The proposed data-based FDI method has the advantage to require only available data: control signals and measured outputs. The unique information that we have about the system is its structure but the parameters values are supposed to be unknown. This data-based residual method was described, by the same authors in previous publication, for linear systems. In this paper we apply this method to bilinear structure models. A particular focus is made on computational complexity reduction. It will be shown that a part of the system dynamics may be neglected with the consequence to simplify the on-line residual computation. This method is illustrated on a simulated Activated Sludge Process.