Enhanced anammox through endogenous partial denitrification/anammox (EPDA) granular sludge: Granule formation, treatment performance, and multi-perspective microbial analysis

The direct application of anammox to wastewater treatment is valuable, but the enrichment of anammox bacteria (AnAOB) is complicated due to the malignant competition of heterotrophic bacteria. Here, we propose the use of endogenous partial denitrification/anammox (EPDA) granular sludge to enhance anammox while achieving simultaneous removals of nitrogen and phosphorus. After operation for 330 days, the EPDA granules were successfully cultured with the mean size of 0.89 mm. Nitrogen and phosphorus were efficiently removed with the effluent total nitrogen (TN) and PO43--P of 6.68±0.41 mg/L and 0.06±0.03 mg/L, respectively. Anammox took a leading role in the removal of TN with a contribution of 75±3%. Moreover, high-throughput sequencing (16S rRNA) results showed that AnAOB could coexist harmoniously with glycogen accumulating organisms (GAOs) and polyphosphate accumulating organisms (PAOs) in the EPDA granules, whereas mature EPDA granules could promote in situ enrichment of AnAOB. Microbial stratification analysis of the EPDA granule found that the inner layer was resided with AnAOB, while the outer layer was employed with GAOs and PAOs. Metagenomic and metatranscriptomic analyses found that the nitrate reductase (1150.56 RPM) was 4.6 times of nitrite reductase (249.98 RPM) at the transcriptome level. Moreover, the key enzymes related to anammox (hydrazine synthase and hydrazine dehydrogenase) were much higher than the other nitrogen conversion functional genes at the transcriptome level, confirming the main role of AnAOB in the removal of nitrogen. This study presents a novel strategy for the application of anammox from EPDA granular sludge.