The breakdown of organic matter (OM) in water consumes oxygen through processes that are facilitated by aerobic (needing oxygen) organisms. The amount of oxygen required to break down the organic material in a volume of water at a certain time and temperature is referred to as the biochemical oxygen demand (BOD). The standard oxidation test for calculating BOD measures the oxygen consumed in a water sample after five days, this is known as the five-day biochemical oxygen demand (BOD5). BOD accounts for the oxygen consumption of all biochemical process, including nitrification as well as breakdown of carbon based molecules. When the oxygen demand is required only for the breakdown of organic carbon molecules to carbon dioxide, it is referred to as the carbonaceous biochemical oxygen demand (CBOD). Similar to BOD, CBOD standard tests are five days since this is when most of the organic carbon oxidation occurs and is known as the five-day carbonaceous oxygen demand (CBOD5).
When water contains excess OM content the BOD/CBOD will be high, resulting in low dissolved oxygen (DO) concentrations; this has negative repercussions on aquatic systems. The reduction of DO is one of the main concerns when loading excess OM into a system. In aquatic systems with low DO concentrations the OM may be decomposed by anaerobic (do not require oxygen) organisms. The anaerobic decomposition of organic material produces hydrogen sulfide gas, which can be toxic to wildlife and humans. High OM loads can also cause difficulties with irrigation and plumbing due to biofilm formation and clogging, and they can reduce the efficiency of disinfection systems, which can impact food safety.
When water contains excess OM content the BOD/CBOD will be high, resulting in low dissolved oxygen (DO) concentrations; this has negative repercussions on aquatic systems. The reduction of DO is one of the main concerns when loading excess OM into a system. In aquatic systems with low DO concentrations the OM may be decomposed by anaerobic (do not require oxygen) organisms. The anaerobic decomposition of organic material produces hydrogen sulfide gas, which can be toxic to wildlife and humans. High OM loads can also cause difficulties with irrigation and plumbing due to biofilm formation and clogging, and they can reduce the efficiency of disinfection systems, which can impact food safety.
Figure 1 BOD effects on DO and fish populations [Source: http://gvsu.edu/cms3/assets/6BDDB6FE-EF92-1DFF-13B97ABEB2F2651C/lowgrand_wit/oxygen_depletion.jpg]
Major sources of organic waste to water are pulp and paper, municipal sewage, and agriculture. Vegetable washwater containing high OM, whether from the vegetables themselves or the soil they were grown in, also tend to have high BOD/CBOD levels. Treatment options are available and necessary for the discharge, and reuse of washwater.
For more information on Dissolved Oxygen, see 'Defining Dissolved Oxygen'.
References
For more information on Dissolved Oxygen, see 'Defining Dissolved Oxygen'.
References
- Dorcey, A. & Freedman, B. 2013. Water Pollution. Historica Canada. http://www.thecanadianencyclopedia.ca/en/article/water-pollution/
- CCOHS. 2010. CHEMINFO: Hydrogen Sulfide. Canadian Centre for Occupational Health and Safety. http://www.ccohs.ca/products/databases/samples/CHEMINFO.html#TOC2
- Delzer, G.C. & McKenzie S.W. 2003.7.0 Five Day Biochemical Oxygen Demand. USGS TWRI Book 9–A7 (Third Edition). United States Geological Survey. Reston, VA, USA. http://water.usgs.gov/owq/FieldManual/Chapter7-Archive/chapter7.2/7.2.html
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