A landmark sustainable building project in lower Manhattan is back on track after being delayed due to the terrorist attacks of Sept. 11. Construction resumed recently on 20 River Terrace, one of the world’s first green residential skyscrapers, which will be located in Battery Park City a stone’s throw from Ground Zero. The 27-story tower, which will include solar panels, purified air, super-insulated walls and windows, and a rainwater collection system, was planned long before Sept. 11, but is setting an example for a sustainable future in Lower Manhattan. The building is designed to use 35 percent less energy than a conventional structure and is being made from recycled materials in order to qualify for a New York State green-building tax credit. To reduce pollution from truck transportation, at least 40 percent of the components will be manufactured no farther than 500 miles from the job site.