Combined bioaugmentation with heterotrophic nitrification–aerobic denitrification bacteria and elevated C/N ratio enhances semiconductor wastewater treatment and reshapes microbial communities in Bardenpho process

Semiconductor wastewater contains high nitrogen and sulfate with low carbon-to-nitrogen ratios (C/N), limiting denitrification in conventional Bardenpho systems. This study evaluated bioaugmentation of a 4-stage Bardenpho process treating real semiconductor wastewater using a defined three-strain heterotrophic nitrification–aerobic denitrification (HNAD) consortium composed of Agrobacterium sp., Stutzerimonas sp., and Acinetobacter sp. Baseline operation at C/N 2 achieved stable nitrification but incomplete denitrification, yielding effluent total nitrogen (TN) of 40.4 mg-N/L (TN removal: 71.2%), a total nitrogen removal rate (TNRR) of 96.5 g-N/m 3 /d, and sulfate removal of 8.6%. After HNAD bioaugmentation with elevated C/N to 10, effluent TN decreased to 8.7 mg-N/L, TN removal increased to 94.0%, and TNRR rose to 132.4 g-N/m 3 /d, with complete nitrate elimination and an increase in sulfate removal to 18.2%. The nitrogen-specific sludge yield increased from 4.16 to 11.31 kg mixed-liquor-suspended-solids (MLSS)/kg-N, indicating enhanced nitrogen assimilation. Reducing the C/N ratio to 5 initially impaired nitrification, but operational adaptation restored performance, maintaining effluent TN 8.6 mg/L, TN removal 93.7%, and a nitrogen-specific yield of 7.04 kg-MLSS/kg-N, while decreasing sulfate removal to 9.0%. Microbial analysis showed that Agrobacterium and Hydrogenophaga dominated under C/N 10 and drove HNAD-mediated nitrogen removal, whereas at C/N 5 reinforced denitrifiers including Simplicispira , Thauera , and Devosiaceae , together with autotrophic nitrifiers, complemented conventional nitrogen removal pathways and sustained high TN removal. qPCR analysis supported phase-dependent changes in nitrogen transformation–related genetic potential. These results demonstrate that HNAD-enhanced Bardenpho process can achieve robust nitrogen removal from semiconductor wastewater when carbon supply and microbial adaptation are balanced. • HNAD and elevated C/N were tested in Bardenpho treating semiconductor wastewater. • HNAD with C/N 10 achieved 94% TN removal and 18.2% sulfate removal. • TN removal sustained at C/N 5, but sulfate removal reverted to the control level. • Agrobacterium , Acinetobacter , Hydrogenophaga drove HNAD at C/N 10. • At C/N 5, reinforced denitrifiers complemented conventional nitrogen removal.