Biochemical Oxygen Demand (BOD) , Chemical Oxygen Demand (COD)

Definition and Characters of BOD

  • Biochemical Oxygen Demand (also called Biological Oxygen Demand ) is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period.
  • Oxygen dissolved in water can come from the atmosphere or as a byproduct of the photosynthesis of aquatic plants.
  • BOD help to determine either the water is portable or not.
  • If the number of organism is increases then the Amount of BOD will decrease.
  • Aerobic microorganisms used BOD to oxidize the organic component in sewage water.
  • The strength of water is depend on the BOD value and the magnitude of BOD is depend on the organic substances in sewage water.
  • BOD test is used to determine the presence of organisms in water.

Reaction and Formula

When biodegradable organic matter is released into a water body, microorganisms feed on the wastes, breaking them into simpler organic and inorganic substances. When this decomposition occurs in aerobic environment the process produces non-objectionable, stable end products like CO2, SO4, PO4 and NO3. A simplified form of Aerobic decomposition is 
O.M + O2 + Microorganisms
CO2 + H2O + C5 H7 NO2 (New Cells) = stable Products like NO3; PO4; NO3)
When sufficient O2 is not available Anaerobic decomposition occurs by different microorganisms. They produce end products that can be highly objectionable, including H2S; NH3 and CH4.
The reaction is O.M + Microorganisms
CO2 + H2O + C5 H7 NO2 (New Cells) = Unstable Products (NH3; H2S; CH4
CALCULATIONS
To determine the value of the BOD in mg/L, use the following formula:
BOD, mg/L = [(Initial DO ‑ Final DO) x 300]/mL sample
For example:
Initial DO = 8.2 mg/L
Final DO = 4.4 mg/L
Sample size = 5 mL BOD mg/L = [(8.2 ‑ 4.4) x 300]/5 = (3.8 x 300)/5 = 1140/5 = 228 mg/L

Chemical Oxygen Demand (COD)

  • COD is the total amount of oxygen required by the organisms to oxidize the biodegradable and non biodegradable substances in water
  • COD value is lower than the BOD value in sample water.
  • COD result are available much sooner than the BOD test.
  • IT can Standardize more easily.

Formula

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