Pilot and full scale applications of floating treatment wetlands for treating diffuse pollution.

Floating treatment wetlands (FTW) are nature-based solutions for the purification of open water systems such as rivers, ponds, and lakes polluted by diffuse sources as untreated or partially treated domestic wastewater and agricultural run-off. Compared with other physicochemical and biological technologies, FTW is a technology with low-cost, simple configuration, easy to operate; has a relatively high efficiency, and is energy-saving, and aesthetic. Water remediation in FTWs is supported by plant uptake and the growth of a biofilm on the water plant roots, so the selection of the macrophyte species is critical, not only to pollutant removal but also to the local ecosystem integrity, especially for full-scale implementation. The key factors such as buoyant frame/raft, plant growth support media, water depth, seasonal variation, and temperature have a considerable role in the design, operation, maintenance, and pollutant treatment performance of FTW. Harvesting is a necessary process to maintain efficient operation by limiting the re-pollution of plants in the decay phase. Furthermore, the harvested plant biomass can serve as a green source for the recovery of energy and value-added products.

Water treatment and reclamation by implementing electrochemical systems with constructed wetlands.

Seasonal or permanent water scarcity in off-grid communities can be alleviated by recycling water in decentralized wastewater treatment systems. Nature-based solutions, such as constructed wetlands (CWs), have become popular solutions for sanitation in remote locations. Although typical CWs can efficiently remove solids and organics to meet water reuse standards, polishing remains necessary for other parameters, such as pathogens, nutrients, and recalcitrant pollutants. Different CW designs and CWs coupled with electrochemical technologies have been proposed to improve treatment efficiency. Electrochemical systems (ECs) have been either implemented within the CW bed (ECin-CW) or as a stage in a sequential treatment (CW + EC). A large body of literature has focused on ECin-CW, and multiple scaled-up systems have recently been successfully implemented, primarily to remove recalcitrant organics. Conversely, only a few reports have explored the opportunity to polish CW effluents in a downstream electrochemical module for the electro-oxidation of micropollutants or electro-disinfection of pathogens to meet more stringent water reuse standards. This paper aims to critically review the opportunities, challenges, and future research directions of the different couplings of CW with EC as a decentralized technology for water treatment and recovery.

Pyrite-based denitrification combined with electrochemical disinfection to remove nitrate and microbial contamination from groundwater.

Nitrate and microbial contamination of groundwater can occur in countries that face intense urbanization and inadequate sanitation. When groundwater is the main drinking water source, as is often the case in such countries, the need to remove these contaminants becomes acute. The combination of two technologies is proposed here, a biological step to denitrify and an electrochemical step to disinfect the groundwater, thereby aiming to reduce the chemical input and the footprint of groundwater treatment. As such, a pyrite-based fluidized bed reactor (P-FBR) was constructed to autotrophically denitrify polluted groundwater. The P-FBR effluent was disinfected in an electrochemical cell with electrogenerated Cl2. Nitrate was removed with 79% efficiency from an initial 178 mg NO3- L-1 at an average denitrification rate of 171 mg NO3- L-1 d-1, with 18 h hydraulic retention time (HRT). The electrochemical unit achieved a 3.8-log reduction in total coliforms with a 41.7 A h m-3 charge density.

Continuous H2/CO2 fermentation for acetic acid production under transient and continuous sulfide inhibition.

Waste gas fermentation powered by renewable H2 is reaching kiloton scale. The presence of sulfide, inherent to many waste gases, can cause inhibition, requiring additional gas treatment. In this work, acetogenesis and methanogenesis inhibition by sulfide were studied in a 10-L mixed-culture fermenter, supplied with CO2 and connected with a water electrolysis unit for electricity-powered H2 supply. Three cycles of inhibition (1.3 mM total dissolved sulfide (TDS)) and recovery were applied, then the fermenter was operated at 0.5 mM TDS for 35 days. During operation at 0.5 mM TDS the acetate production rate reached 7.1 ± 1.5 mmol C L-1 d-1. Furthermore, 43.7 ± 15.6% of the electrons, provided as H2, were distributed to acetate and 7.7 ± 4.1% to butyrate, the second most abundant fermentation product. Selectivity of sulfide as inhibitor was demonstrated by a 7 days lag-phase of methanogenesis recovery, compared to 48 h for acetogenesis and by the less than 1% electrons distribution to CH4, under 0.5 mM TDS. The microbial community was dominated by Eubacterium, Proteiniphilum and an unclassified member of the Eggerthellaceae family. The taxonomic diversity of the community decreased and conversely the phenotypic diversity increased, during operation. This work illustrated the scale-up potential of waste gas fermentations, by elucidating the effect of sulfide as a common gas impurity, and by demonstrating continuous, potentially renewable supply of electrons.

Homoacetogenesis and microbial community composition are shaped by pH and total sulfide concentration.

Biological CO2 sequestration through acetogenesis with H2 as electron donor is a promising technology to reduce greenhouse gas emissions. Today, a major issue is the presence of impurities such as hydrogen sulfide (H2 S) in CO2 containing gases, as they are known to inhibit acetogenesis in CO2 -based fermentations. However, exact values of toxicity and inhibition are not well-defined. To tackle this uncertainty, a series of toxicity experiments were conducted, with a mixed homoacetogenic culture, total dissolved sulfide concentrations ([TDS]) varied between 0 and 5 mM and pH between 5 and 7. The extent of inhibition was evaluated based on acetate production rates and microbial growth. Maximum acetate production rates of 0.12, 0.09 and 0.04 mM h-1 were achieved in the controls without sulfide at pH 7, pH 6 and pH 5. The half-maximal inhibitory concentration (IC50 qAc ) was 0.86, 1.16 and 1.36 mM [TDS] for pH 7, pH 6 and pH 5. At [TDS] above 3.33 mM, acetate production and microbial growth were completely inhibited at all pHs. 16S rRNA gene amplicon sequencing revealed major community composition transitions that could be attributed to both pH and [TDS]. Based on the observed toxicity levels, treatment approaches for incoming industrial CO2 streams can be determined.

Electrochemical treatment of industrial sulfidic spent caustic streams for sulfide removal and caustic recovery.

Alkaline spent caustic streams (SCS) produced in the petrochemical and chemical manufacturing industry, contain high concentrations of reactive sulfide (HS-) and caustic soda (NaOH). Common treatment methods entail high operational costs while not recovering the possible resources that SCS contain. Here we studied the electrochemical treatment of SCS from a chemical manufacturing industry in an electrolysis cell, aiming at anodic HS- removal and cathodic NaOH, devoid of sulfide, recovery. Using a synthetic SCS we first evaluated the HS- oxidation product distribution over time, as well as the HS- removal and the NaOH recovery, as a function of current density. In a second step, we investigated the operational aspects of such treatment for the industrial SCS, under 300 A m-2 fixed current density. In an electrolysis cell receiving 205 ±â€¯60 g S L-1 d-1 HS- over 20 days of continuous operation, HS- was removed with a 38.0 ±â€¯7.7 % removal and ∼80 % coulombic efficiency, with a concomitant recovery of a ∼12 wt.% NaOH solution. The low cell voltage obtained (1.75 ±â€¯0.12 V), resulted in low energy requirements of 3.7 ±â€¯0.6 kW h kg-1 S and 6.3 ±â€¯0.4 kW h kg-1 NaOH and suggests techno-economic viability of this process.

Anode materials for sulfide oxidation in alkaline wastewater: An activity and stability performance comparison.

Electrochemical sulfide removal can be attractive as a zero-chemical-input approach for treatment of waste streams such as spent caustics coupled to caustic recovery. A key concern is possible decline in catalytic activity, due to passivation from deposited elemental sulfur (S0) on the anode surface and stability limitation, due to sulfide oxidation under highly alkaline conditions. In this study, six commercially available electrode materials (Ir Mixed Metal Oxide (MMO), Ru MMO, Pt/IrOx, Pt, PbOx and TiO2/IrTaO2 coated titanium-based electrodes) were tested to investigate the impact of the electrocatalyst on the process efficiency in terms of sulfide removal and final product of sulfide oxidation, as well as to determine the stability of the electrocatalyst under high sulfide concentrations (50 mM Na2S) and high alkalinity (pH > 12). Short-term experiments showed that the catalyst type impacts the anode potential and the sulfide oxidation reaction products. Longer-term experiments under current densities up to 200 A m-2 showed a high differentiation in stability performance among the catalysts. Ru MMO was the most active towards sulfide oxidation with a coulombic efficiency of 63.2 ±â€¯0.5% at an average anode potential of 0.92 ±â€¯0.17 V vs SHE. Ir MMO was the most stable, preserving 100% of its original catalyst loading during the tests. The results demonstrated that Ru MMO and Ir MMO were the most suitable electrode materials for sulfide oxidation under highly alkaline conditions, while the need for establishing a good trade-off between activity, stability and cost still persists.