4.2.1 Physical characteristics of liquid wastes

Solids

Wastewaters may contain particles of solid material carried along in the flow. These may be settleable solids or suspended solids. Settleable solids sink to the bottom (settle out) when the speed of flow is reduced, for example, when the wastewater is stored in a tank. Suspended solids are small particles that remain in suspension in the water; they do not dissolve in the wastewater but are carried along in it. The solids content can be measured by filtering out and weighing the solids in a given volume of water. The laboratory procedure is to weigh a filter paper, pour a measured volume of water through the paper, then dry it and weigh again. The difference in mass equals the mass of solids which can be expressed in terms of milligrams of solid matter per litre of water, in units of mg l^-1.

Temperature

Wastewaters are generally warmer than the ambient temperature. This is because warm or hot water may be included in the waste stream from domestic activities such as showering or from industrial processing. The temperature is given in degrees Celsius (^oC).

Odour

Wastewaters can have an odour, usually due to generation of gases as a result of biodegradation in the wastewater. Biodegradation is the breaking down (decomposition) of organic substances by bacteria and other micro-organisms. Organic matter is any substance that is derived from living organisms, such as human and animal wastes, food waste, paper and agricultural wastes. Detecting odour tends to be a subjective process but it is possible to measure it in terms of odour units.