Nitrogen removal performances of Downflow Hanging Sponge (DHS) reactors for wastewater treatment: A critical review

Nitrogenous wastewater causes significant environmental challenges, including eutrophication, biodiversity loss, and ecosystem degradation. Achieving desired nitrogen discharge standards remains a critical task for conventional wastewater treatment processes. The Downflow Hanging Sponge (DHS) reactor, an advanced iteration of the trickling filter (TF) concept, has emerged as a promising technology for biological wastewater treatment. Its low energy consumption, high efficiency, and operational simplicity make it suitable for both developed and developing nations. This review critically evaluates the nitrogen removal performance of DHS reactors, focusing on the underlying mechanisms, operational parameters, and microbial ecology. Key influencing factors, including hydraulic retention time (HRT), organic loading rate (OLR), sludge retention time (SRT), external carbon supplementation, and recirculation strategies, are analyzed to elucidate their impact on nitrogen removal efficiency. A comparative analysis of previous studies highlights performance trends, revealing the DHS reactor’s strength in achieving high nitrogen removal rates under optimized conditions. However, technological gaps, such as inconsistent performance under varying environmental conditions and limited scalability, pose challenges. By synthesizing current knowledge, this study aims to guide future developments in DHS technology, fostering its broader application for sustainable nitrogen removal from wastewater and mitigating environmental impacts. • The DHS reactor efficiently removes nitrogen from wastewater using attached microbial biofilms. • DHS reactors support simultaneous nitrification and denitrification without requiring external aeration. • HRT and OLR significantly affect the nitrogen removal efficiency. • The sponge media enhances microbial retention, improving treatment performance and reducing sludge production. • DHS technology offers a low-cost, energy-efficient alternative for wastewater treatment.