Wastewater Treatment
Treatment train
Preliminary (screens, grit), primary settling, secondary biological treatment, disinfection, then solids handling. Activated sludge control variables are MLSS, F/M, MCRT (sludge age), dissolved oxygen, RAS rate, and sludge volume index.
Biology and control
F/M = food (BOD load) divided by microorganism mass. Low F/M means long SRT, nitrification, and stable sludge; high F/M means young sludge and poor settling. SVI around 80-150 mL/g settles well; above 200 suggests filamentous bulking. Nitrification needs D.O., alkalinity, and long SRT; denitrification needs an anoxic zone and carbon.
Worked example: SRT
MLSS 2500 mg/L, aeration volume 1.0 MG, WAS 5000 mg/L at 0.05 MGD, effluent solids neglected. Aeration solids = 2500 x 8.34 x 1.0 = 20,850 lb. WAS = 5000 x 8.34 x 0.05 = 2,085 lb/day. SRT = 20,850 / 2,085 = 10 days.
Traps
8.34 converts mg/L x MG to pounds; do not mix MGD with gallons. Clarifier symptoms: high sludge blanket and solids washout from overload; floating sludge from denitrification. Permit limits are NPDES values; effluent quality is measured, not assumed. Watch for the trick of asking for F/M when the given numbers are already a loading rate.
Solids
Thickening, digestion, dewatering. Know percent solids, volatile solids reduction, and why polymer dose is set by jar or bench test.