The B/C ratio is the ratio of BOD to COD!

BOD(Biochemical Oxygen Demand) : It refers to the mass concentration of dissolved oxygen required for the biochemical process in which microorganisms in water decompose organic matter under aerobic conditions. To ensure the comparability of BOD detection values, a time period is generally stipulated, and the consumption of dissolved oxygen in water is measured. Usually, a five-day period is adopted, which is called the five-day biochemical oxygen demand and denoted as BOD5. The five-day biochemical oxygen demand is often used. The larger the BOD value, the more organic matter is contained in the water, and thus the more serious the pollution is.

BOD is an environmental monitoring indicator used to monitor the pollution of organic matter in water. Organic matter can all be decomposed by microorganisms, and this process requires oxygen consumption. If the dissolved oxygen in the water is insufficient to meet the needs of microorganisms, the water body will be treated in a polluted state.

COD(Chemical Oxygen Demand) : It is the amount of oxidant consumed when treating water samples under certain conditions with a certain strong oxidant. It reflects the degree of pollution by substances in water. The greater the chemical oxygen demand, the more serious the pollution by organic matter in the water. COD is expressed in mg/ L. The COD value detected by water quality monitoring instruments can be classified into five major categories. Among them, the COD of Class I and Class II is ≤15mg/L, which basically meets the drinking water standard. Water with a value greater than Class II cannot be used as drinking water. Among them, Class III COD≤20mg/L, Class IV COD≤30mg/L, and Class V COD≤40mg/L are classified as polluted water quality. The higher the COD value, the more severe the pollution.

Why are BOD and COD chosen as pollution indicators?

Due to the wide variety of organic substances in wastewater, some wastewater contains tens, dozens, or even hundreds of organic substances. If each organic substance in the wastewater is qualitatively and quantitatively analyzed one by one, it will not only consume time but also a lot of chemicals. Then, can only one pollution indicator be used to represent all the organic substances in the wastewater and their quantities?

Environmental science researchers have found through their studies that all organic substances share two commonalities: first, they are all composed of at least carbon and hydrogen; Second, the vast majority of organic substances can be chemically oxidized or oxidized by microorganisms. Their carbon and hydrogen respectively react with oxygen to form harmless carbon dioxide and water. Organic substances in wastewater consume oxygen both during the chemical oxidation process and the biological oxidation process. The more organic substances in the wastewater, the more oxygen is consumed. There is a direct proportional relationship between the two. Therefore, chemical oxygen demand (COD) and biochemical oxygen demand (BOD) are used to represent the content of reductive substances in wastewater!

What is the relationship between BOD and COD?

During the sewage treatment process, there are hundreds of organic substances. Analyzing each of these organic substances one by one is not only time-consuming but also consumes a lot of chemicals. After research, it was found that all organic substances share two commonalities: first, they are all composed of carbon and hydrogen; second, the vast majority of organic substances can be chemically oxidized or oxidized by microorganisms, and their carbon and hydrogen respectively react with oxygen to form harmless carbon dioxide and water. Organic matter in sewage consumes oxygen both during the chemical oxidation process and the biological oxidation process. The more organic matter there is in the wastewater, the more oxygen is consumed. There is a direct proportion between the two. Therefore, the amount of oxygen consumed by the oxidation of sewage with chemical agents is called COD(Chemical Oxygen Demand), and the amount of oxygen consumed by the oxidation of microorganisms in sewage is called BOD(Oxygen Demand for Gas).

As COD(Chemical Oxygen Demand) and BOD(Oxygen Demand for Gas) can comprehensively reflect the quantity of all organic matter in water, there are many such detection instruments, and the detection methods are simple. The test results can be obtained in a relatively short time. Therefore, they are widely used in water quality detection and analysis, becoming important indicators for water quality monitoring and also an important basis for environmental monitoring of water bodies. In sewage treatment, we all hear about it quite often.

In fact, COD(Chemical Oxygen Demand) does not merely reflect organic matter in water; it can also represent inorganic substances with reducing properties in water, such as sulfides, ferrous ions, sodium sulfite, etc. For instance, if ferrous ions in the sewage are not completely removed in the neutralization tank, the presence of ferrous ions in the effluent from biochemical treatment may cause the COD(Chemical Oxygen Demand) of the effluent to exceed the standard.

Some of the organic substances in sewage can be biologically oxidized (such as glucose and ethanol), some can only be partially biologically oxidized and degraded (such as methanol), and there are also some organic substances that cannot be biologically oxidized and degraded and have certain toxicity (certain surfactants). In this way, the organic matter in the sewage can be divided into two parts: biodegradable and non-biodegradable organic matter. Conventionally, COD(Chemical Oxygen Demand) basically represents all the organic matter in wastewater, while BOD(Oxygen Demand for gas) is the biodegradable organic matter in wastewater. Therefore, the difference between COD and BOD can indicate the non-biodegradable organic matter in wastewater.

What does the B/C ratio indicate?

Usually, we use the BC ratio (BOD/COD) to represent the biodegradability of wastewater. When the BOD/COD ratio is greater than 0.3, it is generally considered that the wastewater is biodegradable!

Ratio relationship

The higher the BOD5/COD ratio, the higher the biodegradability assessment of the wastewater. Under anaerobic and anoxic conditions, purification is achieved by the digestion of organic matter in the wastewater by anaerobic bacteria. In antibiotic wastewater, as antibiotics contain a large number of bacteria and fungi, they can also digest the organic matter in the wastewater, thereby achieving purification. It is generally believed that only wastewater with a ratio greater than 0.3 is suitable for biological treatment.

The BOD5/COD index is the ratio of the five-day biochemical oxygen demand to the chemical oxygen demand, and it is an indicator of the biodegradability of wastewater. The formula is expressed as: BOD5/COD=(1-α)×(K/V)

In the formula:

α is the ratio of the biologically refractory CODNB to COD. K is the ratio of BOD5 to the final biochemical oxygen demand BODU, which is a constant.
It can be seen from the formula that the BOD5/COD value decreases as α increases. Therefore, this ratio can reflect the function of the biodegradability of wastewater.

When B/C > 0.58, it is completely biodegradable. The B/C ratio is 0.45 to 0.58, indicating good biodegradability. B/C=0.30-0.45 biodegradable; 0.1<B/C < 0.3 Difficult to biodegrade; B/C < 0.1, non-biodegradable.
The lower limit of the biodegradability of wastewater is usually set at BOD5/COD=0.3.