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

Transcription

1 ENV 4001: ENVIRONMENTAL SYSTEMS ENGINEERING Fall 2018 Quiz #1 Wednesday, September 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. 2. This quiz contains four questions. Answer question 1. Then choose any two of the last three problems. 3. If you attempt all the problems (not recommended), make sure you clearly indicate which ones you want me to grade. If it is not clear, I will grade whichever ones I think I can grade the fastest. That might not work in your favor. 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 is provided on the back of this page. 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 as appropriate. 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, H2O: g/mole Density of water at 25 C: g/ml = 997 kg/m 3 Viscosity of water at 25 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 µg 1 kg = lbmass 1 t (metric tonne) = 1000 kg = 2207 lbmass 1 ton (English ton) = 2000 lbmass Length: 1 km = 1000 m = 10 5 cm = 10 6 mm = 10 9 µm 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 Al = g/mole K = g/mole Ca = g/mole Fe = g/mole Cu = g/mole p 2/9

3 1. (10 pts) Write down the number and the title of two of the United Nations Sustainable Development Goals that are particularly relevant for ENV For one of these two goals, also write down the U.N. s brief description of the goal. (For that second part, you can choose which one to describe, but be clear which one you are choosing.) p 3/9

4 2. (25 pts) I had a water sample that contained 20 mg/l of calcium (Ca 2+ ), and I wanted to decrease the concentration of calcium in the water. Professor Trotz told me that if I raised the ph of the water, I could probably get some of the calcium to precipitate. I raised the ph of the water from 7.0 to 8.0, at which point a precipitate just started to form. Then I wondered what the precipitate was. I figured it had to be either CaCO3 or Ca(OH)2, but I wasn t sure which one. I quickly took a sample of the water and measured the concentration of HCO3 and it was 89 mg/l. Based on this information, is my precipitate CaCO3, Ca(OH)2, or a mixture of both? You must show your work to get credit for this question no credit for just flipping a coin or making a lucky guess! more space to work on problem 2 p 4/9

5 2. continued more space to work on problem 2 p 5/9

6 3. (25 pts) A factory produces a waste stream that contains Chemical X, which is hazardous. The waste stream contains mol/l of Chemical X. The factory s operating permit stipulates that they need to remove 90% of Chemical X from the waste stream before they are allowed to discharge it into a nearby river. The environmental engineers that work for the factory know that Chemical X will react with Chemical Y to form Chemical Z, which is harmless. X + Y Z This reaction follows second-order kinetics: R = k2[x][y], where R is the reaction rate in mol/(l s), k2 is a second-order reaction rate coefficient equal to 5.0 L/(mol s), and [X] and [Y] are the molar concentrations of those two chemicals. To treat the waste stream, the engineers design a completely-mixed flow reactor with an average hydraulic residence time of 40 min. They figure out that if they add the proper concentration of Chemical Y to the waste stream just before it enters the reactor, it will react with Chemical X, and they can meet the permit requirement. (a) (15 pts) Write material balances for the mass of both Chemical X and Chemical Y in the reactor (one mass balance for each chemical). Make sure that the meaning of each term in your equations is clear if I don t understand your equations, I can t award you credit. You may assume the system operates at steady state. problem 3 continues p 6/9

7 3. continued more space to work on part (a) b. (10 pts) To meet the permit requirements, what concentration of Chemical Y should the engineers add to the waste stream just before it enters the reactor? Hint: solve the equations you wrote in part (a). p 7/9

8 4. (25 pts) The Bettsville River is pretty clean, but it is slightly contaminated. At point A in the river, the concentration of dissolved oxygen is 8.0 mg/l, and the concentration of the contaminant, expressed as oxygen demand, is 7.0 mg/l. Biodegradation of the contaminant is known to follow first-order kinetics, with a rate coefficient of 0.30 d 1. Point B in the river is 20 km downstream of point A, and point C is another 30 km downstream of point B. The river flows at 0.1 m/s. For this problem, you can treat the river as a plug-flow reactor. (a) (6 pts) Suppose I collected a sample of river water from point A and I brought it to the lab. I measured both BOD3 and BOD5 for that sample of water. Estimate/calculate the results that you would expect from these measurements. (b) (9 pts) Now suppose I collected a sample of river water from point B, and I repeated the same measurements as before, i.e., BOD3 and BOD5. Estimate/calculate the results that you would expect from these measurements. problem 4 continues p 8/9

9 4. continued (c) (10 pts) As dissolved oxygen is depleted in the river, it is replenished by re-aeration from the atmosphere. We will learn about that soon. But for now, suppose we ignored reaeration, and we only considered the consumption of oxygen that occurs when the bacteria in the river degrade the contaminant. Estimate/calculate the concentration of dissolved oxygen that you would expect at point B and at point C. END OF QUIZ p 9/9