CHAPTER 4

WATER QUALITYPrepared by Bereket.T

Physical, chemical and biological characteristics of water

• Pure water is a chemical compound with each of its molecules containing two hydrogen atoms and one oxygen atom [H2O].

• pure water can never be available in nature. Quality of water for different uses• Domestic use: requires the highest degree of purity.• Civic use: is characterized by large water requirement and any

degree of impurity is acceptable unless mixed with sewage and other refuses.

• Trade and/or Business use: Purity depends upon the nature of trade/business.

• Commercial or Industrial use: Water should be chemically pure.

Impurities• Impurities in water could be classified as organic and inorganic.• Impurities could further be grouped into these three classes as:1. Suspended impurities,2. Dissolved impurities, and3. Colloidal impurities.

• To insure safety to public health, economy and utility in industries and other uses, it is authoritative for planners and designers of the public water supply schemes.

Properties of water

• X’crs of Water

Physical x’crs

Chemical x’crs

Biological x’crs

Properties of water• Physical Characteristics: primarily affect the acceptability of

water from the aesthetic considerations.

• Chemical Characteristics: presence of chemicals that alter water

characteristics.

• Biological Characteristics: presence of micro-organisms, viruses, other living organisms and their remains.

Physical CharacteristicsColor• A character of pure water regarding color is colorless.• The presence of color in water is not objectionable from health

point of view, but from aesthetic and psychological point of view it may be objectionable.

Taste and odor • Objection to taste and odor in drinking water is very subjective

in nature and different persons react differently to them.• It is frequently difficult, to identify the specific cause of taste

and odor.

Turbidity

• Turbidity is an expression of the optical property of water that causes light to be scattered and absorbed rather than transmitted in straight lines.

• If a large amount of suspended material such as clay, silt are present in water, it will be muddy or cloudy or turbid in appearance.

• The turbidity of raw water must be measured and then reduced by treatment to permissible values so as to make it almost invisible to naked eye.

PH• pH is a measure of acidity and alkalinity using a scale of 0 to 14,

7 being the neutral point.• Without proper alteration of pH raw water may be corrosive and

adversely affect treatment processes.• Generally, alkalinity is caused by the presence of bicarbonates of

calcium and magnesium.

Temperature• Temperature of water at user's end depends on the temperature

of water source as well as the processes the water has undergone by way of treatment and distribution.

• Generally ground water is cooler than surface water.

Chemical characteristics• Chemical in water May be toxic and may constitute a danger to health even at a

very low concentration. E.g. Arsenic, Cadmium, Cyanide, Lead, Mercury and Selenium.

May affect health or may interfere with the acceptability of the water depending upon their concentration. E.g. Fluoride, Nitrate, Sulphate, Chloride, Iodine.

May cause hardness of water and cardiovascular diseases. This is caused by ions principally calcium and magnesium.

May cause medicinal test and persistent odor problems.

Hardness

• Hard water is one that requires considerable amounts of soap to

produce foam or lather. • surface waters are softer than ground waters.• Principally hardness is caused by Mg and Ca.• Hardness of water is derived largely from contact with the soil

and rock formation. • Hard water originates in areas where the topsoil is thick and

lime stone formations are present. • Hardness is commonly expressed as equivalent concentration of

calcium carbonate and is reported in mg/l of CaCO3.

Con’tThe application of hardness data is that• It is useful in determining the suitability of water for

domestic and industrial uses.• The engineer uses it as a basis for recommending the

need for softening process.• The relative amounts of Ca and Mg hardness and of

Carbonate and non-carbonate hardness present in water are factors in determining the most economical type of softening process to use, and become important consideration in design.

Phenolic Compounds

• Phenols are released into water during the metabolic processes of aquatic organisms.

• Decaying vegetation, for example, fallen leaves of some species of trees may release phenol in water.

• Other sources of Phenolic compounds are: From the distillation of coal and wood, oil refineries, chemical plants, livestock dips microbial degradation of pesticides and human and animal

wastes.• Concentrated solutions of phenol, when ingested, give rise to

severe pain, shock and probably death.• Even at very low concentrations they cause serious taste and

odor problems in water supplies.

Con’t• In the oil-bearing areas the ground water often contains a

higher concentration of phenols.• When it is present in water it can react with chlorine which is

added for disinfecting in the final treatment of water and form chloro-phenol, which give medicinal tastes and cause persistent odor problems in the distribution system.

Microbiological Analysis of water• Unfortunately not all water helps human being to survive.• By definition pathogens are organisms which cause disease to

other organisms and these are fungi, bacteria, protozoa, viruses, etc.

• Such pathogens occur in a drinking water supply when there is feacal pollution to the source.

• The process of detection is time consuming and uneconomical and detection and estimation needs selective media of pure culturing and bio-chemical test.

• Indicators are used to detect the presence of faecal pollution and thus the pathogens in the water supply system.

• i.e., the presence of indicators means the presence of pathogens.

The use of testing for indicators is:

• To detect recent contamination;• To evaluate the efficiency of treatment systems;• To examine the disinfections process;• To choose the best source; and• Generally it serves as a quality control. Thus the types of indicator organisms used in water quality

analysis:• Coli-forms• Streptococci• Anaerobic spore forms Of these, coli-forms are the most widely used indicators.

Con’t• The simplest and most recent method adopted for

detecting and measuring the presence of coli-form bacteria is to filter the water sample through a sterile membrane of special design (i.e., 80 % porosity, pore size 5 to 10m ).

• The process is called culturing.Water Quality Examination• Physical examination: are done for physical

characteristics of water.• Chemical examination: are measurements of the amount

of metals, alkalinity and hardness, intensity of acidity or alkalinity, nitrogen, chlorides, dissolved oxygen, BOD, and Chlorine remaining in the water after treatment.

Con’t• Bacteriological examination: determines the probable number

and kind of bacteria present in the water.• Microscopical examination: provide information on the variety

and number of microscopic organisms with special reference to the probable source of taste and odor.

Water quality standards• Municipal water required for domestic uses particularly the

water required for drinking must be:• colorless,• odorless and tasteless• Free from turbidity, and excessive toxic chemical compounds.• Harmful microorganisms must be absent.

• World Health Organization (WHO) has laid down its international standards, specifying the minimum water quality requirements.