Nitrogen conversion performance of a polypropylene carrier designed to promote biofilm formation through foaming

The performance of plastic carriers in a moving bed biofilm reactor used for water treatment is strongly determined by the carrier material and its shape. Only a few reports have described the effect of foaming on biofilm formation and nitrogen conversion by polypropylene (PP) carriers. Here, we investigated a PP foam carrier for its biofilm forming ability and the resulting biofilm’s ability to remove nitrogen compounds. The results revealed that foaming increased the amounts of denitrifying bacterial biofilms attached to PP by approximately 44 times, indicating that unevenness by foaming promoted biofilm formation. Biofilms were also formed using activated sludges from nitrification and denitrification of a wastewater treatment facility. Amplicon sequencing analysis of the biofilms revealed that the families Xanthomonadaceae and Comamonadaceae, including nitrifying bacteria, were enriched in nitrifying sludge-derived biofilms, whereas Dechloromonas sp. and family Pseudomonadaceae, including denitrifying bacteria, were enriched in denitrifying sludge-derived biofilms. Furthermore, nitrifying sludge-derived biofilms completely removed ammonia, and its removal ability was superior to that of the original sludge. Denitrification by denitrifying sludge-derived biofilms was promoted by the addition of fumaric acid. Both nitrifying and denitrifying sludge-derived biofilms could remove nitrogen compounds from 1.4 mM ammonia-containing synthetic wastewater. Finally, the effect of biomass addition on the carrier was investigated. The addition of composted seaweed waste to PP foam carriers enhanced the formation of denitrifying sludge-derived biofilms by approximately 2.2 times, and the denitrification reaction by biofilms was promoted. These results revealed that waste biomass can be used to further enhance PP foam carrier performance.