ENV 4001: ENVIRONMENTAL SYSTEMS ENGINEERING Spring 2015 Final Examination Monday, April 27, 2015 University of South Florida Civil & Environmental Engineering Prof JA Cunningham Instructions: 1. You may read these instructions, but do not turn the page or begin working until instructed to do so. 2. This exam contains 4 questions, all with multiple parts. Pick any three. 3. The total number of points possible is 120. Point values for each question are indicated. 4. Answer the exam questions on separate sheets of paper (i.e., not on these exam sheets). Make sure your name is on each piece of paper, and that you submit all pages you want graded. At the end of the exam period, staple your papers together in the proper order. You may use as many sheets of paper as you need (though, in the spirit of ENV 4001, I encourage you not to be wasteful). 5. To maximize your partial credit, show all your work and state your assumptions clearly. I can only assign you partial credit if I can follow what you are doing. 6. Report your answers to a reasonable number of significant digits and with proper units. 7. You are allowed to use your text book, your course notes, or other printed materials. You may not receive help from another person. 8. The back of this page contains some constants and unit conversions that you might find helpful. I am also including two additional pages, scanned from a previous year s text book, that have some helpful tables on them. 9. A hand-held calculator is recommended. Other electronic devices are not permitted. 10. Time limit: 120 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. 11. Don t cheat. Cheating will result in appropriate disciplinary action according to university policy. More importantly, cheating indicates a lack of personal integrity. 12. 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. 1
Potentially useful constants: Ideal gas constant, R: 8.314 Pa m 3 mol 1 K 1 = 82.06 10 6 atm m 3 mol 1 K 1 Gravitational acceleration, g: 9.81 m/s 2 Molecular weight of water, H2O: 18.01 g/mole Density of water at 18 C: 0.9985 g/ml = 998.5 kg/m 3 Viscosity of water at 18 C: 1.06 10 3 Pa sec Density of air at 18 C: 1.21 kg/m 3 Viscosity of air at 18 C: 1.81 10 5 Pa sec Potentially useful conversion factors: Pressure: 1 atm = 760 mm Hg = 760 torr = 101325 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 = 2.207 lbmass 1 t (metric tonne) = 1000 kg = 2207 lbmass 1 ton (English ton) = 2000 lbmass Length: 1 km = 1000 m = 10 6 mm = 10 9 µm 1 ft = 12 in = 30.48 cm = 0.3048 m Temperature: 25 C = 298.15 K Volume: 1 m 3 = 1000 L = 10 6 ml = 10 6 cm 3 1 gal = 3.785 L Work/Energy: 1 BTU = 1.055 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 = 1.008 g/mole C = 12.011 g/mole N = 14.007 g/mole O = 15.999 g/mole P = 30.974 g/mole S = 32.06 g/mole Cl = 35.453 g/mole Br = 79.904 g/mole Na = 22.99 g/mole Mg = 24.31 g/mole Ca = 40.08 g/mole Fe = 55.85 g/mole 2
from Principles of Environmental Engineering and Science, 2nd edition, by Davis and Masten 3
from Principles of Environmental Engineering and Science, 2nd edition, by Davis and Masten 4
1. (40 pts) Biochemical oxygen demand, oxygen depletion in a river, reactor theory A factory discharges its liquid waste into the Bellhorn River. During the summer, the temperature of the river is 22 C. Upstream of the discharge point, the flow rate of the river is 4.6 m 3 /s, and the water is pretty clean: BOD5 = 4.0 mg/l, and the concentration of dissolved oxygen is 8.00 mg/l. The flow rate of the factory s wastewater discharge is 0.4 m 3 /s, and it does not contain any dissolved oxygen. Bacteria in the river degrade the organic matter from the factory s wastewater. The firstorder rate coefficient for degradation of the organic matter is 0.40 d 1. You may assume that this is also the deoxygenation rate coefficient in the river. Downstream of the factory s discharge, the width of the river is 10 m, the depth of the river is 2.0 m, and the re-aeration rate coefficient is 0.70 d 1. a. (5 pts) Estimate/calculate the BODult in the river upstream of the factory s discharge. b. (25 pts) We want to be sure that the oxygen concentration in the river is at least 6.00 mg/l at a distance 30 km downstream of the discharge, because that is believed to be a part of the river where fish spawn during the summer. Estimate/calculate the maximum allowable BODult in the factory s wastewater to ensure adequate oxygen at this location. c. (10 pts) The factory measured the BODult in their wastewater and found that they needed to remove 70% of the organic matter in the waste stream to protect the health of the river. They will install a completely mixed flow reactor that can remove the organic matter with a first-order rate coefficient of 1.0 hr 1. Estimate/calculate the required volume of the reactor. Report your answer in units of m 3. 5
2. (40 pts) Material balances, wastewater treatment At a certain wastewater treatment plant, secondary treatment is performed via the activated sludge process, which we studied this year in ENV 4001. There is an aeration basin in which bacteria metabolize dissolved organic carbon (DOC), and there is a clarifier in which the biomass is separated from the treated effluent. Sludge from the bottom of the clarifier is split into a return activated sludge (RAS) stream, which returns to the aeration basin, and a waste activated sludge (WAS) stream, which goes to anaerobic digestion. The volume of the aeration basin is 8,000 m 3. In the aeration basin, active bacterial cells are produced at a net rate of 120 kg/hr. (Note that net rate accounts for both bacterial growth and bacterial death.) a. (20 pts) Consider the following process diagram for the secondary treatment. The table below gives the volumetric flow rate and the concentration of active bacterial cells in some of the streams shown in the diagram. You may assume that the density of every stream is the same (1000 kg/m 3 ); this assumption is not perfect, but it is close enough. Stream Flow rate Bacterial conc. (L/s) (mg/l) ---------- ---------- ---------- A 438 7 B 584 C 5 D E 4 F Your job is to complete the table by estimating/calculating the remaining flow rates and solids concentrations. You must show your work to receive credit. 6 problem 2 continues
2. continued b. (10 pts) The bacteria in the aeration basin that metabolize the DOC have the following biological properties: half-velocity coefficient KS = 60 mg/l BOD5 bacterial death rate coefficient kd = 0.08 d 1 maximum specific growth rate coefficient µmax = 4.0 d 1 yield coefficient Y = 0.72 mg biomass produced per mg BOD5 consumed Estimate/calculate the concentration of DOC (expressed as BOD5) exiting the system in the treated effluent stream (stream C in the process diagram). Report your answer in units of mg/l. Hint #1: Dissolved organic carbon (DOC) is the soluble substrate metabolized by the bacteria. Hint #2: Make use of the net rate of production of active bacterial cells. Hint #3: How is the concentration of DOC in the treated effluent stream related to the concentration of DOC in the aeration basin? c. (10 pts) Estimate/calculate the concentration of DOC (expressed as BOD5) in the stream coming from primary treatment (stream A). What is the fractional removal of DOC in the secondary treatment process? Hint: use a material balance around the aeration basin. 7
3. (40 pts) Environmental chemistry, water treatment A farmer has a pond on his farm. The pond has a volume of 15,000 m 3 and a surface area of 5,000 m 2. It has one stream flowing in at a rate of 3,000 m 3 /hr, and one stream flowing out at the same rate. The temperature is 18 C. The pond has too much phosphate (PO 4 3 ) in the water, because when it rains, fertilizer from the fields gets washed into the pond. The phosphate leads to algae blooms in the pond, which the farmer hates. To control the phosphate concentration, the farmer plans to add alum, Al 2(SO 4) 3, to the influent stream of the pond. This will release Al 3+ and cause the phosphate to precipitate as AlPO 4. Then the AlPO 4 particles will settle to the bottom of the pond, removing the phosphate. This is a good plan as long as the Al 3+ from the alum reacts with the phosphate to form AlPO 4. If too much of the Al 3+ instead forms Al(OH) 3, then the phosphate will not be removed. The solubility product constant (K sp) for Al(OH) 3 is 10 33. The solubility product constant (K sp) for AlPO 4 is 10 20. a. (3 pts) Estimate/calculate the average hydraulic residence time of the pond. b. (10 pts) Suppose the ph of the pond is 8.5. What will be the equilibrium concentration of Al 3+, in units of mol/l? Hint: use the K sp for Al(OH) 3. c. (10 pts) Under this ph condition, what will be the equilibrium concentration of PO 4 3 remaining in the pond water? Report your answer in mg/l as P. Hint: use your result from part (b). Can the farmer reduce his phosphate concentration to below 0.1 mg/l (as P) with this strategy? d. (7 pts) Repeat your calculations from parts (b) and (c) if the ph of the pond is 6.5. Now can the farmer reduce his phosphate concentration to below 0.1 mg/l as P? What might the farmer might want to add to the pond in addition to alum? e. (10 pts) The farmer does not want a lot of AlPO 4 particles flowing out of the pond in the effluent stream. He wants the AlPO 4 particles to settle out while they are still in the pond. What particle diameter is required for the AlPO 4 particles to ensure that at least 90% of the particles are removed before exiting the pond? (It is OK to assume that the flow in the pond is relatively calm, mimicking the behavior of a sedimentation basin at a drinking-water treatment plant.) I do not know the density of the AlPO 4 particles, but let s guess 2.0 g/cm 3 = 2000 kg/m 3. [Note: sedimentation of the AlPO 4 particles will be aided by the formation of some Al(OH) 3 flocs, which will sweep up the AlPO 4 particles and help them settle. You just don t want *too* much Al(OH) 3 because then you won t form AlPO 4 for the phosphate removal.] 8
4. (40 pts) Risk assessment, air pollution, solid waste management A small city of 200,000 people recycles 25% of its municipal solid waste and sends the rest to a waste-to-energy facility. A resident who lives 5 km away from the waste-to-energy facility is worried about the emissions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, also frequently just called dioxin ) from the facility. TCDD is one of the most toxic compounds known to man. It is formed in small quantities when plastic (especially PVC) is burned. If the waste-to-energy facility does not have adequate air pollution controls, then it might be emitting hazardous levels of 2,3,7,8-TCDD into the air. The resident under consideration is a 45-year-old woman who will live in the town for 30 years. She spends about 12 hours per day at her house. The wind patterns in the area are such that her house is downwind of the waste-to-energy facility about 2 days per week on the other 5 days per week, the wind blows away from her house. a. (5 pts) What is an acceptable chronic daily intake (CDI) of TCDD if the woman wants to keep her incremental cancer risk to an acceptable level? State your assumptions and/or show your work, as necessary. Report your answer with the proper units for CDI. b. (10 pts) What is an acceptable concentration of 2,3,7,8-TCDD in the air to ensure that she does not exceed the CDI you found in part (a)? State your assumptions clearly and show your work. Report your answer in units of mg/m 3. c. (13 pts) Suppose that a typical day is one in which the wind speed is 5.5 m/s (at a height of 10 m above the ground) and both the daytime and nighttime conditions are thinly overcast. The effective stack height of the waste-to-energy facility is 35 m. What is an acceptable emission rate of TCDD from the facility to protect the woman s health? Report your answer in units of mg/d. Show your work and state your assumptions as appropriate. d. (12 pts) I read that TCDD is formed at a rate of 2 10 5 mg per kg of newspaper burned, 0.43 mg per kg of PVC burned, and about 0.001 mg per kg of other plastics burned [Shibamoto et al., 2007, Dioxin formation from waste incineration, Rev. Environ. Contam. Toxicol., 190:1 41.] Based on these, estimate the emissions rate of TCDD from the waste-to-energy facility. Does the emission rate that you estimate in part (d) exceed the acceptable emission rate in part (c)? If so, what fractional removal of TCDD would be required by the facility s air pollution control system? Do you think the resident is right to worry about living 5 km from a waste-to-energy facility? Explain very briefly. Hint: use your text book to look up the composition of MSW. END OF EXAMINATION 9