BOD & COD Reduction in Wastewater
Technical guide to the most effective methods for reducing BOD and COD in industrial wastewater — from aerobic biological treatment and anaerobic digestion to advanced chemical oxidation and membrane processes — with removal efficiencies and selection criteria for Indian industries
Overview
About BOD & COD Reduction in Wastewater
Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are the two primary organic pollution parameters monitored in industrial wastewater — and the two parameters most commonly violated by Indian manufacturing plants failing CPCB/SPCB compliance tests. CPCB General Standards require treated industrial effluent to meet BOD ≤30 mg/L and COD ≤250 mg/L for discharge to inland surface waters. For manufacturing plants generating effluent with BOD of 500–8,000 mg/L and COD of 1,000–15,000 mg/L — typical of food processing, pharmaceutical, textile, and chemical industries — achieving these standards requires a carefully designed and operated multi-stage treatment system.
Not all organic matter in wastewater behaves the same way in treatment. The BOD/COD ratio is the key indicator of how responsive an effluent will be to biological treatment. Effluent from food processing, dairy, and beverage manufacturing typically has BOD/COD ratios of 0.5–0.7 — most of the organic matter is readily biodegradable and will respond well to aerobic biological treatment alone. Pharmaceutical API effluent, textile dye effluent, and paper mill bleaching effluent may have BOD/COD ratios of 0.1–0.3 — a large fraction of COD is refractory (non-biodegradable) and will not be removed by aerobic biological treatment, requiring advanced chemical or physical treatment processes.
Spans Envirotech's process engineers design treatment systems that combine the most effective combination of biological, physio-chemical, and advanced treatment methods for each specific effluent profile. This guide describes the key BOD and COD reduction methods available, their applicable ranges, achievable removal efficiencies, and selection criteria — to help industrial engineers understand the treatment options and select the right approach for their wastewater challenge.
Process
BOD/COD Reduction — Treatment Methods
Aerobic Biological Treatment — MBBR and Activated Sludge
The most widely used and cost-effective BOD/COD reduction method for biodegradable effluent. Aerobic bacteria oxidise dissolved organic matter (BOD) using dissolved oxygen, producing CO₂, water, and new biomass (sludge). MBBR achieves 85–97% BOD removal and 70–90% COD removal for effluent with BOD/COD > 0.4. Activated sludge (including extended aeration and oxidation ditch configurations) achieves similar efficiency with larger footprint. MBBR is preferred for industrial applications due to compact footprint, resistance to variable loads, and ability to retrofit existing tanks. Design organic loading rates: MBBR 1–3 kg BOD/m³/day; activated sludge 0.2–0.6 kg BOD/m³/day.
Anaerobic Pre-Treatment — UASB and Plug-Flow Reactors
For high-strength effluent (COD > 2,000 mg/L, BOD > 1,000 mg/L), anaerobic pre-treatment ahead of aerobic biological treatment provides 60–80% COD reduction without requiring oxygen input — dramatically reducing aeration energy costs. Upflow Anaerobic Sludge Blanket (UASB) reactors are the most common configuration for food and agro-processing effluent, treating effluent with COD up to 15,000 mg/L. Anaerobic treatment produces biogas (60–70% methane) that can be captured for heat or power generation — partially or fully offsetting ETP energy costs. Not suitable for toxic or inhibitory effluent (heavy metals, solvents, high sulphate).
Physio-Chemical Treatment — Coagulation, Flocculation, DAF
Coagulation and flocculation using aluminium sulphate (alum), ferric chloride, or polyelectrolytes destabilises and aggregates colloidal and suspended organic matter, removing 30–60% of COD associated with particulate organics in primary treatment. Dissolved Air Flotation (DAF) combined with coagulation/flocculation removes fats, oils, greases, and suspended organics — critical for food industry, meat processing, and dairy effluent with high FOG loads. DAF achieves 60–80% BOD reduction and 70–85% TSS removal as a primary treatment step. Also used as a polishing step after biological treatment for high-quality effluent.
Advanced Oxidation — Fenton, Ozone, UV
For refractory COD that is not removed by biological treatment (BOD/COD ratio < 0.3 in treated effluent), advanced oxidation processes (AOPs) generate hydroxyl radicals that non-selectively oxidise persistent organic compounds. Fenton oxidation (H₂O₂ + FeSO₄) at pH 3–4 achieves 50–90% COD reduction for refractory pharmaceutical, textile, paper, and chemical effluent. Ozone-based oxidation achieves colour and COD reduction for textile and paper effluent. UV/H₂O₂ photocatalysis is effective for trace pharmaceuticals, endocrine disruptors, and other micro-pollutants. AOPs are typically applied after biological treatment as polishing steps, treating the refractory fraction that biological processes cannot address.
Activated Carbon Adsorption
Powdered Activated Carbon (PAC) or Granular Activated Carbon (GAC) filters adsorb dissolved organic compounds — including refractory, low-BOD/COD organics — from treated effluent. GAC filters achieve 40–70% COD reduction from biologically treated effluent and near-complete removal of trace organics, colour, and odour compounds. Activated carbon is highly effective for pharmaceutical effluent with API residues, food industry effluent with taste and odour compounds, and any effluent requiring polishing to very low COD levels (< 50 mg/L). GAC media requires periodic regeneration or replacement (typically 6–18 months depending on loading).
Membrane Processes — UF and RO
Ultrafiltration (UF) membranes in MBR systems provide complete solids separation and achieve very low BOD/COD effluent quality (BOD < 5 mg/L, COD < 30 mg/L) from biodegradable effluent by combining biological treatment with membrane separation. Reverse Osmosis (RO) membranes reject >95% of dissolved organics and inorganic salts, producing near-deionised permeate from ETP-treated effluent. RO is used as a tertiary treatment step in ZLD systems where very high water recovery is required. Nanofiltration (NF) membranes reject divalent salts and larger organic molecules while passing monovalent ions — used for selective COD removal in pharmaceutical and food industry applications.
Benefits
Key Advantages
- Multi-stage treatment combinations address both biodegradable and refractory COD fractions
- MBBR + DAF combination achieves >95% BOD removal for food, dairy, and FMCG effluent
- Anaerobic pre-treatment reduces aeration energy by 40–60% for high-strength effluent
- Fenton oxidation handles refractory pharmaceutical and textile COD that biological treatment cannot remove
- Activated carbon polishing provides final effluent quality assurance for sensitive discharge locations
- MBR technology achieves BOD < 5 mg/L and COD < 30 mg/L for water reuse applications
- ZLD systems incorporating RO + evaporation provide complete organic removal and water recovery
- Use the Spans BOD/COD Calculator to estimate load and treatment requirements online
- Process selection tailored to BOD/COD ratio, industry type, and regulatory discharge standard
- 30+ years of BOD/COD treatment system design across Indian manufacturing industries
Applications
Industries & Use Cases
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