Water, the home of most bacteria, contains oxygen in two forms. The first form, free oxygen, is the most readily available form. Free oxygen is basically the same as dissolved oxygen oxygen from the atmosphere that has become dissolved in water. Aerobic bacteria require free oxygen in order to survive. Bacteria may be aerobic, anaerobic or facultative. Aerobic bacteria require oxygen for life support whereas anaerobes can sustain life without oxygen. Facultative bacteria have the capability of living either in the presence or in the absent of oxygen. Aerobic bacteria live and multiply in the presence of free oxygen. Facultative bacteria always achieve an aerobic state when oxygen is present. While the name “aerobic” implies breathing air, dissolved oxygen is the primary source of energy for aerobic bacteria. The metabolism of aerobes is much higher than for anaerobes. The by-products of aerobic bacteria are carbon dioxide and water. If there is very little oxygen in water, then the water is very attractive to oxygen. But when water has a high concentration of DO, then the water is saturated, meaning that the water contains as much oxygen as it can hold at that temperature.
Saturated water is not very attractive to oxygen. The process of oxygen moving from an area with a high oxygen concentration to an area with a low oxygen concentration is known as diffusion. Water temperature is very important in determining the amount of oxygen that will become dissolved in water. Cold water is able to hold more oxygen than warm water.
The oxygen can be ripped out of the water molecule by anaerobic bacteria, but it takes much more energy to break apart food and water in search of oxygen than it does to simply use free oxygen. Anaerobic bacteria are the major cause of hydrogen sulfide production in the collection system. Through anaerobic respiration, sulfur-reducing bacteria produce hydrogen sulfide gas (H2S), which is an inhalation hazard and causes corrosion of collection system equipment. Anaerobic bacteria release hydrogen sulfide as well as methane gas, both of which can create hazardous conditions. Even as the anaerobic action begins in the collection lines of a sewer system, deadly hydrogen sulfide or explosive methane gas can accumulate and be life threatening. The bacteria work in the sludge layer, eating as much of the edible portions of the waste as possible. Anaerobic bacteria live and reproduce in the absence of free oxygen, and produce carbon dioxide and methane. They utilize compounds such as sulfates and nitrates for energy and their metabolism is substantially reduced. In order to remove a given amount of organic material in an anaerobic treatment system, the organic material must be exposed to a significantly higher quantity of bacteria and/or detained for a much longer period of time.
Most of the bacteria that absorb the organic material in a wastewater treatment system are facultative in nature. This means they are adaptable to survive and multiply in either anaerobic or aerobic conditions. The nature of individual bacteria is dependent upon the environment in which they live. Usually, facultative bacteria will be anaerobic unless there is some type of mechanical or biochemical process used to add oxygen to the wastewater. When bacteria are in the process of being transferred from one environment to the other, the metamorphosis from anaerobic to aerobic state (and vice versa) takes place within a couple of hours. The third type of microorganisms, those which are facultative, have properties of both aerobic and anaerobic organisms. They can live with or without free oxygen. When the oxygen content of water is high, facultative bacteria consume food very quickly using the free oxygen in the water. In low oxygen concentrations, facultative bacteria are still able to consume organic material, although they do so much more slowly.