Secondary (Biological) Treatment
Activated sludge
Mixed liquor is a managed ecosystem. Useful numbers: MLSS 2,000–4,000 mg/L, DO 1.5–2.5 mg/L in the aeration tank, F/M about 0.2–0.4 lb BOD per lb MLVSS per day for conventional systems, and SVI of 100–150 mL/g for good settling. MCRT (sludge age, SRT) is set by wasting and controls which organisms survive; nitrification requires a longer SRT than BOD removal, especially in cold water.
Fixed film and ponds
Trickling filters are rated by organic loading (lb BOD per 1,000 ft³ per day) and hydraulic loading; recirculation dilutes strength and keeps media wet. Ponding and odor indicate organic overload or clogged media; seasonal sloughing is normal. Lagoons may be facultative (aerobic surface, anaerobic bottom), aerated, or anaerobic, and are loaded in lb BOD per acre per day.
Nutrient removal
Nitrification is two-step (Nitrosomonas, then Nitrobacter) and consumes oxygen and alkalinity. Denitrification converts nitrate to nitrogen gas in an anoxic zone with readily degradable carbon. Biological phosphorus removal cycles PAOs through anaerobic (VFA uptake, phosphorus release) and aerobic (phosphorus uptake) zones; alum or ferric provides reliable chemical backup but adds solids.
Traps
Do not confuse SRT with hydraulic detention time. RAS rate changes solids inventory but wasting is what truly controls MCRT. Rising sludge in the clarifier often means denitrification, not poor settling.
Worked example
Aeration basin 0.5 MG at MLSS 2,500 mg/L holds 2,500 × 0.5 × 8.34 = 10,425 lb of solids. Wasting 0.02 MGD at 8,000 mg/L removes 8,000 × 0.02 × 8.34 = 1,334 lb/day. SRT ≈ 10,425 ÷ 1,334 ≈ 7.8 days.