Problems Caused by Various Impurities in Water

PROBLEMS CAUSED BY VARIOUS IMPURITIES IN WATER

 

The quality of water and types of impurities present in the water is a very important parameter to be determined in order to decide the type of application (or) treatment required. The quality of water varies to place to place and seasons.

The followings are some important problems, caused by various impurities, which affect the quality of water.

 

 

1. Colour

Colour is a shade imparted by organic or inorganic material, which change the appearance of the water.

Colour is found mostly in surface water. The colours of natural water range from pale straw through yellowish‒brown to dark brown. The colour of natural waters is mainly due to the presence of dissolved (or) colloidal organic (or) inorganic impurities.

Sources

1. Organic sources

 => Algae, tannins, humic compounds organic dyes, etc.

2. Inorganic sources

 ⇒ Fe and Mn compounds, chemicals and inorganic dyes from various industries.

Sanitary Significance

1. The colours and the materials, which produce colour, are often objectionable in which the water and the manufactured product come into contact.

e.g., Dyeing, scouring and laundering

2. Variation in colour of water from the same source with time serves as index of quality of the water.

e.g.,

(a) Yellowish tinge⇒ indicates the presence of 'Cr' and organic matter.

(b) Yellowish red ⇒ indicates the presence of iron.

(c) Red‒brown ⇒ indicates the presence of peaty matter.

Removal of colour

 Colour and colour producing materials are removed by coagulation, settling, adsorption and filtration.

 

 

2. Tastes and Odours

Taste

Taste is the sensation of flavour perceived in the mouth and throat on contact with a substance.

Odour

Odour is a smell (or) scent caused by one (or) more volatilized chemical compounds that are generally found in low concentration.

Sources

Organic sources: Algae and decaying vegetation, etc.

Inorganic sources: Mercaptans, amines and sulphides, etc.

The tastes and odours observed in chlorinated waters are due to chloro‒organic compounds formed by the reaction between chlorine and organic matter present in the water.

How to Evaluate the Odour?

It is impossible to isolate and identify clearly the odour causing materials. Evaluation of odour depends on the olfactory senses of the testing person and on his ability to distinguish between different types of odours.

Significance

1. Disagreeable odours and tastes are objectionable for various industrial processes such as food products, beverages, textiles, paper, pulp. Most of the odours in natural waters are organic in nature, except H₂S.

2. Tast helps us to decide what to eat and influences how efficiently we digest these foods.

3. Senses of smell and taste are vital in identification of valuable nutrients in the environment.

4. Taste enables the evaluation of foods for toxicity.

Removal of Tastes and Odours

(i) Organic tastes and odours may be removed by aeration (or) activated carbon treatment.

(ii) Inorganic tastes due to H₂S (or) Iron may be be removed by chemical methods like oxidation, chlorination (or) precipitation.

 

 

3. Turbidity and sediments

Turbidity is the reduction of clarity of natural water due to the presence of finely divided, insoluble impurities suspended in water.

Sources

1. Inorganic sources

Clay, silt, silica, ferric hydroxide, calcium carbonate, sulphur, etc.,

2. Organic sources

Finely divided vegetable or animal matter, oils, fats, greases, micro‒organisms, etc.,

Problems Caused by Turbidity

(i) Presence of turbidity and sediments in boiler water and cooling water system cause problems.

(ii) Water‒softening processes cannot be carried out.

(iii) Due to deposition of these organic impurities, disinfection efficiency gets reduced.

Significance

(i) Turbidity affects the taste and odour of drinking water.

(ii) As turbidity affects the disinfection process, it must be removed.

(iii) Turbidity have many negative effects on aquatic life, it block light to aquatic plants, aquatic organisms.

(iv) Turbidity affects the growth rate of algae.

(v) It increases water temperature because suspended particles absorbs more heat.

Removal of Turbidity and Sediments

Turbidity of water may be removed by sedimentation followed by

(a) Coagulation and filtering

(b) Coagulation and settling

(c) Coagulation, settling and filtering.

Example

1. Turbidity caused by suspended silt and mud is objectionable in boilers and in cooling‒water systems.

2. Turbidity caused by colloidal or dissolved organic matter will interfere with water‒softening processes.

 

 

4. pH

The hydrogen ion concentration is represented by the pH value, which is defined as

pH = − log₁₀[ H⁺ ]

pH is defined as negative logarithm of hydrogen ion concentration. The pH value ranges from 0‒14 as

0 <‒‒‒‒‒‒‒‒‒> 7 <‒‒‒‒‒‒> 14

Acidic <‒‒‒‒‒‒‒‒‒‒ Neutral ‒‒‒‒‒‒‒‒‒> Basic

Generally pH of natural waters lies in the neutral range. For drinking water recommended pH = 6.5 to 8.5. For irrigation recommended pH = 6.0 to 9.0. Some surface waters passing over areas rich in sodium and potassium posses alkaline pH. The rain water, contaminated by the dissolved gases such as SO₂ and NO_x, will have acidic pH.

Significance of pH

(i) pH determines the solubility (amount that can be dissolved in water).

(ii) It also determines the biological availability (amount that can be utilized by aquatic life).

(iii) A rise (or) fall in pH can indicate chemical pollution (or) acid rain. Many animals cannot live sin in water at a pH level below 5 (or) above 9.

 

 

5. Alkalinity

Alkalinity of water is a measure of its acid‒neutralising ability. The natural alkalinity in waters is imparted by the hydroxides, carbonates and bicarbonates.

Sources

1. Hydroxides, carbonates and bicarbonates of alkalimetals, borates, silicates, phosphates. ⇒ Industries like fertilizer, detergent, leather and paint.

Sanitary significance

1. Very high values of alkalinity are harmful to aquatic organisms.

2. Alkalinity in boiler feed water causes caustic embrittlement of pipes.

Removal of Alkalinity

Alkalinity in water, can be removed by adding limited amount of HCl.

 

 

6. TDS

TDS stands for Total Dissolved Solids. It is defined as the measure of all inorganic and organic substances present in water.

Significance

(i) TDS impacts the salinity of water.

(ii) TDS in water may not be ideal for your health and must be filtered out before intake.

(iii) Water with TDS level higher than 300 ppm may not be potable as it can taste salty.

(iv) Water with high level of TDS (>1200) impacts the color, odour and taste dramatically.

(v) Water with high level of TDS may not be suitable due to excessive scaling caused by it in water pipes, heaters, boilers and household appliances.

 

 

7. Fluoride

Fluoride is an important impurity, found in ground water as a result of dissolution from geologic formulations. Surface waters generally contain much smaller concentration of fluoride.

Sources

Fluoride containing minerals ⇒ Fluorapatite (Ca₁₀ F₂ (PO₄)₆),

cryolite (Na₃ AlF₆) and igneous rocks containing fluosilicates.

Contaminated domestic sewage, run‒off from agricultural lands ⇒ Phosphate fertilizers.

Sanitary significance

1. Optimum fluoride concentrations, prescribed in public water supplies, are in the range of 0.7 to 1.2 mg / lit.

2. Beneficial health effects have been observed where the fluoride levels are optimum.

3. If the fluoride concentration is low in drinking water it causes dental caries in children.

4. If the fluoride concentration is high it causes fluorosis.

Removal of fluoride (Defluoridation)

1. Precipitation using aluminium salts in alkaline media.

2. Using strongly basic anion exchange resin.

3. By adsorption on activated carbon.

 

8. Arsenic

Arsenic is a metallic element that forms a number of poisonous compounds. It is found in nature at low levels, mostly in compounds, with oxygen, chlorine and sulfur.

Source

Arsenic can get into drinking water from natural deposits (or) runoff from agriculture, mining and industrial processes.

Significance

(i) Long‒term intake of arsenic contaminated water leads to arsenic poisoning with cancer of skin, bladder, kidney.

(ii) Association of arsenic contaminated water produces diabetes, hypertension and reproductive disorders.

(iii) Children may have more exposure to arsenic in drinking water. As a result, children may be at greater risk of illness when higher levels of arsenic are present.

 

 

9. Chemical Oxygen Demand (COD)

COD is defined as, "the measure of amount of oxygen required to chemically oxidise all the oxidisable impurities present in the sewage using an oxidising agent like acidified K₂Cr₂O₇".

Significance of COD

(i) Determination of COD is carried out only in 3 hours, but determination of BOD is carried out after 5 days.

(ii) It measures both the biologically oxidisable and biologically inert organic matter.

(iii) COD test is used to monitor water treatment plant efficiency.

(iv) COD is used to measure pollutants in water, waste water and aqueous hazardous wastes.

(v) It provides an index to assess the effect of discharged waste water on the environment.

 

 

10. Biological Oxygen Demand (BOD)

BOD is defined as, "the amount of free oxygen required by bacteria for the biological oxidation of the organic matter under aerobic conditions at 20°C for a period of 5 days"

Significance of BOD

(i) It indicates the amount of decomposable organic matter present in the sewage.

(ii) It enables us to determine the degree of pollution at any time in the sewage stream.

(iii) Lesser the BOD, better is the quality of water. ie. the water sample with BOD of less than 3 ppm is considered as pure water, whereas the water more than 4 ppm is considered as polluted water.

 

11. Hardness of Water

Hardness is the property (or) characteristics of water, which does not produce lather with soap.

Types of hardness

Depending upon the types of dissolved salts present in water, hardness of water can be classified into two types

1. Temporary hardness.

2. Permanent hardness.

1. Temporary hardness (or) Carbonate hardness (CH) (or) Alkaline hardness

This is due to the presence of bicarbonates of calcium and magnesium. It can be removed by (i) boiling the water (ii) adding lime to the water.

2. Permanent hardness (or) Non‒carbonate hardness (NCH) (or) Non‒alkaline hardness

This is due to the presence of chlorides and sulphates of calcium and magnesium. It cannot be removed by boiling the water. But, it can be removed by (i) Lime‒soda process (ii) Zeolite process.

Significance of Hardness

1. Hardness affects the amount of soap that is needed to produce foam (or) lather.

2. Hardness is very important in industrial uses, because it forms scale in heat exchange equipment boilers and pipe lines.

3. Hardness mitigates metal toxicity because Ca²⁺ and Mg²⁺ help keep fish from absorbing metals such as lead, arsenic and cadmium into their blood stream.

Expression of hardness in terms of equivalents of CaCO₃.

The concentration of hardness producing salts are usually expressed in terms of an equivalent amount of CaCO₃. CaCO₃ is chosen as a standard because

(i) Its molecular weight (100) and equivalent weight (50) is a whole number, so the calculations in water analysis can be simplified.

(ii) It is the most insoluble salt, that can be precipitated in water treatment.

If the concentration of hardness producing salt is x mgs/lit, then

Amount equivalent to CaCO₃ = [ x × 100 ] / Molecular weight of hardness producing salt

i.e., Amount equivalent to CaCO₃ = [Amount of hardness producing salt × molecular weight of CaCO₃ ] / Molecular weight of hardness producing salt

(or)

Amount equivalent to CaCO₃ = [ Amount of hardness producing salt × Equivalent weight of CaCO₃ ] / Equivalent weight of hardness producing salt

Example

If the concentration (or) weight of CaSO₄ is 43 mgs/lit, then weight equivalent to CaCO₃ = 43×100 / 136 (mgs/lit)