What is BOD in wastewater treatment?
BOD (Biochemical Oxygen Demand) is a vital indicator in the field of wastewater treatment.
Definition and meaning
BOD refers to the amount of dissolved oxygen consumed by microorganisms when decomposing organic matter in water under certain conditions. It reflects the content of biodegradable organic matter in water. Simply put, the more organic matter there is in the water, the more oxygen microorganisms need to decompose these organic matter, and the higher the BOD value. For example, domestic sewage contains a large amount of organic matter, such as food residues, feces, etc., and its BOD value is usually high; while pure water after deep treatment has a very low BOD value.
Measurement method
The most commonly used is the five-day biochemical oxygen demand (BOD5) determination method. This method is to fill the culture bottle with water sample, which should be airtight after plugging, and then culture it in the dark at (20±1)℃ for 5 days. The dissolved oxygen content in the water sample is measured before and after culture, and the difference between the two is BOD5. This method is based on the fact that most organic matter can be well decomposed by microorganisms after 5 days at a temperature of about 20℃.
For example, in a water sample, the initial dissolved oxygen is 8mg/L, and after 5 days of cultivation, the dissolved oxygen is 3mg/L, then the BOD5 value of the water sample is 8-3=5mg/L.
Importance in wastewater treatment
Reflecting the degree of pollution: BOD value is an important indicator to measure the degree of organic pollution in wastewater. It can intuitively tell us the amount of biodegradable organic matter in the wastewater. Generally speaking, the higher the BOD value, the more serious the pollution of the wastewater. For example, the BOD value of untreated paper industry wastewater may be as high as thousands of mg/L, while the BOD value of treated wastewater that meets the discharge standard is generally less than 30mg/L (different regions and discharge standards may vary).
Determine the treatment process: According to the BOD value of the wastewater, it can help to select the appropriate wastewater treatment process. If the BOD value is high, biological treatment processes such as activated sludge or biofilm may be required to use microorganisms to decompose organic matter. Moreover, by comparing the BOD values before and after treatment, the effectiveness of the treatment process can be evaluated. For example, a sewage treatment plant uses activated sludge to treat sewage, and the influent BOD value is 200mg/L. After treatment, the effluent BOD value is reduced to 20mg/L, indicating that the treatment process has a good effect on the removal of organic matter.
Measure the self-purification capacity of water bodies: In natural water bodies, BOD is also an important parameter. It can be used to measure the self-purification capacity of water bodies. If the BOD value of wastewater discharged into the water body is too high and exceeds the self-purification capacity of the water body, it will cause a large amount of dissolved oxygen in the water body to be consumed, thereby affecting the survival of aquatic organisms. For example, in a river, if a large amount of sewage with a high BOD value is discharged, the dissolved oxygen in the water will drop rapidly, and aquatic animals such as fish may die due to lack of oxygen.
What is the difference between BOD and COD in sewage treatment? What is the standard of BOD value in wastewater treatment? What are the methods to reduce BOD in wastewater treatment?