Nitrogen removal from decentralized domestic wastewater in gravel-bed coupled with wood chip under variable hydraulic loading rate

The performance of two gravel beds with wood chip as carbon source with different hydraulic loading rates (HLR) (0.5 m 3 /(m 2 ·d) of R1 and 0.3 m 3 /(m 2 ·d) of R2) was evaluated in this study. Results indicated that, during the stable operation period, R2 showed a 25% increase in total nitrogen removal efficiency (TNRE) compared to R1 and achieved 80.09% efficiency in R2. The analysis of the microbial community found that the higher HLR in R1 improved bacterial alpha diversity, of which Shannon index increased 41.07% and Richness increased 70.68% (p < 0.05). Moraxellaceae , a typical saprophytic bacterial community with high relative abundance (23.90% in R1, 57.86% in R2) in both systems, demonstrated that wood chips used was involved in the denitrification process in gravel bed by releasing carbon source. Furthermore, the functional genes prediction indicated that there was not a significant difference in the biological metabolic pathways of nitrogen in the process (p > 0.05), yet the lower HLR in R2 had a potential partial nitrification-partial denitrification process. This study confirms the feasibility of gravel bed coupled with wood chip as a sustainable solution for rural wastewater treatment, where HLR is critical for balancing microbial diversity and nitrogen removal efficiency.