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CWEA Activated Sludge Process Control Exam
Practice Questions And Correct Answers||Verified
Exam!!|| (Verified Answers) Plus Rationale 2026/2027
Q&A| Instant Download Pdf
What is COD? - Answer-a measure of the amount of
oxygen needed to stabilize the organic and some of the
inorganic compounds in wastewater.
What is the difference between COD and BOD? - Answer-
BOD only measures the readily biodegradable organic
matter, while COD is more complete.
COD test takes 3-4 hours.
How can you use BOD as a check on the efficiency of a
plant process? - Answer-Influent flow (Q) should have a
high BOD and the effluent Q should have a significantly
lower BOD.
Mixed Liquor Suspended Solids (MLSS) - Answer-The
amount (mg/L) of suspended solids in the mixed liquor of
an aeration tank.
Composed of inorganics and MLVSS.
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What is MLVSS composed of? - Answer-Living organic
matter and nonliving organic matter
Why is MLVSS only an estimation of living organisms? -
Answer-A portion of the measurement is actually nonliving
organic matter.
What is BOD? - Answer-The amount of oxygen used by
microorganisms as they biologically decompose organic
matter into wastewater. (the organic strength of
wastewater)
What should you notice about BOD from influent to
effluent? - Answer-There should be a decease in BOD. (%
removal)
Why is BOD used? - Answer-Is often used as a measure
of organic loading and plant efficiency because it is a good
indicator of readily biodegradable organic matter.
Why is BOD not a useful process control indicator? -
Answer-The test takes five days to complete, so will not be
ready fast enough to apply changes to the process.
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Why is it better to have the MLVSS over the MLSS -
Answer-MLSS consists of all the solids in the activated
sludge.
What is the range of MLVSS to MLSS? - Answer-MLVSS
is normally 70-80% of the MLSS.
What will happen if you calculate using MLSS instead of
MLVSS - Answer-You will get a higher solids inventory,
and your F/M ratio will be lower.
When is it a problem using MLSS instead of MLVSS? -
Answer-It is only a problem when you get a change in the
incoming wastewater characteristics. An increase in silt
load could lower the volatile part of MLSS. MLSS will not
show the microorganism concentration if you get high
inorganic loading, which will not show a decrease in solids
inventory or a higher F/M ratio.
What is the weight (lbs) of a gallon of water? - Answer-
8.34 lbs/gal
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What does solids inventory measure? - Answer-Measures
the volatile suspended solids in the AB
Typical TSS for stages a conventional activated sludge
plant - Answer-Influent, 150-400 mg/l
Primary effluent 60-160 mg/l
Mixed Liquor 1000-4500 mg/l
Return sludge 2000-10000 mg/l
Final effluent 10-30 mg/l
Typical oxygen demand values for municipal wastewater -
Answer-BOD COD
Raw influent 100-300 mg/l 200-500 mg/l
Primary effluent 50-150 mg/l 150-300 mg/l
Secondary effluent 15-30 mg/l 50-100 mg/l
F/M - Answer-Food to microorganism ratio. Difficult to
measure, approximated by BOD (or COD) for F and SS
(or MLVSS) for M in aeration tank. Also known as the
loading on the system.
CWEA Activated Sludge Process Control Exam
Practice Questions And Correct Answers||Verified
Exam!!|| (Verified Answers) Plus Rationale 2026/2027
Q&A| Instant Download Pdf
What is COD? - Answer-a measure of the amount of
oxygen needed to stabilize the organic and some of the
inorganic compounds in wastewater.
What is the difference between COD and BOD? - Answer-
BOD only measures the readily biodegradable organic
matter, while COD is more complete.
COD test takes 3-4 hours.
How can you use BOD as a check on the efficiency of a
plant process? - Answer-Influent flow (Q) should have a
high BOD and the effluent Q should have a significantly
lower BOD.
Mixed Liquor Suspended Solids (MLSS) - Answer-The
amount (mg/L) of suspended solids in the mixed liquor of
an aeration tank.
Composed of inorganics and MLVSS.
,2|Page
What is MLVSS composed of? - Answer-Living organic
matter and nonliving organic matter
Why is MLVSS only an estimation of living organisms? -
Answer-A portion of the measurement is actually nonliving
organic matter.
What is BOD? - Answer-The amount of oxygen used by
microorganisms as they biologically decompose organic
matter into wastewater. (the organic strength of
wastewater)
What should you notice about BOD from influent to
effluent? - Answer-There should be a decease in BOD. (%
removal)
Why is BOD used? - Answer-Is often used as a measure
of organic loading and plant efficiency because it is a good
indicator of readily biodegradable organic matter.
Why is BOD not a useful process control indicator? -
Answer-The test takes five days to complete, so will not be
ready fast enough to apply changes to the process.
,3|Page
Why is it better to have the MLVSS over the MLSS -
Answer-MLSS consists of all the solids in the activated
sludge.
What is the range of MLVSS to MLSS? - Answer-MLVSS
is normally 70-80% of the MLSS.
What will happen if you calculate using MLSS instead of
MLVSS - Answer-You will get a higher solids inventory,
and your F/M ratio will be lower.
When is it a problem using MLSS instead of MLVSS? -
Answer-It is only a problem when you get a change in the
incoming wastewater characteristics. An increase in silt
load could lower the volatile part of MLSS. MLSS will not
show the microorganism concentration if you get high
inorganic loading, which will not show a decrease in solids
inventory or a higher F/M ratio.
What is the weight (lbs) of a gallon of water? - Answer-
8.34 lbs/gal
, 4|Page
What does solids inventory measure? - Answer-Measures
the volatile suspended solids in the AB
Typical TSS for stages a conventional activated sludge
plant - Answer-Influent, 150-400 mg/l
Primary effluent 60-160 mg/l
Mixed Liquor 1000-4500 mg/l
Return sludge 2000-10000 mg/l
Final effluent 10-30 mg/l
Typical oxygen demand values for municipal wastewater -
Answer-BOD COD
Raw influent 100-300 mg/l 200-500 mg/l
Primary effluent 50-150 mg/l 150-300 mg/l
Secondary effluent 15-30 mg/l 50-100 mg/l
F/M - Answer-Food to microorganism ratio. Difficult to
measure, approximated by BOD (or COD) for F and SS
(or MLVSS) for M in aeration tank. Also known as the
loading on the system.
