COD Removal Techniques in Industrial Wastewater Treatment

COD compliance failure — effluent COD exceeding the CPCB limit of 250 mg/L (or the stricter limit in your CTO) — is the most common compliance problem reported by industrial ETP operators. The frustrating aspect is that COD cannot be reduced by a single treatment stage: different COD fractions require different treatment approaches. Attempting to "fix COD" by running the biological reactor harder will work for biodegradable COD but has zero effect on the recalcitrant fraction. Understanding which fraction is causing your non-compliance is the first step to solving it.

Understanding COD Fractions: Biodegradable vs Recalcitrant

Total COD in industrial wastewater can be divided into fractions that respond differently to treatment:

• Settleable suspended COD: Coarse particles that settle in a primary clarifier; removed by screening and gravity settling (20–30% of total COD in food industry wastewater)
• Colloidal and emulsified COD: Fine particles and emulsified fats that don't settle but can be coagulated and removed by DAF (20–40% of total COD in high-FOG food/dairy wastewater)
• Biodegradable dissolved COD (bsCOD): Dissolved organic compounds (sugars, organic acids, proteins) that biological organisms can oxidise; measured as soluble BOD; responds to biological treatment (30–50% of total COD in food industry wastewater)
• Non-biodegradable dissolved COD (nbsCOD): Dissolved recalcitrant compounds that pass through biological treatment unchanged; requires advanced oxidation or activated carbon (varies by industry; 10–30% in food industry; up to 70% in pharma API manufacturing)

Measuring COD before and after biological treatment on filtered samples reveals the nbsCOD fraction — if filtered COD after biological treatment is still above 150–200 mg/L, the non-biodegradable fraction is a compliance problem requiring advanced treatment.

Primary COD Removal: DAF and Coagulation

The first and most cost-effective COD reduction step is removing suspended and colloidal COD before it reaches the biological stage. For food and dairy wastewater with high FOG and suspended solids, this stage alone removes 40–70% of total inlet COD:

Coagulation-flocculation: Adding ferric chloride or PAC (20–60 mg/L) followed by polyelectrolyte (1–3 mg/L) destabilises colloidal particles and emulsified fat droplets, allowing them to aggregate into settleable or floatable flocs. The coagulant dose must be optimised by jar test — overdosing has no additional COD removal benefit and increases sludge production.

DAF: The most effective primary treatment for food industry wastewater — removes 70–90% of FOG and 40–70% of total suspended COD in a single stage. Well-sized and operated DAF sets the biological stage up for reliable COD compliance. A DAF that is undersized, under-chemically-dosed, or mechanically failing is the most common root cause of persistent COD non-compliance in food industry ETPs.

Biological COD Removal: Aerobic and Anaerobic

Biological treatment is the primary route for removing dissolved biodegradable COD. Aerobic treatment (MBBR, ASP, SBR) achieves 85–95% removal of biodegradable dissolved COD at adequate HRT and DO. Key operating parameters:

• DO maintained 2.0–3.0 mg/L throughout the aeration tank
• HRT adequate for the COD loading: 6–12 hours for food industry wastewater at 1,000–3,000 mg/L COD; 12–24 hours for high-strength or partially inhibitory wastewater
• MLSS in 3,000–4,500 mg/L range with adequate sludge age (SRT 10–15 days for food industry)
• Nitrogen and phosphorus supplementation if COD:N:P ratio is imbalanced

Anaerobic pre-treatment (UASB or anaerobic MBBR) before aerobic biological treatment removes 60–80% of high-strength biodegradable COD as biogas — dramatically reducing the aerobic stage oxygen demand. For inlet COD above 3,000 mg/L, an anaerobic stage significantly reduces total treatment cost through biogas energy recovery and smaller aerobic system sizing. See our guide to biogas from wastewater treatment for design details.

Tertiary Polishing for Residual COD

When biological treatment alone cannot achieve the COD discharge standard, tertiary polishing options target different residual COD fractions:

Coagulation-flocculation polishing: Adding coagulant (20–40 mg/L PAC) to biological effluent precipitates residual colloidal COD as a settleable floc. Effective when residual COD is partly colloidal (turbid effluent), but less effective for truly dissolved COD. Simple and inexpensive.

Activated carbon (GAC) adsorption: Granular activated carbon filter removes dissolved non-biodegradable organics, colour, and trace compounds. Effective for polishing biological effluent from 100–200 mg/L COD to 30–80 mg/L. Carbon replacement or regeneration cost is the main OPEX consideration.

Ozonation: Ozone at 5–15 mg/L residual oxidises refractory organics, decolourises effluent, and eliminates odour. Particularly effective for textile and pharmaceutical effluent. CAPEX for ozone generator; OPEX for electricity (typically 3–5 kWh per kg ozone applied).

Fenton's reagent: Hydroxyl radical oxidation degrades recalcitrant compounds; 50–70% COD reduction for resistant organics. Used as a pre-treatment before biological stage (to improve BOD:COD) rather than as a polishing step for the biological effluent.

Troubleshooting: Why COD Remains High

A systematic diagnosis of persistent high effluent COD:

1. Measure COD at each stage: Inlet → post-DAF → post-biological → final. Identify which stage is not achieving expected removal. This eliminates guesswork and focuses the investigation.
2. Measure BOD:COD ratio at biological stage inlet: If below 0.3, recalcitrant fraction is the problem; biological treatment upgrades will not help.
3. Check DAF performance: Is the DAF producing a defined float layer? Is coagulant dose optimised (jar test in past 3 months)?
4. Check biological stage DO: DO below 1.5 mg/L indicates aeration limitation — not enough oxygen to biodegrade the COD load. Measure at multiple points across the aeration tank.
5. Check for inhibitory discharge: Sudden COD increase after apparent biological crash suggests a toxic shock — check process upstream for disinfectant use, pH excursions, or unusual discharge.

Compliance Strategy by Inlet COD Level

The appropriate treatment train depends on inlet COD concentration and biodegradability:

• COD 500–2,000 mg/L, BOD:COD >0.5: DAF + biological (MBBR/ASP) + sand filter. Standard two-stage treatment. Expected final COD: 50–150 mg/L.
• COD 2,000–8,000 mg/L, BOD:COD 0.4–0.6: DAF + MBBR/ASP + tertiary coagulation or GAC. Expected final COD: 80–200 mg/L.
• COD >8,000 mg/L: Anaerobic pre-treatment (UASB) + aerobic biological + polishing. Expected final COD: 80–200 mg/L depending on biodegradability.
• Any COD with BOD:COD <0.3: Advanced oxidation (Fenton pre-treatment) to improve biodegradability, then biological, then GAC polishing.