Sewage is made up of water plus other materials, and it should be treated before being discharged into a river.
(a)[2]
Describe two reasons why sewage should be treated before it is discharged into a river.
(b)[1]
State the name of one additional stage in sewage treatment.
(c(i))[1]
State the name of the type of organism found on the gravel surface.
(c(ii))[1]
Describe what these organisms do.
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