Removal of nitrate nitrogen by aerobic denitrification granular sludge under strongly alkaline conditions: Performance and mechanism

[Display omitted] •Aerobic denitrifying granular sludge (ADGS) was successfully cultured at pH 11.0.•EPS promoted the formation of AGDS under strongly alkaline conditions.•The alkali-tolerant aerobic denitrifiers contributed to the removal of nitrate.•ADGS responds to a strongly alkaline environment via complex regulatory mechanisms. Aerobic granular sludge (AGS) technology faces challenges in treating strongly alkaline wastewater because high pH affects sludge settleability and microbial metabolic activity. In this study, aerobic denitrification granular sludge (ADGS) was cultivated with strongly autoaggregating aerobic denitrifying bacteria as the sludge source at an influent pH of 11.0 under aerobic conditions. The formation characteristics of ADGS and its pollutant removal efficiency under strongly alkaline conditions were explored. Compared with ADGS cultivated under neutral conditions, ADGS cultivated under strongly alkaline conditions resulted in superior particle formation rates, settleability, particle strengths and particle stabilities. Nitrate nitrogen was almost completely removed at a pH of 11.0 under aerobic conditions. Protein, polysaccharides and humic acid in extracellular polymeric substances (EPS) under strongly alkaline conditions were involved in the ADGS formation process. In addition, the presence of metallic elements in the EPS and inorganic nuclei in the sludge facilitated the formation of ADGS. The strongly alkaline environment enriched the alkali-resistant aerobic denitrifying bacteria, such as unclassified_f__Rhodobacteraceae, Tessaracoccus, Halomonas, and Pseudofulvimonas, as well as specialized alkali-resistant genera. Furthermore, the strongly alkaline environment increased the abundance of key enzymes involved in carbon metabolism and upregulated the expression of nitrogen metabolism genes in the ADGS to maintain their metabolic activity. In addition, the microorganisms upregulated genes that encoded reverse transporter protein genes (mnhACEFG, nhaACP, and phaACDEFG) and K+ uptake protein genes (trkAHG) and secreted greater amounts of acidic functional groups and free amino groups to help maintain intracellular and extracellular osmolality and acid–base homeostasis under strongly alkaline stress. This research provides a possible option and innovative strategy for the management of highly alkaline nitrate-containing wastewater.