CE 60330 – Environmental Biotechnology
Homework No. 3
Due October 3, 2007
1. Complete the first example in the AQUASIM tutorial on page 6, Part A and Part B.
Partial copies of the tutorial were provided in class. The complete tutorial and users
manual can be downloaded from
http://www.aquasim.eawag.ch/e_aquasim_manual.html. The problem statement is
copied below:
Part A: In a stirred batch reactor with a volume of 10 L, a substance A is degraded
by a first order process with a rate of
rA = kACA
and a rate constant of kA = 1 h-1 (rA denotes the absolute value of the process rate,
CA denotes the concentration of substance A). The initial concentation of
substance A is 1 mg/l. Plot the concentration of substance A in the reactor as a
function of time for a time interval of 10 h.
Part B: Substance A is not completely degraded, but it is converted to substance B
with a fixed stoichiometry of 2:1 (1 mg of substance A is converted to 0.5 mg
of substance B). Substance B is converted to substance C with the nonlinear
transformation rate:
rB 
rmax, B C B
K B  CB
according to Monod, with a maximum conversion rate of rmax,B = 0.25 mg/L/h and
a half-saturation concentration of KB = 0.5 mg/L (rB denotes the absolute value of
the process rate, CB denotes the concentration of substance B). The stoichiometric
ratio of this conversion is 1:2 (1 mg of substance B is converted to 2 mg of
substance C). The initial concentrations of the substances B and C are zero, the
process rate and initial concentration of substance A is the same as in part A. Plot
the concentrations of the substances A, B and C as functions of time for a time
interval of 10 h.
2. A wastewater is treated in an aerobic CSTR with no recycle. The hydraulic detention
time is 5 days and the following kinetic parameters apply:
qmax = 15 g/gVSS-d
Y = 0.6 gVSSa/g
K = 12 mg/L
b = 0.15 1/d
If the initial conditions in the tank are So = 1000 mg/L and Xao = 30 mg/L,
a) Show the mass balance equations, the finite differences solutions, and use a
spreadsheet to show the S and Xa concentrations over time. Provide the plot.
b) Set up an AQUASIM model for the above, and provide a plot and the data file
printout.
3. For problem 2, use the steady state model to determine the following
a) What are the steady-state effluent concentrations of substrate (S) for influent
concentrations (So) of 1000, 500, and 100 mg/L?
b) What are the steady-state concentrations of biomass (Xa) for each So?
c) Is Xa directly proportional to So? Is this necessarily the case?
d) For each So value, what is the minimum hydraulic retention time below which
washout (complete loss of active biomass) occurs?
e) What is the minimum effluent concentration achievable in this reactor? To what
hydraulic retention time does this condition correspond?
4. Tabulate the value of S, Xa, Xi, SMP, and S + SMP when θx = 1, 10, and 20 days.
Use the following data:
qmax = 15 g/gVSS-d
Y = 0.5 gVSSa/g
K = 8 mg/L
b = 0.15 1/d
fd = 0.8
Xio = 25 mgVSSi/L
So = 500 mgCOD/L (totally biodegradable)
SMP kinetic parameters
k1 = 0.12 gCODP/gBODL
k2 = 0.09 gCODP/gVSSa-d
qmaxUAP = 1.8 mgCODP/mgVSSa-d
qmaxBAP = 0.1 mgCODP/mgVSSa-d
KUAP = 100 mgCODP/L
KBAP = 85 mgCODP/L