By now, most people have heard that fossil fuels are a limited resource, but with other resources we are still behaving as if we had a few more planets up our sleeves. One of the most limiting resources is phosphorus, which currently comes from mining in only a small number of countries. Experts predict that peak production may come as soon as 2034, or even sooner if oil peaks before that date.
Therefore, recycling schemes aiming to recover phosphorus from sewage residue should be treated as a high priority. I have looked at the situation and the recycling options in a news feature which appears in today's issue of Current Biology (complete with a nice reproduction of "The Alchymist" by Joseph Wright of Derby, showing Brandt's discovery of P):
Fears over phosphorus supplies
Current Biology, Volume 20, Issue 9, R386-R387, 11 May 2010
summary and restricted access to pdf file