NAME GROUP CEE 330 Spring 2015 Exam 3 Closed book closed notes no cheat sheets nothing from class or book stored on calculators no laptop Question 1 Question 2 Question 3 Question 4 Question 5 Question 6 20 5 15 20 15 25 Question 1 25 points Town A wants to discharge wastewater into a river 6 km upstream of town B The state standard level for dissolved oxygen is 3 mg L Other data pertinent to this problem are given in the following table Parameter Town A discharge River 3 Flow m s 0 280 0 877 Ultimate BOD at 28 C mg L 6 44 7 00 DO mg L 1 00 6 00 1 Kd at 28 C day Not available 0 46 Kr at 28 C day 1 Not available 0 85 Velocity m s Not available 0 650 temperature 28 C 28 C DOsat at28 C 7 92 mg L 7 92 mg L a Will the dissolved oxygen level after mixing at town A be reduced below the state standard 5 points b Find the ultimate BOD after mixing 5 points c Find the deficit and DO at town B 15 points Solution a DO at point of discharge due to mixing QDO Qriver DOriver Qwaste DOwaste DO Qriver DOriver Qwaste DOwaste Q m3 mg m3 mg 0 280 1 0 877 6 s L s L m3 m3 0 280 0 877 s s No DO will be above state standard after mixing Grading rubric For the mass balance equation 2points 1 4 79mg L Right calculation 2points Right interpretation 1point b Ultimate BOD at point of discharge due to mixing QL0 Qriver L0river Qwaste L0waste L0 Qriver L0river Qwaste Lwaste 0 Q m3 mg m3 mg 0 280 6 44 0 877 7 s L s L m3 m3 0 280 0 877 s s 6 86mg L Grading rubric For the mass balance equation 2 5points Right calculation 2 5points c Travel time 6km 1000m km t 0 11day 0 65m s 24 3600s day Initial Deficit 7 92 4 79 3 13 mg L Deficit at Town B k L k t k t k t D d o e d e r Do e r kr kd 0 46 1 day 6 8645 mg L 0 85 0 46 1 day e 0 051 e 0 094 3 13 mg L e 0 094 3 17mg L DO 7 92 3 17 4 75mg L Grading rubric Right calculation for time 2points Initial deficit 2points Deficit at town B 4points DO at town B 2 points Question 2 5 points You work in a water quality lab A sample of wastewater has a kinetic rate constant of 0 1 day 1 The initial dissolved oxygen DO reading is 10 mg L The DO reading after 2 days without any additional oxygen being added is 6 mg L Find the ultimate BOD L0 Solution BOD t L0 Lt L0 1 e kt DO0 DOt 2points 10 6 L0 1 e 0 1t 4 2points L0 22 mg L 1points 2 Question 3 15 points Suspended solids in municipal wastewater are typically removed during settling in tanks called clarifiers Assume that the average wastewater flow of a municipal wastewater plants is 3 2 MGD 1 2 104 m3 day The total volume of a clarifier is 2 000 m3 with total surface area of 600m2 1 What is the hydraulic retention time in hrs and overflow rate aka critical settling velocity in m3 m2 day 2 What is the minimum size particle that would be completely removed by settling to the bottom of the chamber during passge through the chamber The density of particles is 2 65 g cm3 and viscosity of water is 0 01185gcm 1s 1 Solution 1 hydraulic retention time q V Q 2000 m3 1 2 104 m3 day 24 hr day 4 hr Overflow rate vA Q A 1 2 104 m3 day 600 m2 20 m3 m2 day Grading correct equation 4 point correct equation correct calculation 4 5 point correct equation correct calculation correct unit 5 point 2 The maximum time for a particle to settle to the bottom is the retention time Within this time the minimum velocity for a particle to settle down to the bottom is vc H q V Aq 2000 m3 600 m2 4 0 hr 1hr 3600s 0 000231 m s Apply Stoke s law to find out the minimum size g r p r d p2 vs vc 18m dp g m 18 0 01185 0 000231 18 m vc cm s s 0 0017 cm 17 m m g r p r m g g 9 8 2 2 65 3 1 3 s cm cm Check Stoke s law assumption Re d p vs 17 10 6 m 2 3 10 4 m s 2 g g 4 m 0 01185 1 3 10 cm s cm cm 2 0 003 1 Grading defining vs as vc in this problem 2 point correct equation 1 point correct calculation 1 point considering the stoke s law assumption with correct stoke s law equation 0 5 point correct Re number 0 5 point Question 4 25 points Chlorine is used for primary disinfection at a water treatment plant that treats 0 2 m3 s of reservoir water Giardia is the primary pathogen of concern and the inactivation rate constant for 3 this pathogen using chlorine is 0 5 min 1 The plant achieves 4 log removal of Giadia at pH 6 9 in a plug flow contact chamber using chlorine concentration of 2 mg L a Find the volume of this contact chamber 5 points b If a complete mixed chamber is used instead of this plug flow what would be the required volume 5 points c Now instead of one chamber 5 smaller complete mixed chambers with equal volumes are constructed and connected in series Write the mass balance equations for chambers 1 2 and 5 10 points d Find the volumes of all 5 chambers in part c that give 3 log removal of Giardia 5 points 4 Solution a Volume of PFR dN V 0 0 Vk N first order decay reaction dt dN k N dt N out V exp k t exp k N in Q Q N in 0 2m3 s V ln ln 10000 211m3 1 k N out 0 5min min 60s The answer is 221 m3 not 211 m3 b Volume of complete mixed Qin Qout Q Mass balance on pathogens 0 QN in QN k VN QN in QN k NV N N Q Q k V N in N in N Q Q N in 0 2m3 s 1 0 0001 V 24 000m3 1min N 0 5min 1 0 0001 k 60s N in The answer is 24 0x104 m3 not 24 000 m3 c Mass balance 5 Chamber 1 0 QN 0 QN1 Vk N1 and N1 QN 0 Q Vk Chamber 2 QN1 QN 2 Vk N 2 QN1 QN 0 N0 N0 Q 1 Q Vk Q Vk Q Vk 1 V …
View Full Document
Unlocking...