Characteristics of water

Characteristics of water

Physical characteristics: 

relate to the quality of water for domestic use and are associated with the appearance of water such as color or turbidity, temperature, and taste and odor. Substances can exist in water as suspended, colloidal or dissolved solids. Suspended solids are defined as those solids that can be filtered by a glass fibre filter disc.Suspended solids can be removed from water by physical methods such as sedimentation, filtration, and

centrifugation.

Colloidal particles are in the size range between dissolved substances and suspended particles (from 0.001 to 0.45 μm). They are in a solid state and can be removed by high-force centrifugation or filtration through membranes. Colloidal particles in water Treatment are removed by chemical coagulation, sedimentation, and filtration.

The presence of suspended material such as clay, silt, fine organic material, and other particulate material is water is known as turbidity. Colloidal particles in water can reflect light. The degree to which a colloidal suspension reflects light at 90o angle to the entrance beam is measured in terms of Nephelometric Turbidity Unit (NTU). Particles of different size and shape will scatter light differently and can interfere with one another.Turbidity measurements tend to be more sensitive to particles in the range 0.3 – 0.7 μm for

visible light range. Increases in turbidity measurements are often used as an indicator for increased concentrations of water impurities such as bacteria, Giardia cysts, and Cryptosporidium oocysts. Drinking water limit is less than 1 NTU.

Colour  describes the state of a solution and is measured by the ability of the solution to absorb light. Colour is caused by dissolved organic material from decaying vegetation and certain organic matter. Excessive algal blooms or growth of aquatic microorganisms may also impart colour. Apparent colour is a measure indicative of the dissolved and colloidal materials present in water.

Taste and odour usually caused by organic compounds, inorganic salts, or dissolved gases. They may come from domestic, agricultural, or natural sources.

Chemical characteristics of water & Health Effect

Chloride presence is caused by pollution from seawater, or industrial or domestic wastes. Concentrations in excess of 250 mg/L usually produce a noticeable taste in drinking water. Drinking water should contain less than 100 mg/L of chloride.

Fluoride may sometimes be found in natural waters; otherwise it is added into treated water to provide beneficial health effects in preventing tooth decay.

Natural water usually contains a small amount of iron. Its presence is objectionable as it imparts a brownish colour to laundered goods and affects the tastes of beverages such as tea and coffee.

Prolonged ingestion of lead is harmful to the body and can result in illness or death. Lead taken into the body in excess quantity can accumulate in the body.

Manganese  Like iron, it imparts a brownish color to water and to laundry washed in it. It gives a medicinal taste.

Zinc can be found in some natural waters. Not detrimental to health, but can impart an undesirable taste to drinking water.

Toxic inorganic substances like nitrates, cyanides, and heavy metals constitute the major classes of inorganic substances of health concern. The toxic heavy metals include arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver. They have a wide range of health effects, from acute poisoning to producing chronic diseases.

Toxic organic substances: these include pesticides, insecticides, and solvents and their effects may be acute or chronic.

Microbiological characteristics of water


Microbiology is important in water treatment as it is responsible for public health problems and it also affects physical and chemical properties of water. The major groups of microorganisms of interest include bacteria, viruses, algae, fungi, protozoa, and worms.

Common bacterial diseases associated with contaminated waters are cholera, salmonellosis, dysentery, and typhoid fever. Actinomycetes bacteria cause musty and earthy tastes and odors in water supply while hydrogen sulphide-producing cause a rotten egg smell. Iron and manganese bacteria cause a severe clogging or corrosion of pipes resulting in taste, coloration and staining problems. Some bacteria can also develop as a slime layer on pipe wall and create a chlorine demand.

Viruses are present in far fewer numbers than bacterial pathogens in natural waters. They enter water sources via direct contamination from humans or animals or indirectly via sewage or urban runoff.  Virus removal by activated carbon adsorption is low and the most effective method is by disinfection.

Algae are produced when there are sufficient nutrients in a water body. Some algae are pathogenic to man as they can produce endotoxins that cause gastroenteritis. Although 90-95% of algae can be removed effectively by coagulation and sedimentation, the remaining that pass through can still clog filter and significantly shorten filter run time. In slow sand filtration, an algae layer over the sand layer can actually aid in the filtration treatment.

Protozoa: Like algae, they are large in size ranging from 10 to 100 μm. Some can cause disease: Giardia lamblia and Cryptosporidium, the symptoms of which are diarrhoea, nausea, fever, and vomiting. The cyst of these organisms can survive a long time, even in the presence of chlorine residual. Poor cyst removal is normally due to insufficient coagulant dose or turbidity breakthrough during filtration.

Fungi: There are aerobic microorganisms frequently forming dense and slimy mats on walls of treatment plants that can clog filters and other treatment plant equipment. Besides producing a musty taste and doors, they also produce colour and turbidity.

Helminths are parasitic worms which cause many diseases mostly on the skin. The worms and eggs are transmitted via drinking contaminated water or vegetables or body contact with polluted waters. They can be removed by controlling turbidity through effective coagulation and filtration.







reff :

•  (Crittenden et al., Water Treatment Principles and Design, John Wiley & Sons, 2005)
• (Tchobanoglous and Schroeder, Water Quality, Addison Wesley, 1985)