What Is BOD ? | YASA ET Tips

Updated: Oct 31

Biochemical oxygen demand, or BOD, is a chemical procedure for determining the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period.

High BOD in your water, is it a bad sign? What are its implications? A simple explanation by YASA ET.

Biochemical oxygen demand is an important water pollution parameter. In wastewater, wastewater treatment plant effluent and polluted water, the oxygen required by microorganisms to grow and reproduce by using organic matter is the oxygen equivalent of degradable (can be used by microorganisms) organic matter.

The pollutants in the ground water consume the dissolved oxygen in the water during the oxidation process mediated by microorganisms. The dissolved oxygen consumed is called the biochemical oxygen demand, which indirectly reflects the amount of biodegradable organic matter in the water. It indicates the total amount of dissolved oxygen in the water consumed by the oxidative decomposition of organic matter in the water due to the biochemical action of microorganisms, making it inorganic or gasifying. The higher the value, the more organic pollutants in the water and the more serious the pollution.

Hydrocarbons, proteins, oils, lignin, etc. existing in domestic sewage and industrial wastewater such as sugar, food, paper, and fiber in a suspended or dissolved state are organic pollutants, which can be decomposed by the biochemical action of aerobic bacteria , due to the consumption of oxygen in the decomposition process, it is also called aerobic pollutants. If such pollutants are discharged into the water body too much, it will cause a lack of dissolved oxygen in the water.

At the same time, the organic matter will cause corruption through the decomposition of anaerobic bacteria in the water, and produce malodorous gases such as methane, hydrogen sulfide, mercaptan and ammonia, which will deteriorate the water body. Smelly.

It takes about 100 days for the complete oxidative decomposition of various organic substances in sewage. In order to shorten the detection time, the general biochemical oxygen demand is represented by the oxygen consumption of the tested water sample in five days at 20 °C. It is the five-day biochemical oxygen demand, referred to as BOD5, and for domestic sewage, it is about 70% of the oxygen consumption of complete oxidative decomposition.

Although biochemical oxygen demand is not an accurate and quantitative test, because it indirectly reflects the relative content of organic substances in water, BOD has been widely used as an environmental monitoring index for a long time. It is not realistic to consider each compound separately, and BOD is also used to simulate the change of organic matter in water.

The ratio of biochemical oxygen demand to chemical oxygen demand (COD) can indicate the proportion of organic matter that is difficult to biochemically decompose in water, and the organic pollutants that are difficult to decompose by microorganisms are more harmful to the environment. It is generally considered that biochemical treatment is suitable when this ratio is greater than 0.3 in wastewater.

In the measurement of BOD, it is usually stipulated to use the test conditions of 20 ° C and 5 days, and the result is expressed in mg/L of oxygen, which is recorded as the five-day biochemical oxygen demand, symbol.

Generally, the five-day biochemical oxygen demand of a clean river does not exceed 2mg/L, and if it is higher than 10mg/L, it will emit a foul odour. Industrial, agricultural and aquatic water use requires that the biochemical oxygen demand should be less than 5mg/L, and the drinking water should be less than 1mg/L. For general domestic sewage organic wastewater, the nitrification process can be significantly carried out after 5-7 days, so it will not affect the measurement of organic matter BOD5; for special organic wastewater, in order to avoid the interference caused by oxygen consumption in the nitrification process, it can be used in the sample Inhibitors are added.

BOD Sewage Discharge Standards

The discharge standards for wastewaters are based on different factors. In many countries the maximum allowable concentration of biochemical oxygen demand (BOD) of wastewater from factories is 60 mg/L, and the biochemical oxygen demand of surface water shall not exceed 4 mg/L.

In China for urban sewage treatment plant there are four different grades:

  • Grade A standard 10mg/L

  • Grade B standard 20mg/L

  • Grade II standard 30mg/L

  • Grade III standard 60mg/L

The difference between biochemical oxygen demand (BOD) and chemical oxygen demand (COD, Chemical Oxygen Demand)

Chemical oxygen demand (COD) is a chemical method to measure the amount of reducing substances that need to be oxidized in a water sample. Under certain conditions, the amount of oxidant consumed by oxidizing reducing substances in 1 liter of water sample is used as an indicator, which is converted into the milligrams of oxygen required for each liter of water sample after all the oxidation, expressed in mg/L. It reflects the degree of contamination of water by reducing substances.

This indicator is also used as one of the comprehensive indicators of the relative content of organic matter. The ratio of biochemical oxygen demand to chemical oxygen demand can indicate how much organic pollutants in water are difficult to decompose by microorganisms. Organic pollutants that are difficult for microorganisms to decompose are more harmful to the environment.

How To Measure BOD In Water?

The microbial sensor for measuring the biochemical oxygen demand in water is composed of an oxygen electrode and a microbial bacterial membrane. The role of bacteria in the membrane consumes a certain amount of oxygen, which reduces the quality of oxygen diffused to the surface of the oxygen electrode. When the biodegradable organic matter in the sample diffuses to the bacterial membrane at a constant rate (mass), the mass of oxygen diffused to the surface of the oxygen electrode is also constant, so a constant current is generated.

Since there is a quantitative relationship between the difference of the constant current and the reduction of oxygen, the biochemical oxygen demand in the sample can be converted accordingly. Rapid determination of biochemical oxygen demand in water and sewage by microbial sensors. The biochemical oxygen demand stipulated in this standard refers to the amount of dissolved oxygen consumed by the dissolved biodegradable organic matter in water and sewage under the action of microorganisms.

Biochemical oxygen demand is widely used to measure the pollution intensity of wastewater and the load and efficiency of wastewater treatment structures, and is also used to study the oxygen balance of water bodies (see river self-purification).

The stored sample or the diluted water sample can be incubated for a period of time, and the difference between the dissolved oxygen of the sample before and after storage is its biochemical oxygen demand at 5 days (BOD5). At present, the cultivation time adopted by various countries is 5 days, the temperature is 20°C, and the parameter is called the five-day biochemical oxygen demand, which is represented by the symbol BOD5, 20°C.

Extending the storage time can measure the total oxygen required by microorganisms to degrade organic matter in water, which is called total biochemical oxygen demand. Generally, it is calculated by BOD5 according to the law of biochemical oxygen consumption. The detection of biochemical oxygen demand is not easy to be accurate. The storage, dilution, inoculation and other testing procedures of water samples should be carried out according to standard methods. Special equipment is often used for toxic industrial wastewater treatment, and sometimes it cannot be measured precisely. The BOD5 of high-concentration organic industrial wastewater can reach thousands or millions of mg/L. The BOD5 of urban sewage is around 200 mg/L. In water bodies that are not polluted by wastewater, BOD5 is often lower than 2 mg/L.


If you have any doubts about BOD and your water quality, YASA ET can help to answer your questions.

Kindly get in touch with our team at:

📧 info@yasa.ltd

🌐 www.yasa.ltd(EN)

🌐 www.yashahuanjing.cn (中文)

📱 +86 136 3643 1077

Visit YASA ET official online store, click here