Abstract Anaerobic digestion (AD) is a complex biological system which can be affected by several operational problems. Among them, biological foaming is one of the most difficult to deal with. It has many effects, such as causing gas pipe clogging and probe failures, and it can affect mixing devices, etc. Since the mechanisms involved in biological foaming development are not fully understood, it is not included in standard anaerobic digestion models. For this reason, a knowledge-based risk model to determine the suitable conditions for the development of biological foaming during AD simulation was developed. The resulting knowledgebased system, based on organic loading rate (OLR) and its daily variation, was experimentally validated using real data from a fully instrumented pilot plant (1 m3 upflow fixed bed digester). Results show a good correlation between the knowledge-based risk model and the estimated biological foaming risk from real data.