Different mechanisms for the anaerobic storage of organic substrates and their effect on enhanced biological phosphate removal (EBPR)

The driving force for Enhanced Biological Phosphorus Removal (EBPR) is the presence of an anaerobic zone that enriches the activated sludge for P-accumulating microorganisms (PAOs). According to accepted models, PAOs anaerobically store volatile fatty acids (VFAs) as polyhydroxyalkanoates (PHAs) utilising polyphosphate hydrolysis as the energy source. For substrates other than VFAs, one hypothesis is that other heterotrophs convert them to VFAs, so acting in favour of PAOs. However, particular glycogen accumulating microorganisms have been described (GAOs), that compete against PAOs being able to store anaerobically many substrates into PHAs, by transforming intracellular carbohydrates (glycogen) into PHAs, as the energy source. In this perspective, the paper presents a summary (with new findings) of a long experimental work to study EBPR processes with a lab-scale Sequencing Batch Reactor fed with different organic substrates (peptone, glucose and acetate, separate or in mixtures). Our results show that EBPR can be obtained with substrates other than VFAs with neither their pre-conversion to VFAs nor their storage as PHA. Moreover, in different periods anaerobic uptake of glucose was possible with and without EBPR. In both cases, the stored polymer was glycogen while the energy source was either polyphosphate hydrolysis or lactic fermentation, respectively. These results are not consistent with the reported behaviour of PAOs or GAOs, so showing that many different mechanisms of anaerobic uptake and storage of substrates can act in favour of, or against EBPR. Deeper insight on these mechanisms is needed to improve design and operation of EBPR plants.