What is BOD in water?

What is BOD mean?

Biochemical oxygen demand (BOD), is the decomposition of organic matter in water into inorganic matter by aerobic microorganisms in water at a certain temperature. It is a comprehensive indicator of the content of oxygen-demanding pollutants such as organic matter in water.

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.

Why choose 5 days as a representative?

The pollutants in water consume dissolved oxygen in the water during the oxidation process mediated by microorganisms, and the dissolved oxygen consumed is called biochemical oxygen demand  mg/L), 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 all kinds of organic substances in sewage to be oxidized and decomposed. 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 called the five-day biochemical oxygen demand, or BOD5 for short. For domestic sewage, it is about 70% of the oxygen consumption of complete oxidative decomposition.

The guiding role of BOD

The measurement of BOD5 has a strong lag, because it takes five days to measure a value, and it is already five days later when we get the value. It is for this reason that in many cases, we only pay attention to the value of COD . Usually, when we use BOD, we use the BC ratio (BOD/COD) to represent the biodegradability of sewage. When BOD/COD is greater than 0.3, it is generally considered that the wastewater is biodegradable!

BOD/COD value relationship

The larger the BOD5/COD value, the higher the biodegradability of wastewater. Under anaerobic and anoxic conditions, anaerobic bacteria are used to digest organic matter in wastewater to achieve purification. In antibiotic wastewater, because antibiotics are a lot of bacteria and fungi, they can also digest organic matter in wastewater to achieve purification. It is generally believed that the sewage with this ratio greater than 0.3 is suitable for biological treatment.

The BOD5/COD index is the ratio of 5-day biochemical oxygen demand to chemical oxygen demand, and is an indicator of the biochemical degradability of sewage.

The formula is expressed as   BOD5/COD=(1-α)×(K/V)

where:

(α is the ratio of CODNB to COD, which is difficult to degrade biochemically; 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 with the increase of α, so this ratio can reflect the function of the biochemical degradability of sewage.

When B/C>0.58, it is completely biodegradable;

B/C=0.45~0.58 Good biodegradation;

B/C=0.30-0.45 Biodegradable;

0.1<B/C<0.3 Refractory to biodegradation;

B/C<0.1 is not biodegradable.

Usually, BOD5/COD=0.3 is the lower limit of biochemical degradation of sewage.

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