What is the purpose of the return sludge step in the activated

1CHE 565PROBLEM SET 2Due on 3/4/2015Unless otherwise indicated all problems are from Chapter 8 of Manahan’s book.The use of internet and literature resources is assumed in answering any of thequestions. These would include such things as constants and conversion factors aswell as additional information needed to complete an answer. Please remember toproperly reference the sources of information employed.1.What is the purpose of the return sludge step in the activated sludge process?2.What are the two processes by which the activated sludge process removes solublecarbonaceous material from sewage?3.Discuss some of the advantages of physical–chemical treatment of sewage as opposed tobiological wastewater treatment. What are some disadvantages?4.How many liters of methanol would be required daily to remove the nitrogen from a200 000-L/day sewage treatment plant producing an effluent containing 50 mg/L ofnitrogen? Assume that the nitrogen has been converted to NO3- in the plant. Thedenitrifying reaction is Reaction 8.34.5.Assume that a waste contains 300 mg/L of biodegradable {CH2O} and is processedthrough a 200 000-L/day sewage treatment plant which converts 40% of the waste to CO2and H2O. Calculate the volume of air (at 25°C 1 atm) required for this conversion.Assume that the O2 is transferred to the water with 20% efficiency.Note: Typically air is bubbled to facilitate the biological oxidation of biomass.6.Explain what anaerobic digestion is (Chapter 7). Give the simplified reaction for theanaerobic fermentation of a hypothetical substance {CH2O}.7.If all of the {CH2O} in the plant described in question 5 could be converted to methaneby anaerobic digestion how many liters of methane (STP) could be produced daily?