In Amsterdam, over 200 so called inverted siphons are situated. As these siphons are prone for blockages, it is key that these parts of a sewer system are maintained proactively. This article outlines the basis of a methodology that can be used to estimate the fouling degree of sewers with the use of water level measurements. Even though the measurement period appeared to be of restricted length, the methodology can be considered promising. Furthermore, evidence has been found that the cleaning regime plays a role in the initial accumulation of Fat, Oil and Grease (FOG), which therefore could be the basis of the development of effective measures to prevent blockages.