Degradation of inorganic polyphosphates (polyP). Releases orthophosphate processively from the ends of the polyP chain. [phosphate](n) + H2O = [phosphate](n-1) + H(+) + phosphate Homodimer. Belongs to the GppA/Ppx family. Could be the product of a pseudogene. Compared with other ppx it lacks 200 residues at the C-terminal region.