BOD/COD Ratio and Wastewater Biodegradability

The biodegradability of wastewater is a crucial factor in designing and operating wastewater treatment plants. It indicates how easily the organic matter in the wastewater can be broken down by biological processes.

Understanding BOD and COD

• BODu (Ultimate Biochemical Oxygen Demand): This represents the total amount of oxygen required by microorganisms to biologically degrade all the biodegradable organic matter present in the wastewater under aerobic conditions over a very long period (theoretically infinite time).
• COD (Chemical Oxygen Demand): This measures the total amount of oxygen required to chemically oxidize all organic and inorganic matter in the wastewater using a strong oxidizing agent.

Significance of the \(\frac{{BO{D_u}}}{{COD}}\) Ratio

The ratio of BODu to COD is a key indicator of the biodegradable fraction of the organic matter in wastewater. Since COD measures both biodegradable and non-biodegradable organic matter, while BODu measures only the biodegradable fraction, the ratio provides insight into how much of the total organic content can be removed through biological treatment.

The ratio is calculated as:

$$ \text{Ratio} = \frac{{BO{D_u}}}{{COD}} $$

A higher ratio indicates a larger proportion of the organic matter is biodegradable, making the wastewater more amenable to biological treatment.

Range for Fully Biodegradable Wastewater

Wastewater is considered fully biodegradable when the majority, if not all, of its organic content can be broken down by biological processes. This corresponds to a high \(\frac{{BO{D_u}}}{{COD}}\) ratio, where BODu is close to COD.

For wastewater to be considered fully biodegradable, the \(\frac{{BO{D_u}}}{{COD}}\) ratio typically falls within a specific range. A ratio close to 1 suggests that almost all the organic matter measured by COD is also measurable by BODu, indicating high biodegradability.

The generally accepted range for wastewater considered fully biodegradable is when the \(\frac{{BO{D_u}}}{{COD}}\) ratio is between 0.92 and 1.0.

This range signifies that the ultimate biological oxygen demand is very close to the total chemical oxygen demand, implying that nearly all the chemically oxidizable matter is also biologically oxidizable.

Therefore, the correct range of \(\frac{{BO{D_u}}}{{COD}}\) for a wastewater to be considered as fully biodegradable is 0.92 to 1.0.