Microfiber structure for enhanced immobilization of nitrifying bacteria in a post-nitrification reactor

Ammonia nitrogen in water negatively impacts aquatic life. Nitrifying bacteria are slow-growing and sensitive microorganisms, and technology using biofilm is an attractive option for their support. This study aims to demonstrate an optimal structure for microfilament carriers for growth of nitrifying bacteria in the tertiary stage of wastewater treatment (post-nitrification). Eleven samples of three polymer types (polyester, polypropylene, polyamide) that display different numbers of fibrils, fibril diameters, and structures were chosen for testing. Biofilm growth was monitored weekly using total protein concentrations and optical and fluorescence microscopy aided by image analysis. Final samples were characterized using molecular biological analysis (quantitative polymerase chain reaction and next-generation sequencing). Contents of ammonia-oxidizing bacteria (AOB) were significantly (four- to five-fold) lower than those of nitrite-oxidizing bacteria (NOB) in all tested samples. The relative abundance of NOB dominant strain, Nitrospira, reached >20%, whereas the abundance of Nitrosomonas, the only AOB representative, ranged between 3% and 6%. Polyester material was identified as optimal for support of nitrifying bacteria. This material displays maximum numbers of multifilament fibrils, and diameters of fibril and interspaces were minimal. These parameters play a crucial role for immobilization of nitrifying bacteria. [Display omitted] •Polyester material was most appropriate for nitrifying bacteria support.•Dominant carrier factors are diameters of fibrils and interspaces.•NGS analyze show a high abundance of other bacterial strains in post-nitrification.•Ammonia/nitrite-oxidizing bacteria ratios vary between 1/4 and 1/5.