Prelim #2 key - M PART A C E: Cemplete 4 of the 5 following...

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Unformatted text preview: M PART A C E: Cemplete 4 of the 5 following problems each worth 20 points Problem A1. [19 points] Plug flow reactors can be approximated by a series of completely mixed reactors. A waste stream contains a degradabie substance with flow rate Q = 10 m [min and concentration Co = 40 mg/L The substance decays at a first order rate (R = 2 per hr.). If a plug flow reactor has volume V equal to 500 m3, determine the substance effluent concentration for this reactor. if the plug volume reactor is approximated by 2 completely mixed reactors in series each with volumes 250 or“ determine the effluent substance effluent concentration from the second reactor. Assume steaggzstate. CEE 3S| 0 Environmental Quality Engineering 0 Second Preliminary Exam 0 a Spring 20l0 Page 2 I . PART A Complete 4 of the 5 following problems each worth 20 points Problem A1. [19 points] As noted in class, our model. for the rate of consumption of BOD by bacteria is a simplification as — kxs . in the BOD model, (IL - _ I r — E;- = le , we have assumed cell concentration, X, [S constant, and that S<<Ks so that k] as E- (remember that L is Just 8 expressed in units of oxygen demand). Both of these kinetic 5 models assume that the food concentration is the only environmental parameter that influences the biodegradation. rate. However, at oxygen concentrations below 2 rag/L lack of 02 might slow the rate at which bacteria consume their food. A possible model for the rate of BOD use when the effect of low 02 concentration is considered is: - 111 2 [(11.1 02 . Here: 02 (Kslo2 + 02 dt = dissolved oxygen concentration in mg/L, and (1(5),;3o is the “half velocity” constant for oxygen . . 02 I (1.e., the 02 concentratlon when —«-———-—- = w ). (Ksloz +02 2 a) 0n the diagram on the following page show the STELLA model you would create to describe the disappearance of L in a BOD test bottle when the effect of O; is included as described above. [Hint, in addition to L, you will need a stock for 02. Remember that use of BOD uses 02, and that there is no reaeration in a stoppered 1301) test bottle] b) Write the equation used to model L g?” l ' $391.} c) Write the equation that would be used to model 02 Ace a CEE 35] I Environmental Quality Engineering - Second Preliminary Exam - 0 Spring 2010 Page 2 g} SIELLAG AI: .11" W CEE 35! 0 Environmental Quaiity Engineering - Second Preliminary Exam I 0 Spring 2010 Page 3 ‘ Problem A3. [19 points] - . An industry dumped ten “SO—gallon drums of TCE, which is equivalent to 3000 kg of TCE, into a river 1km upstream from the intake to a drinking-water treatment plant. The river has a cross- seotional area of 100 mg; a velocity of 1000 mfhr and a dispersion coefficient of 10 mzlsec. SinCe the atmosphere has virtually zero TCE, and it’s saturation concentration C* = zero, the mass transfer rate of TCE : 2,9sz (C‘ — C): — sz (he. the loss of to the atmosphere obeys first-order kinetics). dt 1-00-53 ‘3 i like 7’ ' Assume k2= 35 h‘1 . X Determine if the concentration. of TCE at the intake ever reaches concentrations above the MCL of TCE, .which according to EPA regulations, is 0.5ppb. 7015/19/11, 1 57 l0 “3/ L (a em “'4 C? Qfigelc WM 43% Per/v; .. - AW fl-I- 9:515on Ft 509§ var- (q I I a; u I ' nae-roe Mummmid—W t ‘ o 1090 anon aouu Time“) Problem A4. [ 19 points} An industry wishes to discharge 1000 m3/day of a wastewater containing dissolved organic matter to a river. The raw industry waste (before possible treatment) has a BODL concentration, Lw, of 200 mg/L and dissolved oxygen concentration, Cw, of 2 mg/L. The river characteristics before introduction of the waste are as follow: flow, QR == 10,000 m3/day; cross-sectional area, A = 10 1112; BODL concentration, Ln = lmg/L; and dissolved oxygen concentration, CR _= 9 mglL. The rate of BODL removal by biodegradation in the river is 0.] Slday, and the removal of BODL by sedimentation is negligible. The mass transfer rate for reaeration in the river is 0.2fday. The saturation concentration of dissolved oxygen at the stream temperature is 10 mg/L. The water intake for a town is located 4.4 km downstream from the industry. It is desired to haVe a dissolved oxygen concentration of at least 5 mg/L at this point. Show through a calculation whether or not the industry wilt need to treat its wastewater. QR = 10,000 m3/day LR = kl = 0.] Slday kg : 0.2/day A=10mz (3* =10mg/L Lw = 200 mg/L Qw = 1000 m3/day CW = 2 mg/L Desired minimum 02=SmglL " “’9 ,x=4.4 km. LO r ’lxlodooo {19430}! 1 (9'0? MJ/L @ H 000 __ 5 A. ‘— Co "v 61W") r We ‘2- r-éem/t. u» c 60 t t (at. UJOOO pp 2 (D'Erblo not}; ha 4-851, \Lr'zfgt r on: 2.19.] D = if; (e — e woe W? _. m LM F1, ‘OJS‘ 5:: a-D-a"-:~£}T§ H00 3'- OlS’X‘“: Ho's-r34— 6, "’ e/ ) k Oil/"CH5" $1.3" (0.57%? * owns) «FAA—12,6“! ..—I f «D -, r C O _ yes-Tgwrw CEE 35i 0 Environmental Quality Engineering 0 Second Preliminary Exam 0 0 Spring 2010 Page 6 1. torts-“4 “’3-b'j/tr < y'j/V wt, Problem A5. [ 19 points] An aquifer is being continuously recharged with treated domestic sewage to prevent depletion of the aquifer. A recharge well is located 1000 meters upstream from the withdrawal well in the aquifer. At steady-state operation (continuous input and withdrawal) the wells have a water elevation difference of 3 meters. Aquifer characteristics are: Porosity = 0.4, aquifer bulk density = 2.5 g/cm3, hydraulic conductivity = 100 m/day, cross-sectional area of aquifer (perpendicular to flow) = 100 m2. For some unknown reason cadmium concentration in the sewage discharge suddenly increased from zero to I rug/L and stayed at this level. Cadmimn is toxic but adsorbed in the aquifer (K: = 15 cm3/g) . Estimate the time (from when cadmium first appears in the recharge) it will take for cadmium to reach the withdrawal well. Neglect dispersion in the aquifer. . # CEE 351 0 Environmental Quality Engineering 0 Second Preliminary Exam 0 0. Spring 2010 Page 7 PART B - Choose one of the following problems Problem B1 [24 points] An industry in a city sited on a tidal estuary 5 km from the ocean has a spill of 50 kg of organic waste. If the waste decay rate is 0.2/day, the flow velocity past the city is 5 kin/day, the volumetric flow rate is 500,000 m3/day, and the salt concentration in the estuarine water next to the city is 1/50 of the ocean concentration; what will the waste concentration be (in mg/L) at the mouth of the estuary Ge, 5 km downstream from the city) one day after the spill occurs? You can assume that the dispersion caused by tidal action is significant, and that a PFR would be a bad choice of a model for the estuary. Note: 1 m3 = 1,000 L. WATCH UNITS! Salt Conc. = 1/50 ocean salt concentreation City I ’ “#51311 M: Ocean Q=500,000 m3/d $ U= 5 km/d Estuary I r Spill I = 50 kg 5 APR ad! Cami. "inpui‘ a? in tact E C 7' CD ‘6; 3—K /® modfifij r g ~ _ _ 5‘" 28 l' Cl— tame—a 5"— my Mg?) , ~ " 55a Lg: wanes-s “Ml/a CEE 351 0 Environmental Quality Engineering 0 Second Preliminary Exam 0 0 Spring 2010 Page 8 Problem B1. Additional Work Space (1‘ ' “’0 we imm- my ZAWE} “15+ W153 123 14:05; M _ Wfimlflwo “M JV 7”“ {QT—573mm “433 x“?- § km _ a , 2- A";- G/LA "5333: ='00m SD 105 _ [A \- 2 Y—fz ex fimw‘im 5mm" —~ MAM r? C Zuli ’1\‘x(b3°ws€5‘%*\o\ (115+ : Wmmdg Ka/m‘s 7" Di (M4517 W CEE 351 0 Environmental Quality Engineering 0 Second Preliminary Exam 0 0 Spring 2010 Page 9 Problem BZ. [24 points] Assume 10 kg of a pesticide is accidentally spilled into an incremental volume of 5 tn3 in the tributary to the town’s water supply, giving an initial concentration of 2 g/L. The spill occurs 10 km from the town’s reservoir, the tributary’s flow velocity is l km/day, and it’s cross sectional area is 5 m2. The pesticide obeys first order kinetics with a decay rate of 0.2/day. The maximum acceptable drinking water concentration of the pesticide is 10 mm. Assume the tributary can be modeled as a PFR and the reservoir can be modeled as a CSTR, with a volume of 1.5 x 104 In“. How long will the town have to use an alternative water source because the pesticide level exceeds the acceptable level? [You may assume the tributary is the only flow into the reservoir and the water level in the reservoir is constant. When the pesticide reaches the reservoir you may assume that it is added all at once (i.e, the time it takes a volume of 5 m3 to flow from the tributary into the reservoir is negligible).] 1 m3 = 1000 L. WATCH UNITS! L: 0-9‘ 'Xda o .. 00 L Co ' 5‘ 3/4: ’71:? ' 9.0393“; Reservoir ;u=lkmlda ,fit=5m2 Tributar PF R Qonl Pesticide 5pm Water for town Extra work space is provided on the following page. CEE 35] I Environmental Quality Engineering 0 Second Preliminary Exam 0 0 Spring 20!!) Page 10 TOW Mm 61‘ farm/APE: Wm+9f (fir 0&7A_ AWFLL, I ...
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Prelim #2 key - M PART A C E: Cemplete 4 of the 5 following...

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