What is BOD Test?

The BOD test acts as an important criterion for knowing the health of an ecosystem. The BOD Test of Water is done to know how impure or pure the water body is. The BOD test procedure involves the collection of the water sample in an air-tight container which is then kept in incubation for five days at 20 ℃. The sample of water contains microorganisms that have the function of decomposing organic matter. They utilize oxygen present in the water to do so. BOD measures the amount of oxygen that gets consumed.

Definition of BOD

BOD calculates the amount of dissolved oxygen that is consumed in a water body containing microorganisms.

What is a BOD Test?

BOD stands for Biochemical Oxygen Demand. BOD test is mainly done to check the pollution level of a water body. It is a good indicator of the level of pollution found in a water body. If the value of BOD is high, it indicates a high level of infestation of microorganisms in the water. BOD is expressed as milligrams of oxygen/litre of water. For drinking water the value of BOD is 1-3 mg/L. Wastewater treatment plants are used to clean wastewater and sewage. BOD tests help in checking the efficiency of these wastewater plants. The organisms living underwater use the oxygen dissolved in water for their use. BOD aims at calculating the amount of oxygen that is utilised by microorganisms while decomposing organic matter. A high BOD means the water is unhealthy.

Objectives of BOD Testing

The following are the objectives of BOD:

1. BOD is used to measure the level of pollution in a water body. The level of BOD is directly proportional to the amount of pollution.
2. BOD gives an estimate of the overall health of a water body. If the BOD level is High, the organisms are getting deficient levels of oxygen, including plants and animals.
3. Investigating the BOD levels at different points in the water body can help pinpoint the source of pollution.
4. Wastewater treatment aims at improving the water quality of a particular water body. By comparing the BOD value of influent and effluents, it becomes easier to estimate the efficiency of wastewater treatment.
5. Frequent BOD tests help companies and other industries to remain compliant with the pollution levels that are allowed for a water body.
6. BOD Testing helps in research on marine ecosystems and how it gets affected by pollution and other factors.

Principle of BOD Test

BOD mainly aims at calculating the dissolved oxygen levels that are consumed by microorganisms, while decomposing organic stuff. Water is usually collected from a marine body like a lake or a river. The sample is collected in a way to make sure that no air enters the sample water. The initial concentration of the dissolved oxygen is calculated. After that, the sample is kept in an incubator for 5 days at 20℃. In this period, the microorganisms decompose the organic matter in the water. The final DO (Dissolved Oxygen) is calculated after 5 days.

Calculation of BOD

The BOD value is calculated by subtracting the Final DO from the initial DO value. The formula for calculating BOD is

BOD= (Initial DO- Final DO) x Dilution Factor / Volume of Sample

Here,

• DO stands for Diffused oxygen
• Initial DO is the one taken before incubation
• The final DO is the one taken after incubation
• The dilution factor is the ratio of the volume of the sample to the diluted sample.

The value of BOD is expressed in mg/L which denotes the amount of oxygen consumed.

BOD Test Procedure

The BOD test involves multiple equipment and supplies which are listed below: 

• BOD incubator 
• Burette and stand
• 300ml BOD bottles: These bottles have glass toppers which ensure air-tight sealing.
• DO Meter
• Wash bottle
• Conical flask
• Measuring cylinder
• Pipette with an elongated tip
• Gloves and seal starch

The procedure is carried out in the following manner:

Neutralization of Sample

50 ml of water sample is added to 100 ml of water. The pH is adjusted at 7± 0.2 using sulfuric acid or sodium hydroxide according to the current pH.

Removal of Chlorine Content

To neutralize the chlorine, add the prescribed volume of sodium sulphite solution to the sample. The chlorine interferes with the proper measurement of the BOD value. Hence it needs to be neutralized.

Preparation of Phosphate Buffer Solution

The Phosphate buffer solution is prepared by dissolving potassium dihydrogen phosphate, 21.75 grams of dipotassium hydrogen phosphate, disodium hydrogen phosphate, and ammonium chloride in distilled water. Dilute the solution to a volume of 1 litre.

Alkali-Iodide-Azide Reagent

Dissolve sodium hydroxide and sodium iodide (NaI) in distilled water to make a 1000 ml solution. Then add and dissolve sodium azide in the solution.

Procedure to Test Dissolved Oxygen

The following steps are carried out to test for DO:

• Manganous Sulphate solution is added to the sample using a pipette tip along with an alkali-iodide-azide reagent.
• The solutions are kept for some time to allow for their reaction with dissolved oxygen.
• Once the precipitates are formed, sulphuric acid is added to dissolve the precipitates.
• The BOD sample is transferred to an Erlenmeyer flask. Then it is titrated with  Sodium Thiosulfate solution using a starch indicator till the blue hue fades. The burette reading is noted.
• Perform a similar procedure to determine the burette reading for the blank sample. 

BOD Test Measurement Problems

There can be several reasons why the accuracy of the BOD result might be affected such as:

1. The water sample which is collected from the source needs to reach the laboratory as soon as possible. If there is a delay in the analysis or a slight contamination of the water sample, the BOD test will fail.
2. Temperature plays an important criteria in determining the result of the BOD test. If there is a minor change in the incubation temperature, it might hamper the microbial activity inside the sample.
3. If by any chance, there is an addition of organic matter in the sample, the BOD test results will come out elevated.
4. All samples that are collected need to be stored in air-tight bottles to prevent any contamination of the sample by atmospheric oxygen.
5. In some cases, there might be organic compounds that need more incubation time to degrade. This can result in a lower value of BOD being computed.
6. If there are any toxic compounds present in the water sample, the microbial activity of the microorganisms is disturbed, thus leading to reduced BOD values.

Importance of Biochemical Oxygen Demand

BOD acts as an important parameter in the measurement of water quality. If the BOD turns out to be high, it means the water is infested with pollutants and sewage. Knowing the BOD level helps in making insightful decisions regarding new projects that might have an impact on the environment and the marine ecosystem. BOD also helps in analyzing the effectiveness of wastewater treatment plants.

Limitations of the BOD Test

The various limitations of the BOD Test include:

1. The BOD is unable to provide information on the non-biodegradable material present in the water body.
2. The amount of time taken is too long in some situations where a rapid test may be more suitable.
3. The BOD results are very sensitive to temperature fluctuations. A slight change might affect the microbial activity leading to error in results.
4. Sometimes in a single sample, different kinds of microbes are present that utilise oxygen differently. This can generate false results.
5. The water samples sometimes contain inhibitory chemicals that may stop the microbial activity thus falsifying the BOD test.
6. The presence of nitrogenous substances like ammonia or nitrate in the sample can cause alteration to the microbial activity as these too lead to the consumption of oxygen.
7. The type of pollutants in the water cannot be determined using the BOD test.

FAQs- BOD Test

1. What Factors May Effect a BOD Test?

The amount of oxygen consumption is dependent on multiple factors like pH, temperature, the type of organic and inorganic matter present in the water sample, and the type of different microorganisms .

2.What are the Limitations of the BOD Test?

The major limitations of the BOD test are:

• Any impurity in the water will affect the microbial activity of the water sample.
• The BOD does not include the non- biodegradable matter present in the sample and hence a proper estimate of the pollution level cannot be made.

3. What Happens if BOD is Low?

If the BOD value is low, the water is pure in nature as it is full of oxygen and less oxygen is being consumed by the organisms in it.

4. What is the Principle of BOD?

BOD is established on the principle that microorganisms keep consuming oxygen to decompose organic waste till the time it does not get dissipated.

5. What are the Applications of BOD?

The applications of BOD are:

• It tells us about the pollution present in the water bodies and aquatic ecosystems.
• It is used to estimate the efficiency of wastewater treatment plant.