Abstract: The invention relates to apparatus (1) for monitoring condition of a biomass, particularly that naturally occurring in activated sewage sludge, comprising a vessel (2) for holding a liquid in the form of the activated sewage sludge containing a biomass (3), a device in the form of an aeration ring (4) for continuously aerating the sludge (3) in use, a hopper (5) and valve V2 arrangement to dose the biomass (3) in the vessel (2) with an additive in the form of an additional dose or charge of a feed solution, and a dissolved oxygen probe (6) to measure the dissolved oxygen content of the sewage in the vessel (2) whereby to monitor the condition of the biomass. Poisoning of the biomass is detected using the probe and associated electronic equipment (FIG. 3), such poisoning being shown by a sudden reduction in the measurement of the current of oxygen used per dose of sewage feed solution.