Waste water must be cleaned before it returns to the environment
Definition
Organic matter
Material from living organisms or their waste that microorganisms can break down.
Definition
Harmful microbe
A microscopic organism that can cause disease.
- Homes, farms and industry all produce large volumes of waste water that must be treated before it is released.
- Sewage and agricultural waste water need the removal of organic matter and harmful microbes.
- Industrial waste water may also need the removal of organic matter and toxic chemicals.
Key Idea
- Treatment protects rivers and seas by taking out solids, organic matter and microbes before the water is released.
- Different sources of waste water need different treatments depending on what they contain.
Sewage treatment starts by screening, then settling into sludge and effluent
- Screening passes the sewage through a mesh to remove large solids such as grit, plastic and rags.
- Sedimentation then lets the sewage stand in a large settlement tank.
- The heavier solids sink to the bottom as sludge.
- The lighter liquid on top is the effluent.
Note
- Screening removes large objects that would otherwise block or damage the later stages.
- Sedimentation separates the sewage into two streams, sludge and effluent, which are then treated in different ways.
Sludge and effluent are treated in different ways
- The sludge is treated by anaerobic digestion, in which bacteria break it down without oxygen.
- Anaerobic digestion releases biogas, which can be burned as a fuel, and leaves a digested solid that can be used as fertiliser.
- The effluent is treated by aerobic biological treatment.
- Air is bubbled through the effluent so that aerobic bacteria break down organic matter and harmful microbes.
- The treated effluent is then safe to release into rivers or the sea.
Exam technique
- If asked to describe sewage treatment, give the stages in order: screening, sedimentation, anaerobic digestion of the sludge, then aerobic treatment of the effluent.
- State that anaerobic means without oxygen and aerobic means with oxygen; mixing these up is a common way to drop marks.
Comparing sources: waste water takes more treatment than fresh water, less than sea water
Definition
Potable water
Water that is safe to drink but is not necessarily chemically pure.
- Ground water and other fresh water are usually the easiest to make potable, often needing only filtration and sterilisation.
- Waste water needs more treatment, because organic matter and microbes must be removed through several stages.
- Salt water is the hardest to treat, because desalination is needed and it uses large amounts of energy.
- So the order of increasing difficulty and energy cost is usually fresh water, then waste water, then salt water.
Self review
- Why must sewage be treated before it is released into rivers?
- What is the difference between sludge and effluent?
- How is the sludge from sewage treated, and what useful product does this make?
- What does aerobic biological treatment of effluent remove?
- Put fresh water, waste water and salt water in order of how easy they are to make potable.
