MA Combined Wastewater Treatment Plant Operator Grade 5 Exam
Massachusetts Combined Wastewater Treatment Plant Operator Grade 5 Exam
1. What is the primary purpose of municipal wastewater treatment?
ANSWER To protect public health and the environment by removing pollutants
before discharge to receiving waters.
2. What does BOD stand for? ANSWER Biochemical Oxygen Demand.
3. What is the typical BOD of raw domestic wastewater? ANSWER 150–300
mg/L.
4. What does COD stand for? ANSWER Chemical Oxygen Demand.
5. Why is COD typically higher than BOD in wastewater? ANSWER Because
COD measures both biodegradable and non-biodegradable organic matter,
while BOD only measures biodegradable organics.
6. What is TSS? ANSWER Total Suspended Solids.
7. What is the typical TSS of raw domestic wastewater? ANSWER 150–350
mg/L.
8. What is the difference between primary and secondary treatment?
ANSWER Primary treatment removes settleable solids physically; secondary
treatment removes dissolved and colloidal organics biologically.
9. What is tertiary treatment? ANSWER Advanced treatment beyond
secondary, such as nutrient removal, filtration, or disinfection.
10. What is the purpose of preliminary treatment? ANSWER To protect
downstream equipment by removing large debris, grit, and excessive grease.
,11. What is hydraulic retention time (HRT)? ANSWER The average time
wastewater remains in a treatment unit, calculated as volume divided by flow
rate.
12. What is organic loading? ANSWER The amount of organic matter (usually
BOD) applied to a biological treatment process per unit volume or per unit of
microbial mass per day.
13. What is F/M ratio? ANSWER Food to Microorganism ratio, calculated as
pounds of BOD applied per day divided by pounds of MLSS.
14. What is a typical F/M ratio for conventional activated sludge? ANSWER
0.2–0.5 lb BOD/lb MLSS-day.
15. What is sludge volume index (SVI)? ANSWER A measure of the
settleability of activated sludge, expressed as mL of sludge per gram of MLSS
after 30 minutes of settling.
16. What SVI range indicates good settling sludge? ANSWER 50–150 mL/g.
17. What is sludge volume (SV30)? ANSWER The volume of settled sludge in a
1-liter graduated cylinder after 30 minutes, expressed as a percentage.
18. What is MLSS? ANSWER Mixed Liquor Suspended Solids — the
concentration of solids in the aeration tank.
19. What is a typical MLSS concentration for conventional activated sludge?
ANSWER 1,500–3,000 mg/L.
20. What is MLVSS? ANSWER Mixed Liquor Volatile Suspended Solids — the
organic (volatile) portion of MLSS, representing active microorganisms.
21. What is a typical MLVSS/MLSS ratio? ANSWER 0.65–0.80 (65–80%).
22. What is sludge age (mean cell residence time)? ANSWER The average
time microorganisms remain in the activated sludge system, calculated as
pounds of MLSS in the system divided by pounds of solids wasted per day.
23. What is a typical sludge age for conventional activated sludge? ANSWER
3–10 days.
24. What is return activated sludge (RAS)? ANSWER The flow of settled
sludge from the clarifier returned to the aeration tank to maintain the
microbial population.
, 25. What is a typical RAS flow rate as a percentage of influent flow? ANSWER
25–100% of influent flow.
26. What is waste activated sludge (WAS)? ANSWER The excess sludge
removed from the activated sludge system to maintain the desired MLSS
concentration.
27. What is the purpose of a primary clarifier? ANSWER To remove settleable
solids and reduce organic loading on secondary treatment.
28. What is the typical BOD removal efficiency of a primary clarifier?
ANSWER 25–35%.
29. What is the typical TSS removal efficiency of a primary clarifier? ANSWER
50–65%.
30. What is the purpose of a secondary clarifier? ANSWER To separate
biological floc (activated sludge) from treated effluent.
SECTION 2: ACTIVATED SLUDGE PROCESS (Questions 31–60)
31. What are the three basic components of the activated sludge process?
ANSWER Aeration tank, secondary clarifier, and return sludge system.
32. What are the four types of microorganisms in activated sludge? ANSWER
Bacteria, protozoa, rotifers, and metazoa.
33. Which microorganisms are primarily responsible for BOD removal?
ANSWER Bacteria.
34. What is the role of protozoa in activated sludge? ANSWER They consume
free-swimming bacteria, clarify the effluent, and indicate process health.
35. What type of protozoa indicates a young or stressed sludge? ANSWER
Amoebae and flagellates.
36. What type of protozoa indicates a mature, healthy sludge? ANSWER
Ciliates (stalked ciliates like Vorticella).
37. What is the purpose of aeration in activated sludge? ANSWER To provide
oxygen for aerobic bacteria and to mix the contents of the aeration tank.
38. What is dissolved oxygen (DO) and what is the target range in aeration?
ANSWER The amount of oxygen dissolved in water; target is 1.0–3.0 mg/L.
Massachusetts Combined Wastewater Treatment Plant Operator Grade 5 Exam
1. What is the primary purpose of municipal wastewater treatment?
ANSWER To protect public health and the environment by removing pollutants
before discharge to receiving waters.
2. What does BOD stand for? ANSWER Biochemical Oxygen Demand.
3. What is the typical BOD of raw domestic wastewater? ANSWER 150–300
mg/L.
4. What does COD stand for? ANSWER Chemical Oxygen Demand.
5. Why is COD typically higher than BOD in wastewater? ANSWER Because
COD measures both biodegradable and non-biodegradable organic matter,
while BOD only measures biodegradable organics.
6. What is TSS? ANSWER Total Suspended Solids.
7. What is the typical TSS of raw domestic wastewater? ANSWER 150–350
mg/L.
8. What is the difference between primary and secondary treatment?
ANSWER Primary treatment removes settleable solids physically; secondary
treatment removes dissolved and colloidal organics biologically.
9. What is tertiary treatment? ANSWER Advanced treatment beyond
secondary, such as nutrient removal, filtration, or disinfection.
10. What is the purpose of preliminary treatment? ANSWER To protect
downstream equipment by removing large debris, grit, and excessive grease.
,11. What is hydraulic retention time (HRT)? ANSWER The average time
wastewater remains in a treatment unit, calculated as volume divided by flow
rate.
12. What is organic loading? ANSWER The amount of organic matter (usually
BOD) applied to a biological treatment process per unit volume or per unit of
microbial mass per day.
13. What is F/M ratio? ANSWER Food to Microorganism ratio, calculated as
pounds of BOD applied per day divided by pounds of MLSS.
14. What is a typical F/M ratio for conventional activated sludge? ANSWER
0.2–0.5 lb BOD/lb MLSS-day.
15. What is sludge volume index (SVI)? ANSWER A measure of the
settleability of activated sludge, expressed as mL of sludge per gram of MLSS
after 30 minutes of settling.
16. What SVI range indicates good settling sludge? ANSWER 50–150 mL/g.
17. What is sludge volume (SV30)? ANSWER The volume of settled sludge in a
1-liter graduated cylinder after 30 minutes, expressed as a percentage.
18. What is MLSS? ANSWER Mixed Liquor Suspended Solids — the
concentration of solids in the aeration tank.
19. What is a typical MLSS concentration for conventional activated sludge?
ANSWER 1,500–3,000 mg/L.
20. What is MLVSS? ANSWER Mixed Liquor Volatile Suspended Solids — the
organic (volatile) portion of MLSS, representing active microorganisms.
21. What is a typical MLVSS/MLSS ratio? ANSWER 0.65–0.80 (65–80%).
22. What is sludge age (mean cell residence time)? ANSWER The average
time microorganisms remain in the activated sludge system, calculated as
pounds of MLSS in the system divided by pounds of solids wasted per day.
23. What is a typical sludge age for conventional activated sludge? ANSWER
3–10 days.
24. What is return activated sludge (RAS)? ANSWER The flow of settled
sludge from the clarifier returned to the aeration tank to maintain the
microbial population.
, 25. What is a typical RAS flow rate as a percentage of influent flow? ANSWER
25–100% of influent flow.
26. What is waste activated sludge (WAS)? ANSWER The excess sludge
removed from the activated sludge system to maintain the desired MLSS
concentration.
27. What is the purpose of a primary clarifier? ANSWER To remove settleable
solids and reduce organic loading on secondary treatment.
28. What is the typical BOD removal efficiency of a primary clarifier?
ANSWER 25–35%.
29. What is the typical TSS removal efficiency of a primary clarifier? ANSWER
50–65%.
30. What is the purpose of a secondary clarifier? ANSWER To separate
biological floc (activated sludge) from treated effluent.
SECTION 2: ACTIVATED SLUDGE PROCESS (Questions 31–60)
31. What are the three basic components of the activated sludge process?
ANSWER Aeration tank, secondary clarifier, and return sludge system.
32. What are the four types of microorganisms in activated sludge? ANSWER
Bacteria, protozoa, rotifers, and metazoa.
33. Which microorganisms are primarily responsible for BOD removal?
ANSWER Bacteria.
34. What is the role of protozoa in activated sludge? ANSWER They consume
free-swimming bacteria, clarify the effluent, and indicate process health.
35. What type of protozoa indicates a young or stressed sludge? ANSWER
Amoebae and flagellates.
36. What type of protozoa indicates a mature, healthy sludge? ANSWER
Ciliates (stalked ciliates like Vorticella).
37. What is the purpose of aeration in activated sludge? ANSWER To provide
oxygen for aerobic bacteria and to mix the contents of the aeration tank.
38. What is dissolved oxygen (DO) and what is the target range in aeration?
ANSWER The amount of oxygen dissolved in water; target is 1.0–3.0 mg/L.
