The natural stock of mineral phosphorus is diminishing (see also peak phosphorus and the nutrient cycle). Related to this problem, this exercise covers the importance of phosphorus recycling, possible technologies to do so and the primary challenges regarded with these procedures (see e.g. struvite).
Important Weblinks
bioh.wikispaces.com
http://bioh.wikispaces.com/More Elemental Cycles
[Accessed: 23.11.2012]
An overview on the carbon, nitrogen, and phosphorus cycle.