10. The suspended solids concentration is 220 mg/L in the plant influent; 4000 mg/L in the primary sludge, 15,000 mg/L in the secondary sludge and 3000 mg/L exiting the aeration

basin. The concentration of total dissolved solids in the plant influent is 300 mg/L and the concentration of total dissolved solids exiting the aeration basin is 3300 mg/L. The BOD5 is 150 mg/L measured after primary treatment and 15 mg/L exiting the plant. If the F/M ratio is 0.33 lb BODS/lb MLSS-day, estimate the hydraulic retention time of the aeration basins if the total plant flow is 5 million gallons per day.