Wastewater: Activated Sludge & BOD Removal

Quick Answer: Aerate for BOD uptake → clarify → return sludge. Control with F/M and MCRT. Nitrification needs longer SRT and adequate DO.

Wastewater engineering is the largest single FE Environmental block (~14%). Activated sludge is the workhorse secondary treatment process worldwide.

Process Flow

1. Primary treatment — screens, grit, primary clarifiers (optional TSS/BOD removal ~30–40%).
2. Aeration basin — mixed liquor suspended solids (MLSS) oxidize BOD.
3. Secondary clarifier — biomass settles; RAS returned.
4. Disinfection — optional tertiary.

BOD Removal Kinetics

Monod applies to heterotrophs:

[ \mu = \mu_{max}\frac{S}{K_s + S} ]

Engineering uses F/M and θ_c (SRT) operationally rather than solving Monod on every exam.

F/M Ratio

[ F/M = \frac{Q \cdot \text{BOD}_{in}}{V \cdot \text{MLVSS}} ]

Typical 0.2–0.5 lb BOD/(lb MLVSS·day) conventional plug flow. High F/M → fast growth, may not settle; low F/M → endogenous respiration.

MCRT / SRT

[ \theta_c = \frac{V \cdot X}{Q_w X_w + Q_e X_e} ]

Nitrification often needs θ_c > 4–5 days at 20°C (temperature dependent). BOD removal may succeed at shorter SRT (~1–3 days).

Oxygen Requirements

Carbonaceous stage: ~1.0–1.5 lb O₂ per lb BOD removed (include nitrification add-on ~4.6 lb O₂/lb NH₃-N oxidized — use handbook/stem).

Worked example: BOD load 5000 lb/day; 90% removal → ~4500 lb BOD destroyed → ~5000–7500 lb O₂/day with transfer efficiency accounted in blower sizing (problem may simplify).

Aeration Transfer

Standard oxygen transfer rate SOTR corrected for field conditions: altitude, temperature, α (basin type), β (DO saturation). AOR = SOTR × θ_factors.

Clarifier Coupling

Solids loading rate SLR = Q_in × MLSS / Area. Exceeding SLR → solids washout.

SVI = settled volume per gram MLSS — bulking at high SVI (>150 problematic).

Process Variants

Nitrification-Denitrification

Nitrification in aerobic zone; denitrification recirculate nitrate to anoxic zone with carbon source.

Effluent Limits

BOD₅ and TSS often 10–30 mg/L secondary limits (varies). Tertiary filtration polishes for reuse or strict permits.

Troubleshooting Table

FE Problem Pattern

Given V, Q, BOD, MLVSS, WAS flow → compute F/M or θ_c → judge nitrification feasibility or whether change is needed.

Exam trap: Using MLSS when formula specifies MLVSS (volatile fraction ≈ active biomass).

Exam trap: Forgetting RAS recirculation does not count as WAS — SRT uses waste flows.

Activated sludge links microbiology, stoichiometry, and clarifier hydraulics — the defining wastewater topic for Environmental FE candidates.

Oxygen Transfer and Blower Sizing

Field oxygen transfer = SOTR × α × β × (Cs∞ − CL) × θ^(T-20). Blowers sized on peak organic load and nitrification demand, not average alone.

Secondary Treatment Standards

Secondary treatment regulatory definition (~85% BOD/TSS removal) — basis for NPDES technology-based limits before water-quality-based limits.

Nutrient Removal Overview

BNR (biological nutrient removal) adds anoxic and anaerobic zones for denitrification and EBPR — know zone sequence, not full BioWin modeling.

WAS and SRT Control

Increasing WAS lowers MLSS, shortens SRT, risks nitrifier washout — operators balance clarifier solids loading with effluent quality.

Pure Oxygen Activated Sludge

High-purity oxygen systems reduce basin volume and foaming — covered basins with oxygen dissolving turbines; know as alternative to air aeration conceptually.

Membrane Bioreactor Preview

MBR combines biological treatment and membrane filtration — eliminates secondary clarifier footprint; higher energy; see advanced treatment in related topics. On the FE Environmental exam, confirm units before substituting into handbook equations; distractors often differ only by conversion factors. Practice locating handbook sections by keyword during timed drills so lookup takes under one minute. When a stem gives excess data, identify the governing regulation or equation first, then ignore unrelated values. Document assumptions on scratch paper: stability class, waste classification, exposure pathway, or discount rate as applicable. Cross-check magnitude: TCLP thresholds in milligrams per liter, emissions in pounds per hour, and cancer risk slopes in consistent units. NCEES items may supply all required constants in the stem; do not assume every value appears in the FE Reference Handbook. Flag uncertain items and return after a first pass; steady pacing across twelve knowledge areas improves pass probability. Distinguish whether the question tests federal acronyms (RCRA, CAA, EPCRA) or quantitative engineering before selecting an approach.

F/M and SRT Numeric Example

Aeration volume V = 1.5 MG, Q = 5 MGD, BOD_in = 200 mg/L, MLVSS = 2500 mg/L (MLSS 3000, volatile fraction 0.83).

[ F/M = \frac{5 \times 8.34 \times 200}{1.5 \times 8.34 \times 2500} = \frac{8340}{31{,}275} = 0.27\text{ lb BOD/(lb MLVSS·day)} ]

Conventional range — stable settling expected if DO adequate.

WAS = 0.05 MGD at 8000 mg/L MLSS, effluent TSS = 10 mg/L:

[ \theta_c = \frac{1.5 \times 3000}{0.05 \times 8000 + 5 \times 0.01} = \frac{4500}{400 + 0.05} \approx 11.2\text{ days} ]

Supports nitrification at 20°C.

Oxygen Demand Split

5000 lb/day BOD removed carbonaceous at 1.1 lb O₂/lb BOD = 5500 lb O₂/day. Plus 400 lb/day NH₃-N nitrified at 4.6 lb O₂/lb N = 1840 lb O₂/day. Total ≈ 7340 lb O₂/day before transfer efficiency correction.