p. 1/4 ENV4001:ENVIRONMENTAL SYSTEMS ENGINEERINGSpring 2011 University of South Florida Homework #8 Civil & Environmental Eng. Due: Monday, April 4 Prof. J. A. Cunningham Each homework assignment will be graded on a 100-point scale. Point values for this assignment are indicated for each question. Background information: Recall from Homework #6 that a wastewater treatment plant in the city of Nastyville operates with only primary treatment, and they can’t meet the requirements of the Clean Water Act, so now they have to add secondary treatment to their facility. You work for an engineering firm that has been hired to design and build an activated sludge system for secondary treatment at the wastewater treatment plant. To design the activated sludge system, you must specify three things: the volume of the aeration basin, the oxygen input into the aeration basin, and the sludge return (recycle) rate. You also have to find the solids retention timeand indicate how much waste sludgethey will have to handle as a result of your design. (Another engineer in your firm will design a sludge-handling system based on what you tell him/her.) NOTE: In 2011 we will skip the oxygen input.The design flow rate is 150 million gallons per day (6.6 m3/sec) of primary-settled wastewater. Under current operation, the BOD5of the primary-settled wastewater is usually no greater than 165 mg/L. Of that, 85 mg/L of BOD5come from soluble BOD5, and 80 mg/L of BOD5come from solids. (You might recognize the flow rate and the BOD5from homework #6.) Based on your findings in HW #6, the plant’s treated secondary effluent (which they will continue to discharge into the river) must contain no more than 10 mg/L of BOD5(total, including both soluble and suspended forms) and no more than 5 mg/L suspended solids. Based on the information above, we have the following schematic diagram. QI= 6.6 m3/secS0= 85 mg/Lfromprimarysettlingaeration basinV, X, S(QI + QR)X, SclarifierQESXE= 5 mg/Ltreated effluent to riverQWXWSwastesludgerecycle stream (sludge return)QRXRS
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