Biodegradation of Organic Material (SL IB Environmental Systems & Societies (ESS))

Photo by USGS on Unsplash 

Algal booms on Milford Lake in Kansas are an example of excessive biodegradation of organic material

• Biodegradation of organic material refers to the natural process where microorganisms break down organic substances into simpler compounds
  • During biodegradation, microorganisms utilise oxygen for the breakdown of organic matter
  • High levels of organic material can lead to increased microbial activity and oxygen consumption in water bodies

• Excessive biodegradation of organic material can deplete dissolved oxygen levels, leading to anoxic conditions (low oxygen) in the water
  • In anoxic conditions, anaerobic decomposition occurs, carried out by bacteria that do not require oxygen

• Anaerobic decomposition results in the production of gases such as methane (CH₄), hydrogen sulphide (H₂S), and ammonia (NH₃)
  • Methane is a powerful greenhouse gas and contributes to climate change when released into the atmosphere
  • Hydrogen sulphide is highly toxic and can harm aquatic organisms
  • Ammonia, in high concentrations, can be toxic to aquatic life and contribute to nutrient pollution, causing eutrophication and algal blooms

• The presence of these toxic gases can have detrimental effects on water quality, aquatic ecosystems, and the organisms that rely on them
• It is important to manage organic waste properly, promote adequate oxygen levels in water bodies, and prevent the buildup of excessive organic material in order to minimise the occurrence of anoxic conditions and the subsequent formation of toxic gases

Biochemical Oxygen Demand

• Biochemical oxygen demand (BOD) is a measure of the amount of dissolved oxygen required to break down the organic material in a given volume of water through aerobic biological activity
• Aerobic organisms rely on oxygen for respiration
• When there is a higher abundance of organisms or an increased rate of respiration, more oxygen is consumed
• This means that the biochemical oxygen demand (BOD) is influenced by:
  • The quantity of aerobic organisms present in the water
  • The rate at which these organisms respire

• BOD can be used as an indirect measure to evaluate pollution levels in water
  • The introduction of organic pollutants, such as sewage, leads to an increase in the population of organisms that feed on and break down the pollutants
  • This, in turn, results in elevated BOD values
  • Certain species, such as bloodworms and Tubifex worms, show tolerance to organic pollution and the associated low oxygen levels
  • On the other hand, mayfly nymphs and stonefly larvae are typically only found in clean-water environments

Matthias Tilly, CC BY 3.0,via Wikimedia Commons

Tubifex worms are able to withstand quite polluted water

Example of how BOD is used to indirectly measure the amount of organic matter within a sample

• High BOD values indicate a larger amount of organic matter present in the water sample, as more oxygen is needed for its decomposition
• By measuring the decrease in dissolved oxygen levels over a specific incubation period, BOD provides an estimate of the organic load or pollution level in the water
• BOD values are typically expressed in milligrams of oxygen per litre of water (mg/L) or as a percentage of the initial dissolved oxygen level
• The BOD test involves collecting a water sample in a closed container and measuring the dissolved oxygen concentration initially and after a specific incubation period (often 5 days) at a constant temperature
• For example, if a water sample has an initial dissolved oxygen concentration of 8 mg/L and after 5 days, the dissolved oxygen concentration decreases to 2 mg/L, the BOD value would be calculated as 8 mg/L - 2 mg/L = 6 mg/L
  • As the dissolved oxygen levels have decreased substantially, this indicates that the sample has a relatively high organic load