Calculating Oxygen & Air Requirements from BOD and COD in Wastewater

 Calculating Oxygen & Air Requirements from BOD and COD in Wastewater

In practical wastewater treatment, BOD and COD aren't just water quality parameters — they are design drivers for aeration, microbial loading, and treatment process selection. Here’s how we apply them: 1. Oxygen Requirement from BOD: O₂ required (kg/day) = Q × BOD × f / 1000 Where: Q = Flow rate (m³/day) BOD = Biochemical Oxygen Demand (mg/L) f = Oxygen equivalent factor (typically 1.0 for complete oxidation) Example: Q = 500 m³/day, BOD = 300 mg/L → O₂ = (500 × 300) / 1000 = 150 kg/day 2. Air Requirement Calculation: Air required (m³/hr) = (O₂ required × 100) / (O₂ transfer efficiency × O₂ % in air × 1.429) Typical values: O₂ transfer efficiency (E): 15–35% O₂ % in air: 23% 1.429 kg/m³ = Density of air at STP This guides blower sizing and diffuser selection. 3. Diffuser Type Selection: Diffuser Type Bubble Size Efficiency Coarse Bubble 4–6 mm 10–15% Fine Bubble (Disc) 1–3 mm 25–35% Membrane Tube 1–2 mm 25–30% Fine bubble diffusers are preferred for BOD removal due to better O₂ transfer. 4. Estimating Biodegradability from BOD/COD Ratio: BOD/COD > 0.5 → Easily biodegradable BOD/COD 0.3–0.5 → Moderate biodegradability BOD/COD < 0.3 → Poor biodegradability (requires chemical or AOP treatment) 5. Biomass Yield (Y) and Microbial Growth: Y = (VSS produced) / (BOD removed) Used in designing aeration tanks and calculating sludge production. You can also estimate the Sludge Loading Rate (F/M ratio) and MLSS for activated sludge processes. This is how environmental engineering turns water quality data into real-world design.