Ecotoxicity characterization assisted performance assessment of electro-bioremediation reactors for nitrate and arsenite elimination.

The performance of combined reduction of nitrate (NO3 - ) to dinitrogen gas (N2 ) and oxidation of arsenite (As[III]) to arsenate (As[V]) by a bioelectrochemical system was assessed, supported by ecotoxicity characterization. For the comprehensive toxicity characterization of the untreated model groundwater and the treated reactor effluents, a problem-specific ecotoxicity test battery was established. The performance of the applied technology in terms of toxicity and target pollutant elimination was compared and analyzed. The highest toxicity attenuation was achieved under continuous flow mode with hydraulic retention time (HRT) = 7.5 h, with 95%, nitrate removal rate and complete oxidation of arsenite to arsenate. Daphnia magna proved to be the most sensitive test organism. The results of the D. magna lethality test supported the choice of the ideal operational conditions based on chemical data analysis. The outcomes of the study demonstrated that the applied technology was able to improve the groundwater quality in terms of both chemical and ecotoxicological characteristics. The importance of ecotoxicity evaluation was also highlighted, given that significant target contaminant elimination did not necessarily lower the environmental impact of the initial, untreated medium, in addition, anomalies might occur during the technology operational process which in some instances, could result in elevated toxicity levels.

Advancing towards electro-bioremediation scaling-up: On-site pilot plant for successful nitrate-contaminated groundwater treatment.

The potential of nitrate electro-bioremediation has been fully demonstrated at the laboratory scale, although it has not yet been fully implemented due to the challenges associated with scaling-up bioelectrochemical reactors and their on-site operation. This study describes the initial start-up and subsequent stable operation of an electro-bioremediation pilot plant for the treatment of nitrate-contaminated groundwater on-site (Navata site, Spain). The pilot plant was operated under continuous flow mode for 3 months, producing an effluent suitable for drinking water in terms of nitrates and nitrites (<50 mg NO3- L-1; 0 mg NO2- L-1). A maximum nitrate removal rate of 0.9 ± 0.1 kg NO3- m-3 d-1 (efficiency 82 ± 18 %) was achieved at a cathodic hydraulic retention time (HRTcat) of 2.0 h with a competitive energy consumption of 4.3 ± 0.4 kWh kg-1 NO3-. Under these conditions, the techno-economic analysis estimated an operational cost of 0.40 € m-3. Simultaneously, microbiological analyses revealed structural heterogeneity in the reactor, with denitrification functionality concentrated predominantly from the centre to the upper section of the reactor. The most abundant groups were Pseudomonadaceae, Rhizobiaceae, Gallionellaceae, and Xanthomonadaceae. In conclusion, this pilot plant represents a significant advancement in implementing this technology on a larger scale, validating its effectiveness in terms of nitrate removal and cost-effectiveness. Moreover, the results validate the electro-bioremediation in a real environment and encourage further investigation of its potential as a water treatment.

Nitrate electro-bioremediation and water disinfection for rural areas.

Nitrate-contaminated groundwater is a pressing issue in rural areas, where up to 40 % of the population lacks access to safely managed drinking water services. The high costs and complexity of centralised treatment in these regions exacerbate this problem. To address this challenge, the present study proposes electro-bioremediation as a more accessible decentralised alternative. Specifically, the main focus of this study is developing and evaluating a compact reactor designed to accomplish simultaneous nitrate removal and groundwater disinfection. Significantly, this study has established a new benchmark for nitrate reduction rate within bioelectrochemical reactors, achieving the maximum reported rate of 5.0 ± 0.3 kg NO3- m-3NCC d-1 at an HRTcat of 0.7 h. Furthermore, thein-situ generation of free chlorine was effective for water disinfection, resulting in a residual concentration of up to 4.4 ± 1.1 mg Cl2 L-1 in the effluent at the same HRTcat of 0.7 h. These achievements enabled the treated water to meet the drinking water standards for nitrogen compounds (nitrate, nitrite, and nitrous oxide) as well as pathogens content (T. coliforms, E. coli, and Enterococcus). In conclusion, this study demonstrates the potential of the electro-bioremediation of nitrate-contaminated groundwater as a decentralised water treatment system in rural areas with a competitive operational cost of 1.05 ± 0.16 € m-3.

Electrochemical water softening as pretreatment for nitrate electro bioremediation.

Electro bioremediation is gaining interest as a sustainable treatment for contaminated groundwater. Nevertheless, the investigation is still at the laboratory level, and before their implementation is necessary to overcome important drawbacks. A prevalent issue is the high groundwater hardness that generates scale deposition on electrodes that irreversibly affects the treatment effectiveness and their lifetime. For this reason, the present study evaluated a novel and sustainable approach combining electrochemical water softening as a preliminary step for electro bioremediation of nitrate-contaminated groundwater. Batch mode tests were performed at mL-scale to determine the optimum reactor configuration (single- or two-chambers) and the suitable applied cathode potential for electrochemical softening. A single-chamber reactor working at a cathode potential of -1.2 V vs. Ag/AgCl was chosen. Continuous groundwater softening under this configuration achieved a hardness removal efficiency of 64 ± 4% at a rate of 305 ± 17 mg CaCO3 m-2cathode h-1. The saturation index at the effluent of the main minerals susceptible to precipitate (aragonite, calcite, and brucite) was reduced up to 90%. Softening activity plummeted after 13 days of operation due to precipitate deposition (mostly calcite) on the cathode surface. Polarity reversal periods were considered to detach the precipitated throughout the continuous operation. Their implementation every 3-4 days increased the softening lifetime by 48%, keeping a stable hardness removal efficiency. The nitrate content of softened groundwater was removed in an electro bioremediation system at a rate of 1269 ± 30 g NO3- m-3NCC d-1 (97% nitrate removal efficiency). The energy consumption of the integrated system (1.4 kWh m-3treated) confirmed the competitiveness of the combined treatment and paves the ground for scaling up the process.

Electro-bioremediation of nitrate and arsenite polluted groundwater.

The coexistence of different pollutants in groundwater is a common threat. Sustainable and resilient technologies are required for their treatment. The present study aims to evaluate microbial electrochemical technologies (METs) for treating groundwater contaminated with nitrate (NO3-) while containing arsenic (in form of arsenite (As(III)) as a co-contaminant. The treatment was based on the combination of nitrate reduction to dinitrogen gas and arsenite oxidation to arsenate (exhibiting less toxicity, solubility, and mobility), which can be removed more easily in further post-treatment. We operated a bioelectrochemical reactor at continuous-flow mode with synthetic contaminated groundwater (33 mg N-NO3- L-1 and 5 mg As(III) L-1) identifying the key operational conditions. Different hydraulic retention times (HRT) were evaluated, reaching a maximum nitrate reduction rate of 519 g N-NO3- m3Net Cathodic Compartment d-1 at HRT of 2.3 h with a cathodic coulombic efficiency of around 100 %. Simultaneously, arsenic oxidation was complete at all HRT tested down to 1.6 h reaching an oxidation rate of up to 90 g As(III) m-3Net Reactor Volume d -1. Electrochemical and microbiological characterization of single granules suggested that arsenite at 5 mg L-1 did not have an inhibitory effect on a denitrifying biocathode mainly represented by Sideroxydans sp. Although the coexistence of abiotic and biotic arsenic oxidation pathways was shown to be likely, microbial arsenite oxidation linked to denitrification by Achromobacter sp. was the most probable pathway. This research paves the ground towards a real application for treating groundwater with widespread pollutants.