BOD and COD: Measures of Water Pollution and Oxygen Demand

What Is BOD?

BOD Full Name/ Full Form: Biochemical Oxygen Demand,

1. This measures the amount of dissolved oxygen (DO) required by aerobic organisms to decompose the organic material present in a given water sample at a specified temperature and specified time.
2. Since BOD is a biochemical process, it is not an accurate quantitative test. However, BOD is a widely used test method, indicating the organic quality of the water.
3. BOD is determined by incubating a sealed water sample for five days and measuring the loss of oxygen from the beginning of the test. Two measurements must be taken to calculate the BOD of a sample.
4. One is the initial DO and the second is the final DO after five days. The BOD is expressed in milligrams of oxygen consumed per liter of the sample for five days (BOD5) of incubation at 20 ° C. The BOD directly affects the DO of rivers and streams.
5. The sources of BOD are leaves, woody debris, topsoil, animal manure, food processing plants, wastewater treatment plants, feedlots, defective septic systems, urban runoff from rainwater and effluents from pulp and paper mills.
6. The rate of oxygen consumption depends on the temperature, PH present in the microorganisms and the type of organic material in the water.
7. The higher the BOD in a specific water body, the less oxygen will be available to aquatic life forms in that specific water body. Aquatic life forms would be stressed, suffocate and ultimately die due to high BOD

What Is COD?

 

COD Full Name/ Full Form:  Chemical Oxygen Demand,

Which measures the amount of OD required for the decomposition of organic matter and the oxidation of inorganic chemicals such as ammonia and nitrite.

COD measurements are usually made with samples of waste water or natural water, which are contaminated by domestic and industrial waste.

A closed water sample is incubated with a strong oxidizer such as potassium dichromate (K2Cr207) in combination with boiling sulfuric acid (H2SO4) at a specific temperature for a specified period of time.

The COD is related to the DBO. However, COD is the only method for measuring the amount of industrial waste in the water, which cannot be measured in the DBO.

The amount of cellulose in the water is measured only by the COD. Plants that treat wastewater from commercial operations measure COD.

BOD Vs COD

 

Definition

BOD: BOD is the amount of oxygen consumed by bacteria while decomposing organic matter under aerobic conditions.

COD: COD is the amount of oxygen required for the oxidation of total organic matter in water.

Decomposition

BOD: BOD is a biological oxidation process.

COD: COD is a chemical oxidation process.

Test Method

BOD: The common test method for BOD is method 5210B.

COD: The common method for COD is method 410.4.

Test Procedure 

BOD: BOD is determined by incubating sealed water under a specific temperature sample for five days and measuring the loss of oxygen from the beginning of the test.

COD: COD is determined by incubating a closed water sample with a strong oxidant like potassium dichromate (K2Cr207) in combination with boiling sulfuric acid (H2SO4) under specific temperature for a specified period of time.

Time Taken for Determination

BOD: Five days are taken for the determination of the BOD.

COD: COD measurement can be taken from few days.

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.

Values of Measurement

BOD: BOD value is lower than the COD value.

COD: COD value is always greater than the BOD value. Therefore, the more organic material can be oxidized by COD.

Oxidation Ability

BOD: Biological oxidation is capable of oxidizing natural organic detritus and organic waste in the water.

COD: Industrial sewage is only degraded by COD. But, COD does not measure the oxygen consumption of acetate.

Main Difference – BOD vs COD

1. Aquatic organisms depend on oxygen in water or dissolved oxygen (DO) for their respiratory needs.
   The amount of DO in a body of water depends on the temperature of the water, the amount of sediment, the amount of oxygen removed from the system and the amount of oxygen returned to the water.
2. The respiration and the decomposition of the organisms remove oxygen from the system and the photosynthetic organisms, the aeration and the flow of the flow return the oxygen to the water.
3. The bacteria break down natural organic waste and organic waste into the water using DO. BOD and COD are two measures that describe the demand for OD by bacteria in the water.
4. BOD refers to the biochemical demand for oxygen and COD is the chemical demand for oxygen.
5. The main difference between BOD and COD is that BOD is the amount of oxygen that is consumed by bacteria while decomposing organic matter under aerobic conditions.
6. Whereas COD is the amount of oxygen required for the chemical oxidation of total organic matter in water.
7. The higher the BOD/COD the more oxygen stripping capacity the discharged effluent has when discharged into receiving waters (oxygen is used biologically/chemically to break down the organic matter) and the more potential for damage to biological life in those waters.
8. COD is normally higher than BOD because more organic compounds can be chemically oxidized than biologically oxidized.
9. This includes chemicals toxic to biological life, which can make COD tests very useful when testing industrial sewage as they will not be captured by BOD testing.
10. In regards to normal domestic effluent e.g. BOD & Suspended Solids (SS) 300 mg/I, which is usual for package sewage treatment systems, COD is not always stated as part of the discharge license.
11. If it is not part of your license you do not need to get it tested. However, COD does have a big advantage over BOD in that the test only takes approximately three hours, as opposed to the five days required for BOD testing and it is usually possible over a period of time to establish a ratio of BOD: COD, which allows extrapolation of the BOD.

Conclusion of BOD and COD

1. BOD and COD measurements are taken to determine the pollution level of wastewater. COD value is always greater than the BOD value of a particular water body.
2. BOD measures the oxygen demand for the decomposition of organic material by the microbes in the wastewater.
3. COD measures the oxygen demand for the decomposition of both organic and inorganic materials in the wastewater. This is the main difference between BOD and COD.

FAQ: Understanding BOD and COD

What is BOD and COD, and why are they important?

BOD stands for Biochemical Oxygen Demand, measuring the oxygen consumed by bacteria during organic matter decomposition. COD, or Chemical Oxygen Demand, quantifies the oxygen required for both organic and inorganic material oxidation. These metrics are crucial for assessing water quality and pollution levels.

How do BOD and COD differ in their measurement methods?

BOD is determined through a biological oxidation process, involving a five-day incubation period of sealed water samples. In contrast, COD utilizes a chemical oxidation process with strong oxidizers like potassium dichromate and sulfuric acid, providing a quicker assessment within hours.

What are the permissible limits for BOD and COD in water?

The acceptable limit for BOD is generally set at 30 mg/L, while COD limits typically range from 250 to 500 ppm. These standards help gauge the impact of pollutants on aquatic ecosystems and human health.

What factors influence the values of BOD and COD?

Temperature, pH, types of organic materials present, and the presence of industrial waste can affect BOD and COD measurements. Higher values indicate a greater demand for oxygen, potentially harming aquatic life and ecosystem balance.

Why is COD usually higher than BOD?

COD encompasses both biological and chemical oxygen demands, including substances not captured by BOD tests, such as industrial pollutants. This often leads to higher COD values, reflecting a broader range of oxidizable materials.

How do BOD and COD testing impact environmental regulations?

Monitoring BOD and COD levels is essential for compliance with environmental standards and discharge regulations. Understanding these measures helps industries and wastewater treatment facilities manage their effluent outputs responsibly.

Can BOD and COD testing predict water quality trends over time?

By regularly measuring BOD and COD levels, trends in water quality can be identified, aiding in pollution prevention and mitigation efforts. Long-term monitoring facilitates the assessment of environmental impacts and the effectiveness of remedial actions.

Are there any limitations or drawbacks to BOD and COD testing?

While BOD testing provides valuable insights into organic decomposition, its five-day incubation period can be time-consuming. Additionally, COD testing may not capture all pollutants, necessitating complementary assessments for comprehensive water quality evaluation.

How can industries and municipalities utilize BOD and COD data for wastewater management?

By understanding BOD and COD values, stakeholders can optimize treatment processes, minimize pollutant discharge, and protect water resources. Implementing efficient wastewater management practices contributes to environmental sustainability and regulatory compliance.

What future developments or advancements are expected in BOD and COD analysis?

Ongoing research aims to enhance testing methodologies, improve accuracy, and expand the scope of pollutants detected by BOD and COD assessments. Innovations in sensor technology and data analysis may facilitate real-time monitoring and adaptive management strategies for water quality maintenance.