University researchers find hidden treasure in liquid sludge

Where there’s muck there’s brass. Like all the best proverbs it is based in truth, but if the latest research published by the University of Wolverhampton is anything to go by, it could be more literal that you might think.

Earlier this year, the wastewater industry of England and Wales published data on the quantities of sludge being generated at their treatment plants. Between 2016 and 2017 it amounted to around 1.37 million tonnes of solid waste, the equivalent of 41.28 tonnes of raw sewage.

One of the reasons for publicising this data was to encourage some free thinking on how this vast quantity of material could be put to better use. At present, raw sewage sludge goes through a variety of processes that reduce its water content and make it suitable for use in both power generation and as an agricultural fertiliser. But could there be other, more valuable, uses for our waste?

A precious resource

Dr Alaa Hamood is a lecturer at the University of Wolverhampton and led a study into the question. He feels the first step is a philosophical one, in that we need to stop thinking of sludge as a waste product to be processed and dealt with and instead see it as a resource. Currently, when sludge is used as a fertiliser, there is a focus on monitoring the levels of potentially toxic elements such as zinc, nickel, cadmium and so on. Yet these elements can also be of value. And it turns out they are only the tip of the iceberg.

Extractable deposits

The research team postulated that an additional extraction step in the sewage treatment process could yield valuable heavy metals as well as noble metals like silver, gold and platinum. They set about a pilot study, using five samples of wet raw sewage from treatment plants across the UK. These were then filtered to separate the liquid, which, in turn, was analysed by mass spectrometry.

They also obtained a like number of representative dry samples and performed the same analysis. The average concentrations of various elements were then calculated, and from this, estimated quantities present in raw sewage in terms of tonnes per year were extrapolated.

The results make for compelling reading. 2.5 tonnes of silver, 32.3 tonnes of copper and 0.5 tonnes of gold are numbers that stand out. If these figures were from a geological survey, potential investors would be queueing up to get involved in the extraction process. The quantity of gold alone is at the level of a “minimal mineral deposit” – in other words, if it was found in rock, it would be viable to start mining it.

Next steps

Of course, the research is at an early stage, but the findings broadly echo those carried out in parallel study in the USA. The next stage will be a more extensive period of research over a period of at least 12 months to deliver more accurate estimates and really quantify how much precious material we are literally flushing away. This will also identify any geographic, seasonal or even demographic variations.

These further results will lead to a feasibility study that will really home in on the practicality of making extraction a reality. We will be following the project with interest over the coming months.