Secondary Treatment (Activated Sludge and Fixed-Film Systems)
Activated sludge fundamentals
MLSS is the total suspended solids in the aeration tank; MLVSS (roughly 70–80% of MLSS) approximates the living biomass. F/M ratio (lb BOD5 per day per lb MLVSS) describes organic loading per unit biomass; MCRT (sludge age) describes how long solids stay in the system. Typical conventional values: F/M 0.2–0.4, MCRT 5–15 days, DO 1.5–3 mg/L. Low F/M with long MCRT gives old sludge - good nitrification but often pin floc; high F/M gives young, dispersed sludge that settles poorly.
MCRT = lb MLSS in the aeration system ÷ lb solids leaving per day (WAS + effluent solids). SVI = 30-minute settled sludge volume (mL/L) × 1,000 ÷ MLSS (mg/L); 100–150 is good, above 150 indicates bulking, below about 80 suggests dense or pin floc.
Common upsets
Filamentous bulking follows low DO, low F/M, nutrient deficiency, septicity, or high sulfide; correct by raising DO, adding nutrients, or using selectors, chlorine, or peroxide. Rising sludge comes from denitrification releasing nitrogen bubbles in the clarifier - increase RAS rate, lower the blanket, or shorten MCRT. Straggler floc and dispersed growth point to young sludge or toxicity.
Fixed-film basics
Trickling filters, RBCs, MBBRs, and IFAS rely on attached growth. Trickling filters stratify by depth; sloughing is normal, and ponding signals organic overload, media clogging, or inadequate ventilation. Recirculation dilutes loading and maintains wetting.
Rule of thumb
Shorter MCRT equals younger sludge equals poorer settling; longer MCRT equals older sludge equals better nitrification but risk of pin floc and rising sludge.