15. Which of the following statement is not correct?
A.The use of 25-40°C temperatures allows the biogas production to be more stable
B.The use of 25-40°C temperatures does not destroy potentially harmful bacteria
C.The use of 25-40°C temperatures destroys potentially harmful bacteria
D.None of the above

---

16. Oxidation ponds are shallow ponds, generally designed at the depth of
A.2 to 40 feet
B.4 to 6 feet
C.1 to 3 feet
D.5 to 8 feet

---

17.  Which of the following is generally not referred to the sewerage system?
A.Sanitary sewers
B.Storm sewers
C.Combined sewers
D.Solid sewers

---

18. The magnitude of BOD of wastewater is related to
A.bacterial count
B.amount of organic material
C.amount of inorganic material
D.all of the above

---

19. A dense bacterial population caught in a tangled web of fibers sticking to a surface describes
A.coagulation
B.a biodisc
C.a biofilm
D.the membrane filter technique

---

20. Biogas production is
A.a temperature-dependent process
B.a temperature independent process
C.an oxygen dependent process
D.none of the above

---

21. The acetate-utilizing methanogens are responsible for
A.20% of methane produced in a biogas reactor
B.50% of methane produced in a biogas reactor
C.70% of methane produced in a biogas reactor
D.85% of methane produced in a biogas reactor

---