In this work one example of civil engineering research conducted with the Delphi method is shown. The Delphi method is one of the most popular scientific methods for research of problems with lack of known data. For this research, researcher must form the team of experts and analyze their answers on questions asked during the research with controlled feedback. In this article researcher wanted to prove hypothesis for making risk assessment model for planning and design processes of wastewater treatment plants. This research was conducted with Classical Delphi method is shown step by step.