CHM 110 - CHEMISTRY AND ISSUES IN THE ENVIRONMENT

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Carbon Cycle - Metabolism Reaction
Bacterial Decomposition of Organic Waster

 
Metabolism occurs in animals, humans, and in this case Bacteria, after the ingestion of organic plant or animal foods. Organic materials contain at least carbon and hydrogen and may include oxygen. In the cells a series of complex reactions occurs with oxygen to convert for example glucose sugar into the products of carbon dioxide and water and ENERGY. This reaction is also carried out by bacteria in the decomposition/decay of organic waste materials in the water.

The bacterial decay of organic matter occurs naturally to a limited extent in lakes, ponds, and streams. This natural process is designed to decompose dead or decaying plant and animal life naturally present in the water. The amount of organic matter present in water is measured indirectly by by BOD. BOD, Biological Oxygen Demand, measures the oxygen needed by bacteria to metabolize the organic matter during a 5 day period of time. A high BOD level means that there is a high level of organic waste present. In a natural water system the amount of dissolved oxygen in water is limited by solubility and the temperature of the water. A maximum of 10 -12 ppm dissolved oxygen may present in natural water.
 
If municipal sewage waste is not treated but directly discharged into a stream, the BOD or organic waste level is very high and quickly overwhelms the capacity of the natural bacteria and the available oxygen to decompose the waste. At the sewage treatment plant, the activated sludge process is designed to use natural bacteria in a holding tank with the organic waste that is aerated with air (oxygen) until the organic waste is completely decomposed. The water with a low organic level or low BOD can now be safely discharged to a nearby stream.
 
An important summary statement is that during combustion/metabolism of organic waste, oxygen is used and carbon dioxide is a product. The whole purpose of the process is to decompose and breakdown organic waste into carbon dioxide, a gas emitted to the atmosphere, and unreacted solids which may be removed by settling and filtration.