Microbial nitrogen transformation in constructed wetlands treating contaminated groundwater

Phytoremediation performance of mixed planting patterns and the associated rhizosphere microbial community in pilot-scale constructed wetlands

Phytoremediation is a low-cost, environmentally friendly, and sustainable technology that can utilize vegetation and microorganisms to avoid eutrophication and purifying water environment. The ability of five different living aquatic plants of nitrogen (N), phosphorus (P), and chemical oxygen demand (CODcr) removal were investigated in pilot scale constructed wetlands (CWs). Aquatic plant mixes significantly improved CODcr removal and plant tissue uptake of nitrogen and phosphorus. The wetland performance of mixed plantings was also influenced by the specific species. The mixed planting of Phragmites australi, Nymphaea Colorado and Myriophyllum verticillatum (PNM)When assessing pollutant removal in CWs, PNM performed better within mixtures, a possible synergistic effect, while TNV Typha orientalis, Nymphaea Colorado, and Vallisneria natans (TNV) performed poorly, a possible antagonist effect. The nutrient uptake within plant tissues byunder mixed plants planting was always ahad synergistic effect. Mixed plantingAquatic plant mixes significantly increased the rhizosphere microbial diversity and promoted the growth of functional denitrifying flora.

Constructed wetlands for the removal of organic micropollutants from wastewater: Current status, progress, and challenges

In this work, the practical utility of constructed wetlands (CWs) is described as a promising treatment option for micropollutants (MPs) in wastewater with the aid of their eco-friendly, low-energy, economically feasible, and ecologically sustainable nature. This paper offers a comprehensive review on CW technology with respect to the key strategies for MP removal such as phytoremediation, substrate adsorption, and microbial degradation. It explores the important factors controlling the performance of CWs (e.g., in terms of configurations, substrates, plant-microbe interactions, temperature, pH, oxygen levels, hydraulic loading rate, and retention time) along with the discussions on the pivotal role of microbial populations in CWs and plant-microbe cooperative remediation dynamics, particularly in relation to diverse organic MP patterns in CWs. As such, this review aims to provide valuable insights into the key strategies for optimizing MP treatment and for enhancing the efficacy of CW systems. In addition, the process-based models of constructed wetlands along with the numerical simulations based on the artificial neural network (ANN) method are also described in association with the data exploratory techniques. This work is thus expected to help open up new possibilities for the application of plant-microbe cooperative remediation approaches against diverse patterns of organic MPs present in CWs.

The comprehensive evaluation of nitrate origin and transformation pathways in the oxic alluvial aquifer in Serbia

Accurate pollution source identification is essential for establishing adequate water management strategies, particularly in groundwater with slow flow and prolonged recharge process allowing long-term pollution retention. An integrated study based on hydrogeochemical, dual isotopic (δ15NNO3 and δ18ONO3), and microbiological approaches (DN, IRB, and SRB BART tests) along with the statistical data processing was conducted to determine nitrate origin and fate in oxic alluvial groundwater source Ključ in Serbia. The findings from a comprehensive investigation, encompassing 20 groundwater sampling locations during the period 2010-2019, delineated three distinct zones - the hinterland (anthropogenic impact area-untreated sewage inflow), the middle zone (area of mixed influence from fertilizer application, accompanied by a mitigated anthropogenic impact), and the zone of riparian denitrification. Significant linear relationship between anthropogenic impact parameters (Na, Cl, B, NO3-, NH4+, and electrical conductivity) along with the isotopic signatures (δ15N-NO3- ranking from + 10.01 to + 11.18‰ and δ18O-NO3- ranking from + 1.15 to + 6.24‰) and grouped sampling objects by cluster analysis indicated that hinterland is burdened by the nitrates originating from anthropogenic impact. The cross-section of groundwater flow data, concurrent increase of NH4+, and pH levels, along with the highest values of δ15N-NO3- (+ 12.90‰) and δ18O-NO3- (+ 9.70‰), indicated area of fertilizers (urea) impact. BART test results, pH increase, and low oxygen concentration, along with the groundwater flow data in riparian zone, indicated the unfolding of denitrification process. Presented research emphasizes the importance, necessities, and advantages of simultaneous and complementary use of hydrogeochemical, microbiological, and isotopic data.

Nutrient Removal by Floating Treatment Wetlands Under Different Spatial Arrangement Modes: a Field Study

Floating treatment wetlands (FTWs) are a management method to improve urban rivers, but most studies have been carried out at laboratory, micro, and meso levels, so it is necessary to study full-scale FTWs as a method to improve urban water bodies. In this experiment, the purification effects of water temperature (WT), dissolved oxygen (DO), ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), total phosphorus (TP), chemical oxygen demand (COD_Mn), and chlorophyll-a (Chl-a) under staggered arrangement (SA) and centered arrangement (CA) were evaluated. It was found that the DO concentration and removal rate of COD_Mn, Chl-a, and TP in the SA were significantly higher than those in the CA in months with heavy rainfall. However, interestingly, for TP, August showed the opposite trend. The removal rates of NH₄⁺-N and NO₃^--N were significantly different throughout the test period. The biomass growth values of shoots and roots of plants in the FTWs were 0.40 ± 0.03 kg/m² and 1.38 ± 0.07 kg/m² in the SA and 0.32 ± 0.07 kg/m² and 1.26 ± 0.30 kg/m² in the CA. The increments of N absorbed by plants in the SA and CA were 7.08 ± 0.49 g/kg and 6.83 ± 0.07 g/kg, respectively, and the increments of P were 0.57 ± 0.02 g/kg and 0.32 ± 0.07 g/kg, respectively, which indicated that the growth status of plants in the FTWs in the SA was slightly better than that in the CA. In summary, the hybrid-constructed FTWs of both arrangements can effectively improve the water quality of urban rivers, and the effect of the SA was greater than that of the CA. The purification effect of in situ tests under different arrangement modes of hybrid-constructed FTWs was evaluated, which provides guidance and support for the field layout of FTWs in rivers in the future.

Pollutant removal performance and microbial enhancement mechanism by water-lifting and aeration technology in a drinking water reservoir ecosystem

Here, the performance and mechanism of pollutant removal in the Zhoucun reservoir by water-lifting and aeration systems (WLAs) were explored. The hypolimnion anoxic layer disappeared and the reservoir was mixed after the WLAs were operated for approximately 35 days, providing a suitable environment for pollutant removal. Operation of the system enhanced the metabolic activity of the water microbes and their capacity for purification, which contributed to the removal of nitrogen, organic carbon, Fe, Mn, P, and S. Specifically, the total N concentration decreased from 2.55 to 0.48 mg/L, showing an 81.18% removal rate. Microbial metabolism and the diversity index increased following the operation of the WLAs in the Zhoucun Reservoir. Furthermore, the water reservoir clearly inhibited the performance of Fe, Mn, P, and S through the WLA operation, meeting the requirements for class III based on the Chinese Surface Water Environmental Quality Standard (GB3838-2002). High-throughput sequencing analysis revealed increased levels of indicator and keystone operational taxonomic units belonging to Flavobacterium, hgcI_clade, Rheinheimera, Dechloromonas, Pseudomonas, and Rhodocyclaceae, which are related to the degradation of organic carbon and removal of nitrogen and phosphorus. Moreover, total N, ammonia, total P, dissolved oxygen, temperature, and pH were the principal factors affecting the microbial community based on redundancy analysis and the Mantel test. Furthermore, network analysis showed that symbiotic relationships accounted for the major proportion of the microbial network. Our results provide a theoretical foundation for the efficiency of N removal and essential technical support for improving the self-repair capacity of water in drinking water reservoirs.

Elucidating the impact of influent pollutant loadings on pollutants removal in agricultural waste-based constructed wetlands treating low C/N wastewater

In this study, the common agricultural wastes (wheat straw, apricot pit and walnut shell) were exploited as the potential organic substrates in subsurface flow CWs (SSFCWs) with intermittent aeration to investigate the impacts of the varied influent loadings on the removal performance of pollutants. The results indicated that an application of agricultural wastes in CWs presented the different role in improving removal efficiency as influent pollutant loadings increased. In middle and high influent pollutant loadings, CWs with walnut shell strengthened the denitrification process significantly, and could achieve preferable purification of COD (97.36%), NH₄⁺-N (98.83%), TN (51.78%), and TP (80.33%), respectively. However, N₂O emission fluxes were higher in CWs with agricultural biomass and increased clearly as the influent pollutant loadings increasing. The results indicated that combination of agricultural material addition and aeration could be an appropriate intensifying strategy in SSFCWs especially for treating medium-strength low-carbon wastewaters.

The intensified constructed wetlands are promising for treatment of ammonia stripped effluent: Nitrogen transformations and removal pathways

This study investigated the treatment performance and nitrogen removal mechanism of highly alkaline ammonia-stripped digestate effluent in horizontal subsurface flow constructed wetlands (CWs). A promising nitrogen removal performance (up to 91%) was observed in CWs coupled with intensified configurations, i.e., aeration and effluent recirculation. The results clearly supported that the higher aeration ratio and presence of effluent recirculation are important to improve the alkalinity and pollutant removal in CWs. The influent pH (>10) was significantly decreased to 8.2-8.8 under the volumetric hydraulic loading rates of 0.105 and 0.21 d^-1 in the CWs. Simultaneously, up to 91% of NH₄⁺-N removal was achieved under the operation of a higher aeration ratio and effluent recirculation. Biological nitrogen transformations accounted for 94% of the consumption of alkalinity in the CWs. The significant enrichment of δ¹⁵N-NH₄⁺ in the effluent (47-58‰) strongly supports the occurrence of microbial transformations for NH₄⁺-N removal. However, relatively lower enrichment factors of δ¹⁵N-NH₄⁺ (-1.8‰ to -11.6‰) compared to the values reported in previous studies reflected the inhibition effect of the high pH alkaline environment on nitrifiers in these CWs.

Enhanced nitrogen removal of low C/N domestic wastewater using a biochar-amended aerated vertical flow constructed wetland

Recently, vertical flow constructed wetlands (VFCWs) with intermittent aeration have been proven as an efficient technology to enhance removal efficiency of organics and nitrogen for wastewater treatment. However, the low denitrification effect in VFCWs was a problem for treating low carbon source wastewater. In this study, intermittent aeration and biochar, produced by biomass pyrolysis, was used to promote the nitrogen removal in VFCWs for low C/N domestic wastewater. Four systems, including non-aerated with non-biochar VFCW, non-aerated with biochar VFCW, aerated with non-biochar VFCW and aerated with biochar VFCW, were conducted for comparing their treatment performances. The results showed that much higher removal of COD (94.9%), NH₄⁺-N (99.1%), TN (52.7%) and lower N₂O emission (60.54μg·m^-2·h^-1) was obtained in aerated VFCW with biochar addition. The results suggested that adding biochar to intermittent aerated VFCWs could be an effective and appropriate strategy for low C/N wastewater treatment.

Long-term assessment at field scale of Floating Treatment Wetlands for improvement of water quality and provision of ecosystem services in a eutrophic urban pond

Pollution of urban water bodies requires stringent control measures and the development of low-cost and highly efficient alternative technologies. In contrast to Constructed Wetlands, Floating Treatment Wetlands (FTWs) have the advantage of not requiring large surface of land since they operate in situ. However, there is limited information about their long-term evaluation while operating at field scale. The aim of this work was to assess the performance of FTWs using a combination of Pontederia sagittata and Cyperus papyrus for the improvement of the water quality and provision of ecosystem services of a eutrophic urban pond. The FTWs were built with low-cost material easy to acquire and to ensemble. Two FTWs (17.5m² and 33m²) located in Pond 1 within a complex of 4 urban artificial ponds were evaluated for two years. They promoted an increase in the dissolved oxygen (D.O.) within a range of 15 to 67%, a removal of fecal coliforms in the range of 9 to 86% and a nitrate removal in the range of 9 to 76%. The plant productivity reached a maximum of 363g_dmm^-2d^-1 in the FTW1 and 536g_dmm^-2d^-1 in the FTW2 during the period March-June 2016. The TKN and the TP content in the plant were in the range of 18.3 to 28.1 and of 0.05 to 0.196gkg^-1 dry matter, respectively. In conclusion, the tested FTWs have proved to be a very beneficial low-cost technology for the improvement of water quality and provision of ecosystem services.

Enhanced nutrient removal and mechanisms study in benthic fauna added surface-flow constructed wetlands: The role of Tubifex tubifex

This study designed a combined benthic fauna-T. orientalis-substrate-microbes surface-flow constructed wetlands (SFCWs) through the addition of T. tubifex. Results showed that, the removal efficiencies of nitrogen and phosphorus in the tested SFCWs achieved 81.14±4.16% and 70.49±7.60%, which were 22.27% and 27.35% higher than that without T. tubifex. Lower nitrate (2.11±0.79mg/L) and ammonium (0.75±0.64mg/L) were also observed in the tested SFCWs, which were 3.46mg/L and 0.52mg/L lower than that without T. tubifex. Microbial study confirmed the increased denitrifiers with T. tubifex. The lower nitrogen in effluent was also attributed to higher contents of nitrogen storage in sediment and T. orientalis due to the bioturbation of T. tubifex. Furthermore, with T. tubifex, higher proportions of particulate (22.66±3.96%) and colloidal phosphorus (20.57±3.39%) observed promoted phosphorus settlement and further absorption by T. orientalis. The outcomes of this study provides an ecological and economical strategy for improving the performance of SFCWs.

Nitrogen removal performance in planted and unplanted horizontal subsurface flow constructed wetlands treating different influent COD/N ratios

Microcosm horizontal subsurface flow constructed wetlands (HSSFCWs) were used to examine the impacts of vegetation on nitrogen dynamics treating different influent COD/N ratios (1:1, 4:1, and 8:1). An increase in the COD/N ratio led to increased reductions in NO3 and total inorganic nitrogen (TIN) in planted and unplanted wetlands, but diminished removal of NH4. The HSSFCW planted with Canna indica L. exhibited a significant reduction in NH4 compared to the unplanted system, particularly in the active root zone where NH4 removal performance increased by up to 26 % at the COD/N ratio of 8:1. There was no significant difference in NO3 removal between the planted and unplanted wetlands. TIN removal efficiency in the planted wetland increased with COD/N ratios, which was likely influenced by plant uptake. NH4 reductions were greater in planted wetland at the 20- and 40-cm depths while NO3 reductions were uniformly greater with depth in all cases, but no statistical difference was impacted by depth on TIN removal. These findings show that planting a HSSFCW can provide some benefit in reducing nitrogen loads in effluents, but only when a sufficient carbon source is present.

Microbial nitrogen removal pathways in integrated vertical-flow constructed wetland systems

Microbial nitrogen (N) removal pathways in planted (Canna indica L.) and unplanted integrated vertical-flow constructed wetland systems (IVCWs) were investigated. Results of, molecular biological and isotope pairing experiments showed that nitrifying, anammox, and denitrifying bacteria were distributed in both down-flow and up-flow columns of the IVCWs. Further, the N transforming bacteria in the planted IVCWs were significantly higher than that in the unplanted ones (p<0.05). Moreover, the potential nitrification, anammox, and denitrification rates were highest (18.90, 11.75, and 7.84nmolNg(-1)h(-1), respectively) in the down-flow column of the planted IVCWs. Significant correlations between these potential rates and the absolute abundance of N transformation genes further confirmed the existence of simultaneous nitrification, anammox, and denitrification (SNAD) processes in the IVCWs. The anammox process was the major N removal pathway (55.6-60.0%) in the IVCWs. The results will further our understanding of the microbial N removal mechanisms in IVCWs.