Microbial competition for the organic substrates and its impact on EBPR systems under conditions of changing carbon feed

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Abstract
A deep insight into the mechanisms for the anaerobic storage of organic substrates is gained in lab-scale experiments with a Sequencing Batch Reactor. In addition to accepted models using polyphosphate (PP) hydrolysis to take up acetate and store polyhydroxyalkanoates (PHAs), with EBPR (“PAO-like metabolism”) on the one hand, and using glycogen to take up different substrates, storing PHAs as well, without EBPR (“GAO-like metabolism”) on the other hand, other pathways have to be considered. Under anaerobic conditions, glucose can be taken up directly and transferred to glycogen as the storage compound. The organisms can use PP hydrolysis (with EBPR) or fermentation (gaining a competitive advantage) as the energy source. A conceptual model of the different anaerobic mechanisms is given at the end. It is worth pointing out that these mechanisms are assumed to be combined in real systems and possibly can explain the phenomena observed after starvation at a full-scale wastewater treatment plant for Enhanced Biological Phosphorus Removal (EBPR): after periods of low organic carbon loads the effluent phosphate is significantly increased on the following 1-2 days. This is of both quantitative and qualitative relevance, because the average phosphate load in the effluent is increased by about 60% due to this effect. Periods with low COD inlet load lead to a complete cessation of the anaerobic phosphate release and to a subsequent decreased capacity for phosphate uptake. The effect is partially reproduced by a mathematical single-storage-compound model. The depletion of different metabolic pools, an imbalance in the (storage) pathways during and after the starvation of the phosphate accumulating organisms and the interaction of the EBPR with the denitrification that compete for the usable COD and nitrate are assumed to be responsible for these observations.