The bonus question carries 5 marks, over and above the 30 marks for the regular questions.  Marks obtained by the student in the bonus question will be added to the total marks obtained in other questions.



1.      Give brief answers to the following questions with appropriate reasoning:


A.     Why is recycling necessary in activated sludge process                          (2)


B.     All other things remaining constant, if the BSRT in an activated sludge plant is

increased, what will happen to the effluent substrate concentration                      (2)


C.     Physically explain why BSRT is the inverse of specific growth rate (m)   (2)


D.    What are the differences between microbial physiology in high rate and an extended aeration activated sludge processes.                                            (2)


E.     Why a pure oxygen aeration system is often used for aerobic treatment of high strength waste                                                                                                     (2)





2.      1 MLD of 400mg/L Glucose solution (C6H12O6) is to be treated by a completely-mixed activated sludge process (ASP).  Hydraulic detention time in the aeration tank is 4 hours. The effluent from the process should have 20mg/L glucose.  Biological solids retention time (BSRT) of the ASP is 6 days.  Based on this information, calculate the oxygen requirement in the aeration tank in Kg/d.  Assume all necessary nutrients that are required for bio-mass growth are present in excess.  Also assume that BODu = 1.5 x BOD5. YT and Kd values for microbial degradation of glucose are 0.5 mg/mg and 0.05 /d respectively (calculated based on BOD5 values).      (10)





a)      The volume of an aeration tank in an activated sludge plant is 4700 m3.  The oxygen requirement for the process was calculated to be 5400 Kg/d.  It was decided that 50 kW surface aerators should be used for aeration purposes.  Manufacturers of such aerators specify that the oxygen transfer capacity of these aerators is 2.0 Kg O2/kW-h under standard conditions.  Based on this information, calculate the total power requirement for satisfying oxygen requirements in the aeration tank, and hence calculate the number of aerators required.  The operating temperature of the aeration tank is expected to be 30oC.  The steady state dissolved oxygen concentration in the aeration tank should be 1 mg/L.  Saturation concentration of oxygen in water at 20oC is 9.1 mg/L and at 30oC is 7.5 mg/L.                                                          (7)



Hint:    If a 1 kW aerator is used in the above aeration tank under standard conditions:


b)      If the steady state bio-mass concentration (X) in the aeration tank is 3000 mg/L, calculate the power requirement of keep the bio-mass in suspension, and hence the number of aerators required.  Compare results with those of 3(a) and comment on aerator requirements for the aeration tank.                                                       (3)




Star Question:


4.      Raw wastewater influent to the aeration tank of activated sludge processes often has inorganic suspended solids.  This material is completely inert, and is generally removed from wastewater during secondary settling following aeration, and ultimately expelled from the system during sludge wasting.  However, sludge recycling results in maintenance of a certain steady-state inorganic suspended solid concentration (C) in the aeration tank, which is much higher than the influent inorganic suspended solid concentration (Co). 


Based on the above information, derive the expression for steady-state inorganic suspended solids concentration (C) in the aeration tank of an ASP, and hence calculate C, given Co = 200 mg/L, q = 4 hours, and qc = 6 days.                       (5)