STELLA Assignment #2  BOD
1)
In this problem you will be tracking the biochemical oxygen demand (BOD) impact of a
waste discharged from the Watapiti waste facility.
The plant discharges 5 x 10
5
liters/day of
waste (Q
p
).
The raw waste coming into the facility has an ultimate BOD (BOD
L
) concentration
of 250 mg/L.
The concentration leaving the plant in the effluent (BOD
P
) depends on the
efficiency of the plant (E):
BOD
P
= 250 * (1  E) mg/L
E is a fraction between 0 and 1.
(Change the percentage efficiency to a decimal.)
Sadly, the
facility works at a meager efficiency of 25% (E = 0.25).
Once discharged to the stream, the treated wastewater mixes with the stream which has a flow of
Q
s
= 5*10
6
liters/day.
Downstream from Watapiti the total river flow will be Q
s
plus the
effluent from the treatment plant (Q
P
).
As discussed in class, the following equation may be used to solve for BOD downstream:
BOD
t
= BOD
o
*e
kt
where: BOD
t
is the BOD concentration in (mg/L) at t time units away from the source
BOD
o
is the initial BOD in the river at the point of discharge after mixing (mg/L)
t is the time (days)
k (or k
d
) is the degradation constant (day
1
)
The rate constant for BOD loss can be expressed as the sum of different types of BOD loss.
For
our model, we will consider decay due to biological removal (k
1
) and loss by sedimentation (k
3
).
For now we will consider k
1
= 0.23/day and k
3
=
0.04/day.
Assume the velocity, (U), of the
river above and below Watapiti is 5 km/day.
Calculate (using the above equation) the BOD concentration at 0, 10, and 20 km downstream
from the Watapiti treatment plant.
The distance downstream (X) is the velocity (U) multiplied
by the time (t) traveled.
Remember at distance 0 km
to calculate the initial concentration by
mixing the effluent with the stream water.
2) Now you will use STELLA to answer the same BOD problem.
You will want to start with a
copy of the STELLA model you created for decay of a radioactive material.
The introduction to
STELLA and the first STELLA assignment will probably be useful.
1
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View Full DocumentRemember the radioactive source?
The amount of the source was undergoing an exponential
decline, just as the BOD concentration declines exponentially.
In fact, a STELLA model for
BOD would be very similar to the model that you developed for the radioactive source.
Using
STELLA, develop a BOD model.
You will not use the exponential formula for decay of BOD
concentration that you used to answer the preceding question.
STELLA uses a numerical
formula for BOD as follows:
BOD(t) = BOD(tdt) + ( Decay_Rate ) * dt ;
where: Decay_Rate = Fraction_Decaying * BOD
and
Fraction_Decaying = k
1
+ k
3
Thus the change in the BOD concentration with respect to time is found by multiplying the
degradation rate constant times the current BOD concentration.
This is exactly like the
SOURCE amount that was changing at a rate equal to a constant fraction times the current
amount.
To develop the BOD model for the Watapiti plant in Part 1, simply follow the same
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 Spring '10
 Lion
 Exponential Function, Radioactive Decay, BOD concentration

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