The composition of industrial wastewater will vary depending on the type of industry, the raw materials used, and the processes undertaken. Three of the most important producers of industrial wastewater are the food industry, the textile industry and tanneries.
The food industry
Food production plays a major part in the economy of most low-income countries, with factories producing bread, beverages, sugar and several other products. Many of the production processes require large volumes of water and so most of the factories are located near rivers or boreholes.
The volume of clean water required differs between canneries and the products they are preparing, but ensuring cleanliness is obviously essential. For example, it takes about 220 litres of water to produce 10 kilograms of tomato paste. Canning factories that produce tomato paste generate both solid and liquid waste. The quantity of solid tomato waste may be as much as 15-30% of the total quantity of product (Faris et al., 2002). The wastewater from a cannery will contain organic solids, primarily from washing raw materials such as tomatoes, cleaning equipment, spillage and from floor-washing.
Wastewaters are generated at animal yards, slaughterhouses and packing houses. The main sources are animal faeces, urine, blood and water that has been used for washing floors and surfaces. The pollutants in the wastewaters are organic and can decompose rapidly, generating unpleasant odours. If discharged to a water body, they will cause severe environmental pollution. The meat industry utilises thousands of litres of water per day depending on the size of the facility and the number of animals processed.
What will be the effect of the organic waste from meat packaging if it is discharged into a river?
The organic waste will exert an oxygen demand as it is broken down by bacteria. This could deplete the oxygen available for other living organisms in the water such as fish.
Wastewaters from dairies may come from receiving stations (where milk is delivered from individual farms), bottling plants, creameries, ice cream plants, cheese production units and dried milk production plants. The wastewater from spillage, cleaning and washing usually contains milk which has a very high polluting potential. The polluting potential is the potential of the wastewaters to cause pollution, i.e. damage to the condition, health, safety, or welfare of animals, humans, plants or property.
The raw materials for the textile industry are wool, cotton and synthetic fibres. The processing of wool and cotton involves removing natural impurities, such as dust, and imparting particular qualities relating to appearance, feel and durability. Water is used for washing at various stages, producing effluent that is likely to contain suspended solids and organic material from processing the fibres. It may also contain dyes and other chemicals, depending on the specific processes used in the factory. The outputs from these processes are used to make clothing and other textile products (Figure 4.5).
Tanning of animal hides is an important economic activity in many low-income countries (Figure 4.6).
Tannery effluent is highly polluting and is often discharged directly to nearby rivers without adequate treatment. It contains toxic (poisonous) chemicals such as chromium, sulphides and organic acids, as well as organic matter and solids. Chromium is a particular problem because it is an example of a heavy metal. Heavy metals are a group of toxic chemical pollutants that persist in the environment, i.e. they do not get broken down by natural processes. Tannery wastewaters are produced in large volumes and are considerably more polluting than wastewaters from most other industries. The treatment of tannery waste involves removing solids and organic matter from the effluent. These processes are described in Study Session 6.