3.4.2 Chemical Oxygen Demand (COD) & Biochemical Oxygen Demand (BOD)

Chemical Oxygen Demand is typically expressed in milligrams of oxygen per sample (mg/L) litre. This unit represents the amount of oxygen required to chemically oxidise the organic and inorganic substances present in the sample.

COD is applied to measure the organic content (also referred to as “strength”) of the wastewater or (faecal) sludge. It is an indirect indicator of organic matter because it measures the amount of oxygen required to oxidise it chemically and not directly by measuring the organic matter itself.

It is widely used to assess the characteristics of input materials for designing the treatment unit, and as an indicator for organic loading rate, treatment performance and compliance; like VS, COD can also be used to evaluate the effectiveness of the AD process by analysing the absolute volume of biogas produced and the biogas yield.

Biochemical Oxygen Demand is measured in milligrams of oxygen per litre of the sample (mg/L). It quantifies the amount of dissolved oxygen consumed during the biological degradation of organic matter in water.

BOD is a valuable parameter for assessing the biodegradable organic content in a sample, indicating the amount of organic material that microorganisms can readily break down. It measures the oxygen demand required for biological degradation, although it may not directly correlate with biogas production.

Despite its indirect relationship with biogas production, BOD is a specific indicator of the potential impact on aquatic ecosystems. It focuses on measuring biodegradable pollutants, making it an essential consideration in national regulations. As a result, BOD is commonly used as a design parameter, an indicator of treatment performance, an indicator of organic loading rate and for compliance assessment in various wastewater treatment processes.