I'd tend to agree that 30min won't be enough time to make a connection @ ATL. However, it all depends on what gate your DL flight will depart from. NW uses gates near the centerpoint of Concourse D, something like D 14-18. DL also has several gates at the end of D which are used for domestic arrivals/departures and a few Comair flights. However, the lions share of their gates are in Concourses A & B, which are two and three stops, respectively, on the underground train system from D. I don't hold out much hope that you can do it successfully unless your NW flight is early or your DL flight is late. DL has so many flts between ATL-SAV (10 daily, I think) that you should be able to catch the next one, unless you're on the last one of the day.
AtlMan