Research from Arizona State University has suggested that human sewage could be fruitfully 'mined' for gold, silver and platinum (http://www.theguardian.com/science/2015/mar/23/gold-in-faeces-worth-millions-save-environment). They reckon that the precious metals (from shampoos, detergents and clothes) reach the levels indicative of 'a minimal mineral deposit" and could be safely extracted in sewage plants using chemicals termed leachates. This has two advantages namely a) the leachates are not used in the wider environment where they can be very damaging (as in traditional mining) and b) the removed metals render the remaining biomass safe for use as fertilisers or for burning to generate electricity. They reckon that a city of around a million inhabitants currently flushes away almost 9 million pounds worth of precious metals in a year. There's gold in them there hills!
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