In anaerobic digestion and biogas production, we often hear or read about BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand). Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are critical parameters used to quantify the organic load of a substrate. This post will explain simply about those two terms.
Parameter Definitions
Biochemical Oxygen Demand (BOD) is a measure the mass of dissolved oxygen required by aerobic microorganisms to decompose the biodegradable organic matter present in a water sample over a specific period. Meanwhile, Chemical Oxygen Demand (COD) covers both biologically available and inert organic matter (non-biodegradable), and it is measure of the total quantity of oxygen required to chemically oxidize all organic compounds into carbon dioxide and water. Because COD accounts for the total oxidizable organic matter, whereas BOD only accounts for the bio-fraction, COD values are universally higher than BOD values.
How to Estimate BOD and COD Values
BOD is determined empirically by incubating a sealed sample at a controlled temperature (typically 20°C) for five days (BOD5) and measuring the net decrease in dissolved oxygen. On the other hand, COD is determined by rapid chemical oxidation using a strong oxidizing agent under acidic conditions. This process concludes within a few hours, making COD the preferred metric for real-time process monitoring and control in biogas plants.
I hope you find this simple post useful.
References:
- Handbook POME-to-Biogas Project Development in Indonesia 2nd Edition by USAID and Winrock International