This paper details the investigations that determined the root cause of the scale deposition and how this led to the conclusion that scale was the result of primary deposition due to bacterial activity in plant vessels. The root cause analysis was then applied to the development of a robust mitigation and removal strategy. Details of the preventative biocide treatments are given and how, due to the complex nature of the plant and tie backs, this had to be closely managed and monitored. The combined approach of physical and chemical removal of the scale forms the majority of this case history because it was an online dissolver treatment that used a novel iron chelating agent, injected at only a few hundred mg/l, to remove the scale deposit. Discovery of this scale, and then the rapid treatment and removal saved the PWRI plant from certain total blockage and shut down.