With appropriate filtration and frequent water changes a 30gallon could be used to start to grow out some smaller discus (like the 2.5 and 3" discus). However, I would not personally try to raise discus to adult size in anything smaller than an AGA 45 tall tank, and would usually want to lean towards a 4 foot tank like a 75 gallon over that. Especially if you are keeping discus for the first time, the larger the tank, the more forgiving the fish are.
On the other hand, if you over filter with a large canister filter, change water weekly or better yet biweekly, and limit yourself to just a handful of discus - you can always give it a shot. I would expect to have to upgrade the tank eventually though.
-Sam