Suspended substrate differentiates pollutant removal and bacterial community spatial assembly in modular moving bed constructed wetlands (MMB-CWs)

The novel modular moving bed constructed wetland (MMB-CW), which integrates constructed wetlands (CWs) with moving bed biofilm reactor (MBBR) technology, has demonstrated significant potential for treating various low carbon-to-nitrogen (C/N) ratio wastewater and mitigating clogging. However, further investigation is needed to optimize the suspended substrate in MMB-CWs and clarify how substrate type affects system performance. This study applied the innovative MMB-CW technology to treat low C/N ratio wastewater under high-nitrogen and high-phosphorus conditions. Meanwhile, the practical performance of two engineering materials, ceramsite and high-density polyethylene (HDPE) filler, was compared under these identical conditions. Results indicated that under extremely low C/N ratio (0.5) conditions, MMB-CW filled with ceramsite demonstrated relatively better pollutant removal performance compared to those filled with HDPE filler during the system stabilization period. Within the MMB-CW, in situ self-enrichment facilitated the accumulation of nutrients-removing bacteria, including Candidatus_Competibacter (0.9-5.2%), Ferruginibacter (1.7-9.0%), and Nitrospira (0.3-1.5%). Nutrient metabolism-related gene analysis revealed higher abundances of nitrification-associated genes and a higher nosZ /( nirS + nirK ) ratio in the ceramsite-filled MMB-CW than in the HDPE filler-filled MMB-CW. Therefore, ceramsite may be more suitable than HDPE filler as a suspended substrate for MMB-CWs. Notably, within the MMB-CW, the spatial assembly of bacterial communities was jointly governed by stochastic and deterministic processes, with a greater contribution from stochastic processes in the ceramsite-filled system (87.1%) than in the HDPE filler-filled system (76.2%). This study optimized suspended substrates for MMB-CWs, offering a promising approach to overcome conventional CW limitations and mitigate environmental risks from low-C/N wastewater discharge.