21. Which of the following substances are commonly used in a filter?

A. Charcoal
B. Sand
C. Both (a) and (b)
D.  Aluminum chloride

22. Biological oxidation processes usually referred to as biological treatment, are the most common form of

A. primary treatment
B. secondary treatment
C. tertiary treatment
D. all of these

23. The maximum permissible limit (BIS) of turbidity in drinking water is

A. 5 NTU
B. 10 NTU
C. 15 NTU
D. 20 NTU

24. Sedimentation is a physical process used in wastewater treatment to

A. remove particles that are less dense than water
B. remove particles that are denser than water
C. remove the pertinacious material from the water
D. none of the above

25. The ultimate source of water is

A. rivers and lakes
B. dew and forest
C. rain and snow
D. underground and surface