Wastewater Treatment
Core concepts
Secondary treatment is biological. In activated sludge, influent BOD loads the biomass, and you control the process with F/M, MCRT (SRT), dissolved oxygen, and MLSS. F/M = pounds of BOD applied per day ÷ pounds of MLSS under aeration. SRT = pounds of MLSS in the system ÷ pounds of solids leaving per day (wasted plus effluent). They move in opposite directions: waste more and SRT drops while F/M rises.
Know BOD versus CBOD, TSS versus MLSS, SVI as a settleability index (above roughly 150 mL/g suggests bulking), nitrification (needs DO near 2 mg/L, adequate alkalinity, and longer SRT), and denitrification. Know NPDES permit limits, the 85% removal expectation for conventional secondary plants, and effluent disinfection. Also know preliminary and primary units, clarifier hydraulics, return activated sludge rate, and flow equalization.
Traps Questions Love
More MLSS is not automatically better - it can overload the clarifiers. High SVI means poor settling, not good. Watch units in every formula, and remember 8.34 lb/gal converts mg/L × MGD into lb/day.
Worked Example
Flow 0.5 MGD, influent BOD 240 mg/L: load = 0.5 × 240 × 8.34 ≈ 1,000 lb/day. An aeration basin of 0.3 MG at 2,500 mg/L MLSS holds 0.3 × 2,500 × 8.34 ≈ 6,255 lb. F/M = 1,000 ÷ 6,255 ≈ 0.16 lb BOD per lb MLSS-day - slightly low but within the conventional range.