Partial nitritation anammox biofilm colonization
Partial nitritation anammox (PNA) is a biological nitrogen removal process commonly used in wastewater treatment plants during treatment of the warm and nitrogen rich sludge liquor from anaerobic digestion, also known as sidestream wastewater. In these systems biofilms are frequently used to retain biomass from ammonia oxidizing bacteria and anammox bacteria, which together convert ammonium to nitrogen gas. Little is known about how these biofilm communities develop, and whether knowledge about the assembly of biofilm in natural communities can be applied to PNA biofilms. In this study the start-up of full-scale PNA moving bed biofilm reactor was followed for 175 days using shotgun metagenomics. Environmental filtering likely restricted initial biofilm colonization, resulting in low phylogenetic diversity, with the initial microbial community comprised mainly of Proteobacteria. Facilitative priority effects allowed further biofilm colonization, with growth of the initial colonizers facilitating the arrival and growth of anaerobic taxa like methanogens and anammox bacteria. Among the early colonizers were known 'oligotrophic' ammonia oxidizers including commamox and Nitrosomonas cluster 6a. Increasing the nitrogen load in the bioreactor allowed colonization by 'eutrophic' Nitrosomonas cluster 7 and resulted in the exclusion of the initial ammonia oxidizers and nitrite oxidizers. The results of this study not only contribute to our knowledge about biofilm assembly and PNA bioreactor start-up but could also help guide strategies for the successful implementation of PNA bioreactors.