Water has three characteristics, i.e. physical, chemical and biological characteristics. The raw treated water can be checked and analysed by studying and testing these characteristics as explained below:
The turbidity is measured by a turbidity rod or by a turbidity meter with optical observations and is expressed as the amount of suspended matter in mg/l or parts per million (ppm). For water, ppm and mg/l are approximately equal. The standard unit is that which is produced by one milligram of finely divided silica (fuller’s earth) in one litre of distilled water.
Turbidity Rod: The turbidity can be easily measured in the field with the help of a turbidity rod. It consists of an aluminium rod which is graduated as to give turbidity directly in silica units (mg/l) Turbidimeter: The turbidity can be easily measured in the laboratory with the help of a instruments called turbidity meter. In general, a turbidity meter works on the principle of measuring the interference caused by the water sample to the passage of light rays. Jackson’s candle Turbidimeter: The height of water column will therefore be more for less turbid water and vice versa. Longer the light path lower the turbidity. Such a turbidimeter can not measure turbidites lower than 25 JTU. It can be used for natural sources only and can not be used to measure the turbidity of treated water supplies, for which Baylis turbidity meter or modern nephelometers are used. Baylis Turbidimeters One of the two glass tubes is filled with water sample (whose turbidity I to be measured) and the other is filled with standard water solution of known turbidity. The electric bulb is lighted and the blue colour in both the tubes is observed from the top of the instrument. Modern Nephelometer: for low turbidity less than 1 unit. NTU – Nephelometric Turbidity Units FTU – Formazin Turbidity Units Ratio turbidimeter: River water has maximum amount of turbidity.
The presence of colour in water is not objectionable from health point of view, but may spoil the colour of the clothes being washed. The standard unit of colour is that which is produced by one milligram of platinum cobalt dissolved in one litre of distilled water. For public supplies, the colour number on cobalt scale should not exceed 20 and should be preferably less than 10. Colour determined by an instrument is known as tintometer.
The extent of taste or odour present in a particular sample of water is measured by a term called odour intensity, which is related with the threshold odour or threshold odour number. Water to be tested is therefore gradually diluted with odour free water, and the mixture at which the detection of odour by human observation is just lost, is determined. The number of times the sample is diluted represents the threshold odour number. For public supplies, the water should generally free from odour, i.e. the threshold number should be 1 and should never exceed 3.
For potable water, temperature of about about C is desirable. It should not be more than C.
The total amount of dissolved salts present in water can be easily estimated by measuring the specific conductivity of water.
Total solids (suspended solids + dissolved solids) can be obtained by evaporating a sample of water and weighing the dry residue left and weighing the residue left on the filter paper. The suspended solid can be found by filtering the water sample. Total permissible amount of solids in water is generally limited to 500 ppm.
If concentration increases, pH decreases and then it will be acidic. If concentration decreases, pH increases and then it will be alkaline. pH + pOH = 14 if the pH of water is more than 7, it will be alkaline and if it is less than 7, it will be acidic. The alkalinity is caused by the presence of bicarbonate of calcium and magnesium or by the carbonates of hydroxides of sodium, potassium, calcium and magnesium. Some, but not all of the compounds that cause alkalinity also cause hardness. pH Measurement: the pH value of water can be measured quickly and automatically with the help of a Potentiometer. The pH can also be measured by indicators as given below:
| Indicator | pH range of indicator dye | Original color | Final color produced |
| Methyl orange | 2.8 – 4.4 | Red | Yellow |
| Methyl red | 4.4 – 6.2 | Red | Yellow |
| Phenol red | 6.8 – 8.4 | Yellow | Red |
| Phenolphthalein | 8.6 – 10.3 | Yellow | Red |
Permissible pH value for public supplies may range between 6.6 to 8.4. The lower value of pH may cause incrustation, sediment deposits, difficulty in chlorination.
The chloride content of treated water to be supplied to the public should not exceed a value of about 250 ppm. The chloride content of water can be measured by titrating the water with standard silver nitrate solution using potassium chromate as indicator.
Iron – 0.3ppm, excess of these cause discolouration of clothes. Manganese – 0.05ppm Copper – 1.3ppm Sulphate – 250 ppm Fluoride – 1.5 ppm, excess of this effects human lungs and other respiratory organs. Fluoride concentration of less than 0.8 – 1.0 ppm cause dental cavity (tooth decay). If fluoride concentration is greater than 1.5ppm, causing spotting and discolouration of teeth (a disease called fluorosis).
Oxygen gas is generally absorbed by water from the atmosphere but it being consumed by unstable organic matter for their oxidation. Hence, if the oxygen present in water is found o be less than its saturation level, it indicates presence of organic matter and consequently making the waters suspicious.
The extent of organic matter present in water sample can be estimated by supplying oxygen to this sample and finding the oxygen consumed by the organic matter present in water. This oxygen demand is known as Biological oxygen demand (BOD). It is not practically possible to determine ultimate oxygen demand. Hence, BOD of water during the first five days at C is generally taken as the standard demand. = Loss of oxygen in mg/l x dilution factor. The BOD of safe drinking water must be nil.
Five types of parasitic organisms (i.e. bacteria, protozoa, viruses, worms and fungi) are generally known to be infective to main and are found in water.
These are the minute single cell organisms possessing no defined nucleus and having no green material to help them manufacture their own food. They are reproduced by binary fusion and may of various shapes and sizes are 1 to 4 microns, examined by microscope. a) Non-disease causing bacteria – Non pathogenic bacteria. b) Disease causing bacteria – Pathogenic bacteria.
These are single cell animals and are the lowest and the simplest form of animal life. They are bacteria eaters and thus destroy Pathogens. They are counted by microscope.
These are those plants which grow without sunlight and live on other plants or animals, dead or alive.
