Serious question, do you use copper wool or netting in your home-brew set-up?
Another question, what do you do with your spent yeast?
I haven't ever used copper wool or netting. I wasn't sure what you were talking about so took a look and some people do put copper wool in their boil kettle believing that it will increase maillard reactions and such but for me, other than a copper chiller I don't have any copper in the system.
Are you asking about the yeast or about spent grain? I haven't heard yeast referred to as spent yeast.
So this could get a little long...
When yeast finish fermenting the beer, either because there isn't enough sugar left, the alcohol content is too high (around 13% at the top-end for alcohol resilient yeast) or the temperature drops below that strains lower limit or a combination of those factors, they go dormant and begin to fall out of suspension. But there are many other solids that exist in wort and beer, hop debris being the majority in the boil kettle along with proteins called the hot break and cold break. Wort is what beer is called before yeast has been added and before any fermentation has taken place. You can take steps to remove none, some, a lot or almost all of these solids, called trub, before transferring the wort to the fermentation vessel. If you want to harvest your yeast you will typically want to remove most of the trub before transfer, but it isn't completely necessary.
Anyway, once fermentation is over you one way or another transfer the beer off the yeast cake that has collected at the bottom of the fermentation vessel. That yeast cake contains A LOT of living yeast cells. Much more than you added at the beginning of fermentation. You can scoop this up and either use as is, which is easier/better if you didn't transfer too much trub to the fermentation vessel which will be mixed in with the yeast. The hop material in particular will start to break down and could cause issues. If there is a lot of other stuff in the yeast and you aren't going to use it for another batch of beer right away you can do a thing called "yeast washing" which basically uses the difference in density between the yeast and trub to separate them through a series of steps where water is added, the mixture is agitated and then the water is poured off after the yeast have started to collect at the bottom. Connical fermenters can also allow for the collection of yeast during the mid to later stages of fermentation by opening a valve at the bottom of the conical fermenter and collecting the yeast. Typically you need two valves for this. One above a tube or pipe where the yeast collects and one at the bottom of that pipe. You close the top one and open the bottom one and allow the yeast to come out for collection.
When brewing very high ABV beers a technique some brewers use is to actually make a moderate strength beer first, transfer it off the yeast cake when it's finished and then transfer the wort for the much stronger beer onto that fresh yeast cake. That gives them tons of healthy active yeast that will make good work of their "big" beer.
Regardless of technique this harvested yeast is very useful. Almost all breweries harvest yeast instead of buying commercial yeast for every batch. After a few generations this yeast starts to become unique to the brewery and is essentially their "house strain." This harvested yeast can be stored under refrigeration for many many months.
Okay so that's yeast. If you meant to say spent grain. Personally I throw it out, but it can be used to feed chickens, pigs, deer, etc., or can be added to compost. One quick word to the wise, spent grain spoils very quickly. If you hold onto it and then open the container you are at great risk of losing your lunch. It is a horrible horrible smell.
