Effect of plant-self debris on nitrogen removal, transformation and microbial community in mesocosm constructed wetlands planted with Myriophyllum aquaticum

Subsurface wastewater infiltration systems for nitrogen pollution control

Subsurface wastewater infiltration systems (SWISs) are suggested to be a cost-effective and environmentally friendly method for sewage treatment. However, a comprehensive summary of the relevant mechanisms and optimization methods for nitrogen (N) removal in SWIS is currently lacking. In this review, we first summarize the N transformation mechanisms in SWIS. The impact of operational parameters on the N removal efficiency is then delineated. To enhance pollutant removal and minimize resource wastage, it is advisable to maintain a wet-dry ratio of 1:1 and a hydraulic loading rate of 8-10 cm/day. The organic load should be determined based on influent characteristics to optimize the balance between sewage treatment and nitrous oxide (N2O) emission. Finally, various strategies and modifications have been suggested to enhance pollutant removal efficiency and reduce N2O emissions in SWIS, such as artificial aeration, supply electron donors, and well-designed structures. Overall, greater emphasis should be placed on the design and management of SWIS to optimize their co-benefits while effectively controlling N pollution. PRACTITIONER POINTS: SWISs are often considered black boxes with their efficiency depending on hydraulic characteristics, biological characteristics, and substrate properties. Biological nitrification coupled with denitrification is considered to be the major N removal process. Increasing the reduction of N2O to the inert N2 form is a potential mechanism to mitigate global warming. Strategies such as artificial aeration, supply electron donors, and well-designed structures are suggested to improve N removal performance.

Transformation mechanisms of ammonium and nitrate in subsurface wastewater infiltration system: Implication for reducing greenhouse gas emissions

Subsurface wastewater infiltration system (SWIS) has been recognized as a cost-effective and environmentally friendly tool for wastewater treatment. However, there is a lack of knowledge on the transformation processes of nitrogen (N), hindering the improvement of the N removal efficiency in SWIS. Here, the migration and transformation mechanisms of ammonium (NH4+-N) and nitrate (NO3+-N) over 10 days were explored by 15N labeling technique. Over the study period, 49% of the added 15NH4+-N remained in the soil, 29% was removed via gaseous N emissions, and 14% was leaked with the effluent in the SWIS. In contrast, only 11% of the added 15NO3--N remained in the soil while 65% of the added 15NO3--N was removed via gaseous N emissions, and 12% with the effluent in the SWIS. The main pathway for N2O emission was denitrification (52-70%) followed by nitrification (15-28%) and co-denitrification (9-20%). Denitrification was also the predominant pathway for N loss as N2, accounting for 88-96% of the N2 emission. The dominant biological transformation processes were different at divergent soil depths, corresponding to nitrification zone and denitrification zone along the longitudinal continuum in SWIS, which was confirmed by the expression patterns of microbial gene abundance. Overall, our findings reveal the mechanism of N transformation in SWIS and provide a theoretical basis for establishing a pollutant management strategy and reducing greenhouse gas emissions from domestic wastewater treatment.

Nitrogen removal performance of improved subsurface wastewater infiltration system under various influent carbon-nitrogen ratios

Subsurface wastewater infiltration system (SWIS) has been recognized as a simple operation and environmentally friendly technology for wastewater purification. However, effectively removing nitrogen (N) remains a challenge, hindering the widespread application of SWIS. In this study, zero-valent iron (ZVI) and porous mineral material (PMM) were applied in SWIS to improve the soil matrix. Our results suggested that the addition of ZVI and PMM could simultaneously enhance N removal efficiency and reduce nitrous oxide emissions. This could be attributed to the abundant electrons generated by ZVI alleviating the electronic limitation of denitrification and the porous structure of PMM providing solid phase support for microbial growth. In addition, the abundance of microbial functional genes increased in modified SWIS, which could further explain the higher pollutant removal efficiency. Overall, this study provides new insights into the mitigation of wastewater pollution and greenhouse gas emissions in SWIS. PRACTITIONER POINTS: ZVI and PMM can adapt to different C loads and enhance pollutant removal efficiency in SWIS. Increasing C-N ratios positively affected the nitrate removal performance and negatively affected ammonium removal performance in SWIS. The amending soil matrix promoted the reduction of the N2 O to N2 and greenhouse gas emissions were well controlled. The abundance of microbial functional genes increased with the improvement of the soil matrix.

Leaching of soil legacy nitrogen in intact soil columns and significance of soil macropore structure

Excessive application of chemical fertilizers accumulates nitrogen in soil (soil legacy nitrogen), and its release has a long-term impact on environmental quality. The information regarding leaching of soil legacy nitrogen and role of soil pore structure is scarce. Fifteen undisturbed soil cores with a depth of ~200 mm were collected from five paddy fields in a subtropical area of China, and soil pore structure was characterized with X-ray computed tomography. The batch leaching column experiments were conducted to investigate the nitrogen leaching from the soil cores. In the leachate, inorganic nitrogen (NO3--N and NH4+-N) accounted for 73-85 % of total dissolved nitrogen (TDN), which showed small variations among the five sampling sites. The NO3--N, NH4+-N and TDN concentrations in the leachate over the number of leaching times could be well fitted with the Gaussian, exponential or linear models, suggesting that different nitrogen forms showed variable leaching dynamics. NO3--N and TDN leaching losses continued to increase with the number of leaching times, whereas NH4+-N leaching from the soil had a threshold value. Combination of changes in soil nitrogen content after leaching, the lagging effect of soil legacy nitrogen may be partly attributed to the continuous transformation of NH4+-N and organic nitrogen into NO3--N in the soils. Besides the nitrogen content in the soil, the most important factor of soil pore structure controlling NO3--N and TDN leaching loss was the fractal dimension; NH4+-N was mainly controlled by connectivity. The present study highlighted the importance of soil pore structure and nitrogen format transformation in the leaching of soil legacy nitrogen.

Influence of nitrogen sources on wastewater treatment performance by filamentous algae in constructed wetland system

Although filamentous algae have the characteristics of high nutrient assimilation ability, and adaptation to different conditions, studies on their role in water purification of constructed wetlands (CWs) are limited. In this study, the wastewater treatment capacity under different nitrogen sources was explored by constructing a filamentous algal CW (FACW) system. Results confirmed the fast and stable operation efficiency of the FACW system. Ammonia nitrogen was preferred in Cladophora sp. absorption and assimilation. The nutrient consumption rate (NCR) for total nitrogen (TN) of AG was 2.65 mg g-1 d-1, much higher than that of nitrate nitrogen (NG) (0.89 mg g-1 d-1). The symbiosis of bacteria and Cladophora sp. Contributed to pollutant removal. A stable and diverse community of microorganisms was found on Cladophora sp. Surface, which revealed different phylogenetic relationships and functional bacterial proportions with those attached on sediment surface. In addition, temperature and light intensity have great influence on the purification ability of plants, and low hydraulic retention time is beneficial to the cost-effective operation of the system. This study provides a method to expand the utilization of wetland plants and apply large filamentous algae to the purification of wetland water quality.