Essay: Water quality and demand

1.4 Water demand
Domestic demand:
This includes the water required in private building for drinking, bathing, gardening, sanitary purpose, etc. As per IS : 200 I/c/d (with fully flushing system) 135 I/c/d (for weaker sections and LIG).
 
Public demand:
 
Represents the water demand for public utility purpose like washing of public parks, gardening, washing on roads, public fountain.
 
Industrial demand:
 
It represents the water demand of industries which are earlier existing or are likely to be started in the future. As per IS : 50 l/c/d (for normal industries ) 450 I/c/d (industrial cities).
 
Commercial demand:
 
Water requirement for institutions , hotels , schools, colleges, offices. As per IS : 20 I/c/d (for normal commercial area) 50 I/c/d (highly commercial area).
 
Fire demand:
 
In populated or industrial area fires generally breakout and may lead to serious problems. For controlling the situation it requires sufficient quantity water that is called fire demand.
 
1.5 Sources of water and their characteristics
 
Water resources are under major stress around the world. Rivers, lakes, and underground aquifers supply fresh water for irrigation, drinking, and sanitation, while the oceans provide habitat for a large share of the planet’s food supply. Expansion of agriculture, damming, diversion, over-use, and pollution threaten these irreplaceable resources in many parts of the globe.
 
Flowchart of the sources of clean drinking water:
 
 
 
Quantity of water required for a community depends upon:
 
1. Forecasted population.
 
2. Types and variation in demand (e.g. seasonal variation).
 
3. Maximum demand (Per day/Per month).
 
4. Fire demand.
 
5. Rural demand and supplies.
 
6. Appropriate / Available technology.
 
Main sources of water are:
 
Surface water sources: Lakes impounding reservoirs, streams, seas, irrigation canals.
 
Ground water sources: Springs, wells, infiltration wells.
 
Characteristics of water:
 
Raw treated water can be checked and analyzed by studying and testing their physical, chemical and microscopical characteristics as explained below:
 
Physical characteristics of water:
 
(1) Turbidity:
 
Turbidity is measured by a turbidity rod or by a turbidity meter with optical observations. It 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. Standard unit is that which is produced by one milligram of finely divided silica in one liter of distilled water.
 
Turbidity meters:
 
Turbidity Rod:
 
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:
 
Turbidity can be easily measured in the laboratory with the help of a instruments called turbidity meter. It works on the principle of measuring the interference caused by the water sample to the passage of light rays.
 
Jackson’s candle Turbidimeter:
 
Height of the water column will be more for less turbid water and vice versa. Turbidimeter can not measure turbidities lower than 25 JTU. It cannot be used to measure the turbidities of treated water supplies, for which Baylis’s turbidity meter or modern nephelometers are used.
 
Baylis’s turbidimeters:
 
One of the two glass tubes is filled with water sample (whose turbidity I to be measured) and the other is filled with a standard water solution of known turbidity. Electric bulb is lighted and the blue color 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.
 
(2) Colour:
 
The presence of color in the water is not objectionable from health point of view, but may spoil the colour of the clothes being washed. Standard unit of color is that which is produced by one milligram of platinum cobalt dissolved in one liter of distilled water. For public supplies, the colour number of cobalt scale should not exceed 20 and should be preferably less than 10.The colour determined by an instrument is known as Tintometer.
 
(3) Taste and Odour:
 
Extent of taste or odour present in a particular sample of water is measured by a term called odor intensity. Odour intensity is related to the threshold odour or threshold odour number. Water to be tested is gradually diluted with odour free water. Mixture at which the detection of odour by human observation is just lost, is determined. 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.
 
(4) Temperature:
 
For potable water, temperature of about 10°C is desirable. It should not be more than 25° C.
 
(5) Specific Conductivity:
 
The total amount of dissolved salts present in the water can be easily estimated by measuring the specific conductivity of water.
 
Chemical characteristics:
 
(1) Total Solids and Suspended Solids:
 
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.
 
Suspended solid can be found by filtering the water sample. The total permissible amount of solids in water is generally limited to 500ppm.
 
(2) pH value of Water:
 
 
 
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. All the compounds that cause alkalinity will also cause hardness.
 
pH Measurement:
 
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 colour
 
Final colour 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. Lower value of pH may cause incrustation, sediment deposits, difficulty in chlorination.
 
(3) Hardness of Water:
 
Hard waters are undesirable because they may lead to greater soap consumption, scaling of boilers, causing corrosion and incrustation of pipes, making food tasteless etc.
 
Temporary Hardness:
 
If bicarbonates and carbonates of calcium and magnesium are present in the water. Hardness can be removed to some extent by simple boiling or to full extent by adding lime to water. Such a hardness is known as temporary hardness or carbonate hardness.
 
Permanent Hardness:
 
If sulphates, chlorides and nitrates of calcium or magnesium are present in water, they cannot be removed by simple boiling, such water require special treatment for softening. Such a hardness is known as permanent hardness or non-carbonate hardness. It is caused by sulphates, chlorides, nitrates of Ca and Mg.
 
Carbonate hardness = Total hardness or Alkalinity (whichever is less).
 
Non-carbonate hardness = Total hardness – Alkalinity Equal to the total hardness or alkalinity whichever is less.
 
Non-carbonate hardness is the total hardness in excess of the alkalinity. If the alkalinity is equal to or greater than the total hardness, there is no non-carbonate hardness. One French degree of hardness is equal to 10mg/l of CaCO3. One British degree of hardness is equal to a hardness of 14.25mg/l.
 
Water with hardness up to 75ppm are considered soft and above 200ppm are considered hard and in between is considered as moderately hard. Underground waters are generally harder than surface waters.Prescribed hardness limit for public supplies range between 75 to 115ppm.
 
(4) Chloride Content:
 
Chloride content of treated water to be supplied to the public should not exceed a value of about 250ppm. Chloride content of water can be measured by titrating the water with standard silver nitrate solution using potassium chromate as indicator.
 
(5) Nitrogen Content:
 
Presence of nitrogen in water may occur in one or more of the following reasons:
 
Free ammonia:
 
It indicates very first stage of decomposition of organic matter. It should not exceed 0.15mg/l.
 
Albuminous or Organic Matter:
 
It indicates the quantity of nitrogen present in water before the decomposition of organic molten has started. Should not exceed 0.3mg/l.
 
Nitrites:
 
It is not fully oxidized organic matter in water.
 
Nitrates:
 
It Indicates fully oxidized organic matter in water .
 
Nitrites is highly dangerous and therefore the permissible amount of nitrites in water should be nil.
 
Ammonia nitrogen + organic nitrogen = kjedhal nitrogen
 
Nitrates in water is not harmful. Presence of too much of nitrates in water may adversely affect the health of infants causing a disease called mathemoglobinema commonly called blue baby disease. Nitrate concentration in domestic water supplies is limited to 45mg/l.
 
(6) Metal and other chemical substances in water:
 
(i)Iron – 0.3ppm, excess of these cause discolouration of clothes.
 
(ii)Mangnese – 0.05ppm
 
(iii)Copper – 1.3ppm
 
(iv)Sulphate – 250 ppm
 
(v)Fluoride – 1.5 ppm, excess of this affects 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, cause spotting and discolouration of teeth called fluorosis.
 
(7) Dissolved gases
 
Oxygen gas is generally absorbed by water from the atmosphere but it is being consumed by unstable organic matter for their oxidation. If the oxygen present in water is found to be less than its saturation level, it indicates presence of organic matter and consequently making the water suspicious.
 
Biological Oxygen Demand (BOD):
 
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 20°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.
 
BACTERIAL AND MICROSCOPIC CHARACTERISTICS
 
Five types of parasitic organisms (i.e. bacteria, protozoa, viruses, worms and fungi) are generally known to be infectious and are found in water.
 
(1) Bacteria
 
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.
 
(2) Protozoa
 
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.
 
(3)Viruses
 
Acellular (has no cell structure) and requires a living host to survive; Causes illness in its host, which causes an immune response.
 
(4) Worms
 
These are the larva of flies.
 
(5) Fungi
 
These are those plants which grow without sunlight and live on other plants or animals, dead or alive.
 
Classification of bacteria based on oxygen requirement:
 
Aerobic bacteria: Those which require oxygen for their survival.
 
Anaerobic bacteria: Those which flourish in the absence of free oxygen.
 
Facultative bacteria: Those which can survive with or without free oxygen.
 
Pathogenic bacteria
 
It can be tested and counted in the laboratories with great difficulty. Tests are not performed in routine to checkup the water quality. Routine tests are generally conducted to detect and count the presence of coliforms, Their presence or absence indicates the presence or absence of pathogenic bacteria.
 
Methods to measure the presence of coliform bacteria:
 
(1)Membrane filter technique (modern technique).
 
(2)Mixing different dilution of a sample of water with lactose froth and incubating them in test-tubes for 48 hours at C.
 
The presence of acid or carbon dioxide gas in the tube will indicate the presence of coliform bacteria.Most probable number (MPN) represents the bacterial density.
 
Coliform index
 
It may be defined as the reciprocal of the smallest quantity of a sample which would give a positive portion. Coliform sometimes called bacteria coli (B-coli) or Escherichia (E-coli) are harmless aerobic micro-organisms. If not more than 1 coliform is present per 100ml of water, then water is said to be safe for drinking.