ATLANTA — An environmental group is suing Rayonier Inc., alleging that its factory in southeast Georgia is discharging millions of gallons of waste that darken the Altamaha River and make it smell miles downstream. The Altamaha Riverkeeper filed the lawsuit March 26 against the Jesup mill, which turns wood pulp into high-purity cellulose. Other manufacturers use that cellulose to make digital display screens, impact-resistant plastics, paints and cosmetics.