Environmental challenges of the chlor-alkali production: Seeking answers from a life cycle approach

Flow-through electrochemically assisted reverse-osmosis: A new process towards low-chemical desalination

Two-pass reverse osmosis (RO) process is prevailing in seawater desalination, but each process must consume considerable amounts of chemicals to secure product water quality. Caustic soda is used to raise the pH of the first-pass RO permeate (also the second-pass RO feed) to ensure adequate removal of boron in the subsequent second-pass RO, while antiscalants and disinfectants such as hypochlorite are added in the feed seawater for scaling and biofouling control of the first-pass RO membranes. Here, we report for the first time a flow-through electrochemically assisted reverse osmosis (FT-EARO) module system used in the first-pass RO, aiming to dramatically reduce or even eliminate chemical usage for the current RO desalination. This novel system integrated an electroconductive permeate carrier as cathode and an electroconductive feed spacer as anode on each side of the first-pass RO membrane. Upon applying an extremely low-energy (< 0.005 kWh/m3) electrical field, the FT-EARO module could (1) produce a permeate with pH >10 with no alkali dosage, ensuring sufficient boron removal in the second-pass RO, and (2) generate protons and low-concentration free chlorine near the membrane surface, potentially discouraging membrane scaling and biofouling while maintaining satisfactory desalination performance. The current study further elucidated the high scalability of this novel electrified high-pressure RO module design. The low-chemical manner of FT-EARO presents an attractive practical option towards green and sustainable seawater desalination.

Towards renewable hydrogen-based electrolysis: Alkaline vs Proton Exchange Membrane

This paper focuses on the battle for a dominant design for renewable hydrogen electrolysis in which the designs, alkaline and proton exchange membrane, compete for dominance. First, a literature review is performed to determine the most relevant factors that influence technology dominance. Following that, a Best Worst Method analysis is conducted by interviewing multiple industry experts. The most important factors appear to be: Price, Safety, Energy consumption, Flexibility, Lifetime, Stack size and Materials used. The opinion of experts on Proton Exchange Membrane and alkaline electrolyser technologies is slightly skewed in favour of alkaline technologies. However, the margin is too small to identify a winner in this technology battle. The following paper contributes to the ongoing research on modelling the process of technology selection in the energy sector.

Assessment of cross-media effects deriving from the application of lower emission standards for acid pollutants in waste-to-energy plants

The recent release of the new European Commission reference document on the Best Available Techniques (BAT) for waste incineration has set ambitious targets for the control of the emission of pollutants. However, an improved performance of the existing flue gas treatment systems in waste-to-energy (WtE) facilities is usually associated to an increase of cross-media effects, i.e., additional indirect environmental impacts related to the increased consumption of reactants and to the increased generation of process residues/wastewater in flue gas treatment. The present study introduces an innovative approach to assess cross-media effects deriving from more stringent acid gas emission standards in the WtE sector. By coupling simplified process modelling and life cycle analysis, the proposed methodology links the higher removal efficiency required for flue gas treatment to the impacts related to the reactants supply and waste disposal chain. An application to the Italian WtE sector exemplifies the potential of the method. The results evidence that, in case of HCl emission setpoints lower than 1 mg/Nm³, the reduction of acidifying emissions at the WtE stacks can be offset by the increase of global warming and smog formation impacts in the supply chain of flue gas cleaning reactants. In case of setpoints lower than 0.5 mg/Nm³, even within the acidification category the increase of indirect impacts more than compensates the decrease of WtE emissions. The net environmental benefit is strongly affected by the type of acid gas removal technology adopted, with dry systems typically associated with a larger increase of cross-media burdens when required to perform at higher removal efficiencies.

A Sustainable Integration Approach of Chlor-Alkali Industries for the Production of PVC and Clean Fuel Hydrogen: Prospects and Bangladesh Perspectives

The chlor-alkali industries produce caustic soda (NaOH), chlorine (Cl2), and hydrogen (H2) as primary products. In 2021, the global chlor-alkali market was valued at $63.2 billion. The article evaluates the global aspects of chlor-alkali industries and prospects for Bangladesh. The current production capacity of NaOH from the chlor-alkali industries in Bangladesh is around 282,150 metric tons/year (MT/y). The by-products, chlorine (Cl2) of 250,470 MT/y and hydrogen (H2) of 7055 MT/y, are produced domestically. The local demand of Cl2 is 68,779 MT/y. However, there are no systematic utilizations of the residual Cl2 and vented H2, which threatens the sustainability of the chlor-alkali industries. The article prefigures that a 150,000 MT/y PVC plant can utilize 45.2 % of residual Cl2 of chlor-alkali plants, which would be an economical and environmental milestone for Bangladesh. The residual Cl2 can earn revenue of 908 million USD/y, which can be utilized to import ethylene. For the sustainable utilization of vented H2, production of H2O2, fuel cell electric vehicle (FCEV) and H2 fuel-cell-based power plant are the feasible solutions. Thus, for the long-term growth of the chlor-alkali industry in Bangladesh and other developing countries, systematic utilization of Cl2 and H2 is the only feasible solution.

Optimized Operating Conditions for a Biological Treatment Process of Industrial Residual Process Brine Using a Halophilic Mixed Culture

Residual process brine is a sustainable raw material for chlor-alkali electrolysis processes. This study investigates the influence of critical process parameters on the performance of a continuous treatment process for residual process brine using halophilic microorganisms. The goal of the bioprocess is an efficient degradation of the organic impurities formate, aniline, phenol, and 4,4′-methylenedianline from this residual stream. It was shown that formate could be degraded with high efficiencies (89–98%) during the treatment process. It was observed that formate degradation was influenced by the co-substrate glycerol. The lowest residual formate concentrations were achieved with specific glycerol uptake rates of 8.0–16.0 × 10−3 g L−1 h−1 OD600−1. Moreover, a triple-nutrient limitation for glycerol, ammonium, and phosphate was successfully applied for continuous cultivations. Furthermore, it was shown that all aromatic impurities were degraded with an efficiency of 100%. Ultimately, this study proposed optimized operating conditions, allowing the efficient degradation of organics in the residual process brine under various process conditions. Future optimization steps will require a strategy to prevent the accumulation of potential intermediate degradation products formed at high aniline feed concentrations and increase the liquid dilution rates of the system to achieve a higher throughput of brines.

Comparative Gate-to-Gate Life Cycle Assessment for the Alkali and Acid Pre-Treatment Step in the Chemical Recycling of Waste Cotton

The development of textile recycling solutions is an area of intense research and commercialization. Chemical recycling solutions are becoming increasingly popular due to their ability to separate complex blends and retain or improve the value of the original fiber. The chemical recycling of cotton requires a pre-treatment step to reduce the degree of polymerization (DP). The DP can be reduced in a variety of ways, and here, the environmental footprints of two different pre-treatment approaches are examined using life cycle assessment (LCA); sodium hydroxide pre-treatment and sulphuric acid pre-treatment. We find that the acid pre-treatment has a significantly lower environmental footprint across all impact categories calculated. This is attributed to the lower treatment times required and the lower material and energy requirements for the manufacture of chemicals. The results were normalized to show the most significant impact categories for each pre-treatment, and further environmental implications of the pre-treatments are discussed. The findings will aid academia and industry in implementing the most environmentally benign processes in chemical cotton recycling.

Struvite crystallization by using raw seawater: Improving economics and environmental footprint while maintaining phosphorus recovery and product quality

Seawater, as an alternative magnesium source, has the potential to improve the overall economics and environmental footprint of struvite production compared to the use of pure magnesium salts. However, the dilution effect and the presence of other ions in seawater can reduce the phosphorus recovery potential and the simultaneous precipitation of other compounds may reduce the quality of the produced struvite. This work presents a comparative study of seawater and MgCl₂ by performing a series of thermodynamic equilibrium modeling and crystallization experiments. The results revealed that acceptable phosphorus recovery (80-90%) is achievable by using seawater as the magnesium source for struvite precipitation. Further, the simultaneous precipitation of calcium phosphates was successfully controlled and minimized by optimum selection of reaction pH and seawater volume (i.e. Mg:P and Mg:Ca molar ratios). The increase of temperature from 20 °C to 30 °C reduced the phosphorus recovery by 15-20% while it increased the particle size by 30-35%. The presence of suspended solids in reject water did not have significant effects on phosphorus recovery but it made the struvite separation difficult as the obtained struvite was mixed with suspended solids. The experimental results and economic evaluation showed that the use of seawater can reduce the chemical costs (30-50%) and the CO₂-footprint (8-40%) of struvite production. It was concluded that seawater is a potential alternative to pure magnesium sources in struvite production, while studies in larger scale and continuous mode are needed for further verification before full-scale applications.

Potential formation of PCDD/Fs in triclosan wastewater treatment: An overall toxicity assessment under a life cycle approach

Wastewater may contain a diverse group of unregulated pollutants known as emerging pollutants, such as pharmaceuticals and personal care products (PPCPs). Triclosan (TCS) is a personal care product widely used as an antiseptic or preservative in cosmetics, hand wash, toothpaste and deodorant soaps. Advanced oxidation processes (AOPs) have been used as effective and alternative treatments for complex wastewater. However, an important criterion for the assessment of AOPs and their operation conditions could be the potential formation of new toxic secondary products, such as polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs), especially when emerging pollutants are present in the media. If these are omitted from environmental management studies, the real environmental impacts of a WWTPs (wastewater treatment plants) may be underestimated. Consequently, the current study aims to evaluate the environmental impacts derived from electrooxidation (EOX), one of the most effective oxidation technologies, of emerging pollutants using Life Cycle Assessment. The analyses were performed for the treatment of effluents containing TCS, firstly without considering the formation of PCDD/Fs and, thereafter, considering the effects of these compounds. Total toxicity, calculated through different methods and corresponding impact factors, were evaluated for each stage of the process when different electrolytes are used, including PCDD/Fs formation. Finally, a sensitivity analysis was carried out to study i) the effect of the TCS initial concentration on the environmental impacts associated to ecotoxicity for the different life cycle methods and ii) the influence of changing the organic pollutant on PCDD/Fs formation employing 2-chlorophenol (2-CP). As a result, LCIA methods demonstrate that they are not fully adapted to the computation of PCDD/Fs in the water compartment, since only 2,3,7,8-tetraclorodibenzo-p-dioxina (2,3,7,8-TCDD) is present as a substance in the impact categories assessed, ignoring the remaining list of PCDD/Fs.

Operando Laboratory X-Ray Imaging of Silver-Based Gas Diffusion Electrodes during Oxygen Reduction Reaction in Highly Alkaline Media

Operando laboratory X-ray radiographies were carried out for imaging of two different silver-based gas diffusion electrodes containing an electroconductive Ni mesh structure, one gas diffusion electrode composed of 95 wt.% Ag and 5 wt.% polytetrafluoroethylene and one composed of 97 wt.% Ag and 3 wt.% polytetrafluoroethylene, under different operating parameters. Thereby, correlations of their electrochemical behavior and the transport of the 30 wt.% NaOH electrolyte through the gas diffusion electrodes were revealed. The work was divided into two parts. In the first step, the microstructure of the gas diffusion electrodes was analyzed ex situ by a combination of focused ion beam technology and synchrotron as well as laboratory X-ray tomography and radiography. In the second step, operando laboratory X-ray radiographies were performed during chronoamperometric measurements at different potentials. The combination of the ex situ microstructural analyses and the operando measurements reveals the impact of the microstructure on the electrolyte transport through the gas diffusion electrodes. Hence, an impact of the Ni mesh structure within the gas diffusion electrode on the droplet formation could be shown. Moreover, it could be observed that increasing overpotentials cause increasing electrolyte transport velocities and faster droplet formation due to electrowetting. In general, higher electrolyte transport velocities were found for the gas diffusion electrode with 97 wt.% Ag in contrast to that with 95 wt.% Ag.

Design of an In-Operando Cell for X-Ray and Neutron Imaging of Oxygen-Depolarized Cathodes in Chlor-Alkali Electrolysis

Oxygen-depolarized cathodes are a novel concept to be used in chlor-alkali electrolysis in order to generate significant energy savings. In these porous gas diffusion electrodes, hydrophilic and catalytically active microsized silver grains and a hydrophobic polytetrafluoroethylene cobweb structure are combined to obtain the optimum amount of three-phase boundaries between the highly alkaline electrolyte and the oxygen gas phase to achieve high current densities. However, the direct correlation between specific electrode structure and electrochemical performance is difficult. In this work, we report on the successful design and adaptation of an in-operando cell for X-ray (micro-computed tomography, synchrotron) and neutron imaging of an operating oxygen-depolarized cathode under realistic operation conditions, enabling the investigation of the electrolyte invasion into, and distribution inside, the porous electrode for the first time.

LCA-based Comparison of Two Organic Fraction Municipal Solid Waste Collection Systems in Historical Centres in Spain

Municipal solid waste (MSW) collection is an important issue in the development and management of smart cities, having a significant influence on environmental sustainability. Door-to-door and pneumatic collection are two systems that represent a way of arranging waste collection in city´s historic areas in Spain where conventional street-side container collection is not feasible. Since door-to-door collection generates significant direct greenhouse gas emissions from trucks, pneumatic collection emerges as an alternative to the trucking system. While this technology apparently reduces local direct air emissions, it suffers from a large energy demand derived from vacuum production for waste suction. The introduction of new normative frameworks regarding the selective collection of the biodegradable fraction makes necessary a comprehensive analysis to assess the influence of this fraction collection and its subsequent recycling by anaerobic digestion. As a novelty, this work compares both conventional door-to-door and pneumatic collection systems from a life cycle approach focusing on the biodegradable waste. Results indicate that, in spite of the fact electricity production and consumption have a significant influence on the results, the energy savings from the recycling of the organic fraction are higher than the energy requirements. Therefore, the pneumatic collection could be an environmentally-friendly option for MSW management under a circular economy approach in Spanish city´s historic areas, since wastes could be a material or energy source opportunity.

How does intensification influence the operational and environmental performance of photo-Fenton processes at acidic and circumneutral pH

This study evaluates the technical, economical, and environmental impact of sodium persulfate (Na₂S₂O₈) as an enhancing agent in a photo-Fenton process within a solar-pond type reactor (SPR). Photo-Fenton (PF) and photo-Fenton intensified with the addition of persulfate (PFPS) processes decolorize 97% the azo dye direct blue 71 (DB71) and allow producing a highly biodegradable effluent. Intensification with persulfate allowed reducing treatment time in 33% (from 120 to 80 min) and the consumption of chemical auxiliaries needed for pH adjustment. Energy, reagents, and chemical auxiliaries are still and environmental hotspot for PF and PFPS; however, it is worth mentioning that their environmental footprint is lower than that observed for compound parabolic concentrator (CPC)-type reactors. A life-cycle assessment (LCA) confirms that H₂O₂, NaOH, and energy consumption are the variables with the highest impact from an environmental standpoint. The use of persulfate reduced the relative impact in 1.2 to 12% in 12 of the 18 environmental categories studied using the ReCiPe method. The PFPS process emits 1.23 kg CO₂ (CO₂-Eqv/m³ treated water). On the other hand, the PF process emits 1.28 kg CO₂ (CO₂-Eqv/m³ treated water). Process intensification, chemometric techniques, and the use of SPRs minimize the impact of some barriers (reagent and energy consumption, technical complexity of reactors, pressure drops, dirt on the reflecting surfaces, fragility of reactor materials), limiting the application of advanced oxidation systems at an industrial level, and decrease treatment cost as well as potential environmental impacts associated with energy and reagents consumption. Treatment costs for PF processes (US$0.78/m³) and PFPS processes (US$0.63/m³) were 20 times lower than those reported for photo-Fenton processes in CPC-type reactors.

Combined application of Life Cycle Assessment and linear programming to evaluate food waste-to-food strategies: Seeking for answers in the nexus approach

The great concern regarding food loss (FL) has been studied previously, but in an isolated way, disregarding interdependencies with other areas. This paper aims to go a step further by proposing a new procedure to assess different waste management alternatives based on the nexus approach by means of an integrated Water-Energy-Food-Climate Nexus Index (WEFCNI). The environmental profile of the waste management techniques is determined using Life Cycle Assessment (LCA) which, in combination with Linear Programming (LP), explores the optimal aggregation of weighting factors that lead to an aggregated nexus index. The management of residues from the anchovy canning industry in Cantabria (Spain) has been used as a case study, considering the three current applied alternatives: (i) valorisation of FL as animal feed in aquaculture (food waste-to-food approach), (ii) incineration of FL with energy recovery, and (iii) landfilling with biogas recovery. The last two considered the use of energy recovered to produce a new aquaculture product (food waste-to-energy-to-food scenarios). The results indicate that incineration is the best performing scenario when the nutritional energy provided by the valorisation alternative is not high enough and the valorisation technology presents the highest water consumption. Therefore, a minimisation in the consumption of natural resources is suggested in order to improve the application of circular economy within the sector. The use of the nexus index as an environmental management tool is extendable to any food system with the aim of facilitating the decision-making process in the development of more sustainable products.

LCA of greywater management within a water circular economy restorative thinking framework

Greywater reuse is an attractive option for the sustainable management of water under water scarcity circumstances, within a water circular economy restorative thinking framework. Its successful deployment relies on the availability of low cost and environmentally friendly technologies. The life cycle assessment (LCA) approach provides the appropriate methodological tool for the evaluation of alternative treatments based on environmental decision criteria and, therefore, it is highly useful during the process conceptual design. This methodology should be employed in the early design phase to select those technologies with lower environmental impact. This work reports the comparative LCA of three scenarios for greywater reuse: photocatalysis, photovoltaic solar-driven photocatalysis and membrane biological reactor, in order to help the selection of the most environmentally friendly technology. The study has been focused on the removal of the surfactant sodium dodecylbenzenesulfonate, which is used in the formulation of detergents and personal care products and, thus, widely present in greywater. LCA was applied using the Environmental Sustainability Assessment methodology to obtain two main environmental indicators in order to simplify the decision making process: natural resources and environmental burdens. Energy consumption is the main contributor to both indicators owing to the high energy consumption of the light source for the photocatalytic greywater treatment. In order to reduce its environmental burdens, the most desirable scenario would be the use of solar light for the photocatalytic transformation. However, while the technological challenge of direct use of solar light is approached, the environmental suitability of the photovoltaic solar energy driven photocatalysis technology to greywater reuse has been demonstrated, as it involves the smallest environmental impact among the three studied alternatives.

Measuring the Vulnerability of an Energy Intensive Sector to the EU ETS under a Life Cycle Approach: The Case of the Chlor-Alkali Industry

The EU Emissions Trading System (EU ETS), which is a cornerstone of the EU’s policy to combat climate change, has been criticised by its effects on the competitiveness of intensive energy demanding industries, and in particular, of the chlor-alkali sector. The main chlorine application in Europe is the production of polyvinyl chloride (PVC) from ethylene dichloride (EDC) as intermediate. Since chlorine is mainly traded in terms of derivatives, the aim of this work is to assess the vulnerability of the European chlor-alkali industry to chlorine replacement by imported EDC. An Energetic, Economic and Environmental Sustainability Assessment (EEESA) methodology is proposed based on the main variables affecting EDC production. Moreover, the influence of the EU ETS compensation measures and the emission allowance price in the current (mercury, diaphragm and membrane) and emergent (oxygen-depolarized cathodes (ODC)) technologies is studied. The most vulnerable scenarios become mercury and diaphragm technologies due to energy consumption. However, the salt price dependency on the quality requirements substantially influences the EEESA results. This analysis also shows the importance of hydrogen valorisation, whose major impact is observed in ODC scenario.