Biodegradability of wastewater

The B/C ratio of refinery wastewater is generally between 0.2 and 0.3, indicating poor biochemical properties.

Agree with the opinion from the second floor. The microbial degradability of organic matter can be characterized by the value of the BOD/COD ratio. It is generally believed that when the B/C ratio reaches above 0.3 during biological treatment of wastewater, such wastewater is considered to have good biodegradability, and biological treatment can be carried out directly without the need for pretreatment processes such as hydrolysis and acidification to improve its biodegradability. This is experience summarized based on previous research; it can be found in relevant textbooks, but there may be no established guidelines

BOD, or biochemical oxygen demand, refers to the amount of oxygen required for the biological degradation of organic matter in water, and it serves as an indicator of the biodegradability of water. COD, or chemical oxygen demand, measures the amount of oxygen consumed by organic substances in water using potassium dichromate, and it directly reflects the concentration of organic matter in the water. B/C is an abbreviation for the ratio of BOD5 to COD, and this ratio can indicate the biodegradability of wastewater. If CODNB represents the biodegradable portion of COD, then the proportion of organic matter in wastewater that cannot be biodegraded by microorganisms can be expressed as CODNB/COD. There is a relationship between BOD5/COD and CODNB/COD as shown in the table below: CODNB/COD: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8; BOD5/COD: 0.52, 0.46, 0.41, 0.35, 0.29, 0.23, 0.17, 0.12. When BOD5/COD ≥ 0.45, biodegradable organic matter accounts for less than 20% of the total organic matter, whereas when BOD5/COD ≤ 0.2, biodegradable organic matter makes up more than 60% of the total organic matter. Therefore, the BOD5/COD ratio is often used as an indicator to evaluate the biodegradability of organic matter. BOD5/COD 0.45: easily biodegradable; BOD5/COD 0.30: biodegradable; BOD5/COD 0.30: relatively difficult to biodegrade; BOD5/COD 0.20: very difficult to biodegrade. The B/C ratio has very important and practical significance in environmental engineering. Although the BOD/COD ratio of refinery wastewater varies slightly among different manufacturers, it is not very high.

I don’t know the ratio of BOD to COD in refinery wastewater, as we don’t have the means to measure BOD. However, as far as I know, refinery wastewater can be treated biologically; most of the organic substances in such wastewater are large-molecule cyclic structures that are difficult to degrade by ordinary bacteria. There are however certain bacteria specifically designed to handle oily wastewater, and an activated carbon adsorption process may be required beforehand. I’ve also heard this discussed by some people in the industry; since our wastewater contains only small-molecule linear oils, there are no issues with biochemical processes. I didn’t ask any more detailed questions. Please ask the original poster to inquire with oil fields and similar places to see how they handle their wastewater.

Refinery wastewater has poor biodegradability, BOD/COD

Many industrial wastewater streams have poor biodegradability. There are two approaches: one is to achieve clean production in the production area as much as possible during the overall planning, and to decide whether to carry out clean production on-site or discharge the wastewater to a wastewater treatment facility for centralized treatment, based on the biodegradability of each stream； Second, a hydrolysis-acidification unit is designed at the front end of the wastewater treatment process and after the regulation and homogenization unit to improve biodegradability. The former is actually the most crucial and also more controllable

Based on my experience, a BOD/COD ratio of greater than 0.4 indicates good biodegradability. In particular, some organic wastewater with high concentrations and difficult to degrade generally have a value lower than this; such wastewater must be treated first before it can be subjected to biological treatment, otherwise the treatment results will be poor. Coking wastewater and papermaking wastewater fall into this category.

The determination of BOD is affected by many factors. For example, if the inoculated strain has not been domesticated, the values obtained will be lower than the actual values, sometimes by a significant margin. Therefore, it is recommended that the B/C ratio be used only as a reference, as the results from process experiments are more reliable.

The BOD5/COD ratio is used as an indicator to evaluate the biodegradability of organic matter. BOD5/COD 0.45 – Easily biodegradable; BOD5/COD 0.30 – Biodegradable; BOD5/COD 0.30 – Difficult to biodegrade; BOD5/COD 0.20 – Very difficult to biodegrade