NAME Group CEE 330 Spring 2014 Exam 3 Use PEN to write the answers for your exam If you use pencil you CANNOT request regrade Closed book closed notes no cheat sheets nothing from class or book stored on calculators List of relevant formulas and equations is given after the exam questions Question 1 Question 2 Question 3 Question 4 Question 5 Question 6 Question 7 25 0 10 30 20 10 5 Question 1 25 points A stream of flowing water at 20 25 C initially has an ultimate BOD in the mixing zone of 10 mg L The saturated oxygen concentration is 8 9 mg L and the initial dissolved concentration is 8 5 mg L The reaeration rate constant is 2 0 day the deoxygenation rate constant is 0 1 day and the velocity of the stream is 0 11 km min Calculate the concentration of dissolved oxygen in the flowing stream after 160 km To get full credits you should do the following 1 List the governing equation 2 List all known parameters 3 List unknown parameters and all steps of calculating them Solution 1 Oxygen deficit D is changing according to the following equation 7 points dD kd Lo e kd t kr D dt this equation has the following solution k L D d o e kd t e kr t Do e kr t kr kd k L DO DOs d o e kd t e kr t Do e kr t kr kd 2 Known parameters 6 points kd 0 1day 1 kr 2day 1 L0 10mg L DOs 8 9mg L DO0 8 5mg L 3 Unknown parameters 2 points D DO D0 Find D0 1 point D0 DOs DO0 8 9 8 5 0 4 mg L 1 Find the time it takes to travel 160 km 1 point t 160 km 0 11 km min 60 min h 24h day 1 day Find D after 1 day 5 points k L D d o e kd t e kr t Do e kr t 0 46mg L kr kd Find DO after 160 km 3 points D DOs DO DO 8 9 mg mg mg 0 46 8 4 L L L Question 2 5 points What is the critical point in a river receiving waste Describe the change in DO and its deficit when a river receives waste during the times before and after the critical point Solution The waste is mixed with river water resulting in DO deficit Initially i e near mixing point D will increase until the critical point at which the rate of reaeration of oxygen from the air is the same as the rate of DO consumption 4 points After the critical point the river is recovered with reducing D and increasing DO 1 point Question 3 10 points Name 3 main processes used in wastewater treatment Indicate what each process removes 6 points Name at least one wastewater treatment process that consumes energy 2 points Name at least one wastewater treatment process that produces energy 2 points Solution 1 Bar screen to remove large debris 5 mm primary clarifier to remove suspended solids activated sludge to remove BOD Anaerobic digester to process the sludge 2 Bar screen grid chamber activated sludge primary clarifier 3 Anaerobic digester Maybe activated sludge in future Question 4 30 points The graph here shows the disinfection plot of a pathogen with chlorine at a concentration of 5 mg L and pH 7 in a batch reactor 1 Find the Chick s law rate constant k Indicate units 5 points 2 Find the product Ct for 2 log inactivation Indicate the units for C and for t 5 points 3 To achieve a 2 log removal in a well mixed flow tank what volume of the reactor would be needed Assume steady state Use the k rate constant found in part 1 The flow rate is 6 m3 min 5 points 2 4 To achieve a 2 log removal in a plug flow reactor what volume of the reactor would be needed Use the k rate constant found in part 1 and the same flow rate as part 3 5 points 5 Compare the reactor volume found in part 3 with the volume found in part 4 Which volume is smaller Explain why 5 points Log N N0 0 0 5 y 0 0408x R2 0 9875 1 1 5 2 2 5 0 0 20 0 40 0 60 0 80 0 TIme min Solution 1 k 0 0408 min 1 2 3 0 09 min 1 missing unit take off 1 point Note that the slope need to be determined using a ln plot In this case need to convert from log to ln by multiply by 2 3 Only take out 2 points if forget to do the conversion 2 Ct 5 mg L 50 min 250 mg min L missing unit take off 1 point Time is around 50 min This does not have to be exact 50 min 3 Qin Qout Q Mass balance on pathogens correct mass balance 3 points N 0 number concentration of pathogens in the inlet N number concentration of pathogens in the outlet or in the tank well mixed condition IN QN 0 OUT QN DECAY k NV QN 0 QN k NV N Q 1 3 2 N 0 6 m min 1 10 V 6 600 m3 1 2 0 09 min 10 N k N0 4 Mass balance on pathogens dN V 0 0 Vk N first order kinetics reaction dt N dN V k N out exp k t exp k dt N in Q N Q ln in N out 6 m3 min ln100 N in V ln 307 m3 k V 1 Q k 0 09 min N out 3 5 Volume of PFR is smaller because the reaction in PFR occurs at the initial concentration due to the lack of mixing Question 5 20 points A wastewater treatment plant receives a flow of 35 000 m3 day 1 Calculate the minimum required volume m3 for a 3 m deep horizontal flow grit chamber that will remove particles with a specific gravity 1 9 and diameter 0 2 mm Viscosity of the water 1 002 10 3 kg m s Specific gravity is the ratio between the density of a particle and density of water which is 998 kg m3 Assume the condition for Stokes law is satisfied 10 points 2 The chamber designed with the volume found in part 1 is subjected to wastewater containing particles with diameter 0 02 mm The particle density is the same as part 1 Will this chamber remove these particles Explain your answer If not what is your recommendation 10 points Solution 1 The basic design equation is V hQ vs Calculate the particle settling velocity vs using Stokes Law vs g p w d 2 18 9 81m s 1 9 1 0 998kg m 0 2 10 18 1 002 10 kg m s 2 vs 3 3 m 2 3 vs 0 02 m s or 1 728 m day Correct equation …