Wastewater from a primary treatment tank is to undergo secondary treatment in an activated sludge unit. The following data and operational parameters are specified: Parameter Value Description MLVSS 3000 mgVSS/L Biomass concentration in the aeration basin μmax 1 d-1 Maximum specific BOD degradation rate kd 0.08 d-1 Cell decay rate Ks 35 mg/L Half-saturation constant Y 0.5 gVSS/gBOD Theoretical yield So 300 mg/L Initial substrate concentration Qw 10 m3/d Flow rate of waste stream Xe 10 mg/L VSS concentration at the exit stream The wastewater flow rate is 1000 m3/d, and the BOD5 of the effluent suspended solids is 80% of the allowable concentration by EPA's secondary treatment standards (30 mg/L). The MLVSS/MLSS ratio is 0.7. (a) Determine the solid retention time (units: days)