The analysis of water involves the various tests to be carried out for determining the impurities which may be present in water.
The following three types of analyses are carried out for water:
(1) Physical Analysis:
The physical analysis of water involves the physical tests which are carried out to determine the physical impurities and the corresponding physical characteristics of water.
In this analysis tests are carried out for determining colour, taste, odour and turbidity of water which are described below:
ADVERTISEMENTS:
Colour may be imparted to water by the presence of natural metallic ions (iron and manganese), peat (decayed vegetable matter), weeds, humus, plankton, and industrial wastes. An undesirable appearance is produced by colour in water and people may not like to drink coloured water. Further coloured water may spoil the clothes washed in it and it may affect various industrial processes. As such colour should be removed from water to make it suitable for general and industrial purposes.
The colour in water is measured by platinum-cobalt method. In this method colour is measured by visual comparison of the water sample with the standard coloured water prepared by dissolving platinum-cobalt in distilled water. The intensity of colour in water is expressed on the platinum-cobalt scale as the number of colour units. On this scale one unit of colour is the colour produced by 1 mg of platinum-cobalt dissolved in 1 litre of distilled water.
The standard coloured waters of different colour units are prepared in the laboratory for being used for comparison. A simple instrument called tintometer is used for comparing the water to be tested with the standard coloured water. This instrument has an eye-piece with two holes. A slide of standard coloured water is seen through one hole and through the other hole; the slide of the water to be tested is seen.
ADVERTISEMENTS:
If the colours of the two slides differ the slide of standard coloured water is replaced by another slide of different colour units. The process is repeated till the colour of the water to be tested matches with the colour of the standard coloured water. The number of colour units of the matching standard coloured water thus gives the number of colour units of the water sample.
The platinum-cobalt method is, however, not convenient for field use. As such another method is generally adapted to measure the colour in water in the field. In this method the water sample to be tested is compared with special, properly calibrated glass colour discs instead of standard coloured waters. The glass colour discs are calibrated to correspond with the colours on the platinum- cobalt scale. The glass colour discs give results which are in substantial agreement with those obtained by the platinum-cobalt method.
For drinking water the number of colour units on platinum-cobalt scale should not exceed 5. However, the number of colour units in excess of 5 may be tolerated but it should not exceed 25.
ADVERTISEMENTS:
The taste of water may be bitter, salty, sour and sweet. Similarly water may possess odour such as unpleasant, earthy, fishy, grassy, mouldy, peaty and sweetish. Taste and odour are closely related and these may be imparted to water by the presence of dissolved gases such as H2S, CH4, CO2, O2, etc., combined with organic matter, mineral substances like NaCl, iron compounds, carbonates and sulphates of other elements, and phenol and other tarry or oily matter. It is evident that the water to be supplied from a public water supply scheme should not have any undesirable or objectionable taste and odour.
The taste of water is measured by flavour threshold test. In this test the water sample to be tested is diluted with water free from any taste to such an extent that the mixture of the water sample and the added water just becomes taste free.
The volume of the water sample and that of the taste free water added for dilution are measured and the taste of the water sample is expressed in terms of flavour threshold number (FTN) which represents the dilution ratio at which the water sample loses its taste.
The flavour threshold number (FTN) is computed as follows:
ADVERTISEMENTS:
FTN = A + B/A …..(8.1)
Where,
A = volume of water sample in ml, and
B = volume of taste free water (or diluent) added in ml.
ADVERTISEMENTS:
Equation 8.1 shows that the number of times the water sample is diluted to just make it taste free represents the flavour threshold number (FTN). Thus if for diluting 25 ml of water sample 175 ml of taste free water is required to be added to make the water sample to just lose its taste, then the flavour threshold number (FTN) will be 8.
The odour of water is measured by threshold odour test. In this test the water sample to be tested is diluted with odour free water to such an extent that the mixture of the water sample and the added water just becomes odour free.
The volume of water sample and that of the odour free water added for dilution are measured and the odour of the water sample is expressed in terms of Threshold Odour Number (TON) which represents the dilution ratio at which the water sample loses its odour.
The threshold odour number (TON) is computed as follows:
TON = A + B/A … (8.2)
Where,
A = volume of water sample in ml, and
B = volume of odour free water (or diluent) added in ml.
Equation 8.2 shows that the number of times the water sample is diluted to just make it odour free represents the threshold odour number (TON).
The drinking water should not have any objectionable taste and odour and for this the flavour threshold number (FTN) and the threshold odour number (TON) should not exceed 3 and should preferably be 1.
Turbidity in water is caused by suspended matter, such as clay, silt, finely divided organic and inorganic matter, soluble coloured organic compounds, and plankton and other microscopic organism. Turbidity is an important consideration in public water supplies for three major reasons, viz., (i) aesthetic, (ii) filterability, and (iii) disinfection. Turbid water has muddy or cloudy appearance and it is aesthetically unattractive.
Filtration of water is rendered more difficult and costly with the increase in turbidity. The use of slow sand filters has become impractical in the areas having highly turbid water, because high turbidity shortens filter runs and increases cleaning costs.
In cases where turbidity is caused by sewage solids the disinfection of public water supplies may not be effective because many of the pathogenic organisms may be encased in the particles and protected from the disinfectant. As such the water supplied to the consumers from a public water supply scheme should be free from turbidity.
Turbidity is expressed in terms of parts of suspended matter per million parts of water or shortly written as ppm. It may be noted that for water 1 ppm is approximately equal to 1 mg. per litre and hence turbidity is also expressed in terms of mg per litre (or mg/l). The standard unit of turbidity is the turbidity produced by one part of Fuller’s earth which is in the form of finely divided silica in a million parts of distilled water.
The measurement of turbidity in the field may be done with the help of a turbidity rod which is described below:
Turbidity Rod:
A turbidity rod consists of an aluminum rod about 203 mm long which is graduated to give turbidity directly in ppm. At the upper end of the rod a graduated non-stretchable tape about 1220 mm long is attached. At the lower end of the rod a screw containing platinum needle and a nickel ring. The platinum needle is 1 mm in diameter and 25 mm long. The nickel ring is provided to insert a stick so that the rod may be held in vertical position. On the graduated tape there is a mark for eye position.
For measuring turbidity of water the rod is gradually lowered in the water to be tested and the depth at which the platinum needle just ceases to be seen (keeping the eye at the eye mark) under standard light conditions is noted. The corresponding reading on the rod gives the turbidity of water in ppm.
In laboratory turbidity may be measured with the help of instruments called turbidimeters. In general a turbidimeter works on the principle that with the increase in the turbidity of a water sample the interference caused by the water sample to the passage of light rays increases.
(2) Chemical Analysis:
The chemical analysis of water involves the chemical tests which are carried out to determine the chemical impurities and the corresponding chemical characteristics of water. In this analysis tests are carried out for determining total solids presence in water and its pH value, hardness, chloride content, nitrogen content, alkalinity, dissolved gases, and metals or minerals and other chemical substances.
These tests are described below:
The total solids present in water comprise total dissolved solids (TDS) and suspended solids. Out of the two the dissolved solids usually predominate and these mainly consist of inorganic salts and small amounts of organic matter. The suspended solids are usually present in small amounts.
For determining the amount of total solids present in water a measured volume (between 100 ml to 300 ml) of water is placed in a crucible or dish and it is evaporated to dryness in an oven at 105°C, and the weight of the dry residue left is determined. The amount of total solids present in the water in mg/l or ppm is given by the expression-
For determining the total dissolved solids (TDS) present in water, it is first filtered through Whatman filter paper No. 44 so that the suspended solids present in the water are removed. A measured volume of filtered water is placed in a crucible or dish and it is evaporated to dryness in an oven at 105°C, and the weight of the dry residue left is determined. The amount of total dissolved solids present in the water in mgH or ppm is given by the expression-
The suspended solids present in water is then given by the expression
The amount of suspended solids present in water may also be determined by another method in which a measured volume of water is taken and it is filtered through a Whatman filter paper No. 44. The residue left on the filter paper is collected, dried and weighed. The amount of suspended solids present in the water in mg/l or ppm is given by the expression.
The results obtained by this method for determining the amount of suspended solids present in water would be correct only if water contains relatively large amount of suspended solids.
For drinking water the amount of total dissolved solids (TDS) should not exceed 500 mg/I. However, the amount of total dissolved solids present in water in excess of 500 mg/I may be tolerated but it should not exceed 1500 mg/l which is the maximum permissible limit for the total dissolved solids in water to be considered suitable for human consumption.
ii. pH Value or Hydrogen-Ion Concentration:
pH is defined as the logarithm of the reciprocal of hydrogen ion (H+ ion) concentration. The pH value or hydrogen ion concentration of water is a measure of acidity and alkalinity of water.
In general water (H2O) is a combination of positively charged hydrogen ions (H+ ions) and negatively charged hydroxyl ions (OH– ions). In pure water the concentration of H+ ions is equal to that of OH– ions. When some substance is dissolved in pure water the solution formed ionizes (i.e., splits up into H+ ions and OH– ions) and the balance between the concentrations of H+ ions and OH– ions is disturbed.
If the concentration of H+ ions is in excess of the concentration of OH+ ions the water solution becomes acidic and if the concentration of OH– ions is in excess of the concentration of H+ ions the water solution becomes alkaline. For neutral water (i.e., neither acidic nor alkaline) concentrations of H+ ions and OH– ions are equal. It has been found that in a water solution the product of the concentration of H+ ions and the concentration of OH– ions is constant.
This is in accordance with the law of mass action for electrolytes which states that –
The brackets indicate concentration of the ions in moles/litre. A mole is the molecular weight in gm. Since the dissociation of water is very slight the concentration of undissociated molecules of water may be taken as constant.
Thus from equation (i) we obtain –
where Kw = ionization constant or ion product of water
The constant Kw is found to be equal to 10-4 at a temperature of 25°C.
Equation (ii) can be used to calculate H+ ion concentration when OH– ion concentration is known, and vice versa. Since for pure or neutral water concentration of H+ ions and OH– ions are equal, a litre of pure or neutral water at 25°C contains
10-7 gm of H+ ions × 10-7 gm of OH– ions = 10-14 … (iii)
Further pH being the logarithm of the reciprocal of H+ ion concentration, the value of pH for pure of neutral water is obtained as –
As compared to pure or neutral water an acidic water has more concentration of H+ ions and an alkaline water has less concentration of H+ ions, and pH being the logarithm of the reciprocal of H+ ions concentration, the value of pH for an acidic water will be less than 7 and for an alkaline water it will be more than 7. Figure 8.5 shows pH scale from which it may be observed that when the value of pH is equal to zero there is maximum acidity in water and when the value of pH is equal to 14 there is maximum alkalinity in water.
The acidity in water is caused by the presence of mineral acids, free carbon dioxide, sulphates of iron, aluminium etc. On the other hand the alkalinity in water is caused by the presence of bicarbonates of calcium and magnesium or by the presence of carbonates or hydroxide of sodium, potassium, calcium and magnesium.
Measurement of pH:
The pH value of water can be measured by the following two methods:
(a) Electrometric method
(b) Colorimetric method
(a) Electrometric Method:
In this method the pH value of water is measured with the help of a potentiometer which measures the electrical potential exerted by H+ ions and thus indicates their concentration.
(b) Colorimetric Method:
In this method chemical reagents or indicators are added to water and the colour produced is compared with the standard colours of known pH values. A set of sealed tubes containing coloured waters of known pH values is kept in the laboratory for comparison. A wide variety of indicators are in use for different ranges of pH values. These indicators have the characteristic of showing distinct colour changes over a range of pH values differing by approximately two.
A list of these indicators along with the optimum range of pH values and the change in colour produced is given in Table 8.1:
Alternatively instead of standard colour tubes properly calibrated glass colour discs may be used for comparison. The use of the discs would avoid the cumbersome and time consuming process of preparation of a set of standard colour tubes. As such the use of the discs is becoming increasingly popular in the colorimetric method of measurement of pH value of water.
For drinking water the value of pH should be between 7.0 and 8.5. However, water having pH value less than 7.0 or more than 8.5 may also be accepted but the one having pH < 6.5 or > 9.2 is not suitable for human consumption and it should be rejected.
Further from general considerations the waters having lower values of pH (i.e. acidic waters) may cause tuberculation and corrosion, while those having higher values of pH (i.e., alkaline waters) may cause incrustation, sediment deposits, difficulties in chlormation, certain physiological effects in human system, etc.
Hardness is that characteristic of water which prevents the formation of sufficient lather or foam with soap. The hardness of water is caused by the presence of bicarbonates, sulphates, chlorides and nitrates of calcium and magnesium.
The hardness caused by the presence of bicarbonates of calcium and magnesium is known as carbonate hardness. The carbonate hardness is also known as temporary hardness because it can be removed by boiling the water or by adding lime to the water.
On the other hand the hardness caused by the presence of sulphates, chlorides and nitrates of calcium and magnesium is known as non- carbonate hardness. The non-carbonate hardness is also known as permanent hardness because it cannot be removed by simply boiling the water. It requires special treatment of water softening. The total hardness (T.H.) of water is the sum of the carbonate hardness (C.H.) and the non-carbonate hardness (N.C.H.), i.e., T.H. = C.H. + N.C.H.
Further the natural alkalinity in water is due to the presence of carbonates and bicarbonates in water. However, most of the natural alkalinity in water is due to bicarbonates which are produced by the action of groundwater on limestone or chalk as shown by the following equation –
Thus when the total hardness is greater than the total alkalinity of water then the amount of hardness equivalent to the total alkalinity is equal to the carbonate hardness and the amount of hardness in excess of this is equal to the non- carbonate hardness. On the other hand when the total harness is equal to or less than the total alkalinity then the entire hardness is the carbonate hardness and the non-carbonate hardness is absent, i.e.-
The hardness of water is usually expressed in ppm or mg/l of calcium carbonate (CaCO3) present in water.
A scale of hardness showing different levels of hardness of water is as follows:
The hardness of water is also expressed in terms of degree of hardness. As per Clark’s scale water is said to have one degree of hardness when its power of soap destroying is equivalent to the effect of 14.254 mg of calcium carbonate (CaCO3) present in one litre of water. Again for water since 1 mg/l is approximately equal to 1 ppm, one degree of hardness will be equal to 14.254 ppm of CaCO3 present in water. It has been found that each degree of hardness causes wastage of about 0.60 gm of soap.
The above indicated degree of hardness is termed as British degree of hardness. On the other hand one French degree of hardness is equal to 10 ppm of CaCO3 present in water, and one American degree of hardness is equal to 17.15 ppm of CaCO3 present in water.
Measurement of Hardness:
The hardness of water is usually measured either by the soap solution test or the Versenate or EDTA method. The EDTA method is considered to be more accurate. Both these methods are described below.
In the soap solution test the standard soap solution is added to the water sample and it is vigorously shaken for about five minutes and the formation of lather is observed. The hardness of water sample is then determined from the difference between the total amount of soap solution added to the water sample and the lather factor which is the amount of standard soap solution required to produce lather when added to distilled water of zero hardness.
In the EDTA method the total hardness is measured by titrating the water sample against Ethylene diaminetetracetic acid (EDTA) or its sodium salt so as to form stable complex ions with the calcium (Ca++) or magnesium (Mg++) ions in water accordingly the following equation:
Determination of Hardness of Water from the Results of Chemical Analysis of Water:
The hardness of water is normally expressed in terms of calcium carbonate. The results of the chemical analyses for individual ions are usually given in terms of that ion.
It will thus be necessary to convert the analytical results to the common denominator for which the following expression may be used:
(4) For measuring alkalinity, reading of only CO3– – or HCO3– will be required and expressed as percentage. The values of the carbonate and bicarbonate alkalinities can be determined from the following relations-
Total alkalinity as CO3 in mg/I = (Carbonate alkalinity in mg/l) × 0.60;
Total alkalinity as HCO3 in mg/l = (Bicarbonate alkalinity in mg/l) × 1.22
(5) Molecular wt. of HCO3 = [1 + 12 + (3 × 16)] = 61; and
Molecular wt. of CO3 – [12 + (3 × 16)] = 60
The excess hardness of water is undesirable because it causes more consumption of soap, modifies colour if used in dyeing work, carbonate hardness produces scale in boilers, causes corrosion and incrustation of pipes, and makes food tasteless.
For drinking water the total hardness should be less than 200 mg/l. However, water of total hardness more than 200 mg/l may also be used for drinking purposes but the one having total hardness more than 600 mg/l should be rejected.
Chlorides are usually present in water in the form of sodium chloride (common salt). These impart a salty taste to water. The chlorides may be added to water due to solvent power of water dissolving salts from top soils as well as from underground formations, intrusion of sea water into fresh water in the coastal regions, disposal of industrial and domestic wastes, and human excreta into streams or rivers, etc.
The presence of large quantity of chlorides in water indicates its pollution due to sewage, minerals, etc. The chloride concentrations of raw waters being used for public water supplies should therefore be tested regularly, so as to detect any sudden increase in their chloride contents and the possibility of the organic pollution of the sources of water.
The chloride content of water can be measured by titrating the water with standard (N/35.5) silver nitrate (AgNO3) solution using potassium chromate (K2Cr2O7) as indicator. The silver first reacts with all chlorides and silver chloride is formed as indicated by the following equation –
A high chloride content in water may harm metallic pipes and structures as well as growing plants.
For drinking water the chloride content should be less than 200 mg/l. However, water having chloride content more than 200 mg/l may also be used for drinking purposes, but the one having chloride content more than 1000 mg/l should be rejected.
The presence of nitrogen and its compounds in water is an indication of the presence of organic matter in water and the extent to which the organic matter has undergone decomposition (or oxidation) resulting in the pollution of water.
The nitrogen is present in water in the following four forms:
(a) Free ammonia
(b) Albuminoid nitrogen
(c) Nitrites
(d) Nitrates
(a) Free Ammonia:
The presence of free ammonia in water indicates the presence of undecomposed organic matter. Further it also represents the first stage of decomposition of organic matter present in water and thus indicates that the pollution of water has just commenced.
(b) Albuminoid Nitrogen:
The albuminoid nitrogen present in water represents the quantity of nitrogen present in water before decomposition of organic matter has started.
(c) Nitrites:
The presence of nitrites in water indicates that the organic matter present in water is partly decomposed (or oxidised) or in other words it indicates an intermediate stage of decomposition (or oxidation) of organic matter present in water.
(d) Nitrates:
The presence of nitrates in water indicates that the organic matter present in water is fully decomposed (or oxidised), and thus indicates that the water has been undergoing pollution since long time in the past.
The amount of free ammonia present in water may be measured by simply boiling the water and measuring the ammonia gas thus liberated. The amount of albuminoid nitrogen present in water may be measured by adding strong alkaline solution of potassium permanganate (KMnO4) and then boiling it, and measuring the ammonia gas thus liberated which indicates the amount of albuminoid nitrogen present in water.
The amount of nitrites and nitrates present in water may be measured by colour matching methods. For nitrites colour is developed by adding sulphonic acid and napthamine; whereas for nitrates colour is developed by adding phenol-di-sulphonic acid and potassium hydroxide. The colour developed in water is compared with standard colours of known concentrations of these compounds.
For drinking water the amount of free ammonia should not exceed 0.15 ppm and the amount of albuminoid nitrogen should not exceed 0.30 ppm. The presence of nitrites in drinking water is highly dangerous and hence the amount of nitrites in drinking water should be nil.
The amount of nitrates in drinking water should not exceed 45 mg/I (or 45 ppm). The presence of excess amount of nitrates in drinking water may adversely affect the health of infants, causing a disease called ‘methemoglobinemia’ (commonly known as ‘blue baby disease’), which may result in the death of the child in extreme cases.
The alkalinity of water is its capacity to neutralize a standard solution of acid. It is due to the presence of bicarbonate (HCO3–), carbonate (CO3– – and hydroxide (OH–). Out of these bicarbonates present the major form of alkalinity since they are formed in considerable amounts from the action of CO2 upon the basic materials in the soil, as indicated by the following equation –
Water may also contain appreciable amount of carbonate and hydroxide alkalinities particularly surface waters blooming with algae. The algae take up CO2 for its photosynthesis activities and raise the pH. The carbonate alkalinity may be present with either hydroxide or bicarbonate alkalinity, but hydroxide and bicarbonate alkalinity cannot be present in the same sample of water. Thus total alkalinity of water will be equal to the sum of the carbonate and hydroxide alkalinities, or the sum of the carbonate and bicarbonate alkalinities.
According to the pH value of water its alkalinity is usually divided into the following two parts:
(1) Total alkalinity i.e., above pH 4.5
(2) Caustic alkalinity i.e., above pH 8.2
The determination of alkalinity of water is very useful because it provides buffering to resist changes in pH value. The alkalinity of water is determined by titrating water sample with standard N/50 or N/40 solution of H2SO4.
In the titration the following two indicators are commonly adopted:
(i) Phenolphthalein:
Pink above pH 8.2 and colourless below pH 8.2.
(ii) Methyl Orange:
Red below pH 4.5 and yellow-orange above pH 4.5.
The bromcresol green-methyl red indicator may be preferable to methyl orange as the colour change from greenish-blue above pH 4.5 to light pink below pH 4.5 is more definite.
The amount of alkalinity is expressed in terms of CaCO3. If the strength of the titrant solution is N/50, 1 ml of titrant solution will be equal to 1 mg CaCO3 because the equivalent weight of CaCO3 is 50.
Thus
The neutralization of OH– is completed at pH 8.2. The neutralization of CO3– – is only half completed at pH 8.2 and it is fully completed only when a pH value of 4.5 is reached.
Thus alkalinity on pH scale is represented as follows:
(1) The range of total alkalinity is 4.5 to 14.
(2) The range of bicarbonate i.e., HCO3–, alkalinity is from 4.5 to 8.2.
(3) The range of carbonate i.e., CO3 – –, alkalinity is from 8.2 to 10.
(4) The range 0 to 4.5 indicates no alkalinity.
The procedure adopted for determining alkalinity of water is as follows. Take 100 ml of water sample in a conical flask. Add 3-4 drops of phenolphthalein indicator. If no colour is produced, the phenolphthalein alkalinity is absent. If the sample turns pink, titrate with standard N/50 H2SO4 solution till the pink colour disappears.
Record the ml of acid used, designated as P. Add one drop of methyl orange to the titrated mixture and again titrate with the same standard N/50 H2SO4 solution until the first appearance of orange colour is noted. Record the total ml of acid used for the entire titration designated as T.
The phenolphthalein alkalinity and the total alkalinity may be computed as follows:
From the results obtained by this titration the three types of alkalinities may be determined by using Table 8.2:
Further following rules should be kept in mind while determining the alkalinity of water sample:
(1) For simplicity, it is assumed that HCO3 – and OH– alkalies cannot come together in the same sample.
(2) The alkalinities of other than carbonate, bicarbonate and hydroxide origin are absent.
(3) OH– alone gives initial pH of about 10.
(4) CO3– – will be present at pH > 8.2.
(5) OH– – and CO3– – together give initial pH of about 10.
(6) CO3– – and HCO3 – can exist together.
(7) HCO3 – alone gives initial pH < 8.2.
The water contains various gases which are dissolved in it due to its contact with atmosphere and ground surface. The gases which may usually be present in water are nitrogen, methane, hydrogen sulphide, carbon dioxide and oxygen. The contents of these dissolved gases in water may be determined so as to get an idea of the extent of contamination of water and the treatment required for purifying the same.
The presence of nitrogen gas in water is an indication of the presence of organic matter in water. The concentration of methane gas is to be studied for its explosive property. The hydrogen sulphide gas present in water even in small amount may give bad taste and odour to water. The presence of carbon dioxide gas in water indicates biological activity, causes corrosion, increases solubility of many minerals in water and gives taste to water.
Oxygen gas is generally absorbed by water from atmosphere and pure natural surface water is usually saturated with it. Except oxygen the presence of any other gas dissolved in water is not desirable and steps should be taken to remove the same.
The presence of dissolved oxygen in water upto the saturation level is an indication of its purity. However, if unstable organic matter is present in water it would consume oxygen for its oxidation, which would result in the depletion of the dissolved oxygen below its saturation level.
Hence if the amount of dissolved oxygen present in water is found to be less than its saturation level, it indicates the presence of organic matter in water and consequently doubts would be raised about the purity of water.
It is therefore necessary to determine the deficiency of dissolved oxygen in water. The simple test which can be performed to determine the oxygen deficiency in water is potassium permanganate (KMnO4) test.
In this test 10% acid solution of potassium permanganate is added to the water sample and the mixture is exposed for 4 hours at a constant temperature of 27°C. After this period the amount of oxygen used up from KMnO4 is determined. For drinking water this amount of oxygen used up should not exceed 5 to 10 ppm.
The amount of dissolved oxygen (DO) present in water may be determined by Winkler’s method which has been modified by Alsterberg. In this method in approximately 300 ml sample of water, 1.0 ml of manganous sulphate solution, and 1.0 ml of alkaline potassium iodide solution and 2.0 ml of concentrated sulphuric acid are added and mixed thoroughly by vigorous shaking. About 203 ml of this decoction is taken and it is titrated with N/40 sodium thiosulphate (Na2S2O3) solution using starch as indicator till the first disappearance of the blue colour. The ml of titrant used is equivalent to mg/l of dissolved oxygen (DO) present in water.
The extent of organic matter present in a water sample can also be estimated by supplying oxygen to the water sample and determining the oxygen consumed by the organic matter present in the water. The oxygen so consumed is known as biochemical oxygen demand (BOD) and it is to be determined for raw water as well as for treated water.
The BOD of raw water will indicate the extent of treatment required for purifying the water and making it safe and wholesome. The BOD of treated water should be nil which would ensure that it is free from any organic matter.
viii. Metals and Other Chemical Substances:
Water may contain several metals and other chemical substances such as iron, manganese, copper, lead, barium, cadmium, arsenic, selenium, sulphates, fluorides, etc. In drinking water the amount of the various metals and other chemical substances should not exceed the permissible values because their presence in excess of the prescribed limits may be harmful.
The excess concentration of iron and manganese in water may cause discoloration of clothes washed in such waters. Moreover, they may cause incrustation in water mains due to deposition of ferric hydroxide and manganese oxide.
The presence of copper in excess amount is likely to affect human lungs and other respiratory organs.
A high concentration of sulphates may produce laxative effect on human system. Similarly a higher concentration of fluorides may cause spotting and discoloration of teeth.
Lead, arsenic, cadmium, selenium, etc., are the toxic materials and only a very low concentration of these can be tolerated by human body.
Table 8.3 shows the physical and chemical standards for quality of drinking water as recommended by the Environmental Hygiene Committee and given in the Manual on Water Supply and Treatment, Ministry of Urban Development, Government of India.
The physical, chemical and bacteriological standards for quality of drinking water have also been recommended by the following agencies:
(i) Indian Council of Medical Research Committee (I.C.M.R. Committee).
(ii) World Health Organisation International (W.H.O. International).
(iii) United States Public Health Society (U.S.P.H.S).
(iv) American Water Works Association (A.W.W.A)
(3) Bacteriological Analysis:
The bacteriological analysis of water is done to determine the presence of bacteria in water. The bacteria are minute single cell organisms which are universally found in water obtained from any source. They are very small measuring only 1 to 4 micron in length (1 micron = 10-6 m).
As such they cannot be seen with a naked eye and have to be examined under a microscope. Certain bacteria as well as other organisms such as virus are much smaller and cannot be seen even with a microscope. Their presence can, however, be detected by circumstantial evidences or chemical reactions.
The bacteria are usually classified according to their shapes, their oxygen requirements and their effect on mankind.
According to shape the bacteria are classified as cocci (round shaped), bacilli (rod shaped), spirilla (spiral shaped), and trichobacteria (filamentous).
According to oxygen requirements the bacteria are classified as aerobic bacteria, anaerobic bacteria, and facultative bacteria.
Aerobic bacteria are those which require free oxygen for their survival, thus if present in water they consume dissolved oxygen from the water and decompose the organic matter present in water.
Anaerobic bacteria are those which flourish or thrive in the absence of free oxygen.
Facultative bacteria are those which can survive with or without free oxygen.
According to effect on mankind the bacteria may be classified as harmless bacteria and harmful bacteria. The harmless bacteria are called non-pathogenic bacteria or non-pathogens. These bacteria besides being harmless, under certain conditions are beneficial to human beings, animals and crops. The harmful bacteria are called pathogenic bacteria or pathogens.
The pathogenic bacteria are the real foes of mankind, which may cause serious water borne diseases such as cholera, typhoid, disentery, infectious hepatitis, etc. Generally nonpathogenic and pathogenic bacteria occur together, and hence the presence of large amount of non-pathogenic bacteria in a water sample indicates the possibility of the pathogenic bacteria being also present in the water sample.
Since pathogenic bacteria are harmful for human beings it is necessary to test the water sample to detect the presence of pathogenic bacteria. However, the tests for detection of the presence of pathogenic bacteria in water are difficult because pathogenic bacteria have only a short life in water and it is not possible to isolate pathogenic bacteria with the help of laboratory instruments.
As such the usual practice followed is to detect the presence of non-pathogenic bacteria in a water sample which live longer in water than pathogenic bacteria and then to judge the possibility of the presence of pathogenic bacteria in the sample. A particular type of bacteria called coliform bacteria (or coliforms) are commonly present in water.
The coliform bacteria belong to coliform group which derives its name from the colon or large intestines of human beings or animals where coliform bacteria are found to be present and are excreted with their faeces.
The coliform bacteria are non-pathogenic, aerobic bacteria and their presence or absence indicates the presence or absence of pathogenic bacteria in a water sample. In other words it is presumed that if the bacteria of coliform group are not present in water then it will be free from pathogenic bacteria, and if bacteria of coliform group are present in water then it may also have pathogenic bacteria.
Escherichia coli or E-coli are the coliform bacteria which inhabit the intestines of human beings and animals and are thus excreted in large amount with their faeces. As such the water which has been contaminated with sewage will contain E-coli bacteria.
Bacteriological Quality of Drinking Water:
The guideline values for bacteriological quality of drinking water are given in Table 8.4, which has been taken from the “Manual on Water Supply and Treatment”, published by Ministry of Urban Development, Govt, of India, New Delhi.
According to U.S. Public Health Service the number of samples that should be taken per month for checking bacteriological quality of drinking water depends on the population served by the water supply system. It is further stipulated that of all the standard samples of 10 ml quantities examined per month not more than 10% shall show the presence of organisms of the coliform group; and of 100 ml quantities examined per month not more than 60% shall show the presence of organisms of the coliform group.
In addition to bacteria, water may contain other types of undesirable microscopical organisms as indicated below:
(i) Algae;
(ii) Fungi;
(iii) Protozoa;
(iv) Plankton; and
(v) Crenothrix.
A brief description of each of these is given below:
(i) Algae:
Algae are one-celled plants which grow in water. They impart unpleasant taste and odour to water. Green algae which can be seen growing in clear streams and ponds, grow only in the presence of sunlight. Excessive growths of algae in water may be controlled by application of copper sulphate or chlorine.
(ii) Fungi:
Fungi are plants which grow without sunlight. They produce unpleasant taste and odour. At times they will infest water mains and even cause clogging. They may be removed with chlorine treatment.
(iii) Protozoa:
Protozoa are one-celled animals which may cause certain diseases if present in drinking water. The pathogenic protozoa of primary concern in drinking water are Giardia lamblia, Entamoeba histolytica and Cryptosporidium. Giardia lamblia causes giardiasis (a type of diarrhoea or gastroenteritis; Entamoeba histolytica causes amebiasis or amebic dysentery; and Cryptosporidium causes cryptosporidiosis (cholera like diarrhoea and traveller’s diarrhoea).
(iv) Plankton:
Plankton is microscopic plants and animals usually swimming or suspended in water and having little or no resistance to currents. Planktonic organisms predominate in ponds, lakes and oceans. In most of the cases they are not injurious to health but some types of plankton create offensive taste and odour or toxic conditions resulting in animal deaths or human illness. Further they sometimes interfere with the smooth working of rapid gravity filters by preventing the proper accumulation of alum flock on sand.
(v) Crenothrix:
Crenothrix is a genus (or group) of bacteria which possesses characteristics of the larger organisms. Crenothrix thrives in water containing iron in solution. The growth of this organism results in deposits of rust in water mains and at times is quite troublesome. If the deposits break loose, plumbing fixtures and laundered materials will be stained.
1. Collection of Water Samples for Bacteriological and/or Microbiological Tests:
The samples of water for bacteriological and/or microbiological tests should be collected with utmost care, because if certain harmful pathogens which may be present in water but are not indicated in the test results of the collected water samples then the supply of this water may lead to spreading of serious water borne diseases in epidemic form.
On the other hand if the water samples during collection are contaminated due to the carelessness of the sample collector, then the test results may indicate the presence of some bacteria in water which may not be actually present and this may lead to unnecessary elaborate treatment of water with increased cost. As such great care has to be exercised to ensure that a true representative sample of water is collected and there is no danger of any contamination during the collection of water samples.
The procedure for collecting the water samples for bacteriological and/or microbiological tests and the precautions to be taken while collecting the same are as follows:
1. Collect samples in bottles that have been cleansed and rinsed carefully, given a final rinse with distilled water, and sterilized.
2. Keep sampling bottle closed until it is to be filled. Remove stopper and cap as a unit, do not contaminate inner surface of stopper or cap and neck of bottle. Fill bottle without rinsing, replace stopper or cap immediately, and if used, secure hood around neck of bottle.
3. When sample is collected, leave ample air space in the bottle (at least 25 mm) to facilitate mixing by shaking, before testing.
4. Collect samples that are representative of the water being tested, disinfect sample ports, and use aseptic techniques to avoid sample contamination.
5. When water sample is to be taken from a distribution-system tap without attachments, then-
(i) Select a tap that is supplying water from service pipe directly connected with the main, and is not served from a cistern or storage tank;
(ii) Open tap fully and let water run to waste for 2 or 3 minutes, or for a time sufficient to permit clearing the service line;
(iii) Reduce water flow to permit filling bottle without splashing;
(iv) If tap cleanliness is questionable, apply a solution of sodium hypochlorite (100 mg NaOCl per litre) to faucet before sampling and let water run for additional 2 to 3 minutes after treatment; and
(v) Do not sample from leaking taps that allow water to flow over the outside of the tap.
6. When water sample is to be taken from a mixing faucet remove faucet attachment such as screen or splash guard, run hot water for 2 minutes, then cold water for 2 to 3 minutes and collect the sample as indicated above.
7. When the sample is to be taken from a well fitted with a hand pump, pump water to waste for about 5 minutes before collecting sample. If the well is equipped with a mechanical pump, collect sample from a tap on discharge side. If there is no pumping machinery, collect sample directly from the well by means of a sterilized bottle fitted with a weight at the base, take care to avoid contamination of samples by any surface scum.
8. In drinking water evaluation, collect samples of finished water and from distribution sites selected to assure systematic coverage during each month. Carefully choose distribution system sample locations to include dead-end sections to demonstrate bacteriological quality throughout the network and to ensure that localized contamination does not occur through cross-connections, breaks in the distribution lines, or reduction in positive pressure.
9. In collecting samples directly from a river, stream, lake, reservoir, spring or shallow well, do not take samples too near the bank or too far from the point of drawoff, or at a depth above or below the point of drawoff.
10. When collecting samples from a river, stream, lake or reservoir hold the bottle near its base in the hand and plunge it neck downward below the surface. Turn bottle until neck points slightly upward and mouth is directed towards the current. If there is no current as in the case of a reservoir, create a current artificially by pushing bottle forward horizontally in a direction away from the hand. When samples are collected from a boat, obtain samples from upstream side of boat.
If it is not possible to collect sample from these situations in this way, attach a weight to base of bottle and lower it into the water. In any case, take care to avoid contact with bank or stream bed, otherwise water fouling may occur.
11. Start the bacteriological and microbiological tests of water samples immediately after collection to avoid unpredictable changes. However, if samples cannot be tested within 1 hour after collection, use an iced cooler for storage during transport to laboratory. Further the time elapsing between collection and testing should not exceed 24 hours.
The details about the methods of sampling and bacteriological and/or microbiological tests of water are available in “Standard Methods for the Examination of Water and Sewage”, 17th Edition 1989, published by American Public Health Association.