Environmental Engineering Exam 2 – Study Guide, Review & Practice Questions
Define the variables in the mass balance equation.
dm/dt = min-mout + E + mrxn - correct answer ✔✔dm = change in mass
dt = change in time
ṁin = mass flux into system
ṁout = mass flux out of system
E = emission from a source within the system
ṁrxn = mass flux due to precipitation or decay reaction
What is the equation (or value) of the following variables? i. ṁin,ṁout - correct answer ✔✔ṁ =
Q × C, Where Q = flowrate and C = concentration
ṁrxn for: Conservative compound (e.g. chlorine) = ?
Zero-order reaction decay = ?
First-order reaction decay = ? - correct answer ✔✔0
-k*Vol
-k*[C]*Vol
What does CMFR stand for? - correct answer ✔✔Completely Mixed Flow Reactor
What are some examples (manmade or natural) of CMFRs? - correct answer ✔✔Lakes,
atmosphere, a smoky bar, reservoir, wastewater clarifier, anaerobic digester, etc.
What are some advantages of CMFRs? - correct answer ✔✔handles spikes in toxicants well,
distributes toxicants evenly, good for variable flow
, How do you know if a system is at steady state? - correct answer ✔✔No additional flux is added
to the reactor during mixing, concentrations do not fluctuate, flowrate has not changed in a
long time
*Remember to assume ΣQin = ΣQout and Cout = Ctank for this course. Concentrations are NOT
additive, only flowrates are!
What is the detention time (in hours) of a 4,000,000 gal reactor that treats 8 MGD? - correct
answer ✔✔12 hours
What does PFR stand for? - correct answer ✔✔Plug Flow Reactor
What are some examples (manmade or natural) or PFRs? - correct answer ✔✔Rivers, pipes,
flowing groundwater, etc.
What are some advantages of PFRs? - correct answer ✔✔More efficient than CMFRs: less
materials, waste production, energy requirements; good for industries with consistent flow
What are the major differences between a batch reactor, CMFR and PFR? What common
assumptions can you make about each reactor? - correct answer ✔✔Batch: completely mixed
(Cout = C); completely sealed; no flow in and no flow out; dm/dt only depends on the
chemical/biological reaction.
CMFR: completely mixed (Cout = C); YES flow in and flow out; usually Qin = Qout; better at
handling spikes
PFR: NOT completely mixed (no mixing in the axial direction); YES flow in and flow out (uniform
fluid velocity); constant cross-sectional area; usually Qin = Qout; solve by assuming a PFR acts
Define the variables in the mass balance equation.
dm/dt = min-mout + E + mrxn - correct answer ✔✔dm = change in mass
dt = change in time
ṁin = mass flux into system
ṁout = mass flux out of system
E = emission from a source within the system
ṁrxn = mass flux due to precipitation or decay reaction
What is the equation (or value) of the following variables? i. ṁin,ṁout - correct answer ✔✔ṁ =
Q × C, Where Q = flowrate and C = concentration
ṁrxn for: Conservative compound (e.g. chlorine) = ?
Zero-order reaction decay = ?
First-order reaction decay = ? - correct answer ✔✔0
-k*Vol
-k*[C]*Vol
What does CMFR stand for? - correct answer ✔✔Completely Mixed Flow Reactor
What are some examples (manmade or natural) of CMFRs? - correct answer ✔✔Lakes,
atmosphere, a smoky bar, reservoir, wastewater clarifier, anaerobic digester, etc.
What are some advantages of CMFRs? - correct answer ✔✔handles spikes in toxicants well,
distributes toxicants evenly, good for variable flow
, How do you know if a system is at steady state? - correct answer ✔✔No additional flux is added
to the reactor during mixing, concentrations do not fluctuate, flowrate has not changed in a
long time
*Remember to assume ΣQin = ΣQout and Cout = Ctank for this course. Concentrations are NOT
additive, only flowrates are!
What is the detention time (in hours) of a 4,000,000 gal reactor that treats 8 MGD? - correct
answer ✔✔12 hours
What does PFR stand for? - correct answer ✔✔Plug Flow Reactor
What are some examples (manmade or natural) or PFRs? - correct answer ✔✔Rivers, pipes,
flowing groundwater, etc.
What are some advantages of PFRs? - correct answer ✔✔More efficient than CMFRs: less
materials, waste production, energy requirements; good for industries with consistent flow
What are the major differences between a batch reactor, CMFR and PFR? What common
assumptions can you make about each reactor? - correct answer ✔✔Batch: completely mixed
(Cout = C); completely sealed; no flow in and no flow out; dm/dt only depends on the
chemical/biological reaction.
CMFR: completely mixed (Cout = C); YES flow in and flow out; usually Qin = Qout; better at
handling spikes
PFR: NOT completely mixed (no mixing in the axial direction); YES flow in and flow out (uniform
fluid velocity); constant cross-sectional area; usually Qin = Qout; solve by assuming a PFR acts
