Undoubtedly, BOD or Biochemical Oxygen Demand is a key parameter in water treatment because it provides valuable information on the degree of organic pollution, water quality and the efficiency of treatment processes, which is essential to protect the environment and comply with regulations.
Bodies of water are not usually thought of as containing oxygen, but water always contains a small amount of dissolved oxygen. A small amount, but essential for life in water. Biochemical Oxygen Demand (BOD) generally represents how much oxygen is needed to break down organic matter in water.
What is Biochemical Oxygen Demand?
Biochemical oxygen demand (BOD) represents the amount of oxygen consumed by bacteria and other micro-organisms while decomposing organic matter under aerobic conditions (oxygen present) at a given temperature.
When, for example, we look at the water in a lake, we do not notice the presence of oxygen. In a way, we think of water as the opposite of air, but the average lake or stream contains small amounts of oxygen, in the form of dissolved oxygen.
Although the amount of dissolved oxygen is small, up to about ten molecules of oxygen per million of water (10 ppm), it is a crucial component of natural water bodies, because the presence of a sufficient concentration of dissolved oxygen is essential for maintaining aquatic life and aquatic quality, as well as for the aesthetics of streams and lakes.
What is dissolved oxygen?
Dissolved oxygen is the amount of gaseous oxygen dissolved in a water sample. O2 can be absorbed directly from the atmosphere or indirectly from the by-product of photosynthesis of surrounding plants.
As mentioned above, the presence of a sufficient concentration of dissolved oxygen is essential to maintain aquatic life and the aesthetic quality of streams and lakes. Determining how organic matter affects the dissolved oxygen (DO) concentration in a stream or lake is essential for water quality management.
The decomposition of organic matter in water is measured as biochemical or chemical oxygen demand. Oxygen demand is a measure of the amount of oxidisable substances in a water sample that can reduce dissolved oxygen concentrations.
Factors influencing oxygen depletion in water
Certain environmental factors (warm summer temperatures) and other human-induced factors (introduction of excess nutrients into a body of water) can decrease the amount of dissolved oxygen in a body of water, leading to stresses on local aquatic life.
One water analysis used to better understand the effect of bacteria and other micro-organisms on the amount of oxygen they consume when decomposing organic matter under aerobic conditions (in the presence of oxygen) is the measurement of biochemical oxygen demand (BOD).
Determining how organic matter affects the dissolved oxygen concentration in a stream or lake is fundamental to water quality management. BOD is a measure of the amount of oxygen required to remove residual organic matter from water in the process of decomposition by aerobic bacteria (those bacteria that only live in an oxygen-containing environment).
Residual organic matter is stabilised or decomposed by bacterial organisms that require oxygen for their metabolism. BOD is often used in wastewater treatment plants as an index of the degree of organic contamination of water.
Sources of BOD
Sources of biological oxygen demand include leaves and woody debris, dead plants and animals, animal manure, industrial effluents, sewage treatment plants, feedlots and food processing plants, failing septic systems and urban stormwater runoff.
One of the most important nutrients affecting BOD in aquatic systems - especially in recent times - is phosphate pollution from households.
How is BOD determined?
There are a few approved methods for determining biological oxygen demand, although one method is widely used by the analytical community. This is the standard method 5210B.
This method analyses the dissolved oxygen (DO) difference of a sample over five days. The initial DO content of a known volume of sample is recorded and, after a five-day incubation period at 20°C, the sample is removed from the incubator and the final DO content is measured.
The BOD value is then calculated from the depletion and the sample size used. BOD readings are usually expressed in parts per million (ppm) or in mg O2/L. A higher BOD indicates that more oxygen is needed, which means that the water quality is lower. A lower BOD means that less oxygen is removed from the water, so the water is less polluted.
Since cold water retains oxygen better than warm water, the BOD is usually lower in the summer months.
The biggest challenge of the BOD test is related to time, as the holding time of a BOD sample is 48 hours from collection. For the BOD to work properly there must be a sufficient population of healthy bacteria in the bottle.
How is BOD used in wastewater treatment plants?
Biochemical Oxygen Demand is often used in wastewater treatment plants as an index of the degree of organic contamination of water.
Industries discharging wastewater to the municipal sewerage network or to watercourses are always faced with strict regulations on BOD levels. Solid materials in wastewater may consist of organic and/or inorganic materials and organisms, and solids must be significantly reduced by treatment or they may increase BOD when discharged.
What is the typical ratio between BOD and COD in wastewater?
The typical relationship between Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) in wastewater is as follows:
- The COD value is always higher than the BOD value, as COD measures the amount of oxygen needed to chemically oxidise all organic matter, both biodegradable and non-biodegradable, whereas BOD only measures biodegradable organic matter..
- In domestic wastewater, the BOD/COD ratio is usually between 0.3 and 0.8, i.e. BOD represents between 30% and 80% of COD..
- In industrial wastewater, the BOD/COD ratio can vary greatly depending on the type of industry, as some industries generate more non-biodegradable organic matter. Typical values can be between 0.2 and 0.6.
- A low BOD/C COD ratio (< 0.2) indicates that the wastewater contains a high proportion of non-biodegradable organic compounds..
- The BOD/COD ratio is an indicator of the biodegradability of organic matter in water. The closer it is to 1, the more biodegradable the organic matter is..
In summary, in domestic wastewater the BOD usually represents between 30-80% of the COD, whereas in industrial wastewater the ratio is more variable depending on the type of industry. This ratio is a good indicator of the biodegradability of organic matter.
