Under anaerobic conditions, the PAOs use VFAs as a
substrate, and transform it into poly-b-hydroxyalkanoates (PHAs) containing carbon .
The required energy for forming
the PHAs comes from the degradation of polyphosphate
releasing phosphorus into the liquid. In brief, the purpose of
the anaerobic zone is to stimulate the heterotrophic organisms
to transform fermentable COD into VFAs, and make possible
the storing of PHA by PAOs.
The amount of VFAs can be
increased by adding acetate or fermentable wastes to the
anaerobic tank .