Successful and unsuccessful bioaugmentation experiments monitored by fluorescent in situ hybridization

Two nitrifying reactors were operated under the same conditions except that one was twice inoculated with the aerobic denitrifying bacteria Microvirgu la aerodenitrificans. The first bioaugmentation induced a transient nitroge n loss. Fluorescent in situ hybridization revealed that the bioaugmented ba cteria had been rapidly eaten by protozoa. The second massive inoculation u nbalanced the ecosystem and resulted in an overgrowth of protozoa and pertu rbations of nitrification, whereas both parameters remained stable in the n on bioaugmented reactor. To enhance the incorporation of the added bacteria to indigenous flocs, two strategies were then tested. First, coagulating a nd flocculating substances were added to the reactor just after bioaugmenta tion and second, the bacteria were embedded in alginate beads before inocul ation. The latter strategy gave the best results. After break-up of the bea ds, alginate fragments, containing microcolonies of M. aerodenitrificans, w ere found to be incorporated into the existing flocs. Alginate beads offer a temporary protection against grazing and favor the adhesion of the exogen ous bacterial microcolonies to the existing flocs. These beads therefore co nstitute a suitable bioaugmentation vector to incorporate a bacterial strai n into activated sludge flocs.