PHYSICAL WATER QUALITY
The physical quality of water is determined by intrinsic characteristics as well as by dissolved and colloidal substances in the water. Intrinsic physical properties include temperature, viscosity, and surface tension. Other physical properties such as electrical conductivity, colour, taste and odour are determined by the presence of dissolved and colloidal substances in the water. Some characteristics of water are often indicated as physical characteristics, while they are in actual fact chemical in nature, pH being an example. These properties can also be called physico-chemical properties. The general physical properties of water that play a role in treatment are discussed in the following section.
Turbidity gives an indication of the concentration of colloidal particles in water. Turbidity is expressed in nephelometric turbidity units, NTU. It is determined in a Nephelorometer by comparing the intensity of light scattered by the water sample to the intensity of light scattered by a standard reference in the turbidity meter. The turbidity of raw water can be as low as a 1 or 2 NTU in groundwater and up to several hundred in turbid surface water, e.g. after a rain storm. The turbidity of drinking water should be <1, and preferably <0,1 NTU.
The pH of water is measured in pH units. The pH value is a measure of the concentration of hydrogen ions, [H+] in the water expressed as a logarithmic value. The pH gives an indication of how acidic or basic the water is. Because the concentration of [H+] and [OH-] can vary over the very wide range of 100 to 10-14 it is convenient to use a logarithmic scale to express the concentration. For this purpose the pH function was introduced as: pH = – log [H+] where [H+] is the hydrogen ion concentration expressed in moles/l.
Because the pH scale is logarithmic, it means that a change of one pH unit is equal to a 10 times increase in [H+] concentration (See Appendix A1 for a more detailed discussion).
Electrical conductivity is a measure of the ability of the water to conduct an electric current. Since the electric current is conducted through the movement of ions in solution, EC also gives an indication of the concentration of the ions or total dissolved solids (TDS) in the water. Electrical conductivity (EC) of water is measured in units of milli Siemen per metre (mS/m). Other units that are also used include PS/cm, which is numerically equal to Pmho/cm.
Conversion from PS/cm to mS/m is as follows:
mS/m = PS/cm x 0,1.
The EC value may be used to estimate the TDS concentration in mg/l by multiplying the EC by a factor established for the type of water. For surface water the factor is about 6.5 but will vary with different water sources.
Colour of water can be either true colour caused by dissolved substances or apparent colour caused by colloidal substances. Colour is expressed in mg/l platinum or Hazen units (that are numerically equal). The colour in soft coastal water is generally caused by humic and fulvic acids that derive from decaying plant material. Many industrial effluents also contain colour caused by dyes, for example in the textile industry.
Taste and odour of water can be caused by a variety of substances. These include algal products such as geosmin and 2-MIB (2-methyl isoborneol), inorganic salts such as NaCl, and gasses such as H2S. Taste and odour are expressed as threshold taste number (TTN) and as threshold odour number (TON) respectively. The threshold numbers are determined by diluting the sample with taste-free or odour-free water until the least perceptible taste or odour is detected.
TON = (ml sample + ml odour free water) / ml sample.
The microbiological quality of water is determined by the type and numbers of microorganisms present in the water. A variety of micro-organisms can be present even in very good quality domestic waters. Most of these micro-organisms are harmless but if the water is polluted pathogens may be present. Pathogens are disease-causing
micro-organisms such as those causing cholera, gastro–enteritis, hepatitis, etc. It is difficult to determine the presence of all the different pathogenic organisms and therefore certain indicator organisms are used to give an indication of the possible presence of pathogens. Indicator organisms are specific types of micro-organisms that are present in very large numbers in the intestines of people and warm-blooded animals. The presence of these organisms in water serves as an indication of pollution of the water by human wastes. Such water is therefore unsafe to drink and must be disinfected before use.
An ideal indicator of water pollution and therefore of the possible presence of pathogens in water should always be present when the pathogenic organism of concern is present and be absent in clean, uncontaminated water, it should be present in large numbers in faecal material and it should be easily detected by simple and inexpensive laboratory tests (certain other requirements are further discussed in Chapter A2).
The following are the most commonly used indicator organisms:
Total coliforms are a group of closely related bacteria that are most commonly used as indicator organisms for drinking water. Although all coliform types are found in the gut of animals including humans and therefore suitable indicators for pollution by human waste, most of them occur widely in the environment, including water and wastewater. This means that they are not specific to pollution by human waste, but can also originate from other sources.