Reaeration is the physicochemical process by which oxygen is transferred across the surface of a water body. It is a key element within the biochemical oxygen cycle in natural water bodies, the other elements being oxygen production due to photosynthesis by aquatic plants and oxygen consumption by respiration due to animals and plants, including bacterial decomposition of organic matter, within the water column and within the sediment bed. Reaeration is thus an important pathway by which any deficit in dissolved oxygen (DO) concentration gets reduced over time. The reaeration rate depends on the surface flux of oxygen and is controlled by a complex interaction of molecular diffusion and turbulent exchange processes near the interfaces, modified additionally by organic films or microlayers ('surfactants') that often exist in natural waters. A review of these mechanisms subject to the three turbulent driving mechanisms typical for environmental conditions, namely stream-induced turbulence, wind-induced turbulence, and buoyant convection turbulence, is given herein, together with a summary of predictive equations for estimation of reaera-tion rates.

Oxygen reaeration is but one application within a wider class of gas-transfer processes at the air-water interface. Other important gases are climate-controlling gases, notably carbon dioxide, a greenhouse gas that is currently greatly increasing in the atmosphere (by about 0.5% per year). A major sink for CO2 is transfer into the world ocean largely driven by wind field above the ocean. Critical elements for water quality processes may also be volatilizing pollutants, such as organic components like hydrocarbons or polychlorinated biphenyls (PCBs) or inorganic chemicals like mercury vapors.

Furthermore, reaeration processes can be artificially augmented by mechanical devices, such as sprays or underwater air injection (diffusers). These may be installed in streams, lakes, or reservoirs to overcome low oxygen or even anoxic conditions due to density stratification in such weakly flowing environments. Similar devices are employed in process engineering for municipal or industrial sewage treatment plants. These devices are not considered herein.

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