Growth of Microthrix parvicella in nutrient removal activated sludge plants: studies of in situ physiology

The in situ physiological activity of Microthrix parvicella in nutrient removal activated sludge systems was investigated by uptake of labeled substrates and subsequent determination by microautoradiography. M. parvicella had a considerable capacity to take up oleic acid under oxic, anoxic and anaerobic conditions. For the lipid trioleic acid, the most pronounced uptake was found under anaerobic conditions, indicating that M. parvicella benefits from the degradation of lipids under these conditions. Starvation experiments with different electron acceptors available showed that M. parvicella was able to maintain its high uptake capacity for 4, >7 and 1–2 days under oxic, anoxic (with nitrate or nitrite present) and anaerobic conditions, respectively. This indicates that besides oxygen, also nitrate and nitrite could be used as electron acceptors. The limited tolerance to starvation under oxic conditions indicates that highly oxic conditions were toxic for the organism. No autotrophic or mixotrophic growth on [14C]hydrogencarbonate was detected, and the organism was not found to store [33P]-labeled orthophosphate under alternating anaerobic and anoxic/oxic conditions applied for enhanced biological phosphate removal. Based on the information available on the physiology of M. parvicella, a hypothesis for the growth in nutrient removal plants was presented together with possible control measures.