Why Two Gold Tiles Out Of 1,000 Made This Waste Story So Striking
The reason this became famous is that 1.89 kilograms of gold in a 1,000-kilogram ash frame equals 1.89 per 1,000 by mass, which is tiny in everyday material terms but enormous for something containing gold.
Reuters said the unusually high concentration at Suwa was probably linked to nearby precision-equipment manufacturers that used gold. It was a local industrial fingerprint showing up in a waste stream and suddenly it looked more like an urban ore source than a disposal problem.
And for the sake of this site a story about amount to small to be represented by a single percent can help drive home the usefulness of permille-visualization.
This Was Urban Mining, Not Proof That Ordinary Sewage Is Rich In Gold
The important note is that this was ash from incinerated sludge, not raw sewage and not a typical wastewater average. Incineration removes water and much of the combustible material, leaving behind a much more concentrated mineral residue. That means the headline comparison with gold ore is attention-grabbing but not perfectly apples to apples.
The report compared this concentration with that from Japan's Hishikari Mine, which reported a concentration around 20 grams of gold per tonne of ore.
This doesn't say by itself that urban sludge gold mining is economical as the processing and energy requirements are different, but it should make us think critically about metal recycling in urban settings rather than extracting all metals from nature.
Gold: 2 out of 1,000
Raw amount: about 1.89 kilograms of gold in a 1,000-kilogram frame of incinerated sewage sludge ash, based on the Reuters-reported figure of 1,890 grams per tonne at the Suwa facility in Nagano prefecture, Japan. Permille note: 1,890 grams divided by 1,000,000 grams equals 1.89 per 1,000, which is rounded to 2 visible tiles here. Included and excluded: this is gold measured in ash after sludge incineration, not in wet sludge, untreated sewage, or wastewater plants in general. Significance: the number was remarkable enough to be compared with Hishikari, a mine Sumitomo describes as having ore of around 20 grams of gold per tonne.
Everything else in the ash: 998 out of 1,000
Raw amount: about 998.11 kilograms of non-gold material in the same 1,000-kilogram ash frame. Permille note: the exact remainder is 998.11 per 1,000, displayed as 998 tiles after rounding the gold share up to 2. Included and excluded: this bucket combines everything else left in the ash and does not break out silver, copper, phosphorus, silica, or other recoverable or residual materials separately. Significance: nearly all of the ash still was not gold, which is why metal recovery only works when the waste stream is unusually enriched and the extraction process can justify the cost.
