COD vs BOD: Differences and How to Measure Them

In wastewater treatment, two parameters that always become the main focus are COD (Chemical Oxygen Demand) and BOD (Biological Oxygen Demand). These two parameters determine the level of organic load in the wastewater and how difficult it is to treat.

The difference between COD and BOD values provides insight into:

• The level of organic pollution
• How easily the wastewater can be degraded by bacteria
• The most suitable treatment technology
• Potential environmental contamination risks

Therefore, a deep understanding of both is crucial for designing an effective WWTP.

Definition of COD and BOD

COD (Chemical Oxygen Demand)

The amount of oxygen required by chemical oxidants to oxidize all organic matter in wastewater—both easily and poorly biodegradable.

BOD (Biological Oxygen Demand)

The amount of oxygen required by aerobic bacteria to break down biodegradable organic matter over a specific period, typically 5 days (BOD₅).

The difference in time and analytical methods is what makes both parameters offer different perspectives.

Fundamental Differences Between COD and BOD

Table: COD vs BOD

In general, BOD provides a biological perspective of the system, whereas COD acts as a fast indicator for process control.

When COD Is More Important and When BOD Becomes More Determining

Each parameter has its own function:

• COD is more important when industries need fast monitoring results, especially for daily operations.
• BOD is more important when evaluating how biodegradable the wastewater is.
• The COD/BOD ratio helps determine wastewater composition:
   
  • Ratio < 2 → easily treated biologically
  • Ratio 2-4 → requires physico-chemical combination
  • Ratio > 4 → difficult-to-degrade organics, requires AOP/ozone/chemical oxidation
     

This ratio determines WWTP design.

How COD and BOD Are Measured

COD Measurement

• Uses oxidizing reagents (Potassium Dichromate)
• Heated in a digester
• Measured using a spectrophotometer
• Testing time ± 2-3 hours

BOD Measurement

• Sample is incubated for 5 days
• Initial & final oxygen levels are measured
• Requires a stable temperature (20°C)
   

Because BOD requires a long time, COD is more commonly used for daily control.

Factors Affecting COD/BOD Values

Several factors cause COD/BOD values to fluctuate:

• Oil & grease content
• Surfactants or detergents
• Non-biodegradable chemical waste
• Dyes or aromatic compounds
• Extreme pH
• Sudden organic load increase (shock loading)

Operators must understand these variations to prevent biological process disturbances.

Example COD/BOD Values in Industries

Table: Common COD/BOD Values in Various Industries

Textile and hospital industries tend to have high COD/BOD ratios, indicating wastewater that is difficult to biodegrade.

Technologies for Reducing COD/BOD

To reduce COD/BOD, a combination of several treatment methods is typically used:

Physical-Chemical

• Coagulation & flocculation
• DAF
• Activated carbon adsorption

Biological

• Activated sludge
• MBBR
• MBR
   

Advanced Treatment

• Ozonation
• AOP (Advanced Oxidation Process)
• UV-H₂O₂
   

Technology selection depends on wastewater characteristics and effluent standards.

COD and BOD are two main parameters that determine how well wastewater can be treated. COD gives a quick picture of the total organic load, while BOD shows the wastewater's biodegradability. Understanding both is essential for designing an efficient WWTP and ensuring the effluent meets environmental standards.

If you need assistance with COD/BOD analysis, biological process optimization, or designing a wastewater treatment system based on your industrial raw material characteristics, PJL Enviro can provide technical support and solutions tailored to your operational needs.