Low COD Removal in MBBR

Q: How do I test whether residual COD is refractory?

The simplest method is a BOD₅/COD ratio on the effluent. If BOD₅ is very low relative to COD (ratio <0.1–0.2), the remaining COD is predominantly refractory. A more detailed analysis involves filtered (soluble) vs total COD and extended BOD tests to estimate the inert fraction.

Q: Can adding more carriers fix low COD removal?

Only if the problem is surface-area-limited overloading. If the residual is refractory, or if DO, nutrients or mixing are the bottleneck, more carriers will not help. Diagnose the root cause before adding carriers.

Q: Is COD removal less sensitive to temperature than nitrification?

Yes. Heterotrophic carbon removal is less temperature-sensitive (θ ~1.04–1.07) than nitrification (θ ~1.09–1.10). Significant winter COD failures are less common than winter nitrification failures, but they do occur at very low temperatures or with high loading.

High residual COD in MBBR effluent can stem from several distinct causes: the organic load exceeds the biofilm's removal capacity; dissolved oxygen is insufficient for the heterotrophic biofilm; the residual COD is refractory (non-biodegradable) and cannot be biologically removed; or the biofilm itself is compromised. The first diagnostic step is to determine whether the residual COD is biodegradable or refractory — this fundamentally changes the response.

Biodegradable vs refractory COD

Total COD comprises biodegradable COD (bCOD) and refractory COD (rCOD). Biological treatment can only remove bCOD. If the effluent COD is mostly refractory, no amount of biological optimisation will reduce it further — the solution lies in advanced treatment (activated carbon, ozonation) or upstream source control. A simple BOD/COD ratio test on the effluent helps: a low BOD/COD ratio (<0.1–0.2) suggests the residual is predominantly refractory.

Industrial wastewaters in particular may contain significant refractory fractions — dyes, solvents, and complex organics that pass through biological treatment unchanged. Characterising the influent and effluent COD fractions (soluble, particulate, biodegradable, refractory) is essential for realistic performance expectations.

Cause–remedy matrix

Low COD removal: common causes and actions

Parameter	Typical Value	Unit	Notes
Residual is refractory	Advanced treatment needed	—	Biology cannot remove rCOD.
Organic overloading	Reduce load / add carriers	—	Check SARR vs design.
Low DO (<2 mg/L)	Increase aeration	—	Heterotrophs need O₂ too.
Biofilm loss (toxic event)	Identify toxicant; allow regrowth	—	Heterotroph recovery: 1–3 weeks.
Poor mixing	Fix dead zones	—	Carriers in stagnant areas are unproductive.
Nutrient deficiency	Dose N or P if BOD:N:P unbalanced	—	Industrial WW may lack nutrients.

Nutrient balance for carbon removal

Heterotrophic bacteria need nitrogen and phosphorus as nutrients. The classic rule of thumb is BOD:N:P = 100:5:1. Industrial wastewaters with high COD but low N and P (e.g. some food, chemical, and pulp/paper streams) may be nutrient-limited. Dosing urea (N) and phosphoric acid (P) based on the influent BOD can restore removal efficiency.

Frequently Asked Questions

How do I test whether residual COD is refractory?

The simplest method is a BOD₅/COD ratio on the effluent. If BOD₅ is very low relative to COD (ratio <0.1–0.2), the remaining COD is predominantly refractory. A more detailed analysis involves filtered (soluble) vs total COD and extended BOD tests to estimate the inert fraction.

Can adding more carriers fix low COD removal?

Only if the problem is surface-area-limited overloading. If the residual is refractory, or if DO, nutrients or mixing are the bottleneck, more carriers will not help. Diagnose the root cause before adding carriers.

Is COD removal less sensitive to temperature than nitrification?

Yes. Heterotrophic carbon removal is less temperature-sensitive (θ ~1.04–1.07) than nitrification (θ ~1.09–1.10). Significant winter COD failures are less common than winter nitrification failures, but they do occur at very low temperatures or with high loading.

COD Removal

COD removal process.

Aerobic MBBR

Aerobic carbon removal.

MBBR Organic Loading

Design loading rates.

High-COD Wastewater

Industrial high-strength applications.

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