Wastewater from a primary treatment tank is to undergo secondary treatment in an activated sludge unit. the following data and operational parameters are specified: hydraulic detention time= 6h mean cell residence time= 7days bod content at inlet of reactor = 150 mg/l food to microorganism ratio=0.3gm bod/gm vss-day two identical units are designed to operate in parallel and to process a combined wastewater flowrate of 0.7m3/s.

(a) how large must each reactor be (m3)?
(b) what is the rate at which cell mass must be discharged from the units in steady state operation (kg/day)?

What difference(s) did you notice using a pneumatic circuit over hydraulic circuit. explain why the pneumatic piston stumbles when it hits an obstacle.

Air is to be cooled in the evaporator section of a refrigerator by passing it over a bank of 0.8-cm-outer-diameter and 0.4-m-long tubes inside which the refrigerant is evaporating at -20°c. air approaches the tube bank in the normal direction at 0°c and 1 atm with a mean velocity of 4 m/s. the tubes are arranged in-line with longitudinal and transverse pitches of sl- st 1.5 cm. there are 30 rows in the flow direction with 15 tubes in each row. determine (a) the refrigeration capacity of this system and (b) pressure drop across the tube bank. evaluate the air properties at an assumed mean temperature of -5°c and 1 atm. is this a good assumption?

Calculate the bore of a cylinder that has a stroke of 18 inches and an extension time of 6 seconds at a flow rate of 4 gal/min.