Hydraulic Residence Time Research Articles

Effective fluoride removal from brackish groundwaters by flow-electrode capacitive deionization (FCDI) under a continuous-flow mode

Herein, we demonstrated the suitability and effectiveness of utilizing flow-electrode capacitive deionization (FCDI) for treatment of fluoride-contaminated brackish groundwater. By comparing operational modes of short-circuited closed-cycle (SCC), isolated closed-cycle (ICC) and single cycle (SC), it was found that SCC mode was the most advantageous. In SCC configuration, the effects of different parameters on the removal of F− and Cl− were investigated including current density, hydraulic residence time (HRT), activated carbon (AC) loading and feed concentration of coexisting NaCl. Results indicated that the steady-state effluent Cl− concentration dropped with elevated applied current, and the decreasing rate got faster with the increase of HRT or AC loading. However, for the steady-state effluent F− concentration, it dropped to a value under a small applied current and maintained stable in spite of the increase in applied current, and both HRT and AC loading had insignificant effects on the steady-state effluent F− concentration. F− was preferentially removed from the treated water compared with Cl−, and a higher ion selectivity could be obtained at lower applied current and smaller HRT with the trade-off being that operation under these circumstances would generate outlet water with little change in conductivity compared to the influent. The removal efficiencies of F− and Cl− both decreased with increasing feed concentration of coexisting NaCl. This study should be of value in establishing FCDI as a viable technology for treatment of fluoride-contaminated brackish groundwater.

Effect of Hydraulic Residence Time on Removal Efficiency of Pollutants in Subsurface Flow Constructed Wetlands and Analysis of Denitrification Mechanism

This study used vertical and horizontal subsurface constructed wetlands with continuous inflow, to compare and analyze the effects of four hydraulic residence times on the removal efficiency of conventional pollutants. Using the optimal hydraulic retention time, the two types of wetlands were examined in terms of the stromal layer nitrification, denitrification, and ammonia oxidation abundance of functional genes, as well as the intensity of nitrification and denitrification. In addition, redundancy analysis and variance decomposition analysis were used to determine the main factors affecting nitrogen removal in the two kinds of wetlands, so that targeted improvement measures can be suggested. The best removal efficiency of conventional pollutants (COD, TP, TN, and NH4+-N) was achieved with a hydraulic retention time of 24 h, resulting in a removal rate of more than 70%. With a 24 h retention time, the removal rate of NH4+-N and TN and the intensity of nitrification and denitrification exhibited a gradually decreasing trend along the flow direction. Among the three functional genes, the abundance of denitrification functional genes (nirS) was much higher than that of nitrification functional genes (nxrA) and ammonia oxidation functional genes (AOB-amoA). In this study, the nitrogen removal ability of the two subsurface flow constructed wetlands was jointly affected by environmental factors and microbial factors, among which microbial factors contributed the most to nitrogen removal (55% and 48%). In addition, the removal rates of TN and NH4+-N were proportional to DO, specific surface area of substrate, COD concentration, as well as nitrification and denitrification functional genes, but inversely proportional to pH. Therefore, in order to improve the nitrogen removal efficiency of both systems, the amount of dissolved oxygen and carbon sources in the substrate layer should be increased, while the pH value should be appropriately reduced. Moreover, the horizontal subsurface constructed wetland significantly improves the nitrogen removal efficiency of the system, as the substrate layer has a larger specific surface area. This study provides a theoretical basis for the design of constructed wetlands and the selection of an optimal hydraulic residence time. Quantitative analysis of nitrogen removal pathways is of great significance for understanding the nitrogen removal mechanism and improving the nitrogen removal rate in constructed wetlands.

Study on Start-Up Membraneless Anaerobic Baffled Reactor Coupled with Microbial Fuel Cell for Dye Wastewater Treatment.

In this study, the antitoxicity performance of the traditional anaerobic baffled reactor (ABR) and the newly constructed membraneless anaerobic baffled reactor coupled with microbial fuel cell (ABR-MFC) was compared for the treatment of simulated printing and dyeing wastewater under the same hydraulic residence time. The sludge performances of ABR-MFC and ABR were evaluated on the dye removal rate, extracellular polymer (EPS) content, sludge particle size, methane yield, and the surface morphology of granular sludge. It was found that the maximum power density of the ABR-MFC reactor reached 1226.43 mW/m3, indicating that the coupled system has a good power generation capacity. The concentration of the EPS in the ABR-MFC reactor was about 3 times that in the ABR, which could be the result of the larger average particle size of sludge in the ABR-MFC reactor than in the ABR. The dye removal rate of the ABR-MFC reactor (91.71%) was higher than that of the ABR (1.49%). The methane production and microbial species in the ABR-MFC system were higher than those in the ABR. Overall, the MFC embedded in the ABR can effectively increase the resistance of the reactor, promote the formation of granular sludge, and improve the performance of the reactor for wastewater treatment.

Multi-wave UV-photocatalysis system (UVA+UVC+VUV/Cu-N-TiO2) for efficient inactivation of microorganisms in ballast water

Ballast Water Treatment System (BWTS) is a system designed to remove biological organisms from ballast water. However, the existing BWTSs often have problems in practical applications. In this study, a multiwave ultraviolet (UV)-modified TiO2 photocatalyst biological inactivation system (longwave UV, UVA+shortwave UV, UVC+vacuum UV, VUV/Cu-N-TiO2) for microorganism inactivation in ballast water was established. The results showed that the UVA+UVC+VUV/Cu-N-TiO2 system improved the UV light quantum yield and catalyst activity in the photocatalytic reaction and fully utilized the synergistic inactivation effect of Cu-N-TiO2 photocatalyst+ multiwave UV light (UVA, UVC, and VUV) on microorganisms. Compared with 8 other photocatalytic systems, the logarithmic algae removal rate and logarithmic sterilization rate of the UVA+UVC+VUV/Cu-N-TiO2 system increased to 1.78 log and 5.86 log, respectively. The turbidity of the seawater affected the microorganism inactivation to a certain extent. The humic acid concentration should be controlled below 2 mg L−1 for the UVA+UVC+VUV/Cu-N-TiO2 system to inactivate microalgae more effectively. The multiwave UV photocatalytic system could significantly increase the lipid peroxidation products in microbial cells, rapidly reduce superoxide dismutase (SOD) activity, and degrade a large amount of chlorophyll within a short hydraulic residence time (HRT). Severe damage to the microbial cell membrane can destroy the normal functions of cells, resulting in the death of microorganisms. In conclusion, the UVA+UVC+VUV/Cu-N-TiO2 system is a potential new ballast water treatment system.

Biogeochemical and Hydrological Variables Synergistically Influence Nitrate Variability in Coastal Deltaic Wetlands

AbstractCoastal river deltas are centers of surface water nitrate processing, yet the mechanisms controlling spatio‐temporal patterns in nutrient variability are still little understood. Nitrate fluctuations in these systems are controlled by complex interactions between hydrological and biogeochemical drivers, which act together to transport and transform inorganic nutrients. Distinguishing the contributions of these drivers and identifying wetland zones where nitrate processing is occurring can be difficult, yet is critical to make assessments of nutrient removal capacity in deltaic wetlands. To address these issues, we analyze relationships among regional “external” (river discharge, tides, wind) and local “internal” (water level, temperature, turbidity, and nitrate) variables in a deltaic wetland in coastal Louisiana by coupling a process connectivity framework with information theory measures. We classify variable interactions according to whether they work uniquely, redundantly, or synergistically to influence nitrate dynamics and identify timescales of interaction. We find that external drivers work together to influence nitrate transport. Patterns of hydrological and sediment connectivity change over time due to tidal flushing and discharge variation. This connectivity influences the emergence of functional zones where local nitrate fluctuations and temperature and water level process couplings are strong controls on nitrate variability. High vegetation density decreases hydrological process connectivity, even during periods of high river discharge, but it also increases biogeochemical process connections, due to the lengthening of the hydraulic residence time. Based on these results we make recommendations for monitoring nitrate in a wetland.

Evaluation of an outdoor pilot-scale tubular photobioreactor for removal of selected pesticides from water

This work assesses the capacity of a microalgae-based system to remove three highly to medium polar pesticides typically found in freshwater: acetamiprid, bentazone, and propanil. Degradation of the pesticides was firstly studied individually at batch lab-scale reactors and abiotic and heated-killed controls were employed to clarify their removal pathways. At lab-scale, propanil and acetamiprid were completely removed after 7 days whereas bentazone was not removed. Four and two transformation products (TPs) were generated in the biodegradation process for acetamiprid and propanil, respectively. Then, the simultaneous removal of the pesticides was assessed in an outdoor pilot photobioreactor, operated with a hydraulic residence time of 8 days. During the steady-state, high removal efficiencies were observed for propanil (99%) and acetamiprid (71%). The results from batch experiments suggest that removal is mainly caused by algal-mediated biodegradation. Acetamiprid TPs raised throughout the operational time in the photobioreactor, while no propanil TP was detected at the pilot-scale. This suggests complete mineralization of propanil or residual formation of its TPs at concentrations below the analytical method detection limit. Aiming at biomass valorization, diverse microalgae harvesting methods were investigated for biomass concentration, and the effect of residual pesticides on the biogas yield was determined by biochemical methane potential tests. Anaerobic digestion was not inhibited by the pesticides as verified by the digestion performance. The results highlight the potential of microalgae-based systems to couple nutrient removal, biomass production, micropollutant biodegradation, and biofuel production.

Temperate climate energy-positive anaerobic secondary treatment of domestic wastewater at pilot-scale

Conventional aerobic secondary treatment of domestic wastewater is energy intensive. Here we report net energy positive operation of a pilot-scale anaerobic secondary treatment system in a temperate climate, with low levels of volatile solids for disposal (< 0.15 mgVSS/mgCODremoved) and hydraulic residence times as low as 5.3 h. This was accomplished with a second-generation staged anaerobic fluidized membrane bioreactor (SAF-MBR 2.0) consisting of a first-stage anaerobic fluidized bed reactor (AFBR) followed by a second-stage gas-sparged anaerobic membrane bioreactor (AnMBR). In stage 1, fluidized granular activated carbon (GAC) particles harbor methanogenic communities that convert soluble biodegradable COD into methane; in stage 2, submerged membranes produce system effluent (permeate) and retain particulate COD that can be hydrolyzed and/or recycled back to stage 1 for conversion to methane. An energy balance on SAF-MBR 2.0 (excluding energy from anaerobic digestion of primary suspended solids) indicated net energy positive operation (+ 0.11 kWh/m3), with energy recovery from produced methane (0.39 kWh electricity/m3 + 0.64 kWh heat/m3) exceeding energy consumption due to GAC fluidization (0.07 kWh electricity/m3) and gas sparging (0.20 kWh electricity/m3 at an optimal flux of 12.2 L/m2 h). Two factors dominated the operating expenses: energy requirements and recovery cleaning frequency; these factors were in turn affected by flux conditions, membrane fouling rate, and temperature. For optimization of expenses, the frequency of low-cost maintenance cleanings was adjusted to minimize recovery cleanings while maintaining optimal flux with low energy costs. An issue still to be resolved is the occurrence of ultrafine COD in membrane permeate that accounted for much of the total effluent COD.

Legionella and other opportunistic pathogens in full-scale chloraminated municipal drinking water distribution systems

Water-based opportunistic pathogens (OPs) are a leading cause of drinking-water-related disease outbreaks, especially in developed countries such as the United States (US). Physicochemical water quality parameters, especially disinfectant residuals, control the (re)growth, presence, colonization, and concentrations of OPs in drinking water distribution systems (DWDSs), while the relationship between OPs and those parameters remain unclear. This study aimed to quantify how physicochemical parameters, mainly monochloramine residual concentration, hydraulic residence time (HRT), and seasonality, affected the occurrence and concentrations of four common OPs (Legionella, Mycobacterium, Pseudomonas, and Vermamoeba vermiformis) in four full-scale DWDSs in the US. Legionella as a dominant OP occurred in 93.8% of the 64 sampling events and had a mean density of 4.27 × 105 genome copies per liter. Legionella positively correlated with Mycobacterium, Pseudomonas, and total bacteria. Multiple regression with data from the four DWDSs showed that Legionella had significant correlations with total chlorine residual level, free ammonia concentration, and trihalomethane concentration. Therefore, Legionella is a promising indicator of water-based OPs, reflecting microbial water quality in chloraminated DWDSs. The OP concentrations had strong seasonal variations and peaked in winter and/or spring possibly because of reduced water usage (i.e., increased water stagnation or HRT) during cold seasons. The OP concentrations generally increased with HRT presumably because of disinfectant residual decay, indicating the importance of well-maintaining disinfectant residuals in DWDSs for OP control. The concentrations of Mycobacterium, Pseudomonas, and V. vermiformis were significantly associated with total chlorine residual concentration, free ammonia concentration, and pH and trihalomethane concentration, respectively. Overall, this study demonstrates how the significant spatiotemporal variations of OP concentrations in chloraminated DWDSs correlated with critical physicochemical water quality parameters such as disinfectant residual levels. This work also indicates that Legionella is a promising indicator of OPs and microbial water quality in chloraminated DWDSs.

Modeling spatially resolved characterization factors for eutrophication potential in life cycle assessment.

Prior versions of the Tool for Reduction and Assessment of Chemical and other environmental Impacts (TRACI) have recognized the need for spatial variability when characterizing eutrophication. However, the method's underlying environmental models had not been updated to reflect the latest science. This new research provides the ability to differentiate locations with a high level of detail within the USA and provides global values at the country level. In previous research (Morelli et al. 2018), the authors reviewed a broad range of domain-specific models and life cycle assessment methods for characterization of eutrophication and ranked these by levels of importance to the field and readiness for further development. The current research is rooted in the decision outcome of Morelli et al. (2018) to separate freshwater and marine eutrophication to allow for the most tailored characterization of each category individually. The current research also assumes that freshwater systems are limited by phosphorus and marine systems are limited by nitrogen. Using a combination of spatial modeling methods for soil, air, and water, we calculate midpoint characterization factors for freshwater and marine eutrophication categories and evaluate the results through a US-based case application. Maps of the nutrient inventories, characterization factors, and overall impacts of the case application illustrate the spatial variation and patterns in the results. The importance of variation in geographic location is demonstrated using nutrient-based activity likelihood categories of agricultural (rural fertilizer), non-agricultural (urban fertilizer), and general (human waste processing). Proximity to large bodies of water, as well as individual hydraulic residence times, was shown to affect the comparative values of characterization factors across the USA. In this paper, we have calculated and applied finely resolved freshwater and marine eutrophication characterization factors for the USA and country-level factors for the rest of the globe. Additional research is needed to provide similarly resolved characterization factors for the entire globe, which would require expansion of publicly available data and further development of applicable fate and transport models. Further scientific advances may also be considered as computing capabilities become more sophisticated and widely accessible.

Persistence and removal of trace organic compounds in centralized and decentralized wastewater treatment systems

The persistence of trace organic chemicals in treated effluent derived from both centralized wastewater treatment plants (WWTPs) and decentralized wastewater treatment systems (DEWATS) is of concern due to their potential impacts on human and ecosystem health. Here, we utilize non-targeted analysis (NTA) with comprehensive two-dimensional gas chromatography coupled with time of flight mass spectrometry (GC × GC/TOF-MS) to conduct an evaluation of the common persistent and removed compounds found in two centralized WWTPs in the USA and South Africa and one DEWATS in South Africa. Overall, removal efficiencies of chemicals were similar between the treatment plants when they were compared according to the number of chemical features detected in the influents and effluents of each treatment plant. However, the DEWATS treatment train, which has longer solids retention and hydraulic residence times than both of the centralized WWTPs and utilizes primarily anaerobic treatment processes, was able to remove 13 additional compounds and showed a greater decrease in normalized peak areas compared to the centralized WWTPs. Of the 111 common compounds tentatively identified in all three influents, 11 compounds were persistent in all replicates, including 5 compounds not previously reported in effluents of WWTPs or water reuse systems. There were no significant differences among the physico-chemical properties of persistent and removed compounds, but significant differences were observed among some of the molecular descriptors. These results have important implications for the treatment of trace organic chemicals in centralized and decentralized WWTPs and the monitoring of new compounds in WWTP effluent.

Directional production of even carbon volatile fatty acids from peanut meal: effects of initial pH and hydraulic residence time

In this study, peanut meal was used as the substrate for the continuous production of even carbon volatile fatty acids (ECFA) through anaerobic fermentation. First, a continuous stirred reactor (CSTR) was started by adjusting the initial pH to acclimate the related bacteria, and then the hydraulic residence time (HRT) was optimized. The effects of different initial pH and HRT on the potential of ECFA production by peanut meal fermentation, the transformation and utilization of main organic compounds, and the composition and replacement of microbial community structure behind it were investigated. The results showed that ECFA was the dominant factor in the acid production of peanut meal fermentation, and alkaline pH and HRT = 6 d were the more favorable operating conditions for the hydrolysis and acidification of peanut meal. The average concentration of total volatile fatty acids (TVFAs) obtained was 15,249.50 mg/L, and the content of ECFA varied from 88.51% to 96.27%, with an average ratio of 92.55%. This operation condition drove the formation of microbial community structure, mainly including Prepotella, Tepididimicrobium, Anaerosalibacter, and Igatzschineria, which were favorable for ECFA production, reflecting the potential of using peanut meal for targeted production of ECFA.

Effects of design parameters, microbial community and nitrogen removal on the field-scale multi-pond constructed wetlands

Ecological multi-pond constructed wetlands (CWs) are an alternative wastewater treatment technology for nitrogen removal from non-point source pollution. As an important component of nitrogen cycles in the field-scale CWs, microorganisms are affected by design parameters. Nevertheless, the mechanism of design parameters affecting the distribution of microbial community and removal performance remains largely unexplored. In this study, satisfactory nitrogen removal performance was obtained in three multi-pond CWs. The highest mass removal rate per square meter (1104.0 mg/m2/day) and mass removal rate per cubic meter (590.2 mg/m3/day) for total nitrogen removal were obtained in the XY CW system during the wet season. The changes in seasonal parameters accounted for different removal performances and distributions of the microbial community. The combination of wastewater treatment technologies in the XY CW system consisting of ponds, CWs, and eco-floating treatment wetlands enriched the abundances of nitrogen-related functional genera. Correlation network analysis further demonstrated that longer hydraulic residence time and higher nitrogen concentration could intensify the enrichment of nitrogen-related functional genera. Regulating the combination of wastewater treatment technologies, the nitrogen concentration of influent, hydraulic loading rate, and water depth might promote the accumulation of microbial communities and enhance nitrogen removal. Macroscopical spatial/temporal regulation were proposed to enhance the treatment of non-point source pollution. The clarification of driving mechanism on design parameters, microbial community, and removal performance provided a novel perspective on the long-term maintenance of purification performance, practically sustainable applications, and scientific management of field-scale multi-pond CWs.

The impact of hydraulic retention time on the performance of two configurations of anaerobic pond for municipal sewage treatment

ABSTRACT Anaerobic ponds have the potential to contribute to low carbon wastewater treatment, however are currently restricted by long hydraulic residence time (HRT) which leads to large land requirements. A two-stage anaerobic pond (SAP) design was trialled against a single-stage control (CAP) over four HRTs down to 0.5 days, to determine the lowest HRT at which the ponds could operate effectively. No statistical differences were observed in particulate removal between the ponds over all four HRTs, suggesting solids loading is not a critical factor in AP design. Significantly higher biogas production rates were observed in the SAP than the CAP at 1.5 d and 1.0 d HRT, and microbial community profiling suggests the two-stage design may be facilitating spatial separation of the anaerobic digestion process along reactor length. Hydrogenotrophic methanogensis dominated over aceticlastic, with acetate oxidisation a likely degradation pathway. Experimental tracer studies were compared to CFD simulations, with the SAP showing greater hydraulic efficiency, and differences more pronounced at shorter HRTs. Greater flow recirculation between baffles was observed in CFD velocity profiles, demonstrating baffles can dissipate preferential flow patterns and increase effective pond volume, especially at high flow rates. The study demonstrates the potential of APs to be operated at shorter HRTs in psychrophilic conditions, presenting an opportunity for use as pre-treatments (in place of septic tanks) and primary treatment for full wastewater flows. Two-stage designs should be investigated to separate the stages of the anaerobic digestion process by creating preferential conditions along the pond length.

A novel approach to study cellulose digestion kinetics in biogas fermentation applying feed-stop method and artificial medium to investigate effects of saccharides

Anaerobic cellulose digestion kinetics was studied using a novel artificial medium, which allowed cellulose mono-fermentation with a complex biogas biocenosis. Semi-continuous cellulose mono-fermentation was successfully performed with this medium for more than 18 months achieving high organic loading rates of 8.3 gl−1d−1 and short hydraulic residence times of 9.6 days. Cellulose hydrolysis kinetics was studied by monitoring the substrate depletion after short feed-stops using the data obtained to establish a 1st order reaction model. This method was used to assess the cellulose hydrolysis kinetics under the influence of selected carbohydrates, of which a mixture of cellooligosaccharides showed the highest potential. A qPCR-Method was established for quantitative monitoring of selected cellulolytic clostridia based on the extraction of metagenomic DNA and comparison to standards from single cultures. The monitoring indicated consistent growth of these clostridia in the novel cultivation approach.

Simulation and optimization of hydraulic performance of small baffled subsurface flow constructed wetland.

The water body inside the constructed wetland is affected by various factors, and the flow state is relatively complicated. There will always be a certain degree of low velocity area and rapid outflow phenomenon, which makes part of the space in the wetland unable to be effectively used. Based on Computational Fluid Dynamics (CFD) technology, this paper uses Fluent's porous media model and discrete phase model to establish a hydrodynamic model of up and down baffled subsurface flow constructed wetland system. The internal flow field of the wetland is simulated, and the hydraulic performance of different baffle settings and substrate laying methods in the wetland is systematically evaluated. The results show that when the number of baffles is the same, the hydraulic efficiency is higher when the first baffle is located on the lower part of the substrate. Compared with the position of the baffle, the increase in the number of baffles does not significantly improve the hydraulic efficiency of the constructed wetland. The substrate layer thickness ratio has a significant effect on the two parameters of the variance of the hydraulic residence time distribution (σ2) and the flow divergence (σ02). By increasing the thickness of the middle substrate, the low flow rate phenomenon caused by the small porosity substrate area of the upper layer and the rapid outflow phenomenon of the lower substrate can be improved to a certain extent, the utilization efficiency of the middle substrate layer is improved, and the hydraulic performance is increased. The research results are of great significance for improving the utilization of wetland space and ensuring its efficient decontamination and purification function.

Nitrogen removal and nitrous oxide emissions from woodchip bioreactors treating agricultural drainage waters

Woodchip bioreactors are increasingly used as tools to mitigate nitrogen (N) pollution from agricultural drainage water. They consist of a basin filled with woodchip material through which N contaminated drainage water can flow. During the water transport through the filter matrix, oxygen is rapidly depleted and denitrification removes a fraction of the nitrate N present in the water. However, the N removal efficiency of the bioreactors varies significantly both across systems and seasonally. Furthermore, denitrification can also produce nitrous oxide, which is a potent greenhouse gas. Here, we investigated how variation in hydraulic residence time influenced N removal efficiency and nitrous oxide emissions at eight woodchip bioreactors of different flow designs, monitored for 2–4 years. We also characterised the relative abundance of genes involved in the N cycle at three of the bioreactors using metagenomics. Our results showed that total N removal was 17–73% of the yearly incoming N and that it was influenced by hydraulic residence time and water temperature. Nitrous oxide emissions were variable among the different bioreactors and were higher when the hydraulic residence time was less than 60 h. However, the yearly nitrous oxide release did not exceed 2.4% of the nitrate removal (on N atom basis) and the mean among the bioreactors was 0.6%. Although there were marked differences in nitrate removal and nitrous oxide emissions, there were no clear differences in the relative abundance of N-cycling genes among and within three tested bioreactors. Yet, denitrification genes greatly outnumbered genes related to dissimilatory nitrate reduction to ammonium. Overall, our study showed that all eight bioreactors were effective in removing N from agricultural drainage water and that nitrous oxide emissions were low, especially at hydraulic residence times of 60 h or more.

Performance of constructed wetland for selenium, nutrient and heavy metals removal from mine effluents

Passive biological treatment using constructed wetlands has been employed as a cost-effective and environmentally friendly alternative for mine effluents treatment in Canada. The current work aimed to assess the performance of a laboratory-scale constructed wetland for the removal of selenium, nutrients and heavy metals. This work achieved a maximum selenium removal rate of 54.13% corresponding to the highest hydraulic residence time (almost 47 days). Typha Latifolia was efficient for selenium assimilation and removal from mine effluent. 4.4% of Se mass was accumulated in the under and aboveground biomass while 32.2% was volatilized. Levels <1.69 mg of selenate, <1.69 mg of selenite, <3.39 mg of selenomethionine and <3.39 mg of unknown selenium species were distributed in the underground biomass while levels <0.75 mg of selenate, <0.75 mg of selenite, <1.51 mg of selenomethionine and <1.51 mg of unknown selenium species were found in T. Latifolia leaves. Ammonia removal was mainly based on a nitrification process confirmed by the decrease of ammonia (>96%) and nitrite in association with the increase of nitrate concentrations in the outlet compared to the inlet. The concentrations of other nitrogen compounds mainly thiocyanate and cyanate have significantly decreased from initial average values of 163 and 22.7 mg.L−1 in the influent to 1.5 and 0.3 mg.L−1 final concentrations, respectively. Orthophosphate concentrations were also efficiently decreased from an influent average value of 21.5 ± 3.3 mg.L−1 to an effluent concentration of 9.1 ± 4.4 mg.L−1. Understanding the selenium and nutrient removal mechanisms will allow treatment performance enhancement and the development of large-scale constructed wetlands for sophisticated mine effluent treatment.

Efficacy of a woodchip-sediment integrated system in nitrate elimination from wastewater with low C/N condition

Currently, effective methods to eliminate nitrate in wastewater with a low C/N ratio are urgently needed. Considering that sediment is the main nitrogen removal carrier in lakes and that woodchip is an excellent carbon source, this study was undertaken under an integrated system of these two substances to explore the process and efficiency of the denitrification involved. Using methods of kinetics and molecular biology, denitrification processes involving a woodchip-sediment system were investigated in an up-flow bioreactor (UFBR). Performance indices, functional genes, and microbial community changes in this integrated system were explored through water quality monitoring, quantitative PCR, and high-throughput sequencing. When the hydraulic residence time reached above 3.82 days, the nitrate elimination efficiency could achieve an effective level of higher than 93.3% even under the different flow rates. It was also noted that the nitrate elimination rate in this integrated system fitted the Michaelis–Menten kinetics, a combination of zero-order equation (R2 > 0.98) and first-order equation (R2 > 0.95). A combination of zero-order kinetic equations of nitrate elimination and total organic carbon (TOC) release exhibited nitrate elimination efficiency under direct influence of TOC release rate on the C/N ratio. Microbiological analysis indicated co-existence of composting bacteria and denitrifiers as well as aerobic degradation and heterotrophic denitrification, with the former process providing effective carbon sources for the latter process. This study provided a deep insight into the denitrification process of a woodchip-sediment integrated system, which might inspire a potential wastewater treatment method for reduced eutrophication in the aquatic environment.

Vegetation and Residence Time Interact to Influence Metabolism and Net Nutrient Uptake in Experimental Agricultural Drainage Systems

Agricultural drainage networks within the Lower Mississippi River Basin (LMRB) have potential to attenuate nutrient loading to downstream aquatic ecosystems through best management practices. Nutrient uptake (nitrogen, phosphorus), gross primary production (GPP), ecosystem respiration (ER), and denitrification rates were estimated using a combination of sensor measurements and hourly discrete samples for dissolved nutrients and gases at three hydraulic residence times (2, 4, and 6 h) in three vegetated and three unvegetated ditches. We also measured vegetation and soil nutrient content. GPP and ER were significantly higher in vegetated drainages and increasing hydraulic residence time increased respiration rates. Shorter hydraulic residence times were associated with increased uptake rates for both N and P, and vegetation increased N uptake rates in all hydraulic residence time (HRT) treatments. Vegetation and sediment assimilated N and P over the course of the experiment. Overall, our experimental results demonstrate the strong role of emergent vegetation in nutrient retention and removal processes in agricultural drainage ditch networks.

Occurrence of pesticides and their transformation products in headwater streams: Contamination status and effect of ponds on contaminant concentrations

In France, more than 90% of monitored watercourses are contaminated with pesticides. This high contamination level increases at the head of agricultural watersheds, where dilution capacities are low and transport from treated lands is direct. Ponds, numerous around headwater streams, could provide additional protection against pesticide pollution. Because of their long hydraulic residence time and large water volumes, they mitigate pesticide concentrations between upstream and downstream rivers. However, pesticide transformation products may also be responsible for the degradation of environments, owing to their presence at high concentrations and their persistence, but related data are scarce, particularly because of their high level of molecular diversity. We first reported on the state of water contamination in agricultural headwater streams, based on high frequency water sampling. Analysis of 67 molecules (HPLC-ESI-MS/MS) showed pesticides and pesticide transformation product mixtures of up to 29 different compounds in one sample. Regardless of the sampling location, transformation products represented at least 50% of the detected compounds. Then, we demonstrated the capacity of a pond to reduce contaminant concentrations in downstream rivers for 90% of the detected compounds. Upstream from this pond, environmental quality or ecotoxicological standards were exceeded during sampling, with pesticide and transformation product sum concentrations of up to 27 μg/L. Downstream from the study pond, few exceedances were observed, with a maximum total concentration of 2.2 μg/L, reflecting significant water quality improvement.