Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand (BOD) measures how much dissolved oxygen micro-organisms consume while breaking down the biodegradable organic matter in a water sample over a fixed incubation period — in India usually 3 days at 27°C (BOD₃ at 27°C, equivalent to the international BOD₅ at 20°C). It is the headline indicator of organic pollution: the more biodegradable waste in the water, the more oxygen the microbes demand, and the higher the BOD. The inland-surface-water discharge limit is 30 mg/L.

BOD matters because high-organic effluent discharged to a water body causes oxygen depletion — the microbes breaking down the waste consume the dissolved oxygen that fish and aquatic life need, leading to fish kills and the death of the water body. This is the classic mechanism of organic water pollution, and it is why effluent from food, beverage, sugar, distillery and similar industries (very high BOD) is so damaging if released untreated.

For recyclers, BOD is the key parameter wherever the effluent carries organic load. The clearest case is the CBG/biogas sector: the digestate liquor and any process washings are organically rich, and the digester's whole purpose is anaerobic breakdown of organic matter — managing the BOD of the liquid fraction before discharge or land application is central. Plastic washing lines that handle food-contaminated packaging, and any pre-processing of organic-contaminated waste, also generate BOD.

The practical approach is biological treatment: BOD is removed by secondary (biological) treatment — aerobic systems such as activated sludge or anaerobic digestion — which uses controlled micro-organisms to break down the organics before discharge, rather than letting that breakdown consume oxygen in the receiving water. Monitoring effluent BOD against the applicable column (30 mg/L inland surface water) by NABL lab is routine, and for CBG operators the liquid digestate's BOD is also relevant to its safe use as a fertiliser/irrigation input.