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Purple Sulfur Bacteria

The purple sulfur bacteria belong to the phylum Proteobacteria. These bacteria are often found in stagnant waters or where there are high concentrations of Hydrogen Sulfide. In order to conduct photosynthesis, they need anoxic conditions, which mean that they cannot survive where there are high dissolved oxygen concentrations. They are easily identified under the microscope since their particular shape resembles that of “Chex-Mix” snacks. They take the hydrogen sulfide from the water column and metabolize it into granules of elemental sulfur. These sulfur granules may be oxidized to form sulfuric acid. Also, the sulfur granules can be easily seen within the bacterial cells at a 1000x wet mount microscopic slide. In high numbers, these bacteria give the water and the floc a pink or purplish tint. Generally, they are indicators of septicity, and in severe cases, the dispersed purple sulfur bacteria can cause high effluent Total Suspended Solids levels. In the Pulp and Paper Mill industry, purple sulfur bacteria can be helpful in identifying septic zones which is no other than an indication of poor dissolved oxygen concentrations in different sections of a wastewater treatment system. Although they can be helpful in identifying certain problems within wastewater treatment systems, purple sulfur bacteria are not principal Biochemical Oxygen Demand (BOD) degraders and makes them more of a problem within the system when present in high numbers. In order to control purple sulfur bacteria in a wastewater system, it is important to maintain good dissolved oxygen conditions in the wastewater system. This is effectively achieved with the installation of aeration devices in areas where the dissolved oxygen concentrations seem to decrease significantly in the wastewater system. Another way of controlling purple sulfur bacteria is by the addition of oxidizing agents such as hydrogen peroxide or nitrate salts. They are more prevalent in the summer months due to higher temperatures.