Denitrification and Phosphorus Removal Efficiency of Different Pretreated Qingke Straw Carbon Sources and Microbial Response Mechanisms

Abstract

This study aims to explore the efficiency of nitrogen and phosphorus removal in a laboratory-scale SBR reactor under plateau habitats by using different pretreated highland barley straw fermentation broths as external carbon sources. It investigates the evolution of microorganisms and changes in community structure, conducts a correlation analysis between microorganisms and environmental factors, and examines the changes in microbial community structure and six nitrogen metabolism functional modules. The results indicate that the addition of acid- or alkali-pretreated highland barley straw fermentation broth maintains a high removal rate of NH4 + in the reactor, at 89.30% (acid treatment) and 92.26% (alkali treatment) respectively. The addition of acid treatment shows the best COD removal rate (76.96%), slightly higher than that of the S4 device (73.16%). The TP removal rate of each device exhibits significant fluctuations and remains at a relatively low level overall. Acid treatment releases more carbon sources, with a COD content up to 2000 mg·L⁻¹ and a complex composition. Only 25% of the carbon sources can be rapidly degraded. Alkali treatment produces the most carbon, with a COD content above 6000 mg·L⁻¹, but some carbon sources are difficult to utilize, and only three rapidly degradable carbon sources, namely 2-Heptanone, 2-Octanone, and 3-Octanone, account for about 30%. Different pretreatment methods of highland barley straw fermentation broth as external carbon sources result in an increase in the abundance and diversity of microbial communities, affecting the direction of microbial community succession. The top five species at the genus level in each device include: unclassified_f__Comamonadaceae, norank_f__Chitinophagaceae, Thauera, norank_o__Saccharimonadales, and norank_f__Saprospiraceae. Among them, unclassified_f__Comamonadaceae and norank_o__Saccharimonadales have been detected in related studies under plateau habitats. The addition of fermentation broth significantly inhibits three functional modules: denitrification, dissimilatory nitrate reduction, and complete digestion, while enhancing three functional modules: nitrate assimilation and reduction, nitrification, and nitrogen fixation. The nitrogen and phosphorus removal process in plateau habitats requires enhanced treatment for the phosphorus removal process.