Precious metals including gold, silver and platinum have been discovered in raw sewage sludge samples, academics have revealed.
Based on latest findings, it is estimated that around two tonnes of silver, half a tonne of gold and just over half a tonne of platinum could be extracted from sewage in England and Wales each year.
Dr Alaa Hamood, of the University of Wolverhampton, carried out a pilot study to identify and quantify precious materials present in sewage.
He collected five samples from a sewage treatment works over a three-month period, which were filtered and analysed for chemical composition, as well as representative dry solid samples. Samples showed traces of precious metals and substances with high commercial value and industrial applications, such as heavy metals, noble metals and other materials.
Dr Hamood said: "The research is in its early stages but if we assume all the identified elements are chemically available for full extraction, based on the findings, we could be extracting tonnes of potentially valuable materials."
Current treatment methods of raw sewage sludge include applying physical and chemical processes to reduce water content in preparation for the anaerobic digestion process to generate electricity.
The by-product of the digestion process is a stabilised sludge, currently used as agricultural fertiliser.
But Dr Hamood said that this incorporated other valuable materials which are currently being wasted in the farming industry.
"It is time for the sector to explore innovative practices and to recognise the value of raw sewage sludge as a resource rather than a waste product," said Dr Hamood.
"An additional step in the treatment cycle would enable existing valuable materials to be extracted."
The next stage of the study is to carry out wider research on a national scale for a minimum of 12 months and Dr Hamood is seeking further collaboration with colleagues in the water and wastewater industry.