BOD、COD、TOD AND TOC

Organic matter is one of the main pollutants in water, so it is very important to know the content of organic matter at all times. However, organic matter in water cannot be measured directly. Other methods are needed, such as oxidizing organic matter in water, calculating how much oxygen is used in total, and then calculating the content of organic matter in water in reverse.

If microorganisms are used for oxidation, the oxygen consumption measured is called biological oxygen demand BOD:

If chemical oxidation is used, the oxygen consumption measured is called chemical oxygen demand COD;

If high-temperature combustion is used, the oxygen consumption measured is called total oxygen demand TOD;

These are several common indicators in various standards for water treatment.

In addition to judging by oxygen consumption, there is another more direct way of thinking. Since the definition of organic matter is carbon-containing compounds, the total amount of organic matter can be measured by measuring the carbon content.

Through combustion, these carbons will generate carbon dioxide. The carbon content can be calculated by measuring the carbon dioxide production. This is called total organic carbon TOC, which can also indirectly indicate the content of organic matter in water.

Although BOD, COD, TOD, and TOC can all indirectly indicate the content of organic matter in water, different measurement methods determine their respective limitations.

BOD: Generally, the measurement time takes at least 5 days. If the water contains toxic substances, the microorganisms in the water will be poisoned, which will have a great impact on the test results.

COD: The reagents used are potassium dichromate and potassium permanganate. The sewage needs to be pre-treated first. It is easy to be targeted by people to use COD removers, resulting in inaccurate test results. Moreover, chromic acid is a toxic substance and will cause secondary pollution.

TOD: The measurement process oxidizes almost all organic matter casually, but oxygen also reacts with hydrogen, nitrogen, phosphorus, and sulfur. If there are more of these components in the water sample, the measured results will definitely be inaccurate.

TOC: For measurement, it is necessary to first use dilute sulfuric acid to remove inorganic carbon in the water sample, then burn it in a high-temperature oxygen flow, and finally detect the carbon dioxide content through an infrared detector/thermal conductivity detector. However, this process requires very precise instruments. If the instrument accuracy is not enough, errors are likely to occur.

Why use BOD and COD as effluent indicators?

Theoretically, using TOC to indicate the organic matter content in wastewater is the most accurate, but currently most countries in the world use BOD and COD. This is because when the standards were specified decades ago, the technical level of each country was limited and TOC testing could not be popularized. Although the technical level of most countries can now be achieved, BOD and COD have been used as standards for effluent indicators for decades, and it is not easy to replace them all with TOC.

Why do many countries now advocate the use of TOC standards?

The TOC indicator was originally common in the fields of water supply and industrial water use, and is now often mentioned in the field of water treatment because many countries are now vigorously developing smart water systems.

The first step in developing smart water systems is to popularize online monitoring. At this time, measuring COD is very inconvenient. Not only do you need to prepare more reagents, the measurement results take a certain amount of time, and it will also cause secondary pollution.

In contrast, TOC has a shorter measurement time, higher accuracy, no secondary pollution, and low detection cost, and is more suitable for high-frequency online monitoring.