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As the printer is used, "excess" material is dumped into this tank keeping your machine cleaner on the inside.
You need to purchase as CLT-W409 cartridge for your printer and install it. There are ways to attempt to clean out the existing one, but you will most likely harm the printer in the process.
I have attached a link HERE that will direct you to Amazon.com. The cartridge is just under $20 and should last quite awhile.
Just a comment
I have over 20 Samsung printers of different types in my offices and love them and have very little problems with any of them Bought this multifunction machine for my home office to replace a great CLP-600N color laser printer, a fax machine, a scanner and a copier. Got a lot more space, but it seems that they could have designed it with a waste tray or something that I could empty out easily. Now my machine is down, and I have to wait a few days until I get this part. Not very efficient and not at all what I expect from Samsung machines.
I feel for you, and understand your complaint. In an attempt to make things "greener", they have made nearly impossible to use your printer without the wait for another part.
I am in no way affiliated with Samsung.