Answer:
Tertiary Treatment stage
Step-by-step explanation:
There are basically four stages of treatment of sewer/waste water
a) Screening stage – In this stage of treatment, large objects and garbage is removed from the waste water through the use of grit removal equipment
b) Primary treatment stage (Physical treatment)
Physical processes such as aeration, filtration, sedimentation etc. are used to remove the impurities from waste water after the screening stage
c) Secondary treatment stage (Chemical Treatment)
In this stage chemicals are used to coagulate tiny impurities into larger mass so that they can be easily removed through the process of sedimentation, filtration etc.
d) Tertiary treatment stage (Biological Treatment)
In this stage, waste is acted upon by microorganisms that work either in the presence or absence of oxygen to remove organic impurities from the waste water.
Thus, Derek saw the process of municipal wastewater treatment plant at the tertiary stage of waste water/sewer treatment.