sewage treatment - primary treatment Flashcards
What is sewage?
sewage is used to describe the waste that comes from houses, factories, farms, businesses, etc.
What materials does sewage contain?
organic and inorganic waste
Why must raw sewage be treated first?
if raw sewage were allowed into rivers or lakes the water would become very polluted and so it must be treated first
What are the two main aims of sewage treatment?
- its objective is to produce an environmentally safe fluid waste stream (or treated effluent)
and a solid waste (or treated sludge) suitable for disposal or reuse (usually as farm fertiliser)
what is organic waste?
domestic waste or oils and grease from industrial waste
what is inorganic waste?
nitrates and phosphates from domestic and industrial waste
What gases may sewage contain?
gases such as hydrogen sulfide and methane (formed when organic waste decomposes)
What is the essential purpose of a sewage treatment plant?
to reduce the B.O.D. of the sewage before it is discharged into the local river or lake
Is primary treatment mainly a physical or biological process?
this treatment is mainly physical
How does primary treatment begin?
the sewage is screened to remove large debris by passing through steel bars. This is called screening
What is the following step called?
the sewage is screened to remove large debris by passing through steel bars.
This process is called screening
Why is it important to remove twigs, leaves, etc?
these could interfere with the pumps and other equipment later in the treatment process
What happens after the sewage has been screened (floating debris removed)?
it then passes through ‘grit-traps/channels’ to remove smaller debris (like pebbles and grit)
What happens to the sewage after passing through grit traps/channels?
This is then passed into large settlement tanks to allow suspended particles to settle as sludge at the bottom of the tanks (sedimentation)
What is done to any grease or oil that is floating on the water in the settlement tanks?
Grease and oil, which can be floating on the water can be recovered for saponification
