Biochemical Oxygen Demand
உயிரிய வேதியியல் ஆக்சிஜன் தேவை
Do not confuse BOD with Dissolved Oxygen. High BOD is not a sign of purity; it is a death warrant for aquatic ecosystems. If you think organic waste is harmless, you have already lost a mark in the exam.
The idea
Biochemical Oxygen Demand (BOD) refers to the amount of oxygen that would be consumed if all the organic matter in one liter of water were oxidized by aerobic bacteria. It serves as an indirect measure of the organic matter present in a water sample. The greater the BOD of waste water, the more polluting its potential. Microorganisms utilize this oxygen to decompose organic pollutants, thereby reducing the oxygen available for other aquatic life.
Why it matters
It serves as a standard metric to quantify the pollution level of water; higher BOD indicates higher organic pollution.
NEET lens
NEET focuses on the inverse relationship between BOD and water quality. Remember: Higher BOD equals higher pollution. Expect questions matching BOD values to water samples or identifying the role of aerobic bacteria. Do not memorize numbers; understand the process of microbial oxidation.
Clinical significance
High BOD leads to eutrophication and the depletion of dissolved oxygen, causing the mass death of fish and aquatic flora. In human terms, consuming water with high organic load leads to water-borne diseases like cholera and typhoid. The ecosystem's inability to process waste mirrors the body's failure to maintain homeostasis.
Introduction
BOD (biochemical oxygen demand) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. It measures how much organic pollution water carries.
Core concept
The BOD test measures the rate of uptake of oxygen by micro-organisms in a water sample, and thus indirectly the amount of organic matter. The greater the BOD of waste water, the more is its polluting potential. In sewage treatment, primary effluent is passed into aeration tanks; aerobic microbes grow into flocs and consume the major part of the organic matter, which significantly reduces the BOD. Once the BOD of sewage has reduced significantly, the effluent passes into a settling tank, where bacterial flocs settle (activated sludge). A lake rich in organic waste develops high BOD; decomposing microbes use up the dissolved oxygen, causing mortality of fish due to lack of oxygen.
Go deeper
BOD is used as an index of pollution for biodegradable organic waste. It may not be a good index for water bodies receiving non-biodegradable or toxic industrial effluents (e.g. from petroleum industries), whose pollutants are not readily broken down by microbes.
From Class 10
When dirty water full of waste enters a pond, bacteria break the waste down and use up the oxygen in the water, so fish can die.