Biochemical oxygen demand (BOD) quantifies the oxygen microorganisms consume while breaking down biodegradable organic matter. In an MBBR, heterotrophic bacteria growing on the carriers oxidise this material using dissolved oxygen. BOD removal is a fast, robust process, and the effluent BOD achievable is governed primarily by the surface area loading rate: lower loading yields lower effluent BOD.
How Loading Sets Effluent BOD
Heterotrophs grow quickly and form an active biofilm readily, so BOD removal tolerates relatively high loading. At high loading the reactor removes a large share of BOD but leaves a higher residual; at lower loading it drives effluent BOD down to low concentrations. Where a plant must also nitrify, the BOD load must first be reduced enough that nitrifiers can compete for carrier area and oxygen — which is why carbon removal is usually staged ahead of nitrification.
Because biomass is fixed on the carriers, BOD-removal MBBRs handle diurnal load swings and shock loads well, recovering quickly after peaks.
BOD Removal Design Context (indicative literature ranges — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| SALR — normal BOD removal | 5 – 15 | g BOD/m²·d | Lower for stricter effluent |
| SALR — pre-nitrification carbon stage | < 5 | g BOD/m²·d | Frees area for nitrifiers |
| Design DO | 2 – 3 | mg/L | Carbon-only duty |
| HRT | 0.5 – 2 | h | Strength dependent |
BOD vs. COD
BOD measures only the biodegradable, oxygen-consuming fraction over a fixed test period, while COD measures the total chemically oxidisable matter including inert compounds. For municipal wastewater the two correlate closely and BOD is the usual design basis; for industrial effluents with inert organics, COD characterisation is needed alongside BOD. Both are used together to define the carbon-removal duty of an MBBR.
Frequently Asked Questions
How efficiently does MBBR remove BOD?
Why remove BOD before nitrification?
What is the difference between BOD and COD?
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