Mechanistic Insights into Low Level Cd(II) Enhanced Partial Denitrification: Coupled Effects on Granule Formation and CarbonNitrogen Metabolism

Partial denitrification (PD), as an essential step producing nitrite for the cost effective anammox process, yet its performance under heavy metal stress remains underexplored. This study investigated the effects of low level cadmium (Cd(II), 5 10 mg L) on PD processes. Unexpectedly, Cd(II) slightly enhanced PD performance, with the specific nitrite production rate increasing from 97.96 to 108.65 mg N/g VSS/h, while reducing the organic carbon requirement for optimal nitrite accumulation. Cd(II) also promoted the secretion of extracellular polymeric substances (EPS), facilitating granule formation and improving structural stability. Microbial community analysis revealed significant enrichment of denitrifying bacteria, particularly Proteobacteria and Bacteroidota, which dominated the system at 10 mg/L Cd(II), leading to reduced competition for carbon and enhanced substrate utilization. Metagenomic analysis showed that Cd(II) exposure upregulated genes associated with EPS biosynthesis and ATP binding cassette (ABC) transporters, improving the uptake of organic matter and essential trace metals such as Fe and Mo. Furthermore, Cd(II) enhanced central carbon metabolic pathways, including the TCA cycle and glycolysis/gluconeogenesis, and promoted electron transfer through the upregulation of related protein encoding genes. These synergistic effects collectively accelerated denitrification kinetics and improved system resilience. This study provides mechanistic insights into the coupled effects of low level Cd(II) on sludge granulation, microbial function, and carbon nitrogen metabolism, offering promising strategies for the application of PD in treating nitrate rich industrial wastewater containing trace heavy metals.