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

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1 ENV 4001: ENVIRONMENTAL SYSTEMS ENGINEERING Fall 2018 Quiz #3 Wednesday, November 28 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 four questions. Answer all four. 3. 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. 5. 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. 6. Show your work and state any important assumptions you make. I cannot award partial credit if I can t follow what you did. 7. Report a reasonable number of significant digits in your answers. 8. Include units in your answers. An answer without proper units is not correct! 9. You are allowed to use your text book, your course notes, or other printed materials. You may not receive help from another person. 10. A hand-held calculator is recommended. Other electronic devices are not permitted. 11. Please make sure your mobile telephone is off and put away. 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 2O: 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 µg 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 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 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 1. (10 pts) On quiz 1, you had to list the titles of two of the United Nations Sustainable Development Goals (SDGs). On quiz 2, you had to list two targets of one of the goals that is relevant to ENV Now let s complete the picture. List two indicators that are used to evaluate how well the SDGs are being met. Give the number and the wording of the two indicators. 2. EXTRA CREDIT UP TO TWO BONUS POINTS NO PENALTY FOR INCORRECT ANSWERS According to the American Film Institute, what is the #2 (i.e., second-best) mystery movie in the history of American cinema? Which actor was nominated for the 1975 Academy Award for Best Actor for his leading performance in that movie? p 3/7

4 3. (25 pts) Consider the typical activated sludge process that is used for secondary treatment at a wastewater treatment plant. The figure below shows a schematic of the process. On the figure, the symbol Q is used to denote a volumetric flow rate, and X is used to denote the concentration of bacterial biomass. a. (3 pts) In the context of ENV 4001, what is the purpose of this system? One or two sentences should be enough. In class, I told you that the solids retention time (SRT) is a key variable that controls the performance of the process. I also told you that it is possible for engineers or operators to control the SRT by controlling the sludge wasting rate and the sludge recycle rate. To examine this further, let s define two terms, which we will use later in this problem: recycle ratio, R = Q R/Q 0 wasting ratio, W = Q W/Q 0 b. (7 pts) Write a material balance for the mass of bacterial biomass, using the clarifier as the control volume. For the purposes of this quiz, assume that the clarifier works perfectly, i.e., no solids are present in the treated effluent: X E = 0. (This is not true, but X E is pretty low, so for this quiz, let s assume it is OK.) State other assumptions as needed. p 4/7 problem 3 continues

5 3. continued c. (5 pts) Re-arrange your material balance to solve for the ratio X/X W. When you do this, assume that the wasting flow rate (Q W) is much lower than the recycle flow rate (Q R). This is a good assumption. d. (5 pts) Think about the equation that we usually use to express SRT in terms of V, X, Q W, Q E, X W, and X E. Now recall that we are assuming X E = 0. What is the resulting equation for SRT? e. (5 pts) Use your results from parts b, c, and d to show (derive) that SSSSSS = θθ RR WW 1 + RR where θ is the average hydraulic residence time in the aeration basin. This equation shows us that to control SRT, we can adjust either R or W! (This derivation is based on an assumption of X E = 0, which is not valid, but the general idea is still correct.) p 5/7

6 4. (25 pts) (This problem is based on a problem from the text book Environmental Engineering Science by W Nazaroff and L Alvarez-Cohen.) Mercury (elemental symbol Hg) is a toxic metal; some forms of Hg are non-hazardous, but exposure to Hg in certain forms can cause brain damage. Therefore we try to keep Hg out of the environment. A significant source of Hg into the environment is cremation of dead people who have old dental fillings. Newer dentistry doesn t use Hg in fillings, but when I was a kid, it was very common to get amalgam fillings that included Hg. When a corpse is cremated, the mercury is vaporized and is emitted into the air with the flue gas. a. (3 pts) Suppose that a crematorium burns an average of 10 corpses per day, that the average number of mercury fillings is 4 fillings per corpse, and that an average filling contains 0.6 g of Hg. Estimate/calculate the emissions rate of Hg from the crematorium, in units of µg/s. b. (14 pts) Suppose that there is a house 400 m directly downwind of the crematorium. We want to make sure that the concentration of Hg in the air at the house (at ground level) remains below 0.01 µg/m 3. Assume that this neighborhood is usually not very windy (wind speed 1.5 m/s) and not at all sunny. Estimate/calculate the stack height that would be required for the crematorium. For the purposes of this problem, ignore plume rise from the stack; assume the effective stack height equals the physical stack height. Assume perfect reflection from the ground. p 6/7 problem 4 continues

7 4. continued more room to work on part b c. (8 pts) Now suppose that the weather changes from its usual conditions. The wind speed is 5.5 m/s and it is very sunny. What will be the concentration of Hg at the house? Is your design from part (b) acceptable under the new condition? END OF QUIZ p 7/7