In this Study Session, you have learned that:

• The composition of liquid waste depends on its source. The three main sources are residential, commercial, and industrial areas. Stormwater is also a source of liquid waste.
• Liquid waste from domestic sources can be classified as blackwater, which contains excreta, and greywater, which does not.
• Liquid waste from commercial areas is broadly similar to wastewater from residential areas. Fats and oil from restaurants and cafes can be removed using a grease trap.
• The characteristics of industrial wastewaters depend on the type of industry. Some industrial wastewaters are hazardous.
• The characteristics of wastewaters can be described in physical, chemical and biological terms.
• Physical characteristics include the amount of suspended solids, the temperature and odour. The amount of suspended solids is measured by filtering a known volume of wastewater and weighing the solids retained on the filter.
• The quantity of organic matter in liquid waste is an important measure of its polluting potential. If discharged into a river or lake, organic matter exerts an oxygen demand which can reduce the availability of oxygen for fish and other aquatic organisms.
• Organic matter is measured in terms of biochemical oxygen demand or chemical oxygen demand.
• Three examples of industries that produce liquid wastes are food, textiles and tanneries. They each produce polluting liquid wastes that should be treated before being discharged into the environment.