SAQs
Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering these questions.
Rewrite the paragraph below using terms from the list provided to fill the gaps. (Note that some of these terms have been defined in earlier study sessions.)
biochemical oxygen demand, biodegradation, blackwater, chemical oxygen demand, effluent, greywater, hazardous, heavy metals, inorganic, liquid waste, oxygen demand, pathogenic, polluting potential, sewage, sewers, sullage, suspended solids, wastewater.
When we use water for any purpose and it becomes unclean after we used it, we refer to it as ……………… or ……………… In our homes we generate two types of ……………… The type from toilets which contains excreta is called ………………, while other wastewaters (e.g. from clothes washing) that are not contaminated by excreta are referred to as ……………… Another word for the latter type of wastewater is ……………… Domestic wastewater is sometimes referred to as ………………… In many towns and cities around the world domestic wastewater goes into underground pipes called ……………… that take the wastewater to treatment works.
The solids in wastewater that are carried along in flowing water are called ……………… Organic matter in water may cause odours due to ……………… It also creates ………………, which can be measured by its ……………… or ……………… If faecal matter is in the water, there may be ……………… micro-organisms present. ……………… chemicals in wastewaters are likely to include nitrates and phosphates. Industrial wastewaters may have significant ……………… for example, tannery waste frequently contains persistent ……………… pollutants that contain ………………
When we use water for any purpose and it becomes unclean after we used it, we refer to it as wastewater or effluent. In our homes we generate two types of liquid waste. The type from toilets which contains excreta is called blackwater, while other wastewaters (e.g. from clothes washing) that are not contaminated by excreta are referred to as greywater. Another word for the latter type of wastewater is sullage. Domestic wastewater is sometimes referred to as sewage. In many towns and cities around the world domestic wastewater goes into underground pipes called sewers that take the wastewater to treatment works.
The solids in a wastewater that are carried along in flowing water are called suspended solids. Organic matter in water may cause odours due to biodegradation. It also creates oxygen demand, which can be measured by its biochemical oxygen demand or chemical oxygen demand. If faecal matter is in the water, there may be pathogenic micro-organisms present. Inorganic chemicals in wastewaters are likely to include nitrates and phosphates. Industrial wastewaters may have significant polluting potential, for example, tannery waste frequently contains persistent hazardous pollutants that contain heavy metals.
Suggest two ways in which wastewater from residential areas may differ from wastewater from
- Residential wastewater will probably only contain domestic liquid wastewater but in commercial areas there will also be wastes from businesses such as shops, cafes and restaurants. This may increase the proportion of fats and oils in the effluent.
- Both residential and industrial wastewater will contain organic wastes but domestic waste is unlikely to include toxic chemicals, which will be found in many types of industrial waste.
- Worknesh is a laboratory technician. She is analysing a sample of wastewater collected from a pipe that discharges effluent into a river. Name two tests Worknesh could perform to assess the physical characteristics of the effluent.
- As part of the analysis she also does a BOD test on the sample and gets an unusually high result. What does the high BOD tell her about the wastewater? What effect could it have on the river?
- To assess the physical characteristics, Worknesh could perform a suspended solids test. She could also measure the temperature of the sample and assess the odour. (Note that if she was measuring temperature she would have to do this at the point of origin because the temperature could change within a short time.)
- A high BOD test result would tell Worknesh that there was a lot of organic matter in the sample. If this was discharged into the river it would remove oxygen from the water, which would harm fish and other organisms living in the river.
Which of the following statements are false? In each case explain why it is incorrect.
- Effluent from food processing factories is safe to discharge to rivers without treatment.
- The manufacture of clothing material often uses coloured dyes that may be discharged in effluent from the factory.
- Tannery waste is highly polluting because it contains large quantities of suspended solids.
- Wastewater from slaughterhouses contains blood, animal excreta and washing water.
1 is false. Effluent from food processing factories is not toxic but it will probably contain organic matter, which should be treated before the waste is discharged.
3 is false. Tannery waste may contain suspended solids but the reason why it is highly polluting is because it contains toxic chemicals.