Environmental Engg.

Lesson-8 Drinking water quality - chemical and biological properties

Chemical tests

Under this category, tests are carried out to examine water for the following

1. Chlorides

2. Dissolved gases

3. Hardness

4. pH

5. Alkalinity

6. Nitrogen

7. Total solids

Chlorides

            The chloride contents, especially of sodium chloride or salt, are worked out for a sample of water.  The excess presence of sodium chloride indicates pollution of water due to sewage, minerals, etc.  The water has lower contents of salt than sewage due to the fact that salt consumed in food is excreted by body.  For potable water, the highest desirable level of chloride content is 200 mg/litre and its maximum permissible level is 600 mg per litre. 

The measurement of chloride contents is carried out as follows:

1. 50 cc of sample of water is taken by pipette in a porcelain dish.

2. Two or three drops of potassium chromate solution are added to the sample of water

3. The chloride content is then determined by titrating with standard solution of silver nitrate.

            The silver reacts first with all chlorides and silver chloride thus formed then reacts with potassium chromate. The silver chromate appears as reddish precipitate and the amount of silver nitrate required to produce such reddish precipitate determines the amount of chlorides present in water.

 Dissolved gases

            The water contains various gases from its contact with the atmosphere and ground surfaces.  The usual gases are nitrogen, methane, hydrogen sulphide, carbon dioxide and oxygen.  The contents of these dissolved gases in a sample of water are suitably worked out.

The methane concentration is to be studied for its explosive property.  The hydrogen sulphide gives disagreeable odour to water even if its amount is very small.  The carbon dioxide content indicates biological activities, causes corrosion, increases solubility of many minerals in water and gives taste to water.

            Oxygen in the dissolved state is obtained from atmosphere and pure natural surface water is usually saturated with it.  The simple test to determine the amount of dissolved oxygen present in a sample of water is to expose water for 4 hours at a temperature of 27°C with 10% acid solution of potassium permanganate.  The quantity of oxygen absorbed can then be calculated.  This amount, for potable water, should be about 5 to 10 ppm.

Hardness

            The hardness or soap-destroying power of water is of two types – temporary hardness and permanent hardness.  The temporary hardness is also known as carbonate hardness and it is mainly due to the presence of bicarbonates of calcium and magnesium.  It can be removed by boiling or by adding lime to the water.  The permanent hardness is also known as non-carbonate hardness and it is due to the presence of sulphates, chlorides and nitrates of calcium and magnesium.  It cannot be removed by simply boiling the water.  It requires special treatment of water softening. 

Total hardness = carbonate hardness or alkalinity + Non carbonate hardness

            The excess hardness of water is undesirable because of various reasons such as it causes more consumption of soap, affects the working of dyeing system, provides scales on boilers, causes corrosion and incrustation of pipes, makes food tasteless, etc.,

            The hardness is usually measured by the soap solution test.  The standard soap solution is added in the sample of water.  It is then vigorously shaken for about five minutes and formation of lather is observed.  The difference between the total amount of soap solution and the latter factor indicates the hardness of water.

            The water, having hardness of about 5 degrees, is reasonably soft water and a very soft water is tasteless.  Hence, for potable water, the hardness should preferably be more than 5 degrees but less than 8 degrees or so.

Hydrogen ion concentration (pH)

            The acidity or alkalinity of water is measured in terms of its pH value or H-ion concentration.  It is desirable to maintain pH value of water very close to 7.  The acidic water causes tuberculation and the alkaline water causes incrustation.  For potable water, the pH value should be between 7 and 8.50

Following are the two methods which are employed to measure the pH value of water:

Electrometric method

In this method, potentiometer is used to measure the electrical pressure exerted by positively charged H-ions.  The pH value is then correspondingly expressed.

Colourimetric method

In this method, chemical reagents are added to water and the colour produced is compared with standard colours of known pH values.  A set of sealed tubes containing coloured waters of known pH values is kept in the laboratory for ready reference.  This test is simple and hence, it is commonly carried out in public health laboratories.  The usual indicators are Benzol yellow, Methyl red, Bromphenol blue, etc., for acidic range and Thymol blue, Phenol red, Tolyl red, etc. for alkaline range.

Alkalinity

The alkalinity is the capacity of a given sample to neutralize a standard solution of acid.  The alkalinity is due to the presence of bicarbonate (HCO₃), carbonate (CO₃) or hydroxide (OH).

The determination of alkalinity is very useful in waters and wastes because it provides buffering to resist changes in pH value.  The alkalinity is usually divided into the following two parts:

Total alkalinity i.e. above pH 4.5

Caustic alkalinity i.e. above pH 8.2

The alkalinity is measured by volumetric analysis. The commonly adopted two indicators are:

1. Phenolphthalein :  pink above pH  8.5 and colourless below pH 8.2

2. Methyl orange : red below pH 4.5 and yellow orange above pH 4.5

Alkalinity in mg/l as CaCO₃ = (total reading / volume of sample in ml) x 1000

Nitrogen and its compounds

The nitrogen is present in water in the following four forms:

• Free ammonia

• Albuminoid ammonia

• Nitrites

• Nitrates

The amount of free ammonia in potable water should not exceed 0.15 ppm and that of albuminoid ammonia should not exceed 0.3 ppm.  The terms albuminoid ammonia is used to represent the quantity of nitrogen present in water before decomposition of organic matter has started.

The presence of nitrites indicates that the organic matter present in water is not fully oxidized or in other words, it indicates an intermediate oxidation stage. The amount of nitrites in potable water should be nil.

The presence of nitrites indicates that the organic matter present in water is fully oxidized and the water is no longer harmful.  For potable water, the highest desirable level of nitrates is 45 mg per litre.

The free ammonia is measured by simply boiling the water.  The ammonia gas is then liberated.  The albuminoid ammonia is measured by adding strong alkaline solution of potassium permanganate to water and then boiling it.  The ammonia gas is then liberated.  The nitrites and nitrates are converted chemically into ammonia and then measured by comparison with standard colours.

Total solids

In this test, the amounts of dissolved and suspended matter present in water are determined separately and then added together to get the total amount of solids present in water.  The highest desirable level of total solids is 500 mg/litre and its maximum permissible level is 1500 mg/litre.

For measuring suspended solids, water is filtered through a fine filter and dry material retained on the filter is weighed.  The filtered water is evaporated and weight of residue that remains on evaporation represents the amount of dissolved water in water.

Bacteriological tests

The examination of water for the presence of bacteria is very important.  The bacteria are very small organisms and it is not possible to detect them by microscopes.  Hence, they are detected by circumstantial evidences or chemical reactions.  The growth of bacteria takes place by cell division and there are various classifications of bacteria depending upon their shapes, oxygen requirements and effects on mankind.  The last classification is important for the water supply engineer from the view point of public health.  The bacteria may be harmless to mankind or harmful to mankind.  The former category is known as non-pathogenic bacteria and the latter category is known as pathogenic bacteria.  It is not possible to isolate pathogenic bacteria with the help of laboratory instruments.  Their chances of presence in a sample of water are increased in relation to the amount of non-pathogenic bacteria present in the sample of water.

The combined group of pathogenic and non-pathogenic bacteria is designated by bacillus coli or B-coli group.  This group of bacteria is present in the intestines of all living warm-blooded animals.

Following are the two standard bacteriological tests for bacteriological examination of water.

1. Total count or Agar plate count test

2. B-coli test

Total count or Agar plate count test

In this test, bacteria are cultivated on specially prepared medium of agar for different dilutions of sample of water with sterilized water.  The diluted sample is placed in an incubator for 24 hours at 37°C or for 48 hours at 20°C.  These represent the so-called hot counts and cold counts respectively.  The bacterial colonies which are formed, are than counted and the results are computed for 1cc.  For potable water, the total count should not exceed 100 per cc.

B-coli test

This test is divided into the following three parts:

1. Presumptive test

2. Confirmed test

3. Completed test

The presumptive test is based on the ability of coliform group to ferment the lactose broth and producing gas.  The confirmed test consists of growing cultures of coliform bacteria on media which suppress the growth of other organisms.  The completed test is based on the ability of the culture grown in the confirmed test to again ferment the lactose broth.

Presumptive test

Following procedure is adopted in this test:

1. The definite amounts of diluted samples of water are taken in multiples of ten, such as 0.1 cc, 1.0 cc, 10 cc, etc.

2. The water is diluted in standard fermentation tubes containing lactose broth.

3. The tube is maintained at a temperature of 37°C for a period of 48 hours.

4. If gas is seen in the tube after this period is over, it indicates presence of B-coli group and the result of test is treated as positive.  If reverse is the case, it indicates the absence of B-coli group and the result of test is treated as negative.

5. A negative result of presumptive test indicates that water is fit for drinking.

Confirmed test

A small portion of lactose broth showing positive presumptive test is carefully transferred to another fermentation tube containing brilliant green lactose bile.  If gas is seen in the tube after 48  hours, the result is considered positive and the completed test becomes essential.

Completed test

This test is made by introducing or inoculating bacterial colonies into lactose broth fermentation tubes and agar tubes.  The incubation is carried out at 37°C for 24 to 48 hours.  If gas is seen after this period, it indicates positive result and further detailed tests are carried out to detect the particular type of bacteria present in water.  The absence of gas indicates negative result and water is considered safe for drinking.

B-coli index

This is an index or number which represents approximately the number of B-coli per cc of sample of water under consideration.  The presumptive tests are carried out with different dilution ratios of the sample of water with sterilized water.  A number of tests is carried out for each proportion and percentage of positive results is recorded.  The difference between successive percentages is worked out and it is multiplied by the reciprocal of quantity of solution.  The sum of such values indicates B-coli index.  For potable water, B-coli index should be preferably less than 3 and it should not exceed 10 in any case.