Tag Archives: Biochemical Oxygen Demand

Environmental engineering, Water pollution

What Biochemical Oxygen Demand (BOD) Measures?

July 20, 2025 Jefy Jean A 1 Comment

Biochemical oxygen demand (BOD) is a crucial parameter in assessing water quality and monitoring environmental health. In the field of wastewater treatment, understanding BOD in wastewater treatment is essential for evaluating how much organic matter exists in water bodies and the efficiency of purification processes. The BOD test measures the amount of oxygen that microorganisms consume while breaking down organic pollutants, providing a reliable indicator of organic pollution. Elevated BOD levels can signal excessive contamination, which can deplete dissolved oxygen and harm aquatic life.

By regularly conducting BOD measurement, experts can identify sources of water pollution, compare BOD vs COD (chemical oxygen demand), and implement effective strategies for sewage treatment. Ultimately, controlling biological oxygen demand is fundamental for protecting both public health and the environment.

What Biochemical Oxygen Demand measures are a question that all of us have in our minds while learning about water pollution. In this blog, I will walk you through Biochemical Oxygen Demand and, and its significance. The concept of Dissolved Oxygen is described in detail in the upcoming paragraphs. In the next section, I will show you the complete details about what actually the Biochemical Oxygen Demand measures and BOD vs COD

What does the Biochemical Oxygen Demand measure?

Biological oxygen demand (BOD) measures the amount of oxygen that microorganisms require to break down organic matter present in a water sample. It also accounts for the chemical oxidation of some inorganic materials, both of which contribute to the overall depletion of dissolved oxygen. Typically, the BOD test expresses results in milligrams of oxygen consumed per litre over a 5-day incubation at 20°C, offering a reliable estimate of organic pollution in water.

Monitoring BOD in wastewater treatment is essential for assessing the effectiveness of sewage treatment processes and ensuring optimal water quality. Major sources contributing to high BOD values include plant debris, animal waste, food-processing effluents, wastewater from pulp and paper mills, leachate from urban stormwater runoff, and poorly functioning septic systems. Reduction in biochemical oxygen demand is a key factor in evaluating successful wastewater treatment and protecting aquatic environments.

Biochemical Oxygen Demand in wastewater (BOD) — Biochemical Oxygen Demand in wastewater

Biochemical Oxygen Demand has a direct effect on the amount of dissolved oxygen in rivers and streams. The higher the BOD, the faster the rate of oxygen depletion in the stream. This results in higher forms of aquatic life having less oxygen available to them. As a result, they suffocate, and eventually, die.

Total Biological Oxygen Demand Measures

Total biochemical oxygen demand is the quantity of oxygen necessary to totally oxidise organic substances to carbon dioxide and water over generations of microbial development, death, degradation, and cannibalism. It has a greater impact on food webs than water quality.

Biological Oxygen Demand Measures of Drinking Water

A water sample having a BOD₅ between 1 and 2 mg/l indicates very pure water, 3.0 to 5.0 mg/l means moderately clean water and > 5 mg/l indicates a neighbouring pollution source. The biochemical oxygen demand of safe drinking water must be 1-2 mg/l.

Factors affecting Biochemical Oxygen Demand

Temperature, nutrient concentrations, aeration and the enzymes available to indigenous microbial populations affect the BOD measurements. For instance, rapids and waterfalls will speed up the decomposition of organic and inorganic material in stream water. As a result, BOD levels at a sampling location with slower, deeper waters may be higher for a given volume of organic and inorganic material than at a similar site in highly aerated waters.

Chlorine can interfere with the BOD measurements by preventing or killing the microorganisms that break down the organic and inorganic substances in a sample. Therefore, use sodium thiosulfate to neutralise the chlorine while sampling in chlorinated waters, such as those below a sewage treatment plant’s effluent.

Algae in the wastewater affect the Biochemical Oxygen Demand measures by releasing extra oxygen into the wastewater during photosynthesis. Hence, perform the BOD test in complete darkness. Before delving into Biochemical Oxygen Demand measures we should first understand the concept of Dissolved Oxygen (DO), its significance and its measurement.

Dissolved Oxygen (DO)

Aquatic plants and algae release oxygen into the water after performing photosynthesis in the presence of sunlight. The aquatic animals breathe this dissolved oxygen. Also, some oxygen from the atmosphere continuously dissolves into the water through reaeration. These three processes are in equilibrium and maintain the level of oxygen in water bodies at the required levels.

When organic substances or pollutants enter the waterbody, it disturbs the dissolved oxygen balance since microbes utilize dissolved oxygen in its breakdown. In other words, these organic substances exert a demand on the available dissolved oxygen.

Dissolved Oxygen - Aquatic plants — Dissolved Oxygen – Aquatic plants

The greater the oxygen necessary for its breakdown, the greater would be the reduction in the dissolved oxygen in the water body. Pollution occurs when the oxygen demand exceeds the dissolved oxygen availability.

Also read: Wastewater Treatment- Stages and Process full details

Dissolved Oxygen Determination

Winkler’s method helps in determining the dissolved oxygen. The principle behind this method is the reaction between dissolved oxygen and manganese ions to precipitate out manganese dioxide.

Mn²⁺ + O₂ —-> MnO₂

The manganese dioxide then reacts with iodide ions. This reaction liberates iodine in an amount chemically equivalent to the original dissolved oxygen.

MnO₂ + 2I^– + 4H⁺ —-> Mn²⁺ + I₂ + 2H₂O

Titration with sodium thiosulphate gives the amount of iodine liberated and thereby the equivalent dissolved oxygen content. Thus we can measure the amount of Dissolved Oxygen in a given water sample.

Now, you have got a clear idea about Dissolved Oxygen. In the next section, I will show you the complete details about what actually the Biological Oxygen Demand measures.

Next, let’s see the procedure to obtain the Biochemical Oxygen Demand measures.

Calculation of Biochemical Oxygen Demand

We require two samples from a location to measure Biochemical Oxygen Demand. One is immediately tested for dissolved oxygen, while the other is incubated in the dark at 20^oCelsius for 5 days before being examined for the residual dissolved oxygen.

Let me show you the detailed procedure.

Fill two standard 300-ml BOD bottles with the sample wastewater. Seal the bottles properly.
Immediately determine the dissolved oxygen content of one of the samples using Winkler’s method.
Incubate the second bottle at 20 ⁰C for 5 days in complete darkness.
Determine the DO levels after 5 days.

The amount of BOD is the difference in oxygen levels between the first and second tests, measured in milligrams per litre (mg/L). This is the amount of oxygen that the microorganisms require throughout the incubation period to break down the organic materials in the sample container.

DO (mg/L) of first bottle – DO (mg/L) of second bottle = BOD (mg/L)

The dissolved oxygen level may be nil at the end of the 5-day incubation period. This is particularly the case for rivers and streams which have a high pollution load of organic matter. Moreover, it is impossible to determine the BOD level because it is unknown when the zero point was reached. In this situation, dilute the original sample by a factor that yields a final dissolved oxygen content of at least 2 mg/L. The dilutions should be done with special dilution water of high purity.

The dilution water consists of deionised water with sufficient nutrients, phosphate buffer, trace elements and seed organisms (mostly settled domestic sewage). Perform a blank run on this dilution water and subtract its oxygen demand from the results.

BOD₅ (mg/l) = D* [(DO_t=0 – DO_t=5)]_sample – [(DO_t=0 – DO_t=5)]_blank

BOD₅ vs BOD₂₀

During the standard 5-day/20 ⁰Celsius conditions, about two-thirds of the carbonaceous material undergoes degradation. In the 5-day test, compounds that aren’t easily biodegradable or soluble don’t undergo complete digestion. Incubation of 5 days gives the soluble BOD or BOD₅ measures while that of 20 days gives Ultimate Biochemical Oxygen Demand measures or BOD₂₀

It requires nearly 20 days for the complete breakdown. On the other hand, the 20-day Biochemical Oxygen Demand measures greater long-term oxygen demand from insoluble materials including cellulose, long-chain fatty acids, and grease. Always keep in mind that COD > BOD₂₀ > BOD₅.

In spite of its limitations, Biochemical Oxygen Demand analysis has wide applications in monitoring pollution. These days, Chemical Oxygen Demand analysis is gaining wide popularity for research and plant control.

BOD vs COD – Understanding Key Water Quality Indicators

Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are critical indicators for water quality assessment. BOD measures the amount of oxygen that microorganisms need to biologically break down organic matter in water, reflecting the level of biodegradable organic pollution. In comparison, COD quantifies the total oxygen required to chemically oxidize both organic and inorganic substances, providing a broader measure of all oxidizable pollutants . BOD is determined by incubating a water sample for five days, while COD uses strong oxidizing agents for a faster result. COD values are always higher than BOD because COD includes both biodegradable and non-biodegradable materials. Monitoring both parameters is essential in wastewater treatment to evaluate pollution levels and treatment efficiency

Key Takeaways

Biochemical Oxygen Demand (BOD) measures the amount of dissolved oxygen microorganisms need to decompose organic matter in water, indicating the level of organic pollution .
BOD is determined by measuring oxygen levels before and after a 5-day incubation period at 20°C to assess microbial activity and organic load .
High BOD values signal greater organic pollution, leading to depleted dissolved oxygen and potentially harming aquatic life, including fish kills .
Controlling BOD is essential in wastewater treatment to ensure discharged water meets environmental standards and sustains aquatic ecosystems .
Main sources of high BOD include sewage, industrial effluents, plant debris, animal waste, stormwater runoff, and failing septic systems .
Dissolved Oxygen (DO) levels are inversely related to BOD; higher BOD reduces DO, affecting water quality .
BOD vs COD: While BOD measures biologically oxidizable matter, COD includes both biodegradable and non-biodegradable substances; COD values are typically higher and tests are faster .
BOD testing is a standard practice in monitoring and regulating water and wastewater treatment plant performance .
BOD is expressed in milligrams per litre (mg/L), enabling clear comparison and assessment of water samples’ pollution levels .
Both BOD and COD are critical for understanding water pollution and ensuring effective environmental management .

Conclusion

Biochemical Oxygen Demand (BOD) is a fundamental indicator used to gauge the organic pollution in water by quantifying the amount of oxygen required for microbial decomposition . Regular measurement of BOD is essential for environmental health, as it helps identify pollution sources and assess the effectiveness of wastewater treatment systems . High BOD levels are directly linked to reduced dissolved oxygen, which can endanger aquatic organisms and disrupt ecosystem balance . Comparison with Chemical Oxygen Demand (COD) complements this analysis by accounting for total oxidizable substances both biodegradable and non-biodegradable providing a comprehensive picture of water quality . Effective BOD control supports the protection of natural waterbodies and public health, highlighting its critical role in environmental monitoring and water resource management

That’s it about Biochemical Oxygen Demand. Hope you found it insightful. Let us know your queries in the comments.

Environmental engineering, Water pollution

What are Water Pollutants? – Definition, Sources and Types

June 30, 2021 Jefy Jean A Leave a comment

Water Pollutants kill more people every year than from all forms of violence including war, according to the UNDESA. Every day, 2 million tons of sewage, industrial, and agricultural waste reaches water bodies all over the world. With the growth of the human population, industrial and agricultural activities the hydrological cycle got disrupted. As a result, declining water quality has become a global issue of concern.

In this blog, I will take you on a short trip exploring the various water pollutants, its sources and effects.

Water Pollutants Definition

Water Pollutants are the organic or inorganic chemicals and microbes that degrade the water quality of a water body and renders it hazardous for human consumption and the aquatic life thriving in it. Toxic compounds from farms, cities, and factories easily dissolve and combine with it, polluting the water.

Water, also known as a “universal solvent,” can dissolve more chemicals than any other liquid on the planet. It’s also the reason why water gets easily polluted.

Water Pollutants Sources

The sources of water pollutants belong to two categories.

Point Source Water Pollutants
Non-Point source Water Pollutants

Point Source Water Pollutants

Point source water pollution occurs when contamination arises from a single source. Examples of point source water pollutants include contamination from leaking septic systems, chemical and oil spills, effluent released illegally by an oil refinery, or wastewater treatment plant . While point source water pollutants originate in a single location, it has the potential to pollute miles of streams and the ocean.

Non-Point Source Water Pollutants

Water Pollutants arising from dispersed sources is referred to as non point source water pollutants. Agricultural or storm water runoff, pond ash from ash dykes, as well as debris blown into streams from land, are examples. Above all, it’s tough to control because there’s no single, recognisable source.

Water Pollutants Types

Water Pollutants are classified into nine types as shown below.

Oxygen Demanding Wastes
Disease-causing Agents
Synthetic Organic Compounds
Plant Nutrients
Inorganic Chemicals and Minerals
Sediments
Radioactive Substances
Thermal Discharges
Oil

Oxygen Demanding Wastes

Organic wastes which demand a high amount of dissolved oxygen for their microbial decomposition are referred to as oxygen demanding wastes.
Such kind of organic wastes arises from sewage, food processing plants, tanning operations etc.
Biochemical Oxygen Demand or BOD measures the water pollution potential of the organic waste.
The amount of dissolved oxygen (DO) required by aerobic biological organisms to decompose the organic waste present in a given water sample at a particular temperature over a given time period is known as biochemical oxygen demand (BOD).
Oxygen demand is directly proportional to the organic waste concentration in the water.
In other words, the higher the BOD of the wastewater, the higher is the amount of oxygen required for the degradation of waste.
Oxygen demanding wastes pose a hazard to aquatic life by using up the dissolved oxygen in the water for its degradation.

Disease-causing Agents

Sewage and wastes from farms and industries like tanning and meat packaging industries carry pathogens into the water bodies. As a result, water contains bacteria which causes cholera, typhoid, amoebic dysentery and viruses responsible for polio, coxsackie fever. These pathogens enter the human body through drinking water and other activities.

Escherichia coli is a harmless bacteria found in high concentrations in human faeces. They are used to assess the hygienic quality of water. Since the coliforms usually travel together with the pathogens, a high concentration of E. coli indicates faecal contamination and the presence of pathogens.

Synthetic Organic Compounds

Pesticides, insecticides and herbicides used in the crop fields reach the water bodies via surface runoff from agricultural lands and stormwater.
The commonly used chlorinated pesticides include aldrin, dieldrin and DDT.
They are highly stable, volatile and soluble in fats and oils and they accumulate in the bodies of aquatic organisms.
Through biological magnification, it gets more concentrated from one trophic level to the next in the food chain.
Fishes and predatory birds are the victims of pesticide pollution. For instance, dieldrin affects the calcium metabolism in predatory birds and leads to thinning of their eggshells.
Through drinking water and consumption of fishes, pesticide residues enter the human body.
The surfactants present in detergents create foam in water bodies and hamper the oxygen absorption of water. And, higher levels of phosphate act as a plant nutrient and generate eutrophic conditions.

Plant Nutrients

Phosphates, nitrates and ammonia are the major plant nutrients. They find their way into water bodies via effluents from fertilizer, food and textile industries.
As the concentration of these nutrients increases in the water bodies, algae absorbs them and grows excessively, resulting in eutrophication.
The process through which a water body gets enriched in dissolved nutrients is known as eutrophication.
This leads to algal bloom and develops a green slime layer over the surface of the water.
Therefore, sunlight can’t reach the bottom of the water bodies and this hampers atmospheric reoxygenation.
After that, the algal growth dies down and its degradation results in anaerobic conditions.
An anaerobic bacterium, Clostridium botulinum can flourish in this environment. It secretes a powerful toxin, botulinum that kills the algae feeding birds and humans.
Nitrate enters the human body through drinking water. It forms a complex, methaemoglobin which reduces the oxygen-carrying capacity of the blood. This leads to a fatal condition called methaemoglobin anaemia or blue baby disease.

Inorganic Chemicals and Minerals

The water pollutants like inorganic salts, mineral acids, finely divided metals and metal compounds fall under the category of inorganic chemicals and minerals. Municipal and industrial wastewater and mine runoffs are the main sources of these pollutants. Sulphur and coal mining leads to acid mine drainage consisting of sulphuric acid and iron compounds. In addition to this, tanneries, textiles and coke oven operations release alkalies to the water bodies.

Cadmium, Chromium, Lead, Mercury and Silver are metals found in industrial wastewater that requires serious attention.
Effluents from chemical plants, electroplating and textiles generate cadmium. The use of cadmium contaminated water for irrigation may have been the reason for itai-itai disease in Japan.
Wastewaters from aluminium anodizing, paint and dye industries, ceramic and glass industry brings both trivalent and hexavalent chromium to the water bodies.
Lead is present in industrial effluents from battery manufacture, printing and painting operations. It is a cumulative poison and concentrates mainly on the bones.
The most toxic aquatic pollutant is mercury owing to its rapid methylation in the aquatic environment. It builds up in the food chain and reaches toxic levels at the top of the trophic level.
Severe mercury poisoning causes Minamata disease, a neurological disease. Industrial effluents from chlorine and caustic soda, fertilizers and pesticides are the main culprits of mercury pollution.
The main sources of silver in wastewater are electroplating and photographic operations.

Sediments

Soil, sand and mineral particles reaching the aquatic environment through floodwaters constitute the sediments. The presence of sediments increases the turbidity of water bodies. Therefore, sunlight can’t penetrate to the bottom and its availability to aquatic plants decreases. Moreover, they cause the thickening of fish gills and asphyxiation of the fish and sediments also destroy the spawning sites of fish on the river bed.

Radioactive Substances

The radioactive material used in industrial, medical, or scientific activities creates nuclear waste. Uranium and thorium mining and refining are also sources of nuclear waste. Radium is the most important radioactive waste product and is a health hazard in drinking water.

Thermal Discharges

Heat is a water pollutant because it reduces the capacity of water to carry dissolved oxygen and raises the rate of metabolism in fish.
The practice of releasing cooling water from power stations into rivers is a major source of heat since the released water can be up to 15 degrees Celsius warmer than naturally occurring water.
Firstly, some fish species, such as trout, cannot survive in water with very low dissolved oxygen levels. Secondly, their eggs will not hatch at temperatures higher than 14.5 degree Celsius.
Moreover, there is a decline in oxygen saturation percentage with the increase in temperature.
Due to the density difference, hot water forms a separate layer above cold water.
This prevents the reaeration of the cold water underneath, as it has no atmospheric contact.
Due to the normal biological activities in the lower layer, the Dissolved Oxygen level falls rapidly and generates anaerobic conditions.

Oil

Every year, nearly half of the estimated 1 million tonnes of oil that enters marine habitats comes from land-based sources such as factories, farms, and towns. In addition, oil can ooze out from the ocean’s depths and eroded sedimentary rocks. Since oil does not dissolve in water and instead forms a thick sludge, it suffocates fish and sticks to the feathers of marine birds, preventing them from flying. Also, it prevents photosynthetic aquatic plants from receiving sunlight.

To sum up, the concentration of pollutants in water can be reduced by treating the effluents. To know more about wastewater treatment methods, please check our blogs, Wastewater Treatment- Stages and Process full details.