What is Chemical Oxygen Demand (COD) and
Biochemical Oxygen Demand (BOD)?
Biochemical Oxygen Demand (BOD):
BOD (Biochemical Oxygen Demand), also often referred to as biological oxygen demand, is a test performed to measure the potential of wastewater and other waters to deplete the oxygen level of receiving waters.
In other words, the BOD test is performed to determine what effect dirty water, containing bacteria and organic materials, will have on animal and plant life when released into a stream or lake. When there is an abundance of bacteria and organic materials, the bacteria will take in oxygen in order to breakdown these molecules. If bacteria are taking in large amounts of oxygen, this will have a detrimental effect on the surrounding ecosystem. On the contrary, when there are low levels of organic waste in the water, there are fewer bacteria present, the BOD will be lower and the dissolved oxygen levels higher.
In wastewater treatment plants, they often calculate the percentage removal of BOD to determine the efficiency of the treatment process. For this reason, BOD is sometimes referred to as a water contaminant.
The BOD test involves taking an initial dissolved oxygen (DO) reading and a second reading after five days of incubation at 20oC. For this reason, this test is often written as BOD5 for short. MANTECH has developed a variety of systems that have automated the analysis of BOD5 including:
What are the current methods used?
- Standard Methods 21st Edition 5210 B
- EN 1899-1 & 1899-2
- EPA 405.1
- ISO 5815-1
Chemical Oxygen Demand (COD):
COD analysis is a measurement of the oxygen-depletion capacity of a water sample contaminated with organic waste matter. Specifically, it measures the equivalent amount of oxygen required to chemically oxidize organic compounds in water.
Why Measure COD?
COD is used as a general indicator of water quality and is an integral part of all water quality management programs. Additionally, COD is often used to estimate BOD (Biochemical Oxygen Demand) as a strong correlation exists between COD and BOD, however COD is a much faster, more accurate test.
What are the Current COD Methods Used?
The most common COD method is the wet chemistry method. This involves a two hour digestion at high heat under acidic conditions in which potassium dichromate acts as the oxidant for any organic material present in a water sample. Silver sulfate is present as the catalyst and mercuric sulfate acts to complex out any interfering chloride. Following the digestion, the extent of oxidation is measured through indirect measurement of oxygen demand via electrons consumed in the reduction of Cr6+ to Cr3+. This can be done by titration or spectrophotometry.
What are the Alternatives?
MANTECH has developed an automated COD method utilizing a new, rapid and green technology called PeCOD. This method directly measures the amount of oxidizable material in a sample via photoelectrochemical oxidation in a microcell, eliminating the need for time-consuming digestion and hazardous chemicals as only an electrolyte solution is required for analysis. Automated sample homogenization always ensures a representative sample is analyzed, and multi-point calibration guarantees the best accuracy and precision.