I actually have a nano tank that sits on one end of a long kitchen counter ... waste line goes right into the sink

- which is one of the settings. otherwise you can use a container and tell it how large the container is, and it'll keep track of that (just like a DOS keeps track of how much stuff you got left in the reservoirs).
If I didn't have this option, I would just use a very small bucket or a large tupperware instead...
Trident doesn't actually put out too much waste - the reagent containers last about 2 months using 4 containers (alk you change twice as fast as cal/mag) and i think if you add all four together, it's about 1.5 beers (12 oz can) ... add in the tank water that gets used and it's probably another few beers (lol) over that same 2 months - doesn't really drain a ton from the tank
So, you can get away with pretty much anything... only reason i put it into the sink is i am forgetful and clumsy as heck - would probably spill the waste tupperware on the wood floor!
honestly, being able to control AND monitor the tank 24x7 remotely... i am very lucky to be able to afford this, and it is hugely worth it to me... as another one said on this thread, I wouldn't have a reef tank without a trident; guys at my LFS use them as well on all their main tanks (they probably get a nice discount thou!)