From hybrid process to pure biofilm anammox process: Suspended sludge biomass management contributing to high-level anammox enrichment in biofilms

Abstract

The application of mainstream anammox is highly desirable for municipal wastewater treatment. However, enrichment of anammox bacteria (AnAOB) is challenging, particularly given the vicious competition from denitrifying bacteria (DB). Here, suspended sludge biomass management, a novel operational strategy for hybrid process (suspended sludge/biofilm), was investigated for 570 days based on a modified anaerobic–anoxic–oxic system treating municipal wastewater. By successively decreasing the suspended sludge concentration, the traditional hybrid process was successfully upgraded to a pure biofilm anammox process. During this process, both the nitrogen removal efficiency (NRE) and rate (NRR) were significantly improved (P < 0.001), from 62.1 ± 4.5% to 79.2 ± 3.9% and from 48.7 ± 9.7 to 62.3 ± 9.0 g N/(m3·d), respectively. Mainstream anammox was improved in the following: Candidatus Brocadia was enriched from 0.70% to 5.99% in anoxic biofilms [from (9.94 ± 0.99) × 108 to (1.16 ± 0.01) × 1010 copies/g VSS, P < 0.001]; the in situ anammox reaction rate increased from 8.8 ± 1.9 to 45.5 ± 3.2 g N/(m3·d) (P < 0.001); the anammox contribution to nitrogen removal rose from 9.2 ± 2.8% to 67.1 ± 8.3% (P < 0.001). Core bacterial microbiome analysis, functional gene quantification, and a series of ex situ batch experiments demonstrated that the stepwise decreases in suspended sludge concentration effectively mitigated the vicious competition of DB against AnAOB, enabling high-level AnAOB enrichment. This study presents a straightforward and effective strategy for enriching AnAOB in municipal wastewater, shedding fresh light on the application and upgradation of mainstream anammox.