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ECIV 350 Test Problems Questions with Complete
Solutions
(Lecture 3-5) A water contains two nitrogen species. The concentration of NH3 is 30 mg/L NH3, and the concentration of NO3 - is 5 mg/L NO
-. What is the total nitrogen concentration in units of mg N/L? Correct Answer-25.8 mg N/L (Lecture 3-5) What pH is required to reduce a high concentration of dissolved Mg2+ to 43 mg/L? Ksp for the following reaction is 10-11.16. Mg(OH)2(s) Mg2+ + 2OH Ksp = 10-11.16 Correct Answer-Ph=9. (Lecture 3-5) How long will it take the carbon monoxide (CO) concentration in a room to decrease by 99% after the source of CO is removed and the windows are opened? Assume the first-order rate constant for CO removal (due to dilution by incoming clean air) is 1.2/hr. No chemical reaction is occurring. Correct Answer-2. (Hw#1) 3-1 from Textbook. How many grams of NaCl would you need to add to a 1 L water sample (pH = 7) so the ionic strength equaled 0.1M? Correct Answer-5.8g What percentage of total ammonia (i.e., NH3 + NH4 + ) is present as NH3 at pH 7? pKa for NH4 = 9.3. NH4 + NH3 + H+ Correct Answer- 05%
(Hw#2) 4-6 from Textbook. Poorly treated municipal wastewater is discharged to a stream. The river flow rate upstream of the discharge point is Qu/s = 8.7 m3 /s. The discharge occurs at a flow of Qd = 0. m3 /s and has a BOD concentration of 50.0 mg/L. Assuming that the upstream BOD concentration is negligible. (a) What is the BOD concentration just downstream of the discharge point? (b) If the stream has a cross-sectional area of 10 m2 , what would the BOD concentration be 50 km downstream? (BOD is removed with a first-order decay rate constant equal to 0.20/day.) Correct Answer-a) Cd=4.7 mg/L V=0.96 m/s θ=0.6 days b)Cl=4.20mg/L (Hw#2) 4-10 from Textbook. In the winter, a stream flows at 10 m3 /s and receives discharge from a pipe that contains road runoff. The pipe has a flow of 5 m3 /s. The stream's chloride concentration just upstream of the pipe's discharge is 12 mg/L, and the runoff pipe's discharge has a chloride concentration of 40 mg/L. Chloride is a conservative substance. (a) Does wintertime salt usage on the road elevate the downstream chloride concentration above 20 mg/L? (b) What is the maximum daily mass of chloride (metric tons/day) that can be discharged through the road runoff pipe without exceeding the water quality standard? Correct Answer-a) 21.33 mg/L b) 16.0 tons/day
treated at a rate of 29 m3 /min. The volume of the tank is 580 m3. What is the rate constant? Correct Answer-k=0.2min^- An owl eats frogs as a delicacy, and his intake of frogs is directly dependent on how many frogs are available. There are 200 frogs in the pond, and the rate constant is 0.1/day. How many frogs are left at the end of 10 days? Correct Answer-73 frogs remain (Lecture 6-7) (4.2 from textbook) A pipe from a municipal wastewater treatment plant discharges 1.0 m3/s of poorly treated effluent containing 5.0 mg/L of phosphorus compounds (reported as mg P/L) into a river with an upstream flow rate of 25 m3/s and a background phosphorus concentration of 0.010 mg P/L. What is the resulting concentration of phosphorus (in mg/L) in the river just downstream of the plant flow? Correct Answer-0.20mg/L (Lecture 6-7) (4.3 from Textbook) The completely mixed flow reactor (CMFR) shown in Figure 4.1 is used to treat an industrial wastewater, using a reaction that destroys the pollutant according to first-order kinetics, with k = 0.216/day. The reactor volume is 500 m3, the volumetric flow rate of the single inlet and exit is 50 m3/day, and the inlet pollutant concentration is 100 mg/L. What is the outlet concentration after treatment?1. Determine the volume required for a PFR to obtain the same degree of pollutant reduction as the CMFR in Example 4.3. Assume that the flow rate and first-order decay rate constant are unchanged (Q = 50 m3/day, and k = 0.216/day). Calculate the retention times in the CMFR of Example 4.3 and the PFR of Example 4.6. Correct Answer-a)32mg/L
b)264m^ c) 10 days 5.3 days
- A 15 mL wastewater sample is placed in a standard 300 mL BOD bottle, and the bottle is filled with dilution water. The bottle had an initial dissolved-oxygen concentration of 8 mg/L and a final dissolved- oxygen concentration of 2 mg/L. A blank (a BOD bottle filled with dilution water) run in parallel showed no change in dissolved oxygen over the 5-day incubation period. The BOD reaction rate coefficient for the water is 0.4/day. Calculate the 5- day (y5) and ultimate (Lo) BOD of the wastewater. Correct Answer-120 mg O2/L 138 mg O2/L
- A wastewater with a 5-day CBOD (y5) of 200 mg O2/L and a k of 0.1/day is discharged to a river at a rate of 1 m3/s. Calculate the ultimate CBOD (Lo) of the wastewater before discharge to the river. Assuming instantaneous mixing after discharge, calculate the ultimate CBOD of the river water after it has received the wastewater. The river has a flow rate (Q) equal to 9 m3/s and a background ultimate CBOD of 2mg O2/L upstream of the wastewater discharge. Also calculate the ultimate CBOD (Lo) and CBOD5 (y5) in the river 50 km downstream of the point of discharge. The river has a width (W) of 20 m and a depth (H) of 5 m. Correct Answer-5.78 days (Hw#3) 7-37 from Textbook. An industry discharges 0.5 m3 /s of a waste with a 5-day CBOD of 500 mg/L to a river with a flow of 2 m3 /s
(Exam 1) A wastewater with an ultimate CBOD of 508 mg O2/L and a k of 0.1/day is discharged to a river at a rate of 1 m3 /s. Assuming instantaneous mixing after discharge, the ultimate CBOD of the river water after it has received the wastewater is 52.6 mg O2/L. The river has a flow rate (Q) equal to 9 m3 /s and a background ultimate CBOD of 2 mg O2/L upstream of the wastewater discharge. Calculate the ultimate CBOD (Lo) and CBOD5 (y5) in the river 100 km downstream of the point of discharge. The river has a width (W) of 20 m and a depth (H) of 5 m. Correct Answer-16.5 mg/L O 6.49 mg/L O (Lectures 11-15) (Ref Ex 8.2 from Textbook) A conventional mechanical mixing stirred tank is used for rapid mixing in a water treatment plant with a flow rate of 100 x 106 L/day. The water temperature is 10oC. Determine the tank volume and power requirement (kW). Correct Answer-69m^ 73 kW (HW #4) 8-16 from Textbook. Calculate the settling velocity of a 100 μm diameter particle with a specific gravity of 2.4 in 10o C water. Correct Answer-21 m/h .45< (HW #4) A 10 x 106 L/d raw water source is to be softened to reduce the hardness. The mineral analysis of the raw water is given below. The
average raw water temperature and pH were found to be 10 o C and pH 7.0, respectively. Using the given information, determine the total, carbonate, and noncarbonate hardness present in the raw water Correct Answer-TH: 212. CH: 195 mg/L as CaCO NCH: 17.5 mg/L as CaCO (Lecture 16-18) A municipal wastewater treatment plant treats an average flow of 12,000 m3/day and a peak hourly flow of 30, m3/day. Two circular clarifiers are to be designed, using a depth of 4 meters and overflow rate of 40 m3/m2-day. Calculate the area, diameter, volume, and detention time required for each clarifier. Correct Answer- 919m^ D: 13.8 m DTime: 0.99h (Lectures 16-18) (Ref Ex 9.4 from Textbook) Given the following information, determine the design volume of the aeration basin and the aeration period of the wastewater for an activated-sludge treatment process: population = 150,000; flow rate is 33.75 X 106 L/day (equals 225 L/person-day); and influent BOD5 concentration is 444 mg/L (note this is high-strength wastewater). Assume that the regulatory agency enforces an effluent standard of BOD5 = 20 mg/L and a suspended- solids standard of 20 mg/L in the treated wastewater. A wastewater sample is collected from the biological reactor and is found to contain of suspended-solids concentration of 4,300 mg/L. The concentration of suspended solids in the plant influent is 200 mg/L, and that which leaves the primary clarifier is 100 mg/L. The microorganisms in the activated-
b. 0.12 day=2.9h c. 4,515 Kg/day d. if Qw is increased the SRT will decrease e. 0.97 lb BOD/ lb MLVSS-day f. mean cell residence time equals sludge age equals solid retention time =4 days If the F/M ratio is 0.33 lb. BOD5/lb. MLSS-day, estimate the hydraulic retention time of the aeration basins. Assume that the BOD5 is 150 mg/L measured after primary treatment and the concentration of total suspended solids existing the aeration basin is 3,300 mg/L? Correct Answer-7.26/day 3.3h 9-12 from Textbook. Assume a plant flow of 12,000 m3 /day. Determine the actual detention time observed in the field of two circular settling tanks with depth 3.5 m that was designed to have an overflow rate not exceeding 60 m3 /m2 -day and a detention time of at least 2 hour. Correct Answer-42 m^3/m^2-day A bench scale plug flow thermal oxidizer incinerator is being evaluated to operate at a temperature of 225oC and have a gas residence time of 0.3 s. If the pollutant (diethyl peroxide) enters the incinerator at a flow rate of 12.1 L/s and it is desired to remove 99.995 percent of the pollutant, what should the length of the thermal oxidizer be? The first order rate constant for diethyl peroxide removal is 38.3/s at 225oC and
the inside diameter of the thermal oxidizer (which is shaped like a cylinder) is 8 cm. Correct Answer-a. 25,000ft b. 1592 bags c. 199 bags/compartments
