Researchers from the Swiss Federal Institute of Aquatic Science and Technology, known as EAWAG, its German acronym, surveyed 64 wastewater treatment plants across Switzerland and estimated that that up to 43kgs of gold passes through the country’s sewerage system each year – an amount equal to around $1.8 million.

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The concentrations of precious metals are so minor that they do not present any threat to the environment.

However, in some areas, such as Ticino in the south of the country, there are amounts significant enough to warrant a recycling effort.

“At certain sites in Ticino, concentrations of gold in sewage sludge are sufficiently high for recovery to be potentially worthwhile. This can be attributed to the presence of several gold refineries in the region,” the EAWAG said in a statement.

It is thought that the 64 treatment plants also saw a total of around 3,000kgs of silver go through Swiss sewage systems – the equivalent of $1.7 million.

Overall, however, considering the amounts that could be extracted, the recovery of metals from wastewater or sludge is not said to be worthwhile in a country that refines around 70 percent of the world’s gold on average every year.

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Scientists had originally feared a repeat of the critical concentrations of rare earth metals lanthanum and samarium currently found in the Rhine. However, all analysis came within safe limits, although the EAWAG have warned that little is known as yet about the potential toxicological effects of many trace elements.