A hypothesis was advanced that an anaerobic rotating biological contactor (AnRBC) digester, if operated with its discs half rather than fully submerged, should show an enhanced capability to withstand shock loads. Laboratory-scale AnRBC digesters were constructed and their performance under 50% and 100% disc submergence conditions compared. In all cases, under otherwise comparable conditions the performance in the 50% disc submergence mode was better. From measurements, including biogas hydrogen concentration, pH and propionic acid content of the digester liquid, it was concluded that it was the markedly better hydrogen stripping capability of the digester operated with discs only half submerged that accounted for its superior performance. Implications for design and modelling of digesters are discussed. It is concluded that current models of anaerobic digester systems need to reflect more accurately the actual hydrodynamic conditions in digesters if good predictions of digester performance under dynamic conditions are to be realised.