Enhancing the stability of fungal-bacterial aerobic granular sludge using nitrogen-fixing consortia

The stability of aerobic granular sludge can depend on nitrogen availability and changes in cultivation conditions. In this study, model fungal-bacterial aerobic granules were obtained using Penicillium brasilianum , Engyodontium album and Fusarium oxysporum together with a nitrogen-fixing bacterial community comprising representatives of the genera Agrobacterium , Bacillus and Microbacterium . The model systems were cultivated under nitrate-containing and nitrate-free conditions and subjected to hydrogen peroxide stress and prolonged substrate limitation. Aggregate morphology, nitrogen compound transformation, COD and microbial composition were evaluated during cultivation. The presence of the bacterial community was associated with the formation and preservation of denser aggregates during prolonged cultivation and under nitrogen-limited conditions. Stable granules contained both fungal and bacterial components, with fungal CFU accounting for 54–61% and bacterial CFU for 39–46% of the culturable microbial fraction. In nitrate-free systems, up to 0.7 g/L ammonium was transformed within 3–4 days without marked nitrate accumulation, whereas ammonium transformation in nitrate-containing systems was slower and accompanied by nitrate accumulation. COD decreased to almost zero within approximately 4 days in the more stable granular systems. Quantitative PCR confirmed the retention of Agrobacterium , Bacillus and Microbacterium in the model granules. The results show that fungal-bacterial granular structures can be maintained under nitrate-limited conditions. The observed nitrogen transformation may be associated with the combined activity of fungal and bacterial components, which makes such model systems promising for further investigation of nitrogen transformation processes in fungal-bacterial granules.