ENV 4001: ENVIRONMENTAL SYSTEMS ENGINEERING. University of South Florida Civil & Environmental Eng.

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1 ENV 4001: ENVIRONMENTAL SYSTEMS ENGINEERING Fall 2016 Quiz #2 Wednesday, October 26 University of South Florida Civil & Environmental Eng. Prof. J.A. Cunningham Instructions: 1. You may read these instructions, but do not turn the page or begin working until instructed to do so. 2. This quiz contains three questions. Answer any two. 3. If you attempt more than two, make sure that you indicate clearly which two you want me to grade. If it isn t clear, then I will choose which two I feel like grading. 4. Some questions might have multiple parts. In those cases, the point value of each part is indicated. The total number of points possible is Unit conversion factors and other potentially useful information are provided on the back of this page, and on the page after that. 6. Answer each question in the space provided. If you need more space, you can attach additional pages as needed, but make sure to put your name on them. 7. Show your work and state any important assumptions you make. I cannot award partial credit if I can t follow what you did. 8. Report a reasonable number of significant digits in your answers. 9. Include units in your answers. An answer without proper units is not correct! 10. You are allowed to use your text book, your course notes, or other printed materials. You may not receive help from another person. 11. A hand-held calculator is recommended. Other electronic devices are not permitted. 12. Time limit: 60 minutes. Stop working when asked. If you continue working after time has been called, you will be penalized at a rate of 1 point per minute. 13. Don t cheat. Cheating will result in appropriate disciplinary action according to university policy. More importantly, cheating indicates a lack of personal integrity. 14. Please print your name legibly in the space provided below, and turn in this quiz at the end of the period. 15. Hints: Read each question carefully and answer the question that is asked. Watch your units. If you take good care of your units, they will take good care of you. Work carefully and don t rush. Name: p 1/9

2 Potentially useful constants: Ideal gas constant, R: Pa m 3 mol 1 K 1 = atm m 3 mol 1 K 1 Gravitational acceleration, g: 9.81 m/s 2 Molecular weight of water, H 2 O: g/mole Density of water at 20 C: g/ml = 998 kg/m 3 Viscosity of water at 20 C: Pa sec Density of air at 25 C: 1.18 kg/m 3 Viscosity of air at 25 C: Pa sec Potentially useful conversion factors: Pressure: 1 atm = 760 mm Hg = 760 torr = Pa 1 Pa = 1 N/m 2 = 1 kg/(m sec 2 ) Mass: 1 kg = 1000 g = 10 6 mg = 10 9 g 1 kg = lb mass 1 t (metric tonne) = 1000 kg = 2207 lb mass 1 ton (English ton) = 2000 lb mass Length: 1 km = 1000 m = 10 5 cm = 10 6 mm = 10 9 m 1 ft = 12 in = cm = m Temperature: 25 C = K Volume: 1 m 3 = 1000 L = 10 6 ml = 10 6 cm 3 1 gal = L Work/Energy: 1 BTU = kj Power: 1 MW = 10 6 W = 10 6 J/s = 10 6 N m/s Area : 1 ha = 10 4 m 2 Atomic Masses: H = g/mole C = g/mole N = g/mole O = g/mole P = g/mole S = g/mole Cl = g/mole Br = g/mole Na = g/mole Mg = g/mole Ca = g/mole Fe = g/mole Equilibrium Concentrations of Oxygen (O2) in Fresh Water (air/water equilibrium): Temperature Equil. Conc. of O 2 Temperature Equil. Conc. of O 2 ( C) (mg/l) ( C) (mg/l) p 2/9

3 from Principles of Environmental Engineering and Science, 2nd edition, by Davis and Masten p 3/9

4 1. (30 pts) The City of Oldtown has a problem. They recently closed one of their wastewater treatment plants to save money, but the city s sewer system is old and leaky, and now whenever a big rain storm comes through, the wastewater flow rate goes way up because of rainwater infiltration into the sewers. When that happens, the city cannot treat all of the wastewater. Then they have to dump untreated sewage into the Porcello River. (This situation may sound familiar.) Recently, there was a massive weather front that dumped rain on Oldtown for about three weeks straight. Oldtown had to discharge untreated sewage into the river at a flow rate Q. The city reported the discharge to the Department of Environmental Quality (DEQ), but the DEQ thinks the city under-reported the discharge flow rate. Now the DEQ is trying to estimate the flow rate Q of the untreated sewage discharge. You are going to help them. Here is what we know: The temperature in the river during the rain event was 18 C. Upstream of the sewage discharge, the flow rate of the Porcello River is about 14 m 3 /s, the river contains 9.00 mg/l of dissolved oxygen, and the BOD 5 in the river is about 3.8 mg/l. We don t know the flow rate Q of the sewage discharge, but we do know that the BOD 5 of the untreated sewage is about 125 mg/l, and it does not contain any dissolved oxygen. Downstream of the sewage discharge, the river velocity is 28.8 km/d. As we might expect, the oxygen concentration in the river was observed to drop downstream of the sewage discharge. The worst spot in the river is 41.3 km downstream of the discharge. At that location, the concentration of dissolved oxygen is 7.17 mg/l. The first-order rate coefficient for the degradation of organic matter, k L, is about 0.20/d. You can assume that this is also the first-order rate coefficient for deoxygenation in the river. The first-order rate coefficient for reaeration of the river from the atmosphere is about 1.2/d. To estimate the unknown discharge rate Q, proceed as follows. a. (6 pts) Estimate/calculate the BOD ult of the upstream river and of the untreated sewage. Hint: you are given BOD 5 for each of these streams, and you are given some rate coefficients. problem 1 continues p 4/9

5 1. continued b. (18 pts) Estimate/calculate the oxygen concentration in the river just downstream of the sewage discharge. Also estimate/calculate the contaminant concentration at that point, expressed as oxygen demand, BOD ult. Hint: to calculate these two unknowns, you need two equations -- use the given information about the critical point in the river. That will give you the two equations you need. c. (6 pts) Using an appropriate material balance, estimate/calculate Q, the unknown rate of the sewage discharge. Report your answer in m 3 /s and in millions of gallons per day. Hint: there are two possible material balances you could use; they should both give you the same answer. p 5/9

6 2. (30 pts) This problem is based (loosely) on a problem from the text book Environmental Engineering Science by Nazaroff & Alvarez-Cohen. Consider a sedimentation basin at a water treatment plant. Water entering the sedimentation basin contains 180 mg/l of suspended spherical particles. The particle suspension is divided evenly (by mass) among three size classes: 10 m diameter, 100 m diameter, and 1000 m (1 mm) diameter. In other words, there are 60 mg/l of each size class. The water is 20 C and flows at a rate of 20 m 3 /hr. The height of the sedimentation basin is 4.0 m and the width is 3.0 m. a. (10 pts) Estimate/calculate the length of the sedimentation basin that would be required to remove 100% of the 100- m particles. These particles have a density of 2.2 g/cm 3 = 2200 kg/m 3. b. (5 pts) Does your design seem appropriate? Why or why not? If not, what would you do differently? problem 2 continues p 6/9

7 2. continued c. (5 pts) How effective do you think your design would be at removing the other two size classes? Explain briefly. You do not have to compute actual removal percentages it is OK to give a semi-quantitative answer and discussion,. d. (10 pts) You observe that the largest particles (with 1.0 mm diameter) have a settling velocity of 100 cm/s = 0.1 m/s. Estimate/calculate the particle density of these particles, in units of kg/m 3. Hint: check the thing(s) you need to check. It is easy to check because you already know the settling velocity. p 7/9

8 3. (30 pts) Suppose that there is a particular form of cancer that we are concerned about due to chemical exposure. This form of cancer is pretty rare in most instances. In one city (City #1), which has a population of 100,000 people, only about 4 people die each year from this form of cancer. In a neighboring city (City #2), which has a population of 200,000 people, about 6 people die each year from this same form of cancer. a. (10 pts) For both City #1 and City #2, estimate the annual risk and the lifetime risk of dying from this form of cancer. Assume an average lifespan of 70 years in both cities. Which city has a higher risk of this form of cancer? b. (15 pts) Some epidemiologists believe that the higher risk in one of the cities is due to the presence of carbon tetrachloride (CCl 4 ) in the drinking water supply of that city. Estimate the concentration of CCl 4 that would be consistent with the excess incremental risk. State your assumptions clearly. problem 3 continues p 8/9

9 3. continued more room to work on 3(b) c. (5 pts) If you lived in the city with the CCl 4, how concerned would you be about the presence of that chemical in the water supply? Explain briefly, based on the information from this problem. END OF QUIZ p 9/9