Pathways and biological mechanisms of N2O emission reduction by adding biochar in the constructed wetland based on 15N stable isotope tracing

Enhanced nitrogen removal in constructed wetlands with multivalent manganese oxides: Mechanisms underlying ammonium oxidation processes

The ammonium (NH4+) removal efficiency in constructed wetlands (CWs) is often limited by insufficient oxygen. In this study, an extract of Eucalyptus robusta Smith leaves was used to prepare multivalent manganese oxides (MVMOs) as substrates, which were used to drive manganese oxide (MnOx) reduction coupled to anaerobic NH4+ oxidation (Mnammox). To investigate the effects and mechanisms of MVMOs on ammonium nitrogen (NH4+-N) removal, four laboratory-scale CWs (0 %/5 %/15 %/25 % volume ratios of MVMOs) were set up and operated as continuous systems. The results showed that compared to controlled C-CW (0 % MVMOs), Mn25-CW (25 % MVMOs) improved the average NH4+-N removal efficiency from 24.31 % to 80.51 %. Furthermore, N2O emissions were reduced by 81.12 % for Mn25-CW. Isotopic tracer incubations provided direct evidence of Mnammox occurrence in Mn-CWs, contributing to 18.05-43.64 % of NH4+-N removal, primarily through the N2-producing pathway (73.54-90.37 %). Notably, batch experiments indicated that Mn(III) played a predominant role in Mnammox. Finally, microbial analysis revealed the highest abundance of the nitrifying bacteria Nitrospira and Mn-cycling bacteria Pseudomonas, Geobacter, Anaeromyxobacter, Geothrix and Novosphingobium in Mn25-CW, corresponding to its superior NH4+-N removal efficiency. The enhancement of NH4+ oxidation, first to hydroxylamine and then to nitrite, in Mn25-CW was attributed to the upregulation of ammonia monooxygenase genes (amoABC and hao). This study enhanced our understanding of Mnammox and provided further support for the use of manganese oxide substrates in CWs for efficient NH4+-N removal.

Enhanced textile wastewater remediation in Phragmites karka-based vertical flow constructed wetlands using Phragmites-derived biochar

Vertical flow-constructed wetlands (VFCWs) are treatment systems that can be used for the phytoremediation of highly polluted textile wastewater. Using plant-derived biochar to simultaneously improve the contaminant removal performance of CWs and sustainable utilization of harvested plant biomass is an interesting proposition. The present study explored the phytoremediation potential of Phragmites karka and verified the impact of using P. karka-derived biochar as a substrate in VFCWs for the treatment of textile wastewater. For this, three types of VFCWs were designed; (i) non-vegetated (VFCW), (ii) vegetated with P. karka (VFCW-P), and (iii) vegetated with P. karka and amended with P. karka-derived biochar (VFCW-BP) and semi-batch experiments were conducted. The investigation confirmed that wetlands using biochar as substrate were more efficient than other wetlands in pollutant load reduction. The maximum pollutant removal efficiencies were recorded for VFCW-BP vis-à-vis COD (83.61%), color (77.87%), chloride (73.22%), calcium (73.52%), sodium (67.18%), and potassium (75.72%) after five days. Furthermore, biochar addition enhanced the growth conditions for wetland plants by alleviating osmotic and oxidative stresses and hence helped them to perform better while removing pollutants. The maximum reduction of various pollutant parameters was reached within 72 h, after which remediation efficiency was slowed down. The study suggests that VFCW with biochar amendment is a useful strategy for textile wastewater treatment. Because the experimental design satisfies the needs for low-cost wastewater treatment, it may find widespread applications.

Removal of sulfamethoxazole and Cu, Cd compound pollution by arbuscular mycorrhizal fungi enhanced vertical flow constructed wetlands

The combined pollution of antibiotics and heavy metals (HMs) has a serious impact on the water ecological environment. Previous researches mainly focused on the removal of antibiotics or HMs as single pollutants, with limited investigation into the treatment efficiencies and underlying mechanisms associated with their co-occurring pollution. In this study, 16 micro vertical flow constructed wetlands (MVFCWs) were constructed to treat composite wastewater consisting of sulfamethoxazole (SMX), copper (Cu) and cadmium (Cd), involving two different inoculation treatments (arbuscular mycorrhizal fungi (AMF) inoculated and uninoculated) and eight kinds of pollutant exposure (Control Check (CK), SMX, Cu, Cd, SMX + Cu, SMX + Cd, Cu + Cd, SMX + Cu + Cd). The findings of this study demonstrated that the inoculation of AMF in MVFCWs resulted in removal efficiencies of SMX, Cu, and Cd ranging from 18.70% to 80.52%, 75.18% to 96.61%, and 40.50% to 89.23%, respectively. Cu and CuCd promoted the degradation of SMX in the early stage and inhibited the degradation of SMX in the later stage. Cd did not demonstrate a comparable promotive impact on SMX degradation, and its addition hindered Cu removal. However, comparatively, the presence of Cu exerted a more pronounced inhibitory effect on Cd removal. Furthermore, the addition of Cu augmented the abundances of Proteobacteria, Bacteroidetes (at the phylum level) and Rhodobacter, Lacunisphaera and Flavobacterium (at the genus level), and Cu exposure showed a substantially stronger influence on the microbial community than that of Cd and SMX. AMF might confer protection to plants against HMs and antibiotics by enriching Nakamurella and Lacunisphaera. These findings proved that AMF-C. indica MVFCW was a promising system, and the inoculation of AMF effectively enhanced the simultaneous removal of compound pollution.

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.