Biological Oxygen Demand (BOD)
Updated: Dec 26, 2022
What is Biological Oxygen Demand (BOD)?
Biological oxygen demand (BOD) is a measure of the amount of oxygen required by microorganisms to break down organic matter in water. It is used as an indicator of the quality of water, and is often used to assess the effectiveness of wastewater treatment processes.
To perform the BOD test, a water sample is incubated under standardized conditions, and the decrease in dissolved oxygen (DO) over a specific period of time is measured. The BOD is expressed in milligrams of oxygen consumed per liter of water (mg/L) and is a measure of the biodegradable organic matter present in the water.
BOD is an important parameter in the treatment of wastewater because it can indicate the amount of organic matter that needs to be removed or broken down during the treatment process. High BOD levels can indicate the presence of sewage, agricultural runoff, or other sources of organic pollution in a water body. These pollutants can have negative impacts on the health of aquatic ecosystems, including reducing the oxygen levels in the water and disrupting the balance of species.
In addition to its use in wastewater treatment, BOD is also used to monitor the quality of surface water and to assess the potential impacts of human activities on water bodies. It is often used in conjunction with other water quality tests, such as chemical oxygen demand (COD), to get a more complete picture of the health of a water body.
Why is Biological Oxygen Demand (BOD) measurement important?
Biological oxygen demand (BOD) is an important measure of water quality because it can indicate the presence of organic pollutants in water and the potential impact on the oxygen levels in the water. High levels of organic matter in water can be harmful to aquatic life and can also cause problems for human uses of the water, such as drinking or recreational activities.
Elevated BOD levels can indicate the presence of sewage, agricultural or industrial runoff, or other sources of organic pollution in a water body. These pollutants can have negative impacts on the health of aquatic ecosystems, including reducing the oxygen levels in the water and disrupting the balance of species. High BOD levels can also affect the taste and smell of the water and make it unfit for drinking or other uses.
In addition to its use as a measure of water quality, BOD is also an important parameter in the treatment of wastewater. The BOD test is used to determine the effectiveness of wastewater treatment processes and to monitor the progress of these processes. Reducing the BOD of wastewater is important for protecting the environment and for meeting regulatory standards for the discharge of treated wastewater.
Overall, BOD measurement is important for assessing the health of aquatic ecosystems, protecting human health, and ensuring that water is fit for its intended use.
How is Biological Oxygen Demand (BOD) Calculated?
The biological oxygen demand (BOD) of a water sample is calculated based on the decrease in dissolved oxygen (DO) that occurs over a specific period of time when the water sample is incubated under standardized conditions. The following steps outline the general process for calculating BOD:
Collect a water sample: The water sample should be collected in a clean container and transported to the laboratory as soon as possible to minimize the possibility of contamination or changes in the water quality.
Prepare the sample: The water sample should be filtered to remove any large particulates, such as sediment or debris, that could interfere with the test.
Incubate the sample: The water sample is incubated in an airtight container at a standardized temperature (typically 20°C) for a specific period of time (typically 5 days). The container is sealed to prevent oxygen from entering or leaving the water sample.
Measure the initial and final dissolved oxygen levels: The DO level of the water sample is measured at the start of the incubation period (initial DO) and at the end of the incubation period (final DO).
Calculate the BOD: The BOD is calculated using the following formula:
BOD = (Initial DO - Final DO) x Dilution factor
The dilution factor is used to correct for the volume of the water sample and is typically calculated based on the volume of the water sample and the volume of the incubation container. The BOD is expressed in milligrams of oxygen consumed per liter of water (mg/L).
It is important to follow standard laboratory procedures and quality control measures when conducting a BOD test to ensure the accuracy and reliability of the results.
What causes high Biological Oxygen Demand (BOD)?
High biological oxygen demand (BOD) levels in a water sample generally indicate the presence of a high level of organic matter in the water. This can be caused by a variety of factors, including sewage, agricultural runoff, or industrial discharge. High BOD levels can also be caused by natural sources of organic matter, such as algae blooms or decomposing plant material.
Elevated BOD levels can have negative impacts on the health of aquatic ecosystems and on the quality of the water for human uses. High BOD levels can cause oxygen depletion in the water, which can be harmful to aquatic life and can also affect the taste and smell of the water. High BOD levels can also indicate the presence of pollutants that may be harmful to human health, such as pathogens or chemicals.
It is important to monitor BOD levels in water bodies and to take steps to reduce BOD levels if they are elevated. This may involve treating the water to remove or break down the organic matter, or implementing measures to prevent or reduce the introduction of organic pollutants into the water.
What is the difference between chemical oxygen demand and biological oxygen demand?
BOD measures the amount of oxygen required by microorganism to breakdown organic material, whereas Chemical Oxygen Demand (COD) is the amount of oxygen required to breakdown organic material via oxidation.
Click here to learn more about COD
BOD is an indirect representation of the amount of biodegradable organic matter while COD is and indirect representation of the amount of biodegradable and non-biodegradable organic matter.
COD does not differentiate between biologically available and inert organic matter. Because of this, COD value is always higher than BOD value (about 1.3 to 1.5 times higher)
BOD test requires a minimum of 5 day incubation period whereas COD test is a rapid test which only requires 2 hours digestion.
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Reference:
1. YOUNG, J.C. 1973. Chemical methods for nitrification control. J. Water Pollut. Control Fed. 45:637