Response of wastewater treatment performance, microbial composition and functional genes to different C/N ratios and carrier types in MBBR inoculated with heterotrophic nitrification-aerobic denitrification bacteria

[Display omitted] •Operation parameter for MBBR firstly inoculated with HN-AD bacteria was optimized.•Nitrogen removal efficiency of PVA gel was double that of K1 at low C/N ratios.•PVA gel was beneficial to the growth of oligotrophic and HN-AD bacteria.•HN-AD bacteria Paracoccus and Acinetobacter played a key role at low C/N ratios.•DNRA and denitrification were the main nitrogen transforming pathway. To operate the moving bed biofilm reactor inoculated with HN-AD bacteria (B-MBBR) instead of activated sludge for livestock and poultry breeding wastewater (LPBW) disposal in most efficient manner, nitrogen removal (NR) efficiency and microbial composition of two MBBRs with different carrier types under various C/N ratios were explored. Results indicated that the performance on NR greatly various in different carrier types under various C/N ratios. Attributing to the bacterial protection provided by the porous structure of polyvinyl alcohol (PVA) gel, MBBR using PVA gel as the carrier exhibited a more stable NR performance (range from 78.05% to 83.76%) versus that using Kaldnes (K1) as the carrier (range from 78.05% to 83.76%). Besides, microbial analysis indicated that MBBR with PVA gel as the carrier is conducive to the growth of oligotrophic and HN-AD bacteria (Paracoccus and Acinetobacter), and the highest relative abundance was 16.37% at C/N ratio of 6.