In situ fermentation-coupled symbiosis of polyphosphate-accumulating organisms and microalgae for efficient nutrient removal and sludge reduction in low carbon-to-nitrogen ratios wastewater.

Abstract

Microalgae-bacteria consortia (MBC) integrated with polyphosphate-accumulating organisms (PAOs) treat wastewater sustainably, but face excess sludge and light-dark mismatch issues. This study developed in situ fermentation-coupled photo simultaneous nitrification-denitrification phosphorus removal (F/P-SNDPR) systems by incorporating fermentative PAOs into MBC for low carbon-to-nitrogen ratios (C/N) wastewater. The effects of light-dark cycles on nutrient removal, sludge fermentation, and microbial dynamics were investigated. Under an optimal 16 h dark/8h light cycle, the F/P-SNDPR system achieved > 83% nitrogen and > 95% phosphorus removal, with low sludge production (312.21 mgVSS/d) and low net CO2 emissions. Prolonged light and dark phases promoted early microbial apoptosis and subsequent cell lysis, respectively, thereby facilitating fermentation. Combined dark duration and photoinhibition suppress nitrite-oxidizing bacteria, enabling stable partial nitrification. Flow cytometry and metagenomic results identified Candidatus Phosphoribacter as the primary fermentative microorganism. Its fermentation-associated genes, including LivFGHMK and Pta, facilitated volatile fatty acid (VFA) production during the dark phase. The generated VFA supported Candidatus Accumulibacter/Candidatus Competibacter to enhance nutrient removal, driven by key functional genes for polyphosphate metabolism (Ppk and Ppx) and denitrification (NirS, NirK, and NosZ). Overall, The F/P-SNDPR system offers a low-carbon strategy for efficient low C/N wastewater treatment without mechanical aeration or external carbon addition, while reducing sludge production.