Nitrogen removal in improved subsurface wastewater infiltration system: Mechanism, microbial indicators and the limitation of phosphorus

To enhance the nitrogen removal capacity, scrap iron filings and Si-Al porous clay mineral material (PCMW) was used to improve a subsurface wastewater infiltration system (SWIS). The results showed TN and NH4+-N removal efficiencies of improved SWIS were 20.72% and 5.49% higher than those of the control SWIS, respectively. Based on the response of the removal performance, microbial community and function analysis of 16s rRNA amplicon sequencing results, the amending soil matrix substantially enriched the nitrogen removal bacteria (Rhizobiales_Incertae_Sedis and Gemmatimonadaceae), and significantly improved the activities of key enzymes (Hao, NasAB, NarGHI, NirK, NorBC, NirA and NirBD), particularly at co-occurrence zone of nitrification and denitrification (70-130 cm depth). The amending soil matrix not only extended the growth space of microbes, but also provided additional electrons and carbon sources for denitrifying bacteria by regulating the structure and function of the microbial community. In addition, amending soil matrix could enhance phosphate metabolism genes and phosphate solubilizing microbes in the denitrification zone by increasing the phosphorus source, thus strengthening nitrogen metabolism. Nitrospiraceae, Rhizobiales_Incertae_Sedis and Gemmatimonadaceae related to nitrogen removal and Bacillaceae with phosphate-solubilizing ability could be used as microbial indicators of nitrogen removal in SWISs. The reciprocal action of environmental on microbial characteristics exhibited microbial functional were related to DO, Fe-2(+), TOC, TP, TN, NH4+-N and NO3--N. Those could be used as physicochemical and biological indicators for application and monitoring of SWIS. In conclusion, this study provided a low-cost and efficient enhancement approach for the application of SWIS in decentralized domestic sewage treatment, and furnished theoretical support for subsequent applications.

Automated summary

To enhance the nitrogen removal capacity, this study used scrap iron filings and Si-Al porous clay mineral material to improve a subsurface wastewater infiltration system. The results showed that the improved system had higher removal efficiencies for total nitrogen (TN) and ammonium nitrogen (NH4+-N) compared to the control system. The amending soil matrix enriched nitrogen removal bacteria and improved the activities of key enzymes. It also enhanced phosphate metabolism genes and phosphate-solubilizing microbes, strengthening nitrogen metabolism. Nitrospiraceae, Rhizobiales_Incertae_Sedis, and Gemmatimonadaceae were identified as microbial indicators of nitrogen removal. The environmental factors DO, Fe-2(+), TOC, TP, TN, NH4+-N, and NO3--N were found to affect microbial characteristics. Overall, this study provided a low-cost and efficient enhancement approach for subsurface wastewater infiltration systems.