Waste Water Treatment (AQA GCSE Chemistry)
Revision Note
Sources of waste water
Water is used on a daily basis in a domestic environment
For example, washing-up dishes, showers and baths and cooking
When you run water down a drain, it passes through sewers and then finally to sewage treatment plants
Agricultural waste from animal farms and nutrient run-off which is collected from fields produces an abundance of waste water
Both domestic and agricultural sewage needs to be processed to remove organic matter, harmful microbes, particulates and toxins
This can then be safely returned to freshwater sources i.e. lakes and rivers
If this process did not take place, it could potentially pose health risks for the population
Waste water that is produced by the Haber process and other industrial processes needs to be gathered and treated appropriately
Harmful chemicals and organic matter are present in industrial waste
This therefore means that additional treatment has to be in place to ensure it is safe for the environment
Examiner Tips and Tricks
Waste water, sometimes called 'grey water', cannot be allowed to run untreated as it contains dangerous substances and toxins and is detrimental to the environment and health.
Sewage treatment
What happens at a sewage treatment plant?
Screening & Grit Removal
The first stage of treatment removes large materials such as plastic bags and twigs and grit by screening
Sedimentation
Sedimentation comes next which occurs in a settlement tank. The water is allowed to stand still in the tank while heavier solids sink to the bottom creating sewage sludge, whilst lighter matter which is also known as effluent, floats to the top
Aerobic Digestion
The effluent is removed and treated by biological aerobic digestion
This involves pumping air into the water to encourage the breakdown of organic matter and other microbes by aerobic bacteria
Anaerobic Digestion
Anaerobic digestion is then used to break down the sewage sludge from the bottom of the settlement tank. It is firstly removed and placed in large tanks where bacteria break it down
Anaerobic digestion releases methane gas as a by product from the organic matter in the sludge. Methane gas is used as a source of energy and the leftover, digested waste as a fertiliser
Sewage treatment
Diagram showing the stages of waste water treatment
Removal of toxic substances
When there are toxic substances within waste water, it is important to use additional phases of treatment
This can include using membranes, adding additional chemicals e.g. to precipitate metals out of solution, and also U.V. radiation
Sewage water is often treated in areas where there is little freshwater available
Though this process is longer than processing and preparing freshwater, it uses less energy than the desalination of salt water
Examiner Tips and Tricks
Waste water comes from many different sources so the process of waste water treatment is complex and involves various stages.
You may be asked to compare how easy it is to obtain potable water from waste water, ground water and salt water. The treatment of waste water has been covered in these notes.
Depending on the source, ground water may need very little treatment, if any, to make it potable. Sometimes there are natural minerals, such as arsenic, in the water which make it unsafe to drink. These minerals can sometimes be removed by charcoal filters. Boiling will not remove the minerals, but it may kill harmful micro-organisms in ground water.
Obtaining potable water from salt water is called desalination and usually done by distillation or reverse osmosis (forcing water through a semi-permeable membrane that allows only small water molecules to pass through). Distillation is expensive as it requires a lot of energy to heat the water. Desalination is carried in areas that are very scare in rainfall or groundwater, such as some parts of the Middle East.
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?