In this essay the tests which are done during water analysis are: 1. Physical Tests 2. Chemical Tests 3. Biological Tests.
Essay # 1. Physical Tests:
The physical tests include the following tests:
(i) Temperature:
The temperature of water is measured by means of ordinary thermometers. From the temperature the density, viscosity, vapour pressure and surface tension of water can be determined. The saturation values of solids and gases that can be dissolved in water and the rates of chemical, biochemical and biological activity are also determined on the basis of temperature.
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The temperature of surface water is generally same to the atmospheric temperature, while that of ground water may be more or less than atmospheric temperature. The most desirable temperature for public supply is between 4.4°c to 10°C. Temperatures above 26°C are undesirable and above 35°C are unfit for public supply, because it is not palatable.
(ii) Colour:
The colour of water is usually due to presence of organic matter in colloidal condition, but sometimes it is also due to mineral and dissolved organic and inorganic impurities. Before testing the colour of the water, first of all total suspended matter should be removed from the water by centrifugal force in a special apparatus.
After this the colour of the water is compared with standard colour solution or colour discs. The colour produced by one milligram of platinum in a litre of distilled water has been fixed as the unit of colour. The permissible colour for domestic water is 20p.p.m. on platinum cobalt scale. The colour in water is not harmful but it is objectionable.
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(iii) Turbidity:
It is caused due to presence of suspended and colloidal matter in the water. The character and amount of turbidity depends on the type of soil over which the water has moved. Ground waters are generally less turbid than the surface water.
Measurement of Turbidity and Turbidity Units:
There are two types of turbidimetres:
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1. Based on visual method
2. Based on direct (or meter reading) method.
The turbidimeters are based on the following those parts:
a. Light source (candle or lamp)
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b. Sample compartments along with optical ports
c. Device for the measurement of light.
Units:
Turbidity was previously determined by Jackson candle Turbidity units (JTU). This unit is now replaced by more appropriate unit called ‘Nephelometric Turbidity’ unit or (NTU) because of the use of Nephelometric method of measurement of 1000 turbidities.
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Turbidity is a measure of the resistance of water to the passage of light through it. Turbidity is expressed in parts per million (ppm or milligrams per litre mg/l). The turbidity produced by one milligram of silica in one litre of distilled water is the unit of turbidity. In other words, turbidity produced by one part of finely divided silica in million parts of distilled water is the standard unit.
The turbidity of a water sample is commonly determined by the following methods:
(a) By Turbidity Rod or Tape:
Turbidity rod is a graduated aluminium or steel rod. One platinum needle is fixed at the lower end as shown in Fig. 9.1. The eye is placed at the upper end of the rod and a line indicates the position of the eye. Keeping the eye constantly at position marked and watching needle the rod is immersed in the water.
We will go on immersing the rod in water and looking constantly on the platinum needle, till the needle just disappears from the eye-view due to the turbidity of the water. We will directly read the turbidity of the water on the rod at the surface of the water level. The rod is graduated to give direct readings of turbidity in ppm.
(b) Jackson’s Turbidimeter:
It is used for measuring turbidity, above 50 ppm. It essentially consists of a metal stand and metal container for graduated tube. The standard candle or bulb is used as a light source. This light source is placed at fixed distance from the bottom of the metal container.
For measuring the turbidity of water, it is slowly poured in the graduated tube and the light of bulb or candle is constantly seen from the top through the water. A stage will come when the light source will just disappear from the sight.
At this position we will stop the pouring of water and take out the glass tube. As the tube is graduated to give direct readings, the reading at the water level in the glass tube will be the turbidity of water (Fig. 9.2).
(c) By Baylis Turbidimeter:
Fig. 9.3 illustrates a Baylis Turbidimeter. It essentially consists of a closed steel box. Two glass tubes are kept in vertical position side by side, on one side of this box. These tubes can also be directly taken out and again held in the holes provided for the purpose.
Opposite to the side of tubes and electric bulb of 250 W is fixed with a reflector to throw light on the tubes. The tubes at the bottom are supported on white opal glass plate, and are surrounded by blue cobalt plates shown in the Figure.
One tube is filled with sample water, whose turbidity is to be determined and second tube is filled with standard water of known turbidity. In the light of the lamp, both the tubes are observed by the eyes. If there is any colour difference between the tubes, till both the tubes look like same or match will each. Now the turbidity of the standard tube is noted, which is equal to that of the sample water under test.
Now a days Hellige Turbidimeter is also available in the market which also works on the principle of Baylis Turbidimeter.
Fig. 9.4 shows another type of self-reading turbidimeter. It essentially consists of a steel box, on the two parallel longer sides of which mirrors are fixed. One partition slit can be moved along the longer side with a pointer at the top, which moves along a graduated scale which gives the turbidity of the water directly.
On one side of the mirror one hole is kept in the back of which one standard bulb is fixed. When the light enters through the hole it goes to the mirror fixed on the opposite side. After reflection it again comes to the first mirror. The images of light coming from the hole after reflection from both the parallel mirrors are visible in a series of continuous circular light spots, which are seen from the hole provided on the opposite side of the bulb hole.
The sample water whose turbidity is to be determined is filled in the box, bulb is lighted and the image of bulb light coming from the hole is seen from the eye-slit or hole provided for this purpose. The partition plate is moved till it touches the last image of the light. The pointer at the top will indicate the turbidity of the water against the graduated scale.
(d) Hellige Turbidimeter:
The sample is taken in the turbidity cell up to the mark and then viewed through the eyepieces. There are two fields one control field corresponds to the light transmitted through the light scattered from the suspended matter of the sample.
The light intensity of the control spot is balanced with the surrounding field by turning the dial on the right hand side of the instrument. This dial controls the brightness of the control field. The light is balanced over the different intervals on rotating it from lower to higher values until the black spot at the centre of the field just disappears.
(e) ‘HACH’ Laboratory Turbidimeter:
This instrument based on the Nephelometric method. This operates on the principle that a right passing through a substance is scattered by the particles suspended in the substance. The intensity of scattered light serves as a measure of turbidity.
A beam of light is sent in the sample and then the suspended matter scatter the light at. right angle to the beams and then this scattered light is received on the photoelectric cell. The light energy converted into the electrical pulses (or signals) is further amplified using the amplifier and then measured by a meter.
The Hach turbidity meter has the following characteristics:
Accuracy and reproducibility + 2% of full scale
Sensitivity – better than + 0.5% of full scale
Calibration – For mazin
Standardization – Tubic plastic rod with (calibrated with for margin) known turbiding value
Sample requirements – 30 ml for 0.2
01 and .10 NTU range
10 ml for 0-100 and 0-1000 NTU range
The calibration of the instruments is based for mazin, the best known turbidity standard.
Following are the important steps in the preparation of for margin stock standard 400 units:
1. Take 5.0 gm of reagent grade by drazin sulphate N2H4H2SO4 in 400 ml distilled water.
2. Dissolve 500 gm of pure hexamethy lene tetramine in 400 ml distilled water.
3. Add the solutions in litre volumetric flask and dilute to the volume.
4. Allow the solution to stand for 48 hours at 120°C to develop the turbidity precipitate completely.
When made it is uniform in number, size and shape of particles. It has a good stability I to 2 weeks. Thus making it as a ideal turbidity standard. The calibration is made in Jackson turbiding units (JTUs) the standard units of measurement.
(iv) Tastes and Odours:
Tastes and odours in water may be due to the presence of dead or live micro-organisms; dissolved gases such as hydrogen sulphide, methane, carbon dioxide or oxygen combined with organic matter; mineral substances such as sodiumchloride, iron compounds and carbonates and sulphates of other substances.
The tests of these are done by sense of smell and taste because these are present in such small proportions that it is difficult to detect them by chemical analysis.
The odour of water also changes with temperature. The odour may be classified as fishy, mouldy, sweetish, vegetable, greasy etc. The odour of both cold and hot water should be determined. The water having bad smell or odour is objectionable and should not be supplied to the public.
The intensities of the odours are measured in terms of threshold odour number. This number is numerically equal to the amount of sample of water in c.c.s. required to be added to one litre of fresh odourless water. At the stage when the mixture just starts giving typical smell in the sample, the quantity of sample is done first at 20°C and then at 80°C’. The increase in temperature is done to liberate the dissolved gases which cause odours.
(v) Specific Conductivity of Water:
The total amount of dissolved salts present in water can be easily estimated by measuring the specific conductivity of water. The specific conductivity of water is determined by means of a portable dionic water tester and is expressed in micro-mhoes per cm at 25°C. (Mho is the unit of conductivity and equals 1ampr/1volt).
The specific conductivity of water in micro mhoes per cm at 25°C is multiplied by a coefficient (generally 0.65) so as to directly obtain the dissolved salt content in mg/litre or ppm. The exact value of this coefficient depends upon the type of salt present in water.
Essay # 2. Chemical Tests:
In the chemical analysis of water those tests are done that will reveal the sanitary quality of the water. The chemical tests involve the determination of total solids, hardness, pH value, chlorides, residual chlorine, iron and manganese, organic matter etc.
The following are the methods of doing various chemical tests:
(i) Total Solids:
These include the solids in suspension, colloidal and in dissolved form. The quantity of suspended solids is determined by filtering the sample of water through a fine filter, drying and weighing. The quantity of dissolved and colloidal solids is determined by evaporating the filtered water (obtained from the suspended solid test) and weighing the residue.
The total solids in a water sample can be directly determined by evaporating the water and weighing the residue. If the residue of total solids is fused in a muffle-furnace the organic solids will decompose whereas only inorganic solids will remain. By weighing we can determine the inorganic solids and deducting it from total solids, we can calculate organic solids.
(ii) Hardness:
It is the property of water which prevents the lathering of the soap. It is caused due to the presence of carbonates and sulphates of calcium and magnesium in the water. Sometimes the presence of chlorides and nitrates of calcium and magnesium also cause hardness in the water.
Hardness is usually expressed in mg/litre or p.p.m. of calcium carbonate in water. Hardness is generally determined by Versenate Method. In this method, the water is titrated against EDTA salt solution using Erio chrome Black T as indicator solution. While titrating the colour changes from wine red to blue.
In the past the hardness was determined by soap test, in which the standard soap solution was added in the water and it was vigorously shaked to see the formation of lather for 5 minutes. The hardness of water was calculated on the basis of soap solution added and the lather factor.
The measurement of the Hardness can be done by the following formula:
Putting the values of Ca++, Mg—, CaCO3 as 20, 12 and 50 respectively, the above equation will become
Where, Hw = Hardness of water in ppm or mg/litre
Wc = Combination weight of Ca++
Wm = Combination weight of Mg++
After determining the values of Wc and Wm the value of hardness can be determined.
Unit of Hardness:
Sometimes the hardness is represented in degree of hardness. Each British degree of hardness is equal to 14.25 mg/l or ppm. French unit of degree is equal to 10 ppm of hardness (as CaC03 of course). In general, under a normal range of pH values, water with hardness upto 75 ppm are considered soft and those with 200 p.p.m. and above are considered hard. In between, the water are considered as moderately hard.
Underground water are generally harder than the surface water, as they do have more opportunity to come in contact with minerals. For boiler feed waters and for efficient cloth washing in laundries, etc., the water must be soft with hardness less than 75 ppm or so. However for drinking purposes, waters with hardness below 75 ppm are generally tasteless and hence, the prescribed hardness limit for public supplies ranges between 75 ppm to 150 ppm.
(iii) Chlorides:
Sodium chloride is the main substance in chloride water. The natural water near the mines and sea have dissolve sodium chloride. Similarly the presence of chlorides may be due to the mixing of saline water and sewage in the water. Excess of chlorides is dangerous and unfit for use. The chloride can be reduced by diluting the water. Chlorides above 250 p.p.m. are not permissible in water.
The chloride can be determined by titrating the water with silver nitrate and potassium chromate. In this titration process reddish colours will be formed if chlorides are present.
(iv) Chlorine:
Dissolved free chlorine is never found in natural waters. It is present in the treated water resulting from disinfection with chlorine. The chlorine remains as residual in treated water for the sake of safety against pathogenic bacterias. Residual chlorine is determined by the Starch-Iodide test or Orthotolodin test.
In Starch-Iodide test, potassium iodide and starch solutions are added to the sample of water due to which blue colour is formed. This blue colour is then removed by titrating with N/100 sodium thiosulphate solution, and the quantity of chloride is calculated.
On the addition of orthotolodine solution if yellow colour is formed it indicates the presence of residual chlorine in the water. The intensity of this yellow colour is compared with standard colours to determine the quantity of residual chlorine.
The residual chlorine should remain between 0.5 p.p.m. to 0.2 p.p.m. in the water so that it remains safe against pathogenic bacteria.
The amount of chlorine can then be easily ascertained by using the simplified titration equation,
(v) Iron and Manganese:
These are generally found in ground water. If these are present less than 0.3 p.p.m., it is not objectionable, but if exceeds 0.3 p.p.m. the water is not suitable for domestic, bleaching, dyeing and laundering purposes. The presence of iron and manganese in water makes brownish red colour in it, leads to the growth of micro-organism and corrodes the water pipes. Iron and manganese also cause taste and odour in the water.
The quantity of iron and manganese is determined by colorimetric methods. In these methods some colouring agents are added in the water and colours so formed are compared with standard colour solutions.
(vi) pH-Value:
Depending upon the nature of dissolved salts and minerals, the water found in natural sources may be acidic or alkaline. The acidity or alkalinity is usually measured in p.p.m. of the dissolved salts and is expressed in terms of equivalent weight of calcium carbonate.
When acids or alkalies are dissolved in water they dissociate into electrically charged ions of Hydrogen and Hydroxyl respectively. Hydrogen ions are charged with positive charge whereas Hydroxyl ions are charged with negative charge.
The following equations show the dissociation of some of the acids and alkalies in the water:
The net concentration of hydrogen ion H+ will exceed that of hydroxyl ions and will be more than the hydrogen ion concentration of neutral water (i.e., 10 -7) and thus decreasing the pH-Value to less than 7 and thus making the water acidic.
The net amount of OH– ions present in the alkaline water will be more than that present in the neutral water. Thus, it will reduce the hydrogen ions concentration to less than 10-7 and thereby increasing the pH above 7.
Water itself is weakly ionized its equation is:
H2O =H+ + OH–
According to the law of mass action of physical chemistry.
The value of this constant has been found to be 10-14
But the concentration of H+ ions must be equal to the concentration of OH– ions for electrical neutrality and concentration of un-dissociated molecules in pure water is 100%.
The above relationship can be written is:
From the relationship the concentration of OH– ions can be expressed in terms of equivalent hydrogen-ions concentration.
pH value denotes the concentration of hydrogen-ions in the water and it is a measure of acidity or alkalinity of a substance.
In pure water the concentration of H+ ions and OH– ions are equal.
or H+ = OH– = 10-7
Because concentration of H+ ion or OH’ ion per litre of water is 1/107 gms.
But this figure is inconvenient for use; therefore, logarithm of its reciprocal is used for indicating the pH value:
The pH value of neutral water will be equal to:
For pure water, pH is 7. This value will become more if the concentration of hydrogen ions decrease. On the other hand, it decreases if the concentration of hydrogen ions increases.
In other way it can by represented as follows:
Determination of pH-Value:
The p-H-value of a water is generally determined by colorimetric method or Electrometric method. In colorimetric method some indicator is added in the sample of water and colour so formed is compared with standard colour discs or solutions. The standard colour discs and solutions are supplied by the manufacturers, on comparing with them the pH value of water can be directly determined.
Table 9.2 gives the common indicators with their pH-range values which and commonly used in determining the pH-value:
Sometimes when the pH value of water is to be determined after very short intervals, electrometric method is used. This is very quick and automatic method of recording pH value. In this method specially prepared electrodes are dipped in water sample and are connected to dry cell or electric mains. A meter is also connected to the electric circuit which directly indicates the pH value of the water.
For public water supplies, pH-value should be kept as close to 7 as possible. The lower value may cause tuberculation and corrosion, whereas high values may produce incrustation, sediment deposits, difficulty in chlorination and other bad effects on the human using the water.
(vii) Lead and Arsenic:
These are not usually found in natural waters. But sometimes lead is mixed up in water from lead pipes or from tanks lined with lead paint when water moves through them. These are poisonous and dangerous to the health of public. The presence of lead and arsenic is detected by means of chemical tests for it.
(viii) Dissolved Gases:
Usually it has been found that water contains various dissolved gases present in it.
The following are some of the gases mostly found in the natural water:
(a) Oxygen:
Surface waters contain large amount of dissolved oxygen, because they absorb it from the atmosphere, Algae and other tiny plant life of water also give oxygen to the water. The presence of oxygen in water in dissolved form is necessary to keep it fresh and sparkling. But more quantity of oxygen causes corrosion to the pipe materials.
(b) Carbon Dioxide:
The water absorbs carbon-di-oxide from the atmosphere. If water comes across calcium and magnesium salts, the carbon-di-oxide acts on them and converts them into bi-carbonates and causes hardness in the water. The presence of carbon-di-oxide can be easily determined by mixing the lime solution in the water. If it gives milky white colour the carbon-di-oxide is present in the water.
(ix) Nitrogen:
The presence of nitrogen in the water indicates the presence of organic matters in the water.
The nitrogen may be present in the water in one or more of the following forms:
(i) Nitrites,
(ii) Nitrates,
(iii) Free ammonia, and
(iv) Albuminoid nitrogen.
The presence of the nitrites in the water, due to partly oxidised organic matters, is very dangerous. Therefore in no case nitrites should be allowed in the water, their presence must be nil. The nitrites are rapidly and easily converted to nitrates by the full oxidation of the organic matters. The presence of nitrates is not so harmful. But in no case its quantity should increase 45 p.p.m., because excess presence of nitrate will cause “mathemoglobinemia” disease to the children.
The presence of nitrites or nitrate can be determined by colour matching methods. For determining the presence of nitrites, the colour is obtained by adding sulphonilic acid and napthamine. For testing presence of nitrates the colour is obtained if phenol-di-sulphonic acid and potassium hydroxide are added. The colours so developed are compared with standard colours to ascertain the p.p.m. contents.
Free ammonia is obtained from the decomposition of organic matters in the beginning. Therefore if free ammonia is present in the water, it will indicate that the decomposition of the organic matters has started recently. The presence of nitrites indicates partly decomposition of organic matters. Whereas the presence of nitrates indicates fully oxidized organic matters.
The presence of free ammonia in water should not exceed 0.15 p.p.m. the presence of free ammonia can be easily determined by boiling the water and measuring the ammonia gas obtained.
The presence of albuminoid nitrogen in water indicates the pollution of water. Its measurement is done by adding strong alkaline solution of potassium permanganate (KMnO4) to the already boiled water (obtained after determining the free ammonia test). In no case the quantity of albuminoid nitrogen should exceed 0.3 p.p.m.
(x) Metals and Other Chemical Substances:
Water contains various types of minerals or metals such as iron, manganese, copper, lead, barium, cadmium, selenium, fluoride, arsenic etc. Table 9.4 gives the maximum permissible quantity of these metals which can be allowed in the water.
The concentration of iron and manganese should not be allowed more than 0.3 p.p.m. otherwise they will cause dicolouration of clothes during washing. They may also cause incrustation in water mains due to deposition of ferric hydroxide and manganese oxide. As lead and barium are very toxic, a low p.p.m. of these are allowed.
Arsenic, selenium are poisonous and may cause fatality, therefore they must be removed totally. Human lungs are effected by presence of high quality of copper in the water. A laxative effect is caused in the human body due to presence of sulphates in the water. Fewer cavities in the teeth will be formed due to excessive presence of fluoride in water more than 1 p.p.m.
The determination of the presence and quantity of the metals and other substances as stated above can be done by colour matching methods by use of various standard indicators and comparing the colours obtained with the standard colours.
Living Organism in Water:
The natural water contains various types of living organisms. Some organisms are born in water and remain in it due to their natural habits. Some organisms are introduced in the water by man during disposal of sewage etc. in water. Some of the living organisms such as bacteria, viruses and protozoa are infectious to humans and are responsible for the serious outbreak of fatal water-borne diseases.
Algae and other organisms related to plankton are mainly responsible for taste and odour in water and sometimes render the water unfit for humans and cause sudden death of cattle and fish. All the organisms can be broadly classified as macroscopic and microscopic. Some organisms are so small that they cannot be seen even by microscope, the presence of such organisms is detected by means of minutely observing their reactions in various positions.
The following are the main living organisms of water:
(i) Bacteria:
There are two groups of bacteria, the larger bacteria and the lower bacteria. The largest bacteria is known as B. butxchlit, which is about 60µ in length and 5µ in diameter. One is called micron and is equal to 10-3 mm. Dialister pnuemonsintes is one of the smallest organisms having 0.15µ in diameter and 0.3µ in length. Some bacteria are beneficial to the humans and some types are harmful and cause diseases. The disease causing bacteria are known as pathogens.
In the water works process only the pathogens are to be removed or killed. This is done by disinfecting the water.
(ii) Algae:
In fresh water these are formed in the form or microscopic size, but in salty water these are in the form of cells several hundred metres in length. Sometimes these occur in the form of cells. Sometimes these grow in numbers and cover the surface of a body of water.
(iii) Protozoa:
This includes all the unicellular animals. There are various types of protozoas such as Amoeboid, Flagelae and ciliate protozoa. Amoeboid protozoa are irregular in shape, naked or shelled, single or colonial. They move by means of false feet.
Flagellate protozoa have lash-like appendages and occur single or in the colonies. They produce fishy odour in the water. Ciliate protozoa are characterized by hair like appendages. Mostly these are bacteria-eaters and destroy pathogens.
Generally, protozoa form scum or unsightly deposits on the porcelain utensils and bathroom fittings. Their unit is numbers per litre of water. These are directly counted by seeing through a microscope.
Essay # 3. Biological Tests:
In a biological test or bacteriological analysis the following two tests are done:
(i) Total Count of Bacteria:
In this method total number of bacteria present in a millilitre of water is counted. The sample of water is diluted; 1 ml of sample water is diluted in 99 ml. of sterilized water. Then 1 ml of diluted water is mixed with 10 millilitre of agar of gelatine (culture medium). This mixture is then kept in incubator at 37°C for 24 hours or at 20°C for 48 hours.
After it the sample will be taken out from the incubator and colonies of bacteria are counted by means of microscope. Then the product of the number of colonies and the dilution factor will give the total number of bacteria per millilitre of undiluted water sample.
(ii) Bactria Coli (B-Coli) Test:
Sometimes this is also called as E-coli test. There are two tests for B-coli, first is presumptive and second confirmative. In the presumptive test definite amount of diluted sample of the water in standard fermentation tubes containing lactose broth as culture medium is kept in incubator at 37°C for 24 to 48 hours. If some gas is produced in the fermentation tube, it indicates the presence of B-coli, if not vice-versa.
In the confirmation test some sample from the presumptive tube is taken and placed in another fermentation tube containing ‘brilliant green lactose bile’ as culture medium. It is again kept in incubator at 37°C for 48 hours; if there is formation of gas in the tube, it confirms the presence of B-coli and the water is unsafe for use.
Now a days a new technique of finding out the B-coli is developed which is called ‘Membrane Filtre Technique’. This is a very simple method. In this method the sample of water is filtered through a sterilized membrane of special design due to which all the bacteria are retained on the membrane.
The member is then put in contact of culture medium-M- Endo’s medium-in the incubator for 24 hours at 37°C. The membrane after incubating is taken out and the colonies of bacteria are counted by means of microscope. This method is known as “membrane filter technique”.
The detection of these bacteria in the past was done by another method by mixing different dilutions of a sample of water with lactose broth and keeping it in the incubator at 37°C for 48 hours. The presence of acid or carbon dioxide gas in the test tube will indicate the presence of B-coli.
After this the standard statistical tables (Maccardy’s) are referred and the ‘Most Probable Number’ (MPN) of B-coli per 100 ml of water are determined. MPN is the number which represents the bacterial density which is most likely to be present. Coliform Index (C.I.) is defined as the reciprocal of the smallest quantity of a sample which would give a positive B-coli test. But due to the development of the ‘Membrane Filter Technique’ the MPN or C.I. methods are not used.
For the drinkable water it is necessary that it must be free from pathogenic bacteria. For checking it atleast 5 samples of 100 ml each are collected and tested. The number of C-coli colonies per standard sample should not exceed 4 per 100 ml.