HW4 - STELLA2

# HW4 - STELLA2 - STELLA Assignment#2 BOD 1 In this problem...

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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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Remember 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(t-dt) + (- 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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HW4 - STELLA2 - STELLA Assignment#2 BOD 1 In this problem...

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