Effects of C/N ratio and carbon-source composition on nutrient removal in a dual-sludge membrane bioreactor system...

Microbial competition that is driven by substrates plays an essential role in the stable denitrifying phosphorus removal (DPR) of low carbon wastewater treatment. This paper used a dual-sludge membrane bioreactor (A₂N-MBR) to evaluate C/N ratios of 3-6 using acetate, followed by a comparison of acetate, propionate and their mixture at C/N = 5. Stoichiometric analysis estimated substrate allocations among denitrifying polyphosphate-accumulating organisms (DPAOs), denitrifying glycogen-accumulating organisms (DGAOs) and denitrifying ordinary heterotrophic organisms (DOHOs), while 16S rRNA gene sequencing and Pearson correlation analysis characterised community composition and its associations with nutrient removal. At C/N = 5, stoichiometric estimates assigned 53% of anaerobic COD consumption and 82% of anoxic nitrate reduction to DPAOs, with nitrate and phosphate removal efficiencies of 76.93% and 77.00%, respectively. At C/N = 6, the estimated substrate allocations shifted towards DGAOs and DOHOs pathways. Phosphorus removal was not as consistent as nitrate removal. In the subsequent carbon source comparison at C/N = 5, the acetate propionate mixture improved reactor performance. As a result, the maximum nitrate and phosphate removal efficiencies were 91.72% and 88.50%, respectively. Correlation analysis indicated that Thauera and Zoogloea were associated with NO₃^--N and PO₄^3--P removal. Conversely, Nitrospira relative abundance was negatively associated with nitrate removal. All in all, these findings link substrate conditions with community composition and DPR performance within the respective C/N and carbon source comparisons.HighlightsEmployed a novel A₂N-MBR system to investigate anoxic nitrate reduction and phosphate uptake.C/N ratios altered estimated substrate allocations among microbial pathways.Acetate-propionate (1:1) improved nitrate and phosphate removal.