Characteristics of Water
To ensure the quality of safe drinking water (potable water) the water is to be testedfor its physical, chemical and bacteriological characteristics.
(a) Physical Characteristics:
(i)Temperature:Temperature:Temperature:Temperature:Temperature: It can be measured by a thermometer. The temperature should be suitable to the human beings depending upon the climatic and weatherconditions. An average temperature of 15°C is generally suitable.
(ii)Turbidity: The muddy or cloudy appearance of clay or such other particlesthat presents hindrance to the path of light is known as turbidity. It may not be harmful but even then from aesthetical point of view it should not exceedthe allowable value. The turbidity is measured by a turbidity rod or a turbiditymeter with physical observations and is expressed as the suspended matterin mg/l or ppm (part per million). The standard unit of turbidity is that whichis produced by 1 mg of finely divided silica in one litre of distilled water. Theturbidity in excess of 5 mg/l is detectable by the consumer and is henceobjectionable. It is measured in the laboratory by Jackson , Jackson , Jackson , Jackson , Jackson ,BaylisBaylisBaylisBaylisBaylis or suchother turbiditymeters.
(iii):Colour:
The colour is imparted by dissolved organic matters from decayingvegetation or some inorganic materials such as coloured soils (red soil) etc.The algae or other aquatic plants may also impart colour. Again it is moreobjectionable from aesthetics point of view than the health. The standard unitof colour is that which is produced by one milligram of platinum cobaltdissolved in one liter of distilled water. Colour is measured in the labs byNessler’s tubesNessler’s tubesNessler’s tubesNessler’s tubesNessler’s tubes by comparing the sample with the known colour intensities.More precisely tintometer measures it.
(iv)Taste and odour:
Taste and odour:Taste and odour:Taste and odour:Taste and odour: The dissolved inorganic salts or organic matter or thedissolved gases may impart taste and odour to the water. The water must notcontain any undesirable or objectionable taste or odour. The extent of tasteor odour is measured by a term called odour intensity which is related withthreshold odour, which represents the dilution ratio at which the odour is hardly detectible. The water to be tested is gradually diluted with odour freewater and the mixture at which the detection of taste and odour is just lostis determined. The number of times the sample is diluted is known as thethreshold number. Thus if 20 ml of water is made 100 ml (until it just loosesits taste or odour) then the threshold number is 5. For domestic water suppliesthe water should be free from any taste and odour so the threshold numbershould be 1 and should not exceed 3.
(v)Specific conductivity of water:
Specific conductivity of water:Specific conductivity of water:Specific conductivity of water:Specific conductivity of water: The specific conductivity of water is deter-mined by means of a portable dionic water tester and is expressed as micro-mhos per cm at 25°C. Mho is the unit of conductivity and is equal to 1 amper/1 volt). The specific conductivity is multiplied by a co-efficient (generally 0.65)so as to directly obtain the dissolved salt content in ppm.
Chemical Characteristics
Chemical analysis of water is done to determine the chemical characteristics of water.This involves the determination of total solids, suspended solids, pH value, hardness,chlorides, nitrogen content, iron, manganese and other contents, dissolved gases etc.
(i)Total solids and suspended solids:
The total amount of solids can be determined by evaporating a measured sample of water and weighing the dryresidue left. The suspended solids can be determined by filtering the watersample and weighing the residue left on the filter paper. The difference between the total solids and the suspended solids will be the dissolved solids.
(ii)pH of water:
pH is the negative logarithm of hydrogen ion concentrationpresent in water. The higher values of pH mean lower hydrogen ion concen-trations and thus represent alkaline water and vice versa. The neutral waterhas same number of H+ and OH–ions. The concentration of both ions inneutral water is 10^–7
moles per liter. The neutral water will therefore possessa pH equal to
log 10 (1/H + ) = log 10 (1/10 –7 ) = log 10 10^7= 7
If an acid is added to neutral water the number of hydrogen ion increasesand thus the pH reduces. Similarly, if an alkali is added the number ofhydroxyl ion increases thus reducing the hydrogen ion (as their product isconstant, = 10^–14
mole/liter) and the pH increases.Hence, if the pH of water is more than 7 it is alkaline and if it is less than7 it is acidic. Generally, the alkalinity in water is caused by the presence of bicarbonates of calcium and magnesium, or by the carbonates or hydroxidesof sodium potassium calcium and magnesium. Some of the compounds whichcause alkalinity also cause hardness. Acidity is caused by the presence ofmineral acids, free carbon dioxide, sulphates of iron and aluminium etc
The pH value can be measured by a digital pH meter. It can also be measuredwith the help of colour indicators. The indicators are added to sample of waterand the colour produced is compared with the standard colours of knownpH values.For municipal water supplies the pH should be as close to 7 as possible. Thelower pH water (acidic) may harm the pipe lines etc. by reacting with them(tuberculation and corrosion). The alkaline water may produce sedimentation,(scaling) in pipes, difficulties in chlorination ( for disinfection) and adverseeffect on human physiological system.
(iii)Hardness of water: Hardness in water prevents the formation of sufficientfoam when used with soap. It is caused by certain dissolved salts of calciumand magnesium which form scum with soap and reduce the formation offoam which helps in removing the dirt from clothes. These salts keep ondepositing on the surface of boilers and thus form a layer known as scalewhich reduces the efficiency of the boilers. The hardness is known as tem-porary hardness if it is due to the bicarbonates of calcium and magnesiumas this can be easily removed by boiling water or adding lime to it. By boilingthe carbon dioxide gas escapes and the insoluble carbonates are deposited(which cause scaling). If sulphates, chlorides and nitrates are present theycannot be easily removed by boiling and so such water requires water soft-ening methods and this type of hardness is known as permanent hardness.Hardness is measured by titration method (E.D.T.A. method) and is expressedin ppm or mg/l. Generally the underground water is more hard as it dissolvesthe salts in its journey from surface to the ground water table. For boiler feedwaters and for efficient washing of clothes the water must be soft i.e. hardnessshould be less than 75ppm(mg/l).
(iv)Chlorides:
Chlorides are generally present in water in the form of sodiumchloride and their concentration above 250 mg/l produces a salty taste indrinking water. The chlorides can be measured in water by titrating the waterwith standard silver nitrate solution using potassium chromate as indicator.
(v)Nitrogen content
The nitrogen in water may occur in one or more forms ofthe following:
(a)Free ammonia(b)Albuminoid nitrogen(c)Nitrites(d)Nitrates
The free ammonia indicates very fast stage of decomposition of organic matter(thus indicating fresh pollution); albuminoid nitrogen represents the quantity of nitrogen present in water before the decomposition of organic matter hasstarted, the nitrites indicate the partly decomposed organic matter (the con-tinuation of decomposition) and the nitrates indicate the presence of fullyoxidized organic matter (means the prior pollution condition). In potablewater the free ammonia (undecomposed organic matter should not be morethan 0.15 ppm, and the albuminoidal nitrogen should not be more than 0.3ppm. The nitrogen may remain in the form of nitrates but that too shouldnot be more than 45 ppm as a higher concentration causes blue baby diseasein the infants. Actually the nitrates act with the haemoglobin in the blood(which imparts red colour) and reduce it thus converting the colour of skinto blue (impure blood) and thus making them ill and in extreme cases theycan die. Nitrate is measured either by reduction to ammonia or by matchingthe colours produced with phenoldisulphonic acid.
vi)Metals and other chemical substances:Metals and other chemical substances:Metals and other chemical substances:Metals and other chemical substances:Metals and other chemical substances: Various metals and minerals may bepresent in water like iron, manganese, copper, lead, cadmium, arsenic, barium,selenium, etc. The allowable limits for them are as shown in the above tables.If the concentration of these metals and minerals exceeds the permissible limitsthey have certain harmful effects on the human health.Higher concentrations of iron and manganese may cause discoloration ofclothes washed in such waters. They may cause incrustation in water supplypipe lines due to deposition of ferric hydroxide and manganese oxide. Leadand barium salts are toxic and thus very low concentration of these salts ispermissible. Arsenic is a well known poison and as such extremely lowconcentration (0.05 ppm) is permitted. Most of the ground water in Punjabis having arsenic more than the permissible limits because of the industrialwaste water pollution. High quantities of copper may badly affect humanlungs and other respiratory organs.
vii)Dissolved gases:
Various gases like CO2, O2, N2, H2S and CH4 etc. may be present in dissolved form in water. H2S even in small concentration gives bad taste and odour. CO2
indicates biological activity. Oxygen is generally ab-sorbed by water from the atmosphere. Its saturation concentration dependsupon temperature. The organic matter may be present in water due to thedisposal of waste water in it. Organic matter is instable and has a tendencyto become stable i.e. to be inorganic matter. This conversion is known asdecomposition of organic matter and the process is bio-chemical. As it takesplace by bacteria (bio means living) and the conversion is extra cellularenzymatic reaction so it is known as a bio-chemical reaction. The demand ofoxygen imposed by the aerobic (working in presence of oxygen) bacteria isknown as the Bio Chemical Oxygen Demand (BOD). This BOD reduces thedissolved oxygen content of the water. So if the DO of water is found to be
less than the concentration DO it indicates the water pollution. The BOD oftreated water should be nil.After knowing the standards of potable water one should know the ways and means to make the water fit for drinking, i.e. the treatment of water.
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