High levels of toxic substances spread for decades across farmlands have come from fertilizers made from urban sewage. It has helped take sludge out of landfills, is rich in nutrients and its use has been encouraged by the federal government. But now there’s suspicion these chemicals are in crops, are sickening or killing livestock and threatening farmers’ health. Rolf Halden, a professor in the School of Sustainable Engineering and the Built Environment, part of the Fulton Schools, and director of ASU’s Biodesign Center for Environmental Health Engineering, is among experts who assess the situation and risks and challenges it presents.