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Comment by Gustomaximus | Hacker News Reader

> Urban Mushrooms has already stopped six tonnes of coffee grounds from going to landfill.

What happens to the coffee grounds after the mushrooms are done growing? Not trying to be a smartass. Genuinely curious given this is a core claim.

Regarding landfill, I've wondered if these site will be come super valuable in 100 years type thing. There must be huge mineral wealth someone will mine from these one day. Kinda like us mining old mines tailings now our extraction methods are better.

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Spent mushroom substrate is really good in compost (particularly vermicompost). I worked on a small scale gourmet mushroom farm earlier this year, it's easy enough to sell the spent medium to gardening businesses when you're done growing mushrooms.

I think all coffee grounds make decent fertiliser too, as well as decent compost, even without mushroom.

coffee grounds alter the pH of whatever you use them in so you can't just blindly use it as fertilizer. Just an FYI before people start throwing their coffee grounds everywhere. It's worth researching whatever your growing to see what pH it prefers and then treat accordingly.

They also just get moldy in "sterile-ish" environments like pots since there's nothing there to break them down. Works ok in the yard though.

Gourmet mushroom farm? Can you tell more?

It was at What The Fungus - you can check them out on youtube

thanks, great name and sorry for the late reply!

Yes I've wondered the same. 1970s solid-state electronics full of gold, metals lying around everywhere, rare earth metals in old cellphones, recylcable glass and plastics... Wanna co-found a startup that buys old landfills?

It is already being done. Dealing with old piles of trash is somewhat dangerous (all sorts of volatile gasses build up there) but once you get around this you can burn the thrash (releasing quite a bit of energy) and go through the ashes later. The ash can be used for various purposes, such as construction, but needs to be sorted by grain size first. While going through the ashes you do indeed encounter precious metals. I recently read about a company that performs this sorting as a service. They are not paid for the work directly, but get to keep the gold (and possibly other metals?) they find. Apparently you can expect to find a gram or two of gold in a tonne of ash.

That doesn't sound like much? That's less than 100USD per tonne (approx $40/g current gold price)

Is there much other valuable stuff in the ash to make the venture worthwhile?

I dug up the original article for some fact checking, and it seems my previous post has a few mistakes in it. There are "grams" of silver and gold (each or together? The article does not specify.) and "kilograms" of copper in a tonne of ashes. Other metals are also extracted, but their amount or value is not specified.

It further turns out that there is a monetary payment for the service, but it is low because of the valuable metals. The article also tells that the machine can sort around 100000 tonnes of ashes annually.

According to this article, a good open-pit gold mine has deposits of about two grams per tonne.

http://www.mining.com/web/making-the-grade-understanding-exp...

I'm interested! I've done small-scale precious metal reclamation, also silver castings and home forgings. Basically that makes me an expert ;)

As someone with no knowledge of this, how do you do small-scale precious metal reclamation and how did you get into home smithing?

Here is a link to a popular scientist YouTuber who has done significant work in this area

https://www.youtube.com/playlist?list=PLKhDkilF5o69PqPy-oMCC...

Well worth the watch

I built a forge a few years ago with a bucket, plaster of paris, some quartz sand, some iron pipe and a blow dryer. Then I started hammering on steel when it turned red.

Metal reclamation is just chemistry so youtube and grainger are all you really need.

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