Biological media houses bacteria involved in the nitrogen cycle. It provides a larger surface area for beneficial nitrifying bacteria to colonize, where water can pass through the colonies, bringing nutrients and oxygen required for the nitrogen cycle. ... They can accommodate a lot of bacteria in a small space.
Bio mediums are use in a process called biofiltration Which is a pollution control technique using a bioreactor containing living material to capture and biologically degrade pollutants. Common uses include processing waste water, capturing harmful chemicals or silt from surface runoff, and microbiotic oxidation of contaminants in air.