Diffusion and Advection

DIFFUSION and ADVECTION

Diffusion

Diffusion refers to the movement of a substance from an area of high concentration to an area of low concentration. Diffusion is driven by a gradient in the chemical potential of the diffusing species (If one atom or one molecule is identical to another, the are said to belong to the same species). The two main types of diffusion are passive diffusion and facilitated diffusion.

Active diffusion involves the presence of an additional energy to transport a substance from an area of high concentration to an area of low concentration. Active diffusion is also called facilitated diffusion. Passive diffusion does not require the any additional energy to transport a substance across a membrane.

Advection

Advection is similar to convection. It refers to horizontal movement of the solute with the bulk solvent in a macroscopic sense. This includes transport of pollutants.

Diffusion and Advection in wastewater treatment

Transport processes in the environment may be divided into advection and diffusion. Advection refers to transport with the mean fluid flow. For example, if the wind is blowing toward the east, advection will carry any pollutants present in the atmosphere toward the east. Similarly, if a bag of colour is emptied into the center of a river, advection will carry the resulting colour downstream. 

• Diffusion refers to the transport of compounds through the action of random motions.
• Diffusion works to eliminate sharp discontinuities in concentration and results in smoother, flatter concentration profiles. 

Advective and diffusive processes can usually be considered independently.
In the example of a colour in a river, while advection moves the center of mass of the colour downstream, diffusion spreads out the concentrated colour to a larger, less concentrated region.