How to Reduce COD in Wastewater — A Practical Guide for Industrial Plants

High outlet COD is the most common compliance failure in industrial ETPs across India. The CPCB standard is COD <250 mg/L for discharge to inland surface waters. Many plants — even well-designed ones — struggle to maintain consistent compliance. And the diagnosis and fix are almost never the same twice.

This guide covers every practical method for reducing COD in industrial wastewater, from the most basic (increasing HRT, checking aeration DO) to the most advanced (Fenton oxidation, ozonation for recalcitrant compounds). The goal is to give plant engineers and EHS managers a clear decision framework, not a general overview of wastewater chemistry.

What COD Measures and Why It Matters

Chemical Oxygen Demand (COD) measures the total amount of oxygen required to chemically oxidise all organic and inorganic compounds in water. It is a broader measure than BOD (Biological Oxygen Demand), which only measures the oxygen consumed by biological degradation over 5 days. COD includes both biodegradable and non-biodegradable organic compounds — everything that can be oxidised by a strong chemical oxidant (potassium dichromate in the standard test).

In practice: COD is always higher than BOD for the same sample. If COD is 600 mg/L and BOD is 300 mg/L, the BOD:COD ratio is 0.5 — typical for moderately biodegradable organic wastewater. If COD is 1,200 mg/L and BOD is only 150 mg/L, the ratio is 0.125 — indicating that most of the COD is from non-biodegradable compounds that biological treatment alone cannot remove. Understanding this distinction is the first step in choosing the right treatment approach.

First Step: Assess Biodegradability (BOD:COD Ratio)

Before selecting any COD reduction method, measure both BOD (5-day) and COD on a representative composite sample. This gives you the BOD:COD ratio — the single most important parameter for treatment technology selection.

• BOD:COD >0.5: Highly biodegradable. Standard aerobic biological treatment will achieve 80–95% COD reduction. MBBR or activated sludge with proper HRT (6–24 hours depending on load) is sufficient.
• BOD:COD 0.3–0.5: Moderately biodegradable. Biological treatment will work but may require extended HRT, higher MLSS, or anaerobic pre-treatment for high-strength fractions.
• BOD:COD <0.3: Poor biodegradability. Non-biodegradable or toxic compounds dominate. Physico-chemical pre-treatment or advanced oxidation is required before biological treatment can achieve compliance.

For complex industrial effluent, also measure: Total Dissolved Solids (TDS above 5,000 mg/L suppresses biological activity), heavy metal concentrations (toxic to microorganisms at elevated levels), and pH (biological treatment requires pH 6.5–8.5).

Biological Methods for COD Reduction

Aerobic biological treatment is the most cost-effective COD reduction method for biodegradable industrial wastewater. Microorganisms oxidise dissolved organics to CO2, water, and new cell mass — with the microorganisms eventually settling out as sludge.

MBBR (Moving Bed Biofilm Reactor): The preferred technology for variable industrial loads. Plastic carriers in the aeration tank support a biofilm that is more stable under load variation than suspended growth systems. Achieves 80–95% BOD/COD reduction for food, FMCG, and pharma wastewater with BOD:COD >0.4. HRT 4–12 hours typically. Learn more about MBBR →

Anaerobic digestion (UASB or AASP): For very high COD wastewater (>2,000 mg/L) — distilleries (COD 30,000–80,000 mg/L), breweries (COD 3,000– 8,000 mg/L), dairy (COD 2,000–6,000 mg/L) — anaerobic digestion should be the primary treatment stage. Anaerobic digestion converts 60–80% of COD to biogas (methane) at operating cost dramatically lower than aerobic treatment for high-strength waste. The anaerobic effluent (COD still 400–2,000 mg/L) then needs aerobic polishing to achieve discharge compliance. The combination — anaerobic + aerobic — achieves overall COD reduction of 90–97%.

Extended aeration: For smaller installations (<100 KLD) with moderate COD loads (500–2,000 mg/L), extended aeration (HRT 18–36 hours) in an activated sludge system provides robust COD removal. Higher power consumption than MBBR for the same treatment outcome, and more sensitive to shock loads.

Common biological treatment failures that cause high outlet COD:

• Dissolved oxygen (DO) below 2 mg/L in the aeration tank — check diffusers, blowers, and air distribution
• Sludge age too low (below 10 days for aerobic treatment of high-COD industrial waste) — increasing SRT improves COD removal efficiency
• Temperature below 15°C in winter — biological activity drops sharply below this threshold
• Toxic shock load — a single batch discharge of solvent, acid, or disinfectant can crash the biological system for 2–4 weeks
• Insufficient equalisation — hourly COD variation of 10x is common in batch manufacturing; equalisation tank sizing below 6–8 hours HWD leads to hydraulic and organic shock loads

Physico-Chemical Methods

For wastewater with BOD:COD <0.3, or where specific non-biodegradable compounds need removal, physico-chemical methods are required.

Coagulation-flocculation and DAF: Removes colloidal and suspended organic matter (30–60% COD reduction) and fats, oils, greases (>90% FOG removal). Essential primary treatment for food industry effluent before biological stages. Also reduces colour from reactive dyes in textile effluent. A DAF system is the standard approach for food and FMCG ETPs.

Activated carbon adsorption: Removes recalcitrant soluble organics (surfactants, phenols, pesticides, residual pharmaceuticals) through adsorption onto activated carbon surface area. Achieves 50–90% reduction in recalcitrant COD as a polishing step after biological treatment. Operating cost is significant — granular activated carbon (GAC) must be regenerated or replaced as it becomes exhausted. Cost-effective for low-volume tertiary polishing (<50 KLD) but expensive at scale.

Lime/coagulant precipitation: Removes heavy metals and some organic compounds through precipitation. Also raises pH, which can precipitate phosphates and some dissolved organics. Limited direct COD removal but essential for removing toxic metals that inhibit biological treatment.

Advanced Oxidation Processes (AOPs)

AOPs generate highly reactive hydroxyl radicals (•OH) that non-selectively oxidise recalcitrant organic compounds to biodegradable intermediates or to CO2 and water. They are used for wastewater with very low BOD:COD ratios where conventional treatment fails to achieve compliance.

Fenton oxidation (H2O2 + FeSO4): The most widely used AOP in India for industrial ETP applications. Ferrous sulphate catalyses hydrogen peroxide decomposition to generate hydroxyl radicals. Effective at pH 2.5–4. Achieves 40–80% COD reduction on recalcitrant wastewater, and significantly improves BOD:COD ratio. CAPEX is low (reaction tank + dosing systems), but OPEX includes H2O2 and FeSO4 consumption. Generates iron sludge that must be dewatered and disposed of. Most cost-effective AOP for high-strength recalcitrant wastewater at plant scale.

Ozonation: Ozone (O3) is generated on-site by corona discharge and bubbled through the wastewater. Ozone directly oxidises some compounds; in combination with UV or H2O2 (O3/UV or O3/H2O2), it generates hydroxyl radicals for deeper oxidation. Very effective for colour removal (reactive dyes) and pharmaceutical compound destruction. Higher operating cost than Fenton due to ozone generation energy, but produces no sludge. Preferred for applications where Fenton sludge generation is problematic.

UV/H2O2: Useful for trace organic compounds (micropollutants, pharmaceuticals) at low concentrations but is not economical for bulk COD reduction at industrial scale.

Process Modification: Reduce COD at Source

The cheapest way to reduce outlet COD is to reduce the COD entering the ETP. This is an underrated but high-return approach.

Specific measures that have delivered measurable results at Indian food and FMCG plants:

• Dry cleaning before wet cleaning: Sweeping or scraping product residues from floors and equipment before washing reduces wastewater COD load by 30–50% in food factories. One of the highest-return interventions available.
• CIP optimisation: Extending CIP rinse cycles to recover product from lines before caustic washing reduces product loss to drains. Optimising caustic concentration and temperature reduces chemical load while maintaining cleaning efficacy.
• Condensate segregation: Boiler condensate is very low COD and high temperature — keeping it separate from process effluent allows recovery as boiler feedwater and avoids unnecessarily diluting (and then concentrating) the ETP feed.
• Spill prevention: Seal leaks from fermenters, storage tanks, and transfer lines. A single large product spill can overload an ETP biological stage for weeks.
• Flow segregation: High-COD process streams (first rinse water, CIP effluent) are typically 10–20% of total wastewater volume but carry 60–80% of COD. Treating these streams separately or delaying their discharge (batch equalisation) rather than mixing with dilute cooling water allows more efficient treatment.

Practical Checklist for High Outlet COD

If your ETP is producing outlet COD above your consent limit, work through this checklist before investing in additional treatment capacity:

1. Measure DO in the aeration tank — should be 2–4 mg/L throughout. Low DO is the most common cause of poor biological performance.
2. Check HRT — is the actual flow rate matching the design? Many plants are hydraulically overloaded by 30–50% above their design flow.
3. Measure inlet BOD:COD ratio — is the ratio what you assumed during design? Industrial process changes over years often change effluent character.
4. Check for toxic inhibition — any new chemicals, solvents, or cleaning agents introduced recently? Biological sludge can be tested for inhibition using respiration rate measurements.
5. Verify sludge age (SRT) — maintain sludge age of 10–20 days for aerobic treatment of industrial effluent. Too-young sludge washes out the slow-growing organisms needed for complete COD removal.
6. Check equalisation tank performance — is the ETP receiving consistent flow and load, or are there peak discharges from batch production steps?
7. Run a bench-scale treatability study for the current effluent — jar tests for coagulation-flocculation, batch respirometry for biological treatability, activated carbon adsorption isotherms. The results will tell you definitively whether the root cause is process overload, recalcitrant compounds, or toxic inhibition.