What is BOD ?

BOD:- Biological Oxygen Demand or Biochemical Oxygen Demand.Before knowing about it let us know definition of water pollution.

Water pollution:-

Water pollution is defined as the addition or presence of undesirable substances in water such as organic, inorganic, biological, radiological or thermal contaminants,which deteriorate the quality of water to the point where it is unfit for use.

Biological Oxygen Demand (BOD):-

The amount of dissolved Oxygen needed ( i.e demand) by aerobic biological organisms to break down organic materials presence in a given water sample at certain temperature over a specific time period.

Water bodies have specific amount of dissolved oxygen on which aquatic life relies for its respiratory needs. Actually water contains molecular oxygen which is either a result of photosynthesis in aquatic plants or atmospheric oxygen in the dissolved state.

When organic matter present in water body ,to breakdown the oraganic matters aerobic micro organisms use dissolved oxygen ,so more organic matters more oxygen to use by aerobic bacteria to dissolve that.Means less oxygen more BOD.

Testing for BOD :-

A sample of the water is kept at a constant temperature of 20 degree centigrade in the dark for 5 days . After the 5 th day the remaining dissolved oxygen in the water sample is measured.with this oxygen value the BOD level can be calculated.

Note:-

• Drinking water should have after 5 days a BOD of well below 1 mg/liter.
• Acceptable waste water from sewage treatment plant should have a BOD of around 20 mg/liter.

BOD increase due to :-

• Organic waste.
• Nitrates and phosphates.
• Chemical waste and industrial waste.

Dissolved Oxygen in water as per standards (as per ICMR):-

• In summer :- 5-8 mg/ liter.
• In winter :- 16-23 mg/liter.
• In one droplet :- lesser than 5.0 mg/liter effect aquatic life.
• Aerobic bacteria consumes :-02 mg/ liter for to break down organic maters.

To control BOD:-

• Control polluting water.
• Reverse osmosis (RO).
• Anaerobic sludge blanket reactor with up flow (UASD).

Chemical Oxygen Demand (COD):-

Chemical Oxygen Demand is a test that measures the amount of oxygen required to chemically oxidize the organic and inorganic matters dissolved in the water , inorganic matters like Ammonia,Nitrate .

Difference between BOD and COD:-

BOD to break down the organic matters.

COD to break down organic and inorganic matters via oxidation process by adding some chemicals to water.

BOD must be less than <30 for surface,<100 for irrigation,<350 for public severe. As per IS 3025.

COD must be less than <250 for all purposes as per IS 3025.