COD and BOD – The main indicators of wastewater quality
To characterise wastewater quality, numerous physical, chemical and biological parameters are used. Among these, the most important group consists of indicators describing the organic load, of which the most fundamental are COD (Chemical Oxygen Demand) and BOD (Biochemical Oxygen Demand). These two values provide relevant information on the treatability and environmental impact of wastewater and serve as a basis for selecting appropriate treatment technologies and for evaluating the efficiency of existing wastewater treatment plants.
COD – Chemical Oxygen Demand
COD (Chemical Oxygen Demand) indicates how much oxygen would be required to chemically oxidise the organic matter and certain inorganic reducing substances present in wastewater.
Unit: mg O₂/L
Method of determination: laboratory analysis using a strong oxidising agent (typically potassium dichromate)
The advantage of COD is that it can be measured quickly and reproducibly, therefore it is widely used for:
- monitoring influent and effluent water at wastewater treatment plants,
- characterising industrial wastewaters,
- regulatory and technological control purposes.
BOD – Biochemical Oxygen Demand
BOD (Biochemical Oxygen Demand) expresses how much dissolved oxygen is consumed by microorganisms during the biological degradation of biodegradable organic matter present in wastewater.
Unit: mg O₂/L
Most commonly used form: BOD₅ (5-day biochemical oxygen demand at 20 °C)
BOD directly indicates:
- the biological load of the wastewater,
- the oxygen-depleting effect of the water on the receiving surface water,
- the expected efficiency of biological treatment.
Since BOD only considers biologically degradable organic matter, its value is always lower than COD.
Relationship Between COD and BOD
The combined interpretation of these two parameters is particularly important in wastewater treatment. The COD/BOD ratio provides information on the biodegradability of the wastewater:
Low COD/BOD ratio (≈ 1.5–2.5): well biodegradable wastewater (typically municipal)
High COD/BOD ratio (> 3–4): poorly biodegradable or industrial-origin pollution
This ratio helps in selecting the appropriate treatment technology and in designing and operating biological treatment processes. Regular measurement and interpretation of these parameters are essential for the efficient and environmentally safe operation of wastewater treatment plants and are therefore mandatory operational tasks prescribed by regulatory authorities.
