12.4 A wastewater treatment plant consists of primary treatment fol- lowed by an activated-sludge secondary system. The plant processes 7000 m³/d of wastewater with 180 mg/L of BOD, and 150 mg/L of suspended solids. The primary sludge contains 500 kg of dry solids per day with 4.5% solids content. The plant produces 760 kg of total sludge (primary and secondary) per day. Assume 30% removal effi- ciency of the primary clarifier and F/M ratio of 0.2 for the aeration tank. Determine the following: a. Primary sludge volume and removal efficiency of primary clarifier b. Influent and effluent BOD, of the secondary activated-sludge system (#8) c. Volume of secondary sludge with 1% solids content 12/5 Why it is necessary to thicken the primary and secondary skudge sludge before further processing Give examples of different sludge chick- ening processes. Describe the most common methods available for volume reduction of sludge 12.6 In problem 12.4, the primary and secondary sludges are mixed in a gravity thickener and sent for further treatment. The thickened sludge contains 4% solids. Calculate the percent volume reduction in the gravity thickener. 12.7 What are the objectives of sludge stabilization? List the common methods used to stabilize the sludge before disposal. 12.8 Briefly describe the advantages and disadvantages of anaerobic digestion. What factors should be considered for the design of an anaerobic digester? 2.9 The thickened sludge from problem 12.6 is to be digested in a stan- dard-rate mesophilic anaerobic digester (MAD). The sludge has 68% organic content, and approximately 60% of the organic fraction is digested after a 30-d period. The digested sludge has a solids content of 6% and is stored for a period of 90 d. Calculate the volume of the standard-rate digester. (#10) use 12.6 and 12.4 12.10 Rework problem 120 fr