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The natural and organic waste and foliage from the wastewater treatment plants fall in urban runoff, and the whole system acts as the source food for the bacteria borne in water. These Bacterial matters decompose the organic wastes using the dissolved oxygen present in the water. Therefore, Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are the main properties that show the characteristics and power of sewage. Using these BOD & COD, the oxygen demand in wastewater is tested during the Sewage Treatment Plant treatment.
Biochemical Oxygen Demand (BOD)
BOD is the abbreviation of biochemical oxygen demand, which defines the amount of dissolved oxygen used by aerobic organisms to break down organic material present in a given water sample. BOD is a biochemical process; it is not a simple test. But, BOD is a widely used test for indicating water quality. BOD is done by incubating a sealed water sample for five days and measuring the loss of oxygen in it.
Two measurements are to be taken for the calculation of BOD. One is the initial depletion of oxygen and the second is the final depletion after five days. BOD directly affects the DO of rivers and streams.
The common sources of BOD are leaves, woody debris, topsoil, animal manure, food-processing plants, and wastewater treatment plants, feedlots, failing septic systems, urban stormwater runoff, and effluents from pulp and paper mills. The oxygen consumption rate depends on the temperature and pH present in microorganisms. Greater the BOD in a particular water body, the less oxygen available for the aquatic life forms in that particular water body. Aquatic life forms would be more stressed, suffocate and ultimately die due to high BOD.
Chemical Oxygen Depletion (COD)
Chemical oxygen demand is typically performed where a strong oxidising chemical is used. For example, organic matter is oxidised into carbon dioxide and water in acidic conditions. The quantity of organic matter or the oxygen demand is calculated by determining how much oxidising chemical was consumed during the test.
Chemical oxygen demand tests are typically performed on wastewater. The pollution level is calculated by measuring the amount of organic matter in the water. Water with too much organic material can harm the environment in which the wastewater is discharged.
Chemical oxygen demand is similar to biochemical oxygen demand in that they are both used to calculate the oxygen demand of a water sample. The difference between the two is that chemical oxygen demand measures everything that can be oxidised, whereas biochemical oxygen demand only measures the oxygen demanded by organisms.
Difference Between BOD and COD (Based on certain parameters)
1. Definition
BOD: BOD is the test of the amount of oxygen consumed by bacteria while decomposing organic matter under aerobic conditions.
COD: COD is the test where the amount of oxygen required to oxidise total organic matter in water is tested.
2. Decomposition
BOD: BOD is a biochemical oxidation process.
COD: COD is a chemical oxidation process.
3. Test Method
BOD: The most common test method for BOD is method 5210B.
COD: The most common method for COD is method 410.4.
4. Test Procedure
BOD: BOD is determined by incubating sealed water under a specific temperature sample for five days, thus measuring the loss of oxygen from the beginning of the test.
COD: COD is determined by incubating inside a closed water sample with a strong oxidant like potassium dichromate (K2Cr2O7) and boiling sulphuric acid (H2SO4) under a specific temperature for a specified period.
5. Time Taken for Determination
BOD: Five days are taken for the day of determination of the BOD test.
COD: COD measurement test can be taken within a few days.
6. Permissible Limit of Test
BOD: The permissible limit of BOD is 30 mg/L.
COD: The permissible limit of COD is 250 to 500 ppm.
7. Values of Measurement
BOD: BOD value is lower than that of the COD value.
COD: COD value is always greater than that of the BOD value. Therefore, the more organic material can be oxidised through the COD test.
8. Oxidation Ability
BOD: Biological oxidation can oxidise natural organic residues and organic waste in the water.
COD: Industrial sewage is only degraded by COD. But, COD does not measure the oxygen consumption of acetate.
Conclusion
BOD and COD are two parameters to measure water quality and the amount of organic pollution of water. BOD measures the amount of oxygen utilised by aerobic microorganisms to break down organic matter in water. In contrast, COD measures the amount of oxygen needed to oxidise total organic and inorganic pollutants in water chemically without the involvement of microbes. COD uses a strong oxidising agent. It oxidises pollutants completely into CO2 and H2O, unlike in BOD. Furthermore, BOD is a time-consuming test, while COD can finish within a few hours. Thus, this summarises the difference between BOD and COD.
