Wastewater Treatment and Collections
The Wastewater Treatment and Collections domain tests both sides of the system: getting sewage to the plant and treating it to permit limits. Expect questions on preliminary treatment, primary clarification, activated sludge process control (F/M, MCRT, SVI, dissolved oxygen, RAS/WAS), secondary clarification, disinfection, NPDES reporting, collection system hydraulics, lift stations, infiltration/inflow, and SSO prevention.
Process control basics
- Primary clarifiers remove settleable solids; typical BOD removal is 25–40% and TSS removal 50–70%.
- F/M = (Q × BOD × 8.34) / (V × MLSS × 8.34). Conventional activated sludge runs about 0.2–0.4 lb BOD per lb MLSS per day.
- MCRT (sludge age) = solids mass in aeration and clarifiers ÷ solids leaving in waste and effluent per day. Higher MCRT means lower F/M, more nitrification, and more aeration demand.
- SVI = 30-min settled volume (mL/L) × 1000 / MLSS (mg/L). Good settling is roughly 80–150 mL/g; above 200 mL/g suggests bulking.
- Collections: keep minimum velocity near 2 ft/s for self-cleansing; lift stations need wet-well level control and emergency response plans.
Worked example: SVI
A 30-minute settleometer reading is 240 mL/L and MLSS is 2,400 mg/L. SVI = 240 × 1000 / 2400 = 100 mL/g. That is in the good-settling range, so no bulking control action is needed. If the reading were 600 mL/L at the same MLSS, SVI = 600 × 1000 / 2400 = 250 mL/g, indicating filamentous bulking; check dissolved oxygen, F/M, nutrients, and return rates.
Traps
- Mixing units in F/M: use lb/day BOD and lb MLSS, not mg/L alone.
- Using the settleometer volume as a percentage without converting to mL/L.
- Assuming SVI alone diagnoses bulking; low DO, nutrient deficiency, and septic influent also cause poor settling.
- Confusing MCRT with hydraulic detention time.
- Letting collection velocity fall below 2 ft/s, which deposits solids; excessive slope can generate hydrogen sulfide and corrosion.