Earth's Resources and Water » Waste Water Treatment

Why waste water needs treating

Every time you flush a toilet, take a shower or wash up, you make waste water. Factories and farms make huge amounts too. If this water went straight into rivers and seas, it would harm wildlife and spread disease. So it has to be treated first.

The spec says that urban lifestyles and industrial processes produce large amounts of waste water that need treatment before being released into the environment. What has to be removed depends on where the water came from:

Key terms:

• Waste water: water that has been used and is no longer clean enough to release into the environment.
• Organic matter: material that comes from living things, such as food waste and human waste.
• Microbes: tiny living things, such as bacteria. Some microbes are harmful and cause disease.

The four stages of sewage treatment

Sewage treatment is done in a sewage works. The spec lists four stages. Follow them in order.

Stage 1: Screening and grit removal

The sewage first passes through screens. These are grids that catch large objects, such as sticks, cloths and plastic. Next, the water flows slowly through a channel so that grit and sand sink to the bottom and can be removed. This protects the pumps and pipes in the rest of the works.

Stage 2: Sedimentation

The water is left in large tanks. Solid material slowly sinks to the bottom. This settled material is called sewage sludge. The liquid left on top is called effluent. From here, the sludge and the effluent are treated separately.

Stage 3: Anaerobic digestion of the sludge

The sludge is moved to large sealed tanks. Bacteria break down the organic matter in the sludge. Anaerobic means that this happens without oxygen. The process makes a gas called biogas, which contains methane. Biogas can be burned as a fuel, so some sewage works use it to make their own energy.

Stage 4: Aerobic biological treatment of the effluent

Air is pumped through the effluent. Aerobic means that this happens with oxygen. The oxygen helps bacteria to break down the organic matter still in the water. Together with the earlier stages, this leaves the water clean enough to be released into rivers or the sea.

Key terms:

• Sewage: waste water and waste matter from homes and businesses.
• Sewage sludge: the solid material that settles at the bottom of the tanks.
• Effluent: the liquid left after the sludge has settled out.
• Anaerobic: without oxygen.
• Aerobic: with oxygen.

Agricultural and industrial waste water

Farm waste water, such as runoff from animal waste, has the same problem as sewage. It contains organic matter and harmful microbes, so it needs those removed before release.

Industrial waste water is different. As well as organic matter, it may contain harmful chemicals from the factory. These chemicals may not be removed by the same steps as sewage treatment. So extra treatment is needed to take them out. Which extra treatment depends on which chemicals are present.

Waste, ground and salt water: how easy is it to get potable water?

The spec says you should be able to comment on the relative ease of obtaining potable water from waste water, ground water and salt water. Remember that potable water is water that is safe to drink. Ground water is water found underground in rocks.

In a good answer, give a comparison and a reason. For example: "Obtaining potable water from ground water is easier than from salt water, because salt water needs a lot of energy to remove the dissolved salt."

Exam-style question

Describe how sewage is treated at a sewage works. (4 marks)