Computer Models of Stream Oxygenation

One STELLA II Model of Deoxygenation and Reoxygenation:

The concentration of oxygen in water is a function of temperature, pressure, and other dissolved and undissolved materials. However, the temperature and pressure in this model are considered to be constant. The dissolved waste B is only the total waste entering into the stream, and there is no soil corrosion on the banks of the stream. This waste B can be studied to determine the WasteCoef (k'''B) in the laboratory. The growth rate of a micro-organism is considered in the determination of the WasteCoef (see Suggested Exercises and Discussion Questions). In addition, the waste can be easily monitored, and there is not another entry point before or below the mixing point of waste B. At the mixing point, there is a complete mixing between waste B and water. There is also a constant velocity of the water flow in the stream. In general, the stream is an ideal stream.

This model will use eq. 5 to describe the rate of removal of organic waste by bacteria:

WasteEaten=WasteCoef*WastePPM*Time

With eq. 10, the process of deoxygenation is:

DeOx = (wastecoef*WastePPM*time)-(ReOxCoeff/Streamheight*OxygenPPM*time)

The saturation value of oxygen in water is 9.8 ppm. Therefore, the rate of oxygen absorption is only the interphase of the water's surface and air; there is no other mechanical or natural devices to increase the surface area of water. The rate of reoxygenation is:

ReOx = Area*(9.8-OxygenPPM)*ReOxCoeff*time

Figure 1: Deoxygenation and Reoxygenation Model

In this model, the test data are:

Temperature = 17šC
Saturation value of oxygen = 9.8 ppm
k'''B = 7.38x10-5 ppm/sec (WasteCoef)
1k'A = 1.65x10-3 ppm.ft/sec (ReOCoeff)
Initial waste = 0 ppm in the water at time 0 sec
Initial waste amount = 150,000*1000 g
Stream height = 20 ft
Stream volume = 200*20 ft3
Velocity of stream is 2ft/sec

Figure 2: System Equations for Deoxygenation and Reoxygenation

OxygenPPM(t) = OxygenPPM(t - dt) + (ReOx - OxGone) * dt
	INIT OxygenPPM = 9.8
	INFLOWS:
		ReOx =	IF OxygenPPM=9.8 THEN 0 ELSE 
		MIN((Volume/StreamHeight) * (9.8-OxygenPPM) * ReOxCoeff 
		* (time-dt),9.8-OxygenPPM)
	OUTFLOWS:
		OxGone = DeOx

WastePPM(t) = WastePPM(t - dt) + (WasteinPPM - WasteEaten) * dt
	INIT WastePPM = 0
	INFLOWS:
		WasteinPPM = GramsWasteIn*(time)/(Volume*1000)
	OUTFLOWS:
		WasteEaten = WasteCoef*WastePPM*(time+dt)

Area = 200
DeOx = (WasteCoef*WastePPM*(time))-(ReOxCoeff/StreamHeight*OxygenPPM*(time))
GramsWasteIn = IF TIME = 1 THEN 250000*1000 ELSE 0
ReOxCoeff = .0000738
StreamHeight = 20
Volume = Area*StreamHeight
WasteCoef = .00165

By setting running the time options, the students can observe the relationship between the oxygen concentration and waste B concentration (in PPM) (see figure 3).

Figure 3: The Graphical Relationship Between the Oxygen Concentration and Waste B Concentration

Graph of Oxygen and Waste PPM versus Time

Figure 4: The Numeric Relationship Between the Oxygen Concentration and Waste B Concentration

Time	OxygenPPM	WastePPM	OxGone	ReOx	WasteEaten
14	4.73	52.75	1.22	0.97	1.31
15	4.49	51.44	1.27	1.1	1.36
16	4.31	50.08	1.32	1.21	1.4
17	4.21	48.68	1.37	1.32	1.45
18	4.16	47.23	1.4	1.41	1.48
19	4.17	45.75	1.43	1.49	1.51
20	4.23	44.24	1.46	1.56	1.53
21	4.34	42.71	1.48	1.61	1.55
22	4.47	41.16	1.49	1.65	1.56
23	4.36	39.6	1.5	1.68	1.57

Teachers: Dissolved oxygen (DO) and 1k'A can be found in the CRC Chemical and Physical Handbook or Perry's Chemical Engineering Handbook. k"'B for a specific chemical can also be found in Perry's or CRC's, but they can also be easily determined in the laboratory.

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