ENV 4001: ENVIRONMENTAL SYSTEMS ENGINEERING. University of South Florida

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1 ENV 4001: ENVIRONMENTAL SYSTEMS ENGINEERING Spring 2010 Final Examination Friday, May 7, 2010 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 exam contains 5 questions. All students should answer question #1 and question #2; then select any one of the final three problems. The total points possible is 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). 4. Only turn in the problems that you want me to grade. If you turn in any extra problems, I will pick the one that looks like it can be graded the fastest and thereby make my life easier. It is probably not in your interests to submit an extra problem. 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 or might not find helpful. I am also including two additional pages, photocopied from last 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. 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 Molecular weight of water, H 2 O: 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 s 2 ) Mass: 1 kg = 1000 g = 10 6 mg = 10 9 µg 1 metric ton = 1 tonne = 1 t = 1000 kg 1 English ton = 2000 lb mass 1 kg = lb mass 1 lb mass = kg = g Temperature: 25 C = K Volume: 1 m 3 = 1000 L = 10 6 ml = 10 6 cm 3 Energy : Power: 1 gallon = L 1 BTU = kj = 1055 J 1 kwh = 3600 kj 1 W = 1 J/s = 1 N m/s 1 MW = 1000 kw Atomic Masses: H = g/mole C = g/mole N = g/mole O = g/mole F = g/mole Mg = g/mole Cl = g/mole Ca = g/mole Br = g/mole p 2/9

3 1. (10 points) Look at the cartoon below. Explain what major error the cartoonist made. You can probably answer this question in one paragraph. Please note that I am looking for a scientific error, not an error like Batman s utility belt is on backwards or something like that. ( (Note: I thought this cartoon was pretty funny despite the scientific error. It s not that professors don t have a sense of humor; we just have a hard time letting things go when we see a mistake.) 2. (25 points) A common water-treatment process that we did not discuss this semester is water softening. Water softening is the process of removing Ca 2+ and Mg 2+ ions from water so that they don t cause scaling problems in your home. Typically, water softening is accomplished by raising the ph of the water so that Ca 2+ precipitates as CaCO 3 and Mg 2+ precipitates as Mg(OH) 2. This process also typically removes bicarbonate (HCO 3 ) from the water because the bicarbonate precipitates as CaCO 3. Suppose that we analyze some water after it has been softened, and we find that the concentration of Ca 2+ is 4.01 mg/l and the concentration of Mg 2+ is 2.43 mg/l. a. (10 pts) Estimate/calculate the ph of the water based on the information given. Hint: you might have to look something up in your book. b. (15 pts) Estimate/calculate the concentration of bicarbonate that you would expect to be present in this water. Report your answer in units of mg/l. p 3/9

4 3. (85 points) Topics: Environmental chemistry, drinking water treatment, reactor theory Consider a water-treatment plant that treats 10 million gallons per day (mgd). The plant uses chlorine to disinfect its water. Chlorine is added in the form of hypochlorous acid, HOCl. Hypochlorous acid dissociates into hypochlorite ion, OCl. The dissociation can be described by: HOCl <===> H + + OCl pk a = 7.54 The undissociated acid form, HOCl, is about 100 times more powerful as a disinfectant than the OCl ion. In other words, the disinfection rate constant of HOCl is about 100 times higher than the disinfection rate constant of OCl. Let s estimate that the overall rate constant can be described this equation: k overall = f HOCl k HOCl + f OCl k OCl where f HOCl is the fraction of the disinfectant in the HOCl form, and f OCl is the fraction of the disinfectant in the OCl form. The plant is worried about the presence of Giardia in its water. The federal Safe Drinking Water Act requires three-nines removal (three log kills) of Giardia during disinfection. Suppose the plant typically disinfects its water at a concentration of 0.06 mm of total hypochlorite (HOCl plus OCl ). At that dose, the first-order rate constant (base e, not base 10) for disinfection of Giardia by HOCl is 1.8 min 1, and the first-order rate constant for disinfection of Giardia by OCl is min 1. a. (20 pts) Estimate/calculate the overall rate constant, k overall, if the disinfection is performed at a ph of Then repeat your estimation for ph of 6.54, 7.54, 8.54, and Suppose the disinfection is performed in a contact tank that has four chambers. Each chamber of the tank is well-mixed, holds 50,000 gallons of water, and receives the disinfectant dose of 4.0 mg/l indicated above. The water travels through all four chambers, one after the other; in other words, the contact tank works like four completely-mixed flow reactors in series. b. (20 pts) Estimate/calculate the fractional removal of Giardia if the system is operated at a ph of Then repeat the calculation for ph of 6.54, 7.54, 8.54, and c. (5 pts) In what ph range do you recommend that the water treatment plant should operate its disinfection process? Explain why (briefly -- just a sentence or two.) You must consider the Giardia standard, but you can also consider other operational factors that you think might be important. problem 3 continues p 4/9

5 3. continued The same drinking water plant employs (upstream of disinfection, of course) the processes of coagulation, flocculation, sedimentation, and filtration. The sedimentation basin is designed to remove flocs that are approximately 50 µm in diameter with a density of 1.6 g/cm 3. The sedimentation basin is 3.0 m deep. You may assume a water temperature of 25 C. d. (20 pts) How much residence time is required in the sedimentation basin to remove 100% of the flocs? Report your answer in units of minutes. How much residence time is required to remove 50% of the flocs? 25%? 10%? e. (20 pts) Using your answers from part (d), plot N/N 0 versus residence time, where N 0 is the concentration of flocs entering the basin, and N is the concentration of flocs exiting the basin. Examine your graph: if we wanted to model sedimentation as a reaction taking place in a reactor, what is the order of the reaction kinetics? Explain briefly (one sentence should suffice). What is the value of the reaction rate constant? (Be sure you get the units correct!) p 5/9

6 4. (85 points) Topics: material balances, solid waste management, oxygen demand, oxygen depletion in a river, wastewater treatment Imagine an above-ground county landfill operating in fictional Lynn County. The dimensions of the landfill are 1000 m 500 m, and the landfill has a maximum height of 12 m (not including the final cover). The slope of the landfill is G = 2.7. a. (20 pts) If the landfill is supposed to operate for 50 years, how many people can it serve? For the purposes of this problem, you may assume that the volume of daily cover is equal to 20% of the volume of municipal solid waste (MSW). State any other assumptions clearly. In the Lynn County landfill, the leachate is collected by perforated pipes located in the drainage layer, just above the liner. The leachate is then pumped to a treatment system. Landfill leachate is typically very low-quality water. On average, the leachate from the Lynn County landfill has a BOD 5 of 1000 mg/l, a COD of 2300 mg/l, an ammonium concentration of 120 mg/l (as N), and is completely devoid of dissolved oxygen. One day, there was a leak in the pipe, and some of the leachate ran off into a nearby pond. Landfill managers were not sure exactly how much leachate entered the pond, but the best estimate was 4000 gallons. The volume of the pond is about 2100 m 3. Before the spill, the pond was in pretty good health, with a dissolved oxygen concentration of 8.5 mg/l. b. (10 pts) Estimate/calculate the concentration of dissolved oxygen in the pond 5 days after the spill occurred. Does the spill appear to threaten the health of the pond? For the purposes of this problem, you may ignore reaeration of the pond from the atmosphere, and you may assume that organic matter in the pond degrades with a rate constant 0.21 d 1. Hint: account for the dilution of the leachate when it mixed with the pond water. State any assumptions clearly. As a result of the leak in the leachate pipeline, an unethical engineer got a sinister idea. He thought that the landfill could save some money by (illegally) diverting some of the untreated leachate into a nearby stream. He thought nobody would notice if he diverted only 100 gallons per minute (equivalent to m 3 /s). Here is some information about the stream (measured upstream of the landfill): temperature, T = 20 C volumetric flow rate, Q = 0.14 m 3 /s average flow velocity, u = 0.12 m/s stream depth, h = 0.40 m stream width, w = 2.9 m concentration of dissolved oxygen = 9.00 mg/l BOD 5 (at 20 C) = 3.0 mg/l deoxygenation rate const., k 1 (at 20 C) = 0.21/d reaeration rate const., k 2 = 2.3/d problem 4 continues p 6/9

7 4. continued c. (30 pts) The engineer got caught when things started dying in the river downstream of the landfill; this made the nearby residents suspicious, so they went out and took some water samples and had them analyzed for dissolved oxygen (DO). At what distance downstream of the landfill would you expect the DO to be lowest? Estimate the DO concentration at that location. Hint: think carefully about how you want to estimate L 0 -- there is something you have to remember to do. After all the problems with the pipeline leak and the illegal dumping, local regulators really cracked down on the Lynn County landfill. They said that the leachate treatment system had to reduce the BOD 5 of the leachate from 1000 mg/l down to 100 mg/l. Then the treated leachate could be routed to the sanitary sewer system where it would eventually end up at the county s wastewater treatment plant for additional treatment. The landfill engineers (those who didn t get fired for illegal dumping) designed a biological activated sludge treatment process to treat the leachate to meet the 100 mg/l requirement. Their design employs a completely-mixed flow reactor with an average hydraulic residence time of 120 min. The bacteria that remove the BOD 5 from the leachate have the following biological properties: half-velocity coefficient, K S = 60 mg/l BOD 5 bacterial death rate constant, k d = 0.10 d 1 maximum specific growth rate constant, µ max = 3.0 d 1 yield coefficient, Y = 0.60 mg of biomass produced per mg BOD 5 consumed d. (20 pts) What concentration of biomass, X, must be maintained in the reactor to achieve the desired treatment? Hint: use an appropriate material balance to solve this problem. e. (5 pts) What elements would you incorporate in the design in order to ensure that you can maintain the necessary biomass concentration? Explain briefly. p 7/9

8 5. (85 points) Topics: material balances, air pollution, risk assessment, reactor theory In many parts of the United States (but not in Florida), most of the homes have basements or cellars. In these parts of the country, groundwater contamination can be a problem because toxic vapors can intrude into the basements and can make people sick. (This happened in the famous Love Canal case in upstate New York in the late 1970s, and has occurred at many other sites around the country since then.) Imagine a home with a basement that is 15 m long 6 m wide 2.5 m high, i.e., the total air volume of the basement is 225 m 3. The groundwater under the house is contaminated with trichloroethylene (TCE) and tetrachloroethylene (also called perchloroethylene, or PCE). These are common groundwater pollutants. Here is some information about TCE and PCE: TCE PCE molecular formula: C 2 HCl 3 C 2 Cl 4 molecular weight (g/mole): boiling point ( C): Henry s constant, K H, dimensionless form, at 22 C: biodegradation rate constant, k (hr 1 ): In this particular home, TCE seeps up through the basement floor with a flux of 0.80 mg/(m 2 d) and PCE has a flux of 0.24 mg/(m 2 d). (Flux is the rate of intrusion per day per unit area of basement floor.) Under typical conditions, the basement is ventilated at a rate of 10 m 3 /hr. a. (15 pts) Under steady state conditions, what will be the concentrations of TCE and PCE in the basement air, in units of mg/m 3? Hint: use an appropriate material balance. State any assumptions. b. (15 pts) Convert the concentrations of TCE and PCE to units of parts per billion (ppb). You can assume the air in the basement is at atmospheric pressure and a temperature of 22 C. Suppose that a man who lives in this residence has a home office in the basement. He typically spends 4 hr/d in his office, 6 days per week. The man s body mass is 75 kg, he breathes at a rate of 15 m 3 /d, and he is currently 41 years old. c. (20 pts) What is the risk of this man contracting cancer from breathing contaminated air in his home office? Consider a 10-year time frame, i.e., estimate/calculate the risk of his contracting cancer some time in the next 10 years. problem 5 continues p 8/9

9 5. continued d. (20 pts) Suppose you could seal off the basement so that the harmful vapors were no longer intruding. Then, you used a fan to blow clean air through the room. The rate of ventilation is 100 m 3 /hr (i.e., the rate is 10 times higher than under normal, no-fan conditions). How long will it take to reduce the TCE and PCE concentrations to 10% of the values you found in part (a)? Hint: set up a material balance and solve the resulting equation. The company that was responsible for the contamination has agreed to install forcedventilation systems in all the basements in this neighborhood. These systems will flush all the contaminated air out of the basements; the contaminated air is going to be treated by a biofilter. The biofilter will be constructed below land surface in a nearby field which the company purchased as part of the clean-up arrangement. The total air flow rate to be treated will be 1000 m 3 /hr. The maximum area in the field that can be excavated is 2500 m 2. e. (15 pts) If the biofilter has to remove at least 75% of the PCE and 90% of the TCE, how deep must the biofilter be? Which chemical controls the overall design? Why? Explain briefly. END OF EXAMINATION p 9/9

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

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,