Any parameter which can be sensed or felt by any of our senses are referred as physical water quality parameters. Important Physical Characteristics includes
1. Suspended Solids 2. Turbidity 3. Colour
4. Taste and Odour 5. Temperature
1. Suspended solids (SS)
Total solids may be present in the form of
a. Suspended Solids – 10-1 to 10-3 mm size
b. Colloidal solids – 10-3 to 10-6 mm size
c. Dissolved solids – 10-6 mm size
NOTE – Suspended solids and colloidal solids are combindely called dispersed solids.
Suspended Solids is a physical water quality parameter, whereas the dissolved solids is a chemical water parameter.
Suspended Solids in water may come from inorganic particles like clay, silt, sand, gravel, etc. or organic particles like plant fibres, algae, plankton, etc. (algae and plankton are aquatic plants) or may come from immiscible liquids like oil, grease, etc.
- Suspended Solids in water induces psychological impact as they make the water aesthetically displeasing.
- Suspended Solids, if organic in nature may result in the growth of disease-causing micro-organisms.
- Suspended Solids leads to incrustation (deposition) in pipes, thereby reduces there discharge carrying capacity.
- Suspended Solids provides strong adsorption sites to both chemical & biological reagents thereby interfere with the treatment of water.
Measurement of Suspended Solids
- Solids in water are determined using Gravimeter technique (technique in which weight of the parameter is noted)
- Total solids (TS) i.e. all solids (suspended and dissolved) are determined by evaporating the test sample and measuring the residue, where the heating temperature is 104 degree Celsius.
- Suspended Solids are determined by passing the test sample through the filter and heating the residue left over at 104 degree Celsius.
- Organic solids (OS) both in total in suspended form are determined by heating the test sample and residue left over the filter at 600 to 650 degree Celsius respectively, in muffle furnace, the temperature at which organic solids being volatile is converted into vapours leaving behind inorganic solids or non- volatile solids (NVS) or fixed solids (FS).
NOTE – Suspended Solids having the size smaller than the size of the voids of the filter gets measured as dissolved solids hence in order to avoid this, classification of solids is done as filterable & non-filterable solids. Where filterable solids correspond to dissolved solids and non-filterable solids corresponds to suspended solids.
- Total Solids acceptance/permissible limit ≤ 500 mg/l for drinking water and in no cause it should be greater than 2000mg/l.
- As per environmental protection agency (EPA), acceptable limit for Suspended Solids is 30 mg/l for drinking water.
Turbidity is the extent to which light is either scattered or absorbed due to the presence of the suspended solids in water but there is no quantitative relationship between the two.
Turbidity depends up on the fineness, shape, size, refractive index and concentration of particles present in water.
Large amount of fine suspended matters make the water to appear cloudy or turbid in appearance
Mostly turbidity is due to colloidal materials like clay, silt, rock fragment, metal oxides, vegetation fibres and micro-organisms.
- Disinfection of turbid water is difficult because the suspended solids partially shield the organisms from disinfectant.
- Turbidity in natural water body interferes with the penetration of sunlight thereby inhibits photosynthesis reaction during which oxygen is released thereby affect the survival of aquatic life.
Measurement of turbidity
Turbidity is measured photo-chromatically by determining the percentage of the light of a given intensity i.e. either absorbed or scattered.
Turbidity can be measured by any of the following methods:
a. Turbidity rod method
- It is a field method
- Turbidity rod consist of an aluminium rod having platinum needle at its tip, immersed in the sample of the water to be tested.
- The depth at which platinum needle just becomes invisible gives turbidity in ppm.
- As depth of insertion increases, reading will decrease.
- The needle is calibrated to give the turbidity in standard units obtained by the addition of silica as silica dioxide (fullers earth) in powdered form in one litre of pure water and represented as mg/l, ppm or STU (silica turbidity unit)
b. Jackson Turbidity Method
- It is a laboratory method which gives the turbidity of the water if greater than 25 units, hence are not found for the testing of water supplies but are readily used for the testing of the natural water body.
- It consists of a metallic container having a flask bottom which is placed over on the ignited flame.
- The sample of the water to be tested is poured in the container and the height of the water at which the image of the flame ceases to be seen is noted which is further calibrated to give the turbidity in standard units expressed as mg/l, PPM, STU or JTU (Jackson turbidity unit).
c. Baylis Turbid meter and Nephelometer
- Both these test are based on a colour matching technique which can give the turbidity of water of even less than 1 mg/l, hence this method is readily used for the testing of water supplies.
- This test consists of a test sample (standard sample is that where turbidity is known & test sample is that whose turbidity is to be determined) which are being subjected to a source of light and that induces colour in them. The intensity of which is further matched with each other. If the intensity of colour in the test sample is the same as that of the scattered sample, then turbidity of the test sample is also same as that of the standard sample.
- In order to improve the effectiveness of this test intensity of current is now being measured instead of intensity of colour which is been produced by the use of photometer, that is the semiconductor device which is sensitive to light being incident upon it.
- In Baylis turbid meter, photometers are placed behind the sample hence measure the intensity in the direction of incidence, thereby it is based upon absorbtion principle whereas in Nephelometer, photometers are placed at right angles to the sample thereby it measures the intensity of scattered light, hence is based upon scattering principle.
- In Niphlometer turbidity is expressed in terms of standard units which is obtained by the addition of formazine (hexamethylene tetraamine) in one litre of pure water and is represented as mg/l, ppm, FTU (formazine turbidity unit) and NTU (Nephelometer turbidity unit)
Government of India (GOI) gives turbidity in NTU. Acceptable limit is 1 NTU and cause of rejection is 10 NTU.
- Pure water is colourless, but water nature is often coloured by foreign substances.
- Water whose colour is partly due to suspended matters are said to have apparent colour.
- Colour, due to dissolved solids, that remain after removal of suspended matters is called true colour.
- Humic acid gives yellowish colour, manganes oxide gives brownish or blackish colour and iron oxide gives reddish colour.
- Heavy growth of algae in water also may impart colour to the water.
- Coloured water cannot be used for washing and dying of clothes.
- There are certain colour causing compound like iron and manganese which increases the demand of chlorine in water thereby reduces its effectiveness as a disinfectant or increases the cost of disinfection (chlorine is one of the strong oxidizing agent).
- There are certain colour causing organic compounds which reacts with chlorine & leads to the formation of carcinogenic (compounds capable of causing cancer).
- There are certain colour causing organic compounds like phenol which reacts with chlorine and gives taste & odour in water.
Measurement of Colour
Colour in water is measured by a colour matching technique in tintometer using nesseler’s tube and colour is produced by the addition of platinum in the form of chloroplatinate ion in one litre of pure water and intensity is expressed in terms of a standard unit referred as TCU (True colour unit).
The permissible limit for drinking water is 5 TCU and cause of rejection is 25 TCU
NOTE – Testing of colour should be done within 72 hours of the collection of the water sample otherwise biological activity may alter the colour.
4. Taste & Odour
Many substances with which water comes into contact in nature or during use may impart perceptible taste and odour. These include minerals, salts from the soils, metals, ends products from biological reaction and constituents of waste water.
Taste & odour in water may be induced due to inorganic mineral salts (like sodium, calcium, magnesium, etc.) or due to organic matter or may come from dissolved gases (like H2S, NH3, methane, etc.)
Taste and odour causing compounds may be carcinogenic (compounds capable of causing cancer).
- Taste and odour, which is produced due to organics can be measured by gas or liquid chromatography, however, this method is costly and not done in routine.
- Taste and odour in water are also determined by osmoscope by diluting the water sample up to an extent.
- Taste & odour is hardly deductible and is expressed in terms of a standard unit referred as TON (threshold odour number) which represents the dilution ratio at which taste and odour is hardly deductible.
- The acceptable Limit of drinking water is 1 TON and cause of rejection is 3 TON
- In any case, TON cannot be less than 1.
NOTE – Odour can be removed by mechanical aeration, oxidation by chemical like chlorine or its compounds or ozone or permagnate and adsorption of odour by agents such as activated carbon.
The temperature of water affects both chemical & biological reactions and thus affects the water quality.
An average increase of 10 degree Celsius in temperature of water almost doubles the biological activities.
For water supplies temperature should be in the range of 10 to 250 C. and greater than 250 C is objectionable.
By increasing the temperature, solubility of dissolved gases decrease. At 200 C dissolved oxygen (DO) in water is nearly 9.2 mg/l. if DO falls below 4 mg/l, then water species may die such as fishes, lobsters and crabs.
Thermal Shock – If the hot water effluent from power plants and automobile industry is discharged in natural streams then due to rise in temperature, DO level may fall below 4 ppm and which may causes death of water species. Such type of situation is called thermal shock.