Efficient nitrogen removal pathways and corresponding microbial evidence in tidal flow constructed wetlands for saline water treatment

How a constructed wetland within a natural park enhances plankton communities after more than 10 years of operation: Changes over space and time

Constructed wetlands are increasingly used as a solution to treat polluted water in natural environments. Located in the Albufera de València Natural Park, a constructed wetland was built in 2009 as a pilot project to act as an intermediary between low-quality waters and the largest protected coastal lagoon in the Iberian Peninsula. With a unique dataset spanning more than a decade (2009-2023), this study assessed changes in plankton communities both spatially (comparing six sampling sites) and temporally (comparing four periods of years). The results show how the constructed wetland, after nearly 15 years of operation, has not only maintained but also improved its capacity to enhance the biological quality of the water which is released into the protected lagoon, thus fulfilling one of the main aims of its construction. During the last period (2020-2023) of the time series, the constructed wetland outlets had significantly higher zooplankton biomass, particularly filter-feeding cladocerans, compared to the inlets. This clear improvement in the plankton community was due to management interventions (e.g., drying sectors of the constructed wetland during the summers since 2019) and the rise in temperature. These circumstances promoted earlier hatching of cladoceran diapause eggs from the sediments compared to previous years, maintaining their presence throughout all seasons. Consequently, the outlets of the constructed wetland had significantly lower phytoplankton abundance and sestonic chlorophyll-a concentrations than in the past, nearly oligotrophic states, and a reduced biovolume of potentially toxic cyanobacteria in the released waters.

Enhanced removal of PPCPs and antibiotic resistance genes in saline wastewater using a bioelectrochemical-constructed wetland system

Pharmaceuticals and personal care products (PPCPs) are insufficiently degraded in saline wastewater treatment processes and are found at high concentrations and detection frequencies in aquatic environments. In this study, the wetland plant Thalia dealbata was selected using a screening plant experiment to ensure good salt tolerance and high efficiency in removing PPCPs. An electric integrated vertical-flow constructed wetland (E-VFCW) was developed to improve the removal of PPCPs and reduce the abundance of antibiotic resistance genes (ARGs). The removal efficiency of ofloxacin, enrofloxacin, and diclofenac in the system with anaerobic cathodic and aerobic anodic chambers is higher than that of the control system (41.84 ± 2.88%, 47.29 ± 3.01%, 53.29 ± 2.54%) by approximately 20.31%, 16.04%, and 35.25%. The removal efficiency of ibuprofen in the system with the aerobic anodic and anaerobic cathodic chamber was 28.51% higher than that of the control system (72.41 ± 3.06%) and promotes the reduction of ARGs. Electrical stimulation can increase the activity of plant enzymes, increasing their adaptability to stress caused by PPCPs, and PPCPs are transferred to plants. Species related to PPCPs biodegradation (Geobacter, Lactococcus, Hydrogenophaga, and Nitrospira) were enriched in the anodic and cathodic chambers of the system. This study provides an essential reference for the removal of PPCPs in saline-constructed wetlands.

Exogenous compound bacteria enhance the nutrient removal efficiency of integrated bioremediation systems: Functional genes and microorganisms play key roles

With the continuous development of intensive mariculture, the application of the integrated bioremediation system of aquaculture wastewater (IBSAW) is increasingly promoted. However, the process and nutrients removal performance of the IBSAW need to be further optimized due to its immature technologies. In this study, exogenous compound bacteria (ECB) were added to IBSAW to investigate its pollutants removal efficiency and the relevant mechanisms. High-throughput sequencing and Geochip gene array were used to analyze the correlation between nutrients and bacteria, and the abundance of N and P cycling genes were quantified. Multivariable statistics, dimensionality reduction analysis, and network analysis were applied to explore the mechanisms of IBSAW operation. The results showed that the nutrients decreased significantly after adding ECB, with the brush treatment group significantly outperforming the ceramsite in removing NO3- and PO43-. Ceramsite has an advantage in removing NO2--N. The addition of ECB and different substrates significantly affected the composition of bacterial communities. The contents of nosZ and nirKS related to denitrification in the treatment groups were significantly higher than those in the control group, and the contents in the brush treatment group were significantly higher than that of ceramsite. The biomarkers Psychroserpens and Ruegeria on the biofilm of the brush treatment group were positively correlated with nirKS, while Mycobacterium, Erythrobacter and Paracoccus, Pseudohaliea in the ceramsite group were positively correlated with nirS and nirK, respectively. Therefore, it is speculated that the ECB significantly promoted the increase of denitrification bacteria by affecting the composition of bacterial communities, and the ECB combined with functional genera improved the efficiency of nutrients removal in the system. This study provided a reference for understanding the process and mechanism of nutrients removal, optimizing the wastewater purification technology of the IBSAW and improving the performance of the system.

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.

Exploring organic matter conversion pathway and its effect on nitrogen removal in tidal flow constructed wetlands

Achieving effective nitrogen removal remains a significant challenge faced by constructed wetlands. Although organic matter is a crucial factor influencing nitrogen removal, little attention has been paid to the impact of organic matter conversion pathways on nitrogen removal in constructed wetlands. Here, we showed that endogenous microorganisms performing carbon internalization could be easily enriched in tidal flow constructed wetlands (TFCWs) under its special rhythmic cycle of anaerobic/aerobic operational mode. Endogenous microorganisms could translate influent carbon sources into intracellular carbons during the anaerobic stage and supply the carbon source for endogenous denitrification after the aerobic stage (rest period). Based on these findings, an innovative combined TFCW and Nitrifying-CW system was developed, and robust total nitrogen (TN) removal (82% on average) was achieved even under carbon source limiting conditions. This performance was a substantial improvement compared to the conventional single bed TFCW with multiple "tides" (corresponding to the multiple contact/rest periods) with TN removal of only 54% on average. Simultaneous nitrification-endogenous denitrification (SNED) was found to be the major nitrogen removal pathway in the proposed system. Compared with classical nitrification-denitrification, simultaneous nitrification-endogenous denitrification brings high nitrogen conversion rates and significantly reduces the demand for oxygen and organic carbon. Furthermore, microbial community analysis indicated that endogenous microorganisms such as Candidatus_Competibacter and Defluviicoccus were successfully enriched, accounting for 50.73% and 3.46% in CW1, and 25.25% and 1.76% in CW2, respectively. Together, these mechanisms allow the proposed system to achieve efficient TN removal.