below:
(ii) Clark’s Process:
Water is softened by the addition of necessary amount of lime in the form of lime water of milk of lime.
Ca(HCO3)2 is precipitated as CaCO3. In case of Mg(HCO3)2 lime is added in double the quantity to form sparingly soluble Mg(OH)2.
Lime softened water is not stable and causes deposits of calcium carbonate in pipes and filter sands. Addition of alum stabilises it.
Removal of Permanent Hardness:
(i) Soda-Lime Process:
Permanently hard water can be softened by the addition of sodium carbonate (washing soda). It converts the soluble sulphates and chlorides of Ca and Mg into insoluble carbonates of Ca and Mg Water cannot be softened by this process below the hardness value of 21 ppm.
(ii) Ion-Exchange Method:
Water of zero hardness value can easily be produced by this process Zeolites are used as the softener which is complex compounds of sodium, aluminium and silica. These are either artificially prepared or may be obtained from natural deposits and refined. They can easily exchange bases. The principle behind the method is that when hard water (both temporary and permanent) is filtered through zeolite, the calcium and magnesium cations are removed in exchange of sodium.
The filtrate being free from Ca and Mg salts is soft. When the sodium of the zeolite is exhausted, it is regenerated by applying salt solution whereby an exchange is again affected. Softeners of the type are usually constructed like pressure filters.
System of Distribution:
Distribution of water is done either by intermittent or continuous system In the intermittent system, wastage of water may be controlled but the main disadvantage is that during non-supply hours, water in the pipeline rushes towards the lower reaches, thus creating partial vacuum in pipes, resulting in the sucking of impurities and gases in the pipeline. This may cause health hazards. Also during times of fire, intermittent supply system proved to be a great handicap.
Continuous supply, though preferable in every way, but cannot be functional in every town, unless supply to every household is metered to avoid wastage. Grid system of distribution mains where all the pipelines are interconnected is always preferred. It eliminates dead ends.
2. Zoning:
Zoning to the distribution is done for equitable distribution of water throughout the area. It mainly depends on the topography, density of population and type of locality.
Service Reservoirs:
It is of importance for regulation for pressure in the distribution system. . It also regulates the supply in case of failure of electricity, pumps or any other such defects. The elevation of the reservoir should be designed, to maintain minimum residual pressure in the distribution mains. In towns of undulating topography as well as in big towns, several service reservoirs are constructed at suitable places in different zones.
Pipe Size and Residual Pressure:
For towns having population up to 50,000, minimum pipe size shall be 100 mm, whereas for population above 50,000 it should be 150 mm.
Minimum residual pressure at system shall be:
For single storeyed building – 7 m
For two storeyed building – 12 m
For three storeyed building – 17 m
For higher buildings, boosting of water should be done by booster pumps.
Laying and Maintenance of Mains:
For roads wider than 25 meters, distribution pipe shall be laid on both sides provided with suitable valves. Sluice valves should be provided at regular intervals and at junctions to isolate the portions for repair, when necessary. Air valves are provided to release air automatically when pipelines are filled. They are provided at the peak points in the pipeline gradient to release entrapped air.
Absence of air valves may result in deficiency in flow or even bursting of pipes. Scour valves are provided at the valley points. They are opened at regular intervals to cleanse the pipeline while water rushes out to empty the pipeline for repair or replacement. The use of hemp yarn while caulking with lead of cast iron pipe sometimes infects the water. The usual practice is to use bituminous coating as pipe lining materials is identified to increase the concentration of Poly nuclear aromatic hydrocarbon (PAH).
Now-a-days, cement lining inside and zinc coating outside, of the cast iron pipe is gaining importance for smooth surface and longer working life. The latter is high accumulating in nature and known to be teratogenic (a substance which induces a change in the genes). The use of Poly Vinyl chloride (PVC) pipe in public water supply is also not good, because it contributes lead by leaching to the water if not properly controlled. PVC is not good for hot water supply.
Water supply to any individual building is made through house service connection. It is guided by the bye-laws of the concerned municipality.
Wastage of Water:
Wastage may occur in treatment plant, from leakages in service reservoir and pipelines, careless use by consumers through continuous flow in public hydrants, due to theft of cocks and so on. Levels of wastage upto 10% may be considered low but above it, corrective measures are needed from the point of view of economy.
Protection against Pollution near Sewers and Drains:
Water main should be at least 3 m away from the existing or proposed sewer or drain. In case where it has to cross house or sanitary sewer of storm drain the bottom of the main should be at least 0.5 m above the top of the drain or sewer.
Incrustation and Corrosion of Delivery Pipes:
Incrustation and corrosion are the major enemies of distribution pipes and valves, causing tremendous national loss in term of money and loss of output. Incrustation significantly reduces carrying capacity of the water delivery pipes. Considerable amount of energy is lost due to incrustation in boilers.
Similarly the aesthetic value of drinking water is tremendously lost due to incrustation and corrosion of cast iron pipes. The life of shallow hand pump pipes is drastically reduced from average length of 30 years to less than 5 years in regions where water is either acidic or contains exceedingly high proportion of soluble iron.
Keeping in view the tremendous loss, caused mainly by incrustation of corrosion, an urgent need to envisage a proper check and balance method has been felt throughout the world. A laboratory method to ascertain the nature of water, along with easily applicable methods for the control are briefly presented here.
Laboratory Method to ascertain the Corrosive and Incrustating Nature of Water:
Langelier Index – LI = pH – pHs (1)
Where, pH is actual pH of water and pHs = Saturated pH produced by adding calcium carbonate.
The Langelier inded (LI) with zero value indicates that water is neither corrosive nor incrusting in nature i.e. the water is chemically balanced. A positive value indicates the tendency of water toward incrusting. A negative value indicates tendency to eat up the metals i.e. water under examination is corrosive in nature.
The value of the saturation pH in equation (1) may be determined as:
pHs = 9.3 + (A+B) – (C+D) (2)
The co-efficient A, B, C and D indicate the following factors:
(i) Factor A – TDS
(ii) Factor B – Water temperature
(iii) Factor C – Calcium carbonate in ppm
(iv) Factor D – Total range alkalinity in ppm as calcium carbonate.
Prevention of Incrustation:
Incrustation cannot be prevented entirely, but it can be delayed and kept in check by keeping the draw-down as small as possible, as follows:
(i) In order to reduce the head loss to a minimum the well must be developed properly so that aquifer losses are kept at its minimum.
(ii) A screen with large open area and fully penetrating the aquifer should be installed. As such, the high precipitation of iron salts and carbonate will be reduced.
(iii) The pumping rate should be reduced by increasing the pumping period. The required quantity of water may be obtained from several wells rather than pumping from a few large wells at excessively higher rates.
(iv) The screens should be cleaned periodically, at least once in a year, even if the discharge has not fallen-off.
In case of already incrusted wells where treatment is essential, the same may be achieved by using acid, chlorine or other effective agents, depending upon the nature of the incrusted materials. Hydrochloric and sulphuric acids are effective in removing carbonates. Sodium hexa meta phosphate is effective in dispersing iron and manganese oxides. Chlorine is effective in removing bacterial growth and slimes.
3. Corrosion:
The negative Langelier index results in corrosion which in turn, reduces the strength of the pipe, increases the screen size, causing sand pumping and eventual failure of the tubewell. The important reasons for corrosion are galvanic action corrosive water, high velocity and unsaturated waters.
(i) Electrolytic or Galvanic Action:
It is the most important factor leading to corrosion. The principle behind electrolytic corrosion is that when two or more dissimilar metals are dipped into solution (viz. in water) a solution potential is set up which causes the electrons to move from cathode (electrode) to anode (electrode).
Thus the anodic portion is corroded and deposition takes place on the cathode. The elements lying above the hydrogen in electrochemical series, act as anode and those below as cathode. The positions of the elements in electrochemical series determine the rate of corrosion, depending upon their mutual positions. Generally, the greater the difference in their position, more intense corrosion is produced.
For example, if iron and brass are coupled together in water, iron acts as anode and brass as cathode. Thereby, iron is progressively being corroded and brass remains protected. Sometimes, corrosion takes place in the same metal between two path of slightly dissimilar composition. This is known as local corrosion. The protection of iron pipe is achieved by galvanizing the same. In doing so, the zinc start working as anode and iron as cathode. Thus, zinc is corroded and iron is protected from corrosion.
(ii) Corrosive Water:
If the water is acidic, it attacks the metals directly and causes corrosion. Hydrogen sulphide corrodes steel and copper alloys, if it is present in quantities greater than 1 ppm. If the dissolved carbon dioxide concentration is more than 53 ppm and DO is 2 ppm, make the water corrosive. The electrical conductivity value above 1500 ohms/cm makes water corrosive.
(iii) High Velocity of Water:
The high flow of water removes the hydrogen from the cathode, which in turn accentuates the process of corrosion. Very low velocity of water tends to prevent local corrosion but makes it uniform.
(iv) Unsaturated Water:
When water is deprived of the minimum quantity of iron that water can normally contain. It is known as unsaturated water. As such, the water corrodes iron and steel until it becomes saturated. The water with less than 50 ppm of hardness is generally unsaturated and often causes corrosion.
Methods to Control Corrosion:
(i) The use of corrosion resistant materials can effectively control corrosion. Such materials include everdur bronze (16% copper, 3% silicon, 1% manganese) stainless steel, polythene, epoxy-bonded fibre glass etc.
(ii) Galvanizing – coating with plastics etc. offer considerable protection from corrosion.
(iii) Reduction in velocity reduces the rate of corrosion. It can be achieved by increasing the percentage of open area of the diameter of the well pipe or decreasing the rate of pumping or the draw down.
(iv) The corrosiveness of water may be affectively reduced by suitably treating such water. The addition of lime to neutralise the acidity of water is worth suggesting.
(v) Cathode protection.
In this method an alternative anode is selected. Thus, the iron pipe may be protected from corrosion by making zinc as anode.