Example of environmental final exam

College of Engineering Civil and Environmental Engineering Department CIVL375: Water and Wastewater Technology Example Final Exam (9 pts) Select the most appropriate answer Part A through D are related to the table below which shows C•t (mg l-1 min) values for 1.0-Log (90%) inactivation of a certain microorganism Disinfectant Water Temperature 0.5 °C Free chlorine Performed chloramine Chloride dioxide Ozone 70 1300 21 0.97 °C 10 °C 15 °C 20 °C 50 740 8.7 0.63 37 620 7.7 0.48 25 500 6.3 0.32 18 370 5.0 0.24 A The strongest disinfectant among the four listed in the above table is (i) Free chlorine (ii) Performed chloramines (iii) chloride dioxide (iv) Ozone B The value of C•t (mgl-1 min) for free chlorine for 99% inactivation at 20 °C is about (i) (ii) 36 (iii) 24 (iv) 13.5 C The value of C•t (mgl-1 min) for free chlorine for 90% inactivation at 25 °C is about (i) 12.5 (ii) 25 (iii) 50 (iv) 13.5 D A water at 20 °C is to be disinfected using ozone with 90% inactivation For a contact time of 10 min, the applied ozone dose (mg/l) should be (i) 0.024 (ii) 0.24 (iii) 240 (iv) 2400 E One of the disadvantages of using free chlorine for disinfection is (i) it forms THMs (ii) it forms chlorate (iii) it is cheap (iv) it leaves residual F One of the following sentences is correct about lime/soda-ash softening process (i) Magnesium precipitates as MgCO3 (ii) After treatment the water will have a low pH value (iii) Soda ash will be needed when there is not enough alkalinity in the water (iv) The chemical formula of soda ash is Ca(OH) G As the water turbidity increases, one of the following will increase (i) suspended solids (ii) TDS (iii) hardness (iv) alkalinity H In the electrodialysis method for removal of TDS, as the flow rate increases while other conditions remain the same, the removal efficiency (i) increases (ii) decreases (iii) remains the same I One of the followings is not a common coagulant (i) FeCl3 (ii) FeSO4 (iii) Al2Cl3 (iv) Al2(SO4)3 J Reject brine from a distillation unit has 3% dissolved solids Thus, the concentration of dissolved solids in the brine in mg/l is (i) (ii) 300 (iii) 3000 (iv) 30000 K Applied chlorine dose versus chlorine residual of drinking water is shown below The applied chlorine dose (mg/l) needed to have a free available chlorine of 0.4 (mg/l) is (i) 0.4 (ii) 0.5 (iii) 0.9 (iv) 1.4 L From the previous figure, the chlorine demand at an applied chlorine dose of 1.6 mg/l is about (i) 1.6 mg/l (ii) 0.6 mg/l (iii) 1.0 mg/l (iv) 2.2 mg/l (4 pts) What is the minimum diameter of a cylindrical CSTR needed to remove 95% of a contaminant entering the reactor at a flow rate of 20 m3/hr? The reaction rate constant is 9.0/day and the reactor depth should not exceed 4.0 m (4 pts) A reverse osmosis unit (schematically shown below) is to desalinate 100,000 gal of brackish water per day The membrane mass transfer coefficient is 0.035 gal , the pressure difference between the feed and the product water is 350 psi, and ft d psi the osmotic pressure difference between the feed and the product water is 45 psi Feed: Qf=100,000 gal/d Cf= 10,000 mg/l Product: Qp=70,000 gal/d Cp= 250 mg/l Brine A What is the required area (ft2) of the membrane? B What is the concentration (mg/l) of dissolved solids in the brine? (5 pts) Results of water analysis are presented in the following meq/l bar graph 2.2 3.7 4.5 5.2 + ++ ++ + Na Ca Mg K CO2 HCO30.36 Cl- SO4-3.2 4.7 5.2 Answer the following questions a The total hardness of the water in mg/l as CaCO3 is b The alkalinity of the water in mg/l as CaCO3 is Lime and soda-ash treatment is selected to reduce hardness to practical limits Answer the following questions: c The amount of lime (mg/l) needed is d The amount of soda ash (mg/l) needed is e The concentration of Na (mg/l) in the treated water is (4 pts) In a sorption isotherm experiment, 1.5 g activated carbon is mixed with 50 ml methylene blue solution with a concentration of 70 mg/l After equilibrium, the concentration of methylene blue in the solution was found to be 34 mg/l Assuming no adsorption of methylene blue on the container walls and that sorption is linear What is the value and units of the equilibrium sorption distribution coefficient (K)? (4 pts) A single-stage trickling filter receives wastewater with a BOD of 220 mg/l The filter is 180-ft in diameter and 7-ft-deep The design flow rate is 2.0 mgd Calculate the effluent BOD if the wastewater temperature is 25 oC Recirculated flow= 0.5 mgd Q=2.0 mgd BOD= 220 mg/l Q=2.0 mgd BOD= ? mg/l (5 pts) The schematic diagram below shows part of a wastewater treatment plant The primary settling tank removes 30% BOD and 60% suspended solids (SS) while the biological treatment removes 90% BOD and 90% SS The characteristics of the wastewater are indicated at point A •A Q = 400 m3/d BOD5=350 mg/l SS=250 mg/l Primary settling tank •B Aeration tank V=215m3 •C Secondary •D settling tank R =30% •E SS=10,000 mg/l Waste sludge= 7m3/d A The primary settling tank is usually designed assuming (i) Type I settling (ii) Type II settling (iii) Type III settling (iv) Type IV settling B The suspended solids (mg/l) entering the aeration tank (point B) is approximately (i) 100 (ii) 2385 (iii) 2510 (iv) 10100 C The expected BOD (mg/l) at point D (i) 18-25 (ii) 28-35 (iii) zero (iv) above 35 D What is the food-to-microorganisms ratio inside the aeration tank? E What is the sludge age?