Influence of the degree of perlite expansion on immobilization of Acinetobacter junii

Three different forms of perlite (non-expanded, semi-expanded and expanded) were tested as carriers of phosphate-accumulating bacterium Acinetobacter junii. The highest rate of immobilization of A. junii (12.65 × 109 CFU g−1) was obtained for expanded perlite, followed by semi-expanded (5.57 × 109 CFU g−1) and non-expanded (1.68 × 109 CFU g−1) perlite. The number of immobilized A. junii on expanded perlite was the highest among all carriers of Acinetobacter species reported in the literature so far. The immobilization of A. junii on perlite was dependent on the particle size, but not on the zeta potential of particles. The SSA of the perlite samples influenced immobilization of A. junii but was not recognized as the crucial factor. Immobilized bacteria were metabolically active and successfully removed phosphate from the wastewater when incubated in pure culture or in bioaugmented activated sludge. In the system with activated sludge the best carrier of A. junii was the semi-expanded perlite which was well incorporated in the sludge. The expanded perlite was floating on the surface of the wastewater while raw perlite sunk to the bottom of the reactor, diminishing the exposure and consequently phosphate removal of immobilized bacteria in the wastewater.