BOD (BOD)

Biochemical Oxygen Demand (BOD) is the mass of dissolved oxygen consumed by microorganisms when they decompose organic matter in a water sample under standardised conditions — by convention, 5 days at 20°C in the dark, expressed as mg/L (BOD5). It is the oldest and still the most universally applied measure of organic pollution in wastewater, used in every Indian SPCB effluent consent.

The mechanism is biological. A measured aliquot of effluent is diluted with oxygen-saturated, nutrient-buffered water, seeded with mixed microbial culture, sealed in a 300 mL bottle, and incubated. Dissolved oxygen is measured at day 0 and day 5; the decrease is BOD5. The result quantifies how much oxygen the discharged water will steal from a receiving river or pond — high BOD effluent collapses dissolved oxygen, killing fish and shifting the receiving water to anaerobic, malodorous conditions.

Indian discharge limits under the Water Act 1974 are set per the Environment (Protection) Rules 1986, Schedule VI: BOD ≤30 mg/L for discharge to inland surface water, ≤100 mg/L to public sewer (with downstream STP), ≤350 mg/L for land disposal/irrigation, ≤100 mg/L to marine coastal areas. Specific industries have tighter or looser limits — pulp & paper, tannery, dairy, distillery and slaughterhouse have category-specific Schedule VI rows.

For recycling plants, BOD load comes from wash-line effluent (PET, PE, PP wash from agricultural film), biogas plant slurry (CBG), tyre-pyrolysis wastewater, e-waste leachate (from precious-metal acid leaching residue). A typical PET wash line generates 4-7 m³ effluent per tonne of feed at 500-2,500 mg/L BOD, requiring biological treatment — equalisation tank, anaerobic UASB or anoxic-aerobic-clarifier sequence — to hit 30 mg/L. BOD:COD ratio is the design tell — above 0.6 means biodegradable (biological treatment works), below 0.3 means refractory (advanced oxidation or activated carbon needed). CBG digestate carries BOD of 5,000-25,000 mg/L pre-digester and 200-800 mg/L post-digester, still above limit and requiring polishing in an aerobic Effluent Treatment Plant or constructed wetland before discharge.