Understanding Oxygen Demand in Wastewater: BOD, COD, and TOD

Wastewater contains various pollutants that require oxygen for decomposition or oxidation. Different tests are used to quantify this oxygen requirement, giving us parameters like Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Theoretical Oxygen Demand (TOD). Understanding the relationship between these parameters is crucial in wastewater engineering.

What is Biochemical Oxygen Demand (BOD)?

Biochemical Oxygen Demand (BOD) measures the amount of dissolved oxygen required by aerobic biological microorganisms to decompose organic matter present in a sample of water over a specific time period, usually 5 days (BOD₅) at 20°C. It represents the biodegradable portion of the organic matter.

• Measures oxygen consumed by microorganisms.
• Primarily reflects biodegradable organic pollutants.
• Typical test duration is 5 days (BOD₅).

What is Chemical Oxygen Demand (COD)?

Chemical Oxygen Demand (COD) measures the total amount of oxygen required to chemically oxidize all organic (and sometimes inorganic) pollutants in a water sample using a strong oxidizing agent (like potassium dichromate) under acidic conditions. This test oxidizes most organic matter, including both biodegradable and non-biodegradable components.

• Measures oxygen consumed by chemical oxidation.
• Reflects both biodegradable and non-biodegradable organic pollutants.
• Results are typically obtained in a few hours.

What is Theoretical Oxygen Demand (TOD)?

Theoretical Oxygen Demand (TOD) is the calculated amount of oxygen required to completely oxidize a substance to its final stable oxidation products (e.g., $\text{CO}_2$, $\text{H}_2\text{O}$, $\text{NH}_3$ becomes $\text{NO}_3^-$). It is determined based on the stoichiometry of the chemical reaction and the elemental composition of the organic matter. TOD represents the maximum possible oxygen demand.

• Calculated based on chemical composition.
• Represents the total potential oxygen demand if complete oxidation occurs.
• Does not depend on biological or chemical test efficiency.

Comparing BOD, COD, and TOD Values

For a typical wastewater sample containing organic pollutants:

TOD represents the theoretical maximum oxygen required for complete oxidation of all organic and oxidizable inorganic matter present. This is a stoichiometric value.

COD measures the oxygen required for chemical oxidation. Strong chemical oxidants can break down a wider range of compounds than microorganisms. Therefore, COD typically includes oxygen demand from both biodegradable and non-biodegradable organic matter, as well as some inorganic substances that can be oxidized.

BOD measures the oxygen required for biological oxidation of only the biodegradable organic matter. Not all organic matter is easily biodegradable by microorganisms within the typical test period (5 days), and some is resistant to biological degradation.

Considering these points, the amount of oxygen demanded follows a general trend:

• TOD is the theoretical maximum.
• COD accounts for most organic matter (biodegradable + non-biodegradable chemically oxidizable) and some inorganic matter. This is usually less than TOD because chemical oxidation might not be 100% efficient or some substances might not be fully oxidized under test conditions, but it's generally higher than BOD as it includes more components.
• BOD accounts for only the readily biodegradable organic matter. This is typically the lowest value.

Therefore, the general relationship between these three parameters for typical wastewater is:

In practice, for most wastewaters containing a mix of organic compounds, the strict inequality holds:

This order signifies that the theoretical potential oxygen demand (TOD) is the highest, followed by the chemical oxygen demand (COD) which accounts for a broader range of substances than the biochemical oxygen demand (BOD).

Based on this comparison, the correct relation is TOD > COD > BOD.

Revision Table: Wastewater Oxygen Demand Tests

Additional Information: Significance and Limitations

Each oxygen demand parameter provides different information about the wastewater characteristics:

• BOD: Useful for evaluating the impact of discharge on receiving waters and designing biological treatment processes. It indicates the portion of organic matter that can be removed biologically. Limitation: Takes 5 days, doesn't account for non-biodegradable organics, affected by presence of toxic substances.
• COD: Provides a quicker measure of total oxidizable organic content than BOD. Useful for monitoring treatment plant efficiency and assessing overall pollution strength. Limitation: Includes non-biodegradable organics and some inorganics, doesn't distinguish between biodegradable and non-biodegradable fractions.
• TOD: Gives the ultimate, theoretical oxygen demand. Useful for understanding the maximum potential impact and for research purposes or pure substance analysis. Limitation: Requires knowing the exact elemental composition, doesn't reflect actual oxygen consumption under practical conditions (biological or chemical).

The ratio of BOD/COD is an indicator of the biodegradability of the wastewater. A high BOD/COD ratio (e.g., > 0.6) indicates readily biodegradable wastewater suitable for biological treatment. A low ratio suggests a significant portion of non-biodegradable organic matter.