Secondary Treatment (Suspended Growth and Attached Growth)
Core concepts
Suspended-growth (activated sludge) biology converts soluble BOD into floc that settles. Master these control parameters: mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS, the organic fraction, typically 70–80% of MLSS), food-to-microorganism ratio (F/M), mean cell residence time/solids retention time (MCRT/SRT), sludge volume index (SVI), and dissolved oxygen. Typical F/M for conventional aeration is about 0.2–0.4 lb BOD/lb MLSS·day; SVI of 100–150 mL/g indicates good settling, above ~200 suggests filamentous bulking.
Know the process configurations: conventional plug flow, complete mix, extended aeration (low F/M, long SRT, high DO demand, low excess sludge), oxidation ditch, sequencing batch reactors, and membrane bioreactors. Understand return activated sludge (RAS) and waste activated sludge (WAS) - RAS maintains MLSS, WAS controls MCRT.
Attached growth: trickling filters (media, hydraulic loading, organic loading, ponding, filter flies, psychoda), rotating biological contactors, and moving bed biofilm reactors. Nitrification needs longer SRT, higher DO (2+ mg/L), and warmer temperatures; it consumes alkalinity and drops pH.
Traps
Bulking questions often tempt "increase RAS" when the real fix is reducing filamentous growth (lower SRT adjustment, selector, or nutrient correction). Don't confuse high SVI (poor settling) with high MLSS. Also watch DO probes reading low because of probe fouling rather than true oxygen demand.
Worked example / rule of thumb
Aeration basin: 0.5 MG, MLSS 2,500 mg/L. MLSS mass = 0.5 MG × 8.34 × 2,500 ≈ 10,425 lb. If influent BOD load is 3,000 lb/day, F/M = 3,000 ÷ 10,425 ≈ 0.29 - within conventional range. Rule of thumb: if SVI climbs and effluent turbidity rises, suspect filaments before adding polymer.