It's been almost a decade since this thread started. There are now quite a few off-the-shelf ATS systems avaiable now. Most are semi-plug-and-play, with you needing only to plumb it in and give it power.
My tank has struggled with super high nitrates for a while, and I added a sulphur reactor to help. Nitrates came down (I don't remember the exact numbers ... I sent my water to a lab and ideally nitrates should have been close to zero, with many test kits measuring up to 50ppm, mine was way over 100 or so, I can't remember, the numbers are in my wifebane tank journal).
Due to a failed return pump I no longer have fish, so I can now stop feeding the tank, with only BTAs and a clam (and snails) to keep alive. My rocks are coated with algae, I'm pretty sure it's bryopsis.
I run a skimmer (ancient AquaC EV-120) and a sulphur reactor. I've not recently tested nitrates, which I should. But doing some research (and chatting with Rygh) I've decided to get an ATS.
I was going to build one, which would be cheaper, but if I'm honest with myself, I'm not going to ever get around to it...so I ordered a RAIN2 ATS from Marine Depot.
Since my tanks is a disaster now, once it shows up, I look forward to seeing if it helps with algae and nitrates.
max_nano, to answer your question, based on my internet reading, an ATS is simply a subsystem of your sump that grows algae really well, so that you concentrate all the algae growing there instead of in your main tank. There are a few different strategies, from super simple "throw a knitting mesh into the sump" to inline plumbed systems that create a waterfall over a mesh with powerful lights to grow algae.
I settled on the RAIN2 based on my own sump configuration, taking into consideration space under my tank and my plumbing. They aren't cheap, but I also didn't spend thousands to have a display tank to display algae covered rocks either. I hope it works out.
V