Question

what is oxygen demand of water

Answer 1

A high standard of living involves a high demand for water and, at the same time, causes much greater pollution of this essential element for life. The resultant interference in the natural cycle can often overwhelm natural processes of recovery, so that, in addition to products arising from the decomposition of natural substances (e.g., proteins, greases, carbohydrates) there is a build-up of anthropogenous additives such as pesticides, effluents and garbage, which contaminate drinking water supplies with their toxic or hormonal effects. They may also consume such large quantities of oxygen that water resources become fouled. 

To prevent the threat of possible danger to health, or the very existence of certain species, it is essential to determine the quality of a water source before water is drawn off for consumption. 

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Answer 2

Bio chemical oxygen demand is the amount of dissolved oxygen needed( i, e demanded) by aerobic biological organisms to break down organic material present in given water sample at certain temperature over a specific time period