Secondary Treatment (Activated Sludge, Trickling Filters, Ponds)
Activated sludge fundamentals
Mixed liquor flows to the secondary clarifier, where biomass settles and returns as return activated sludge (RAS); the excess is wasted (WAS). Control levers include MLSS/MLVSS, sludge age (MCRT/SRT), food-to-microorganism ratio (F/M), dissolved oxygen (1–3 mg/L in the aeration basin), and RAS rate. Low F/M and long sludge age favor nitrification; high F/M and short age favor BOD removal only and raise bulking risk.
Reading the process
SVI near 80–150 mL/g indicates good settling; above about 200 suggests filamentous bulking from low DO, low nutrients, septicity, or high F/M. Rising sludge in the clarifier is often denitrification releasing nitrogen bubbles, not a solids overload. Pinpoint floc suggests an over-oxidized sludge. Cloudy effluent with a thin blanket may mean short-circuiting or a toxic shock.
Trickling filters and ponds
Trickling filters are rated by hydraulic loading (gpd/ft²) and organic loading (lb BOD/day per 1,000 ft³ of media). Sloughing is normal; ponding means media clogging. Facultative ponds rely on an algae-bacteria symbiosis and are loaded in lb BOD per acre per day at the surface area, not per volume.
Traps
Confusing MCRT with detention time; using MLSS where MLVSS is asked; forgetting that WAS flow is what sets sludge age.
Worked example
A 30-minute settleability test gives 220 mL/L and MLSS is 2,200 mg/L. SVI = (220 × 1,000)/2,200 = 100 mL/g - good settling. At 250 mL/g, investigate DO and septicity before reaching for polymer.