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What Is Biochemical Oxygen Demand (BOD or BOD5) ? | YASA ET Tips

Updated: Jun 25

In this article we explore the meaning of Biochemical Oxygen Demand (BOD or BOD5) and its implications for water and wastewater treatment applications.

BOD Lab Test Samples
BOD Lab Test Samples

What Is Biochemical Oxygen Demand (BOD or BOD5) ?

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.

What is biochemical oxygen demand and why is it important?

Biochemical oxygen demand, or BOD, measures the amount of oxygen consumed by microorganisms in decomposing organic matter in stream water. BOD also measures the chemical oxidation of inorganic matter (i.e., the extraction of oxygen from water via chemical reaction).

A test is used to measure the amount of oxygen consumed by these organisms during a specified period of time (usually 5 days at 20 C). The rate of oxygen consumption in a stream is affected by a number of variables: temperature, pH, the presence of certain kinds of microorganisms, and the type of organic and inorganic material in the water.

BOD directly affects the amount of dissolved oxygen in rivers and streams. The greater the BOD, the more rapidly oxygen is depleted in the stream. This means less oxygen is available to higher forms of aquatic life. The consequences of high BOD are the same as those for low dissolved oxygen: aquatic organisms become stressed, suffocate, and die.

Sources of BOD include leaves and woody debris; dead plants and animals; animal manure; effluents from pulp and paper mills, wastewater treatment plants, feedlots, and food-processing plants; failing septic systems; and urban stormwater runoff.

How to measure BOD?

Winkler Method

The Winkler method involves filling a sample bottle completely with water (no air is left to bias the test). The dissolved oxygen is then "fixed" using a series of reagents that form an acid compound that is titrated. Titration involves the drop-by-drop addition of a reagent that neutralizes the acid compound and causes a change in the color of the solution. The point at which the color changes is the "endpoint" and is equivalent to the amount of oxygen dissolved in the sample. The sample is usually fixed and titrated in the field at the sample site. It is possible, however, to prepare the sample in the field and deliver it to a lab for titration.

Dissolved oxygen field kits using the Winkler method are relatively inexpensive, especially compared to a meter and probe. Field kits run between $35 and $200, and each kit comes with enough reagents to run 50 to 100 DO tests. Replacement reagents are inexpensive, and you can buy them already measured out for each test in plastic pillows.

You can also buy the reagents in larger quantities, in bottles, and measure them out with a volumetric scoop. The advantage of the pillows is that they have a longer shelf life and are much less prone to contamination or spillage. The advantage of buying larger quantities in bottles is that the cost per test is considerably less.

The major factor in the expense of the kits is the method of titration they use eyedropper, syringe-type titrator, or digital titrator. Eyedropper and syringe-type titration is less precise than digital titration because a larger drop of titrant is allowed to pass through the dropper opening and, on a micro-scale, the drop size (and thus the volume of titrant) can vary from drop to drop. A digital titrator or a buret (which is a long glass tube with a tapered tip like a pipet) permits much more precision and uniformity in the amount of titrant that is allowed to pass.

If your program requires a high degree of accuracy and precision in DO results, use a digital titrator. A kit that uses an eye dropper-type or syringe- type titrator is suitable for most other purposes. The lower cost of this type of DO field kit might be attractive if you are relying on several teams of volunteers to sample multiple sites at the same time.

Meter and Probe

A dissolved oxygen meter is an electronic device that converts signals from a probe that is placed in the water into units of DO in milligrams per liter. Most meters and probes also measure temperature. The probe is filled with a salt solution and has a selectively permeable membrane that allows DO to pass from the stream water into the salt solution. The DO that has diffused into the salt solution changes the electric potential of the salt solution and this change is sent by electric cable to the meter, which converts the signal to milligrams per liter on a scale that the volunteer can read.

DO meters are expensive compared to field kits that use the titration method. Meter/probe combinations run between $500 and $1,200, including a long cable to connect the probe to the meter. The advantage of a meter/probe is that you can measure DO and temperature quickly at any point in the stream that you can reach with the probe. You can also measure the DO levels at a certain point on a continuous basis. The results are read directly as milligrams per liter, unlike the titration methods, in which the final titration result might have to be converted by an equation to milligrams per liter.

However, DO meters are more fragile than field kits, and repairs to a damaged meter can be costly. The meter/probe must be carefully maintained, and it must be calibrated before each sample run and, if you are doing many tests, in between samplings. Because of the expense, a volunteer program might have only one meter/probe. This means that only one team of samplers can sample DO and they will have to do all the sites. With field kits, on the other hand, several teams can sample simultaneously.

Laboratory Testing of Dissolved Oxygen

If you use a meter and probe, you must do the testing in the field; dissolved oxygen levels in a sample bottle change quickly due to the decomposition of organic material by microorganisms or the production of oxygen by algae and other plants in the sample. This will lower your DO reading. If you are using a variation of the Winkler method, it is possible to "fix" the sample in the field and then deliver it to a lab for titration. This might be preferable if you are sampling under adverse conditions or if you want to reduce the time spent collecting samples. It is also a little easier to titrate samples in the lab, and more quality control is possible because the same person can do all the titrations.

High BOD in your water, is it a bad sign? What are its implications?

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.

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.

Is Biochemical Oxygen Demand (BOD or BOD5) Accurate?

Although biochemical oxygen demand (BOD) 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.

Five-day Biochemical Oxygen Demand (BOD5)

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 (BOD5), 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.

For example 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.


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