The green thumbs that run the 10-acre Savannah Botanical Gardens on Eisenhower Drive are worried visitors will soon be smelling sewage instead of roses. But city staffers assure them the plan for a sewage treatment plant at the Bacon Park Golf Course behind the gardens is nothing to turn their nose up at. The treatment plant is being planned as the city faces stricter state limits on the amount of groundwater it can withdraw, as well as limits on the treated wastewater it can discharge into the Savannah River.