Solving challenges in electrochemical water treatment for a circular economy

Erweiterte elektrochemische Oxidationsverfahren (engl.: Electrochemical Advanced Oxidation Processes, EAOP) sind vielversprechende Technologien für die dezentrale Wasseraufbereitung und haben das Potential wichtige Elemente bei der Realisierung einer Kreislaufwirtschaft zu werden. Bislang wurde die Technologie trotz ihrer nahezu konkurrenzlosen Reinigungsleistung, aufgrund der Bildung von Hydroxylradikalen, noch kaum im technischen Maßstab angewendet. Daher wurden innerhalb der vorliegenden Arbeit vier Anwendungsfälle in verschiedenen Bereichen betrachtet und fünf Herausforderungen für EAOPs auf Basis von bordotierten Diamantelektroden (BDD) identifiziert, die eine technische Realisierung bislang erschweren: 1. Nur die Anode wird zur Erzeugung eines direkten Oxidationsmittels verwendet 2. Die kathodische Wasserstoffentwicklung trägt zu einem höheren Energieverbrauch und zur Schaumbildung bei 3. Hohe Betriebskosten aufgrund preisintensiver BDDs mit begrenzter Lebensdauer 4. Die Kathodenoberfläche wird während der Elektrolyse alkalisch. Dies führt beim Vorliegen von Härtebildnern in der Wassermatrix zu einer Verkalkung der Kathode, bis hin zur Isolierung der aktiven Oberfläche 5. Chlorierung von organischen Verbindungen wenn Chloridionen im zu behandelnden Wasser vorliegen. Diese Herausforderungen wurden durch zwei neuartige Reaktorsysteme und Prozessführungen gelöst. Durch die Verwendung eines Reaktors auf Grundlage einer BDD der nächsten Generation (auf Basis von Tantal anstatt Niob) in Kombination mit einer Wasserstoffperoxid-bildenden Gasdiffusionselektrode (BDD-GDE-System) wurden die Herausforderungen eins bis drei gelöst. Es konnte gezeigt werden, dass dieser Reaktor bei der Behandlung von künstlichen pharmazeutischen Abwässern eine wesentlich höhere Abbaueffizienz (135 %) bei einem wesentlich geringeren Energieverbrauch (75 %) gewährleistet als dem Stand der Technik entsprechende Zellkonzepte und die konkurrierenden Technologien Ozonierung und Perozonierung. Daneben ermöglichen BDD-GDE-Systeme einen nahezu 100 %-igen Abbau. Die neue Ta-basierte BDD zeigt eine deutlich reduzierte Degradation während des Betriebes auf und es wird eine erhöhte Lebensdauer von 18 Jahren anstelle von 2 Jahren (Nb-basierte BDD-Anoden) erwartet. Dies führt zu einer Reduzierung der Betriebskosten um bis zu 80 %. Dieses System stößt an seine Grenzen, wenn Härtebildner in hohen Konzentrationen im Wasser vorhanden sind (Ca2+, Mg2+). Diese Herausforderung wurde durch die Entwicklung eines neuartigen Zellendesigns auf der Grundlage einer in-situ bewegten Graphit-Polymer-Compound Kathode (BDD-GPC-System) gelöst. Ein Test (120 Stunden) mit künstlichem Zugtoilettenabwasser zeigte die Langzeitstabilität des BDD-GPC-Systems auf und demonstrierte, dass die periodisch magnetisch induzierte Bewegung der GPC-Kathode in-situ Ablagerungen auf ihrer Oberfläche entfernt und den wartungsarmen Betrieb des Systems ermöglicht. Neben der Abwasserreinigung ermöglicht die BDD-GPC-Kombination die elektrochemische Fällung von anorganischen Stoffen und eröffnet neue Anwendungsbereiche wie die elektrochemische Wasserenthärtung (Enthärtungsgrade bis > 90 %) und Metallfällung (Fällungsgrade bis > 98 %). Die fünfte Herausforderung wurde durch die Identifizierung eines neuartigen Betriebspunktes gelöst, der den Abbau organischer Verbindungen ohne die Bildung von Chloremissionen an BDD-Anoden und damit ohne die Bildung chlorierter Kohlenwasserstoffe und Produkte ermöglicht. Um diese chlorfreie Abwasserbehandlung an der BDD zu nutzen, muss der pH-Wert des Anolyts während der Behandlung bei über 14,2 gehalten werden. Mit diesen Entwicklungen kann die Kreislaufwirtschaft in der pharmazeutischen Industrie und in der metallverarbeitenden Industrie, insbesondere zur Wiederverwendung von Wasser, erreicht werden. Die chlorfreie Reinigung von organisch belasteten NaCl-haltigen Prozesswässern aus der Kunststoffproduktion ermöglicht die Wiederverwendung beispielsweise in der Chlor-Alkali-Industrie. Durch den Einsatz in Zügen kann das durchschnittliche Frischwassertankintervall deutlich verlängert werden, da das Abwasser im Zug aufbereitet und als Spülwasser wiederverwendet werden kann. Darüber hinaus ermöglicht ein Wasserenthärtungsreaktor weitere erhebliche Einsparpotentiale - lokale Frischwasserversorgungen der Züge werden von der Wasserhärte entkoppelt.Electrochemical advanced oxidation processes (EAOP) are promising technologies for decentralized water treatment and have the potential to be important components in achieving a circular economy. To date, EAOPs are rarely applied at technical scale despite their nearly unrivaled treatment performance owing to hydroxyl radical formation. Therefore, four use cases in diverse fields were considered and five challenges of boron-doped diamond electrode (BDD) based EAOPs were identified, which impede the application in technical scale: 1. Solely the anode is used for oxidant generation and only one oxidant is directly formed 2. Cathodic hydrogen evolution contributes to a higher energy consumption and foam formation 3. High operation costs due to expensive BDDs with limited lifetimes 4. The local pH at the cathodes surface becomes highly alkaline, which results in a calcification of the cathode in the presence of hardness minerals 5. Chlorination of organic compounds, in case of chloride ions in the wastewater matrix, generating toxic byproducts. These challenges were successively solved by two novel reactor systems and unique process controls. By using a reactor based on a "next-generation" BDD (tantalum-based instead of niobium) combined with a hydrogen peroxide-forming gas diffusion electrode (BDD–GDE system), challenges one to three were solved. Compared to the state-of-the-art cell design and also to the competitive technologies ozonation and peroxone it was shown that this novel reactor ensures much higher degradation efficiency (135 %) with much lower energy consumption (75 %) when treating artificial pharmaceutical wastewater. Next to the low energy demand of BDD–GDE systems, the investigations revealed a treatment efficiency rate of nearly 100 % with the lowest specific energy consumption per mass organic compared to the mentioned technologies and electrochemical processes reported in the literature. The new Ta-based BDDs show drastically reduced degradation during operation and an increased lifetime of 18 years is predicted instead of 2 years for Nb-based BDD anodes. This results in a reduction of up to 80 % of the operational costs. The system reaches its limits in presence of a high concentration of hardness minerals in the water (Ca2+, Mg2+). This challenge was solved by developing a novel cell design based on an in situ moving graphite-polymer-composite (GPC) cathode (BDD–GPC system). A treatment test (120 h) of artificial vacuum toilet wastewater indicated the long-term stability of the BDD–GPC system and demonstrated that the in situ periodically magnetic-induced movement of the GPC cathode removed deposits from its surface and ultimately resulted in a low-maintenance operation of the system. Besides the wastewater treatment, the BDD–GPC combination can be used for electrochemical precipitation of inorganics and opens new application areas, such as electrochemical water softening (softening levels up to > 90 %) and metal precipitation (efficiencies up to > 98 %). Challenge five was solved by experimentally determining an operation point for the purification of organic substances while avoiding the formation of chlorine species at BDD anodes. Therefore, the treated water remained free of chlorinated hydrocarbons. To reach the chlorine-free purification at the BDD, the pH value of the anolyte must be maintained above 14.2 during treatment. With the presented developments, a circular economy for water can be achieved in the pharmaceutical industry and in the metal processing industry. The chlorine-free purification of organic-polluted sodium chloride-containing water from plastics production can led to the reuse of water and brine in the chlor-alkali industry. The application in trains extend the average fresh water tank interval significantly due to an on-train wastewater treatment and reuse of the water as flushing water. For areas with high water hardness levels significant savings for water utilization can be achieved by the use of the above-mentioned water softening reactor

Electrochemical reactors for wastewater treatment

Regarding the treatment of (waste)water, electrochemical processes have various advantages over other methods. They are robust, easy to operate and flexible in case of fluctuating wastewater streams. In addition, a relatively broad spectrum of organic and inorganic impurities can be removed. This contribution provides an overview of electrochemical reactors for water, process water, and wastewater treatment, which are already in technical‐scale operation or subject of research. Some essential basics of electrochemical processes for the treatment of water are presented and examples for applications are given. This is followed by a description of the reactors

Investigation and improvement of scalable oxygen reducing cathodes for microbial fuel cells by spray coating

This contribution describes the effect of the quality of the catalyst coating of cathodes for wastewater treatment by microbial fuel cells (MFC). The increase in coating quality led to a strong increase in MFC performance in terms of peak power density and long-term stability. This more uniform coating was realized by an airbrush coating method for applying a self-developed polymeric solution containing different catalysts (MnO2, MoS2, Co3O4). In addition to the possible automation of the presented coating, this method did not require a calcination step. A cathode coated with catalysts, for instance, MnO2/MoS2 (weight ratio 2:1), by airbrush method reached a peak and long-term power density of 320 and 200–240 mW/m2, respectively, in a two-chamber MFC. The long-term performance was approximately three times higher than a cathode with the same catalyst system but coated with the former paintbrush method on a smaller cathode surface area. This extraordinary increase in MFC performance confirmed the high impact of catalyst coating quality, which could be stronger than variations in catalyst concentration and composition, as well as in cathode surface area

Improved operating parameters for hydrogen peroxide‐generating gas diffusion electrodes

The influence of process parameters on the H2O2 yield of gas diffusion electrodes (GDE) are investigated. The investigated GDEs consist of Vulcan XC72 carbon black and PTFE on gold‐plated nickel wire cloth. An electrolysis cell is used to evaluate the influence of various process parameters, such as temperature, pH value, oxygen pressure and stoichiometric factor, electrolyte flow regime, current density and separator material at steady‐state conditions. It is found that the investigated GDE enables current efficiencies greater than 90 % at up to 2 kA m−2, whereby lower electrolyte temperatures and higher pH values contribute to higher H2O2 yields above 90 % current efficiency

Optimized Process Conditions for Hydrogen Peroxide Generating Gas Diffusion Electrodes

Due to increasingly stringent environmental regulations for dealing with production residues, the purification of persistent residues have moved into research focus. Therefore, the inner recycling of process waters as a secondary raw material is getting continuously more relevant. In search for new and efficient methods to purify process waters, electrochemical processes have moved to an aspiringly research emphasis. Especially electrochemical oxidation process are receiving much attention, in which strong oxidants are generated in situ, either directly on the electrode surfaces or indirectly, for instance through chemical trace substances (hypochlorite for example). An innovative concept for removing trace elements is a combination of a boron-doped diamond electrode and a gas diffusion electrode (GDE) for generating highly oxidative species at both electrodes simultaneously. Such a system is not commercial available yet, which is mainly due to the absence of high efficient hydrogen peroxide generating gas diffusion electrodes working at greater current densities. In addition to the challenging technical GDE design, the operating parameters strongly affect the electrochemical performance of H2O2-GDEs. While many published articles are screening electrodes under untechnical aspects with small Rotating-Ring-Disc-Electrode Systems [1, 2] or potential-controlled small scale electrolyzers [3, 4, 5], more technical-relevant scales under technical galvanostatic operation are missing. Therefore, a technical-relevant scale of laboratory electrolysis with 100 square centimeters respectively was used, to evaluate the influence of process parameters to determine optimized operating points. The impact of current density, electrolytes pH-value, temperature (first results on the H2O2 concentration depending on temperature using 1 molar sodium hydroxide solution for a current density of 0.5 kA/m2 are presented in the diagram below) and O2 volume flow rate and the effect of the difference pressure were measured with respect to the H2O2-yield. Finally, optimum process conditions for H2O2-producing GDE-evaluation are suggested. References J. F. Carneiro, R. S. Rocha, P. Hammer, R. Bertazzoli and M.R.V. Lanza, Applied Catalysis A: General, 517, 161–167 (2016). W. R.P. Barros, R. M. Reis, R. S. Rocha and M. R.V. Lanza, Electrochimica Acta, 104, 12–18 (2013). H. Luo, C. Li, C. Wu and X. Dong, RSC Adv, 5(80), 65227–65235 (2015). R. S. Rocha, R. M. Reis, Beati, André A. G. F., M. R. V. Lanza, Sotomayor, Maria Del Pilar T. and R. Bertazzoli, Quím. Nova, 35(10), 1961–1966 (2012). M. Panizza and G. Cerisola, Electrochimica Acta, 54(2), 876–878 (2008). Figure 1 <jats:p /

Evaluation of a new electrochemical concept for vacuum toilet wastewater treatment – Comparison with ozonation and peroxone processes

New electrochemical advanced oxidation processes (EAOPs) are becoming increasingly attractive for use in wastewater treatment. A reactor with a boron-doped diamond anode and a gas diffusion cathode seems to be a promising approach for water purification, because of the in situ generation of highly reactive species such as anodic •OH radicals and cathodic H2O2.To evaluate the application potential of this EAOP concept, the treatment efficiency and energy efficiency were compared with those of well-established AOPs such as ozonation and peroxone processes (O3 + H2O2). In this study, the innovative electrochemical batch treatment of artificial toilet wastewater showed a COD degradation efficiency of 38.1%, which is higher than that obtained using ozonation (17.0%) or the peroxone process (25.7%). Additionally, the specific energy demand is lower for EAOP (93.6 kWh/kg mCOD) than for ozonation (125.4 kWh/kg mCOD) or the peroxone process (134.5 kWh/kg mCOD). Keywords: Electrochemical wastewater treatment, Boron-doped diamond electrode, Gas diffusion electrod

Investigation and Improvement of Scalable Oxygen Reducing Cathodes for Microbial Fuel Cells by Spray Coating

This contribution describes the effect of the quality of the catalyst coating of cathodes for wastewater treatment by microbial fuel cells (MFC). The increase in coating quality led to a strong increase in MFC performance in terms of peak power density and long-term stability. This more uniform coating was realized by an airbrush coating method for applying a self-developed polymeric solution containing different catalysts (MnO2, MoS2, Co3O4). In addition to the possible automation of the presented coating, this method did not require a calcination step. A cathode coated with catalysts, for instance, MnO2/MoS2 (weight ratio 2:1), by airbrush method reached a peak and long-term power density of 320 and 200–240 mW/m2, respectively, in a two-chamber MFC. The long-term performance was approximately three times higher than a cathode with the same catalyst system but coated with the former paintbrush method on a smaller cathode surface area. This extraordinary increase in MFC performance confirmed the high impact of catalyst coating quality, which could be stronger than variations in catalyst concentration and composition, as well as in cathode surface area.</jats:p

Improved Hydrogen Peroxide Generating Gas Diffusion Electrodes Combined with Boron Doped Diamond Electrodes: A Compact in-Situ Oxidant Production Reactor for Space Application

Electrochemical methods offer the opportunity for in-situ generation of oxidants for wastewater treatment, as disinfectant for apparatuses and production sites as well as rocket fuel. One method well known is the electrolysis of water at boron doped diamond (BDD) anodes enabling formation of highly oxidizing OH-radicals [1]. While the BDD electrode is already used as total organic carbon (TOC) sensor on the International Space Station (https://condias.de/unternehmen/chronik/), its application potential for space is even more versatile. In particular, the combination of the BDD anode and a hydrogen peroxide (H2O2) generating gas diffusion electrode (GDE) enables a simultaneous generation of oxidants at both electrodes. By this approach, the current efficiency can be theoretically improved up to 200 %. Furthermore, the formed oxidizing agents enable the formation of additional oxidation species by consecutive reactions e.g. ozone and other oxygen radicals [2,3]. Their formation is adjustable by process conditions. All together, this can be the basis of a very universal electrolysis cell for space use. BDD electrodes are available in technical dimensions and can be operated at high current densities, but H2O2-GDEs are not. Therefore, within the radar project (http://machwas-material.de/RADAR.html) carbon-based GDEs were developed and optimized in cooperation with Covestro AG, a company manufacturing GDEs for other electrochemical processes in technically relevant dimensions since many years [4]. The activity of carbon materials for the 2-electron step of H2O2 formation is been known for a long time. On the one hand, they enable the two-electron reaction step, on the other hand, they offer large BET surface areas and thus excellent electro catalyst support qualities. Moreover, they are inexpensive and offer sufficient chemical and thermal stability. In our study, the optimization of the GDEs include the variation of composition, catalyst-loading and manufacturing parameters and the performances are correlated with the pore systems. In addition to the technical electrode design, the operation mode has a great influence on H2O2 production. Therefore, the influence of the process parameters on the H2O2 yield was also investigated. During the electrolysis process the pH-value at the cathode shifts to higher values and the conditions for H2O2 formation shift as well. Without controlling the pH at the cathode it is not clear which pH value is the best for optimal H2O2 formation. [5] In our investigation, the pH value was kept constant by balancing the pH shift with controlled dosing of water. The current density was at least 0.5 kA/m2, important for the design of compact technical reactors and particular for usage in space. By this approach stationary operation was reached, allowing to identify the best conditions for H2O2 formation. Our H2O2–GDE investigations show that from 15 °C to 20 °C, H2O2 production changes only slightly, but with further temperature increase from 25 °C to 60 °C, H2O2 current efficiency decrease significantly due to H2O2 self-decomposition. Furthermore, the optimized carbon-based GDE produces H2O2 with current yields greater than 90 % at 15 °C at a current density of up to 2 kA/m2. In addition, the choice of the pH value is critical with respect to attainable H2O2 concentration. An optimized mode of operation is crucial for a high yielding GDE and the identified optimized operating point is achieved by varying the mass flow rate of the dosages. The presented electrolysis cell for simultaneous oxidant generation on both electrodes is solely fed with water on cathode and anode side – a very convenient operation method especially in space. References Samuel P. Kounaves, Total Organic Compound (TOC) Analyzer (US8216447B2) (2012). B. Marselli, J. Garcia-Gomez, P.-A. Michaud, M.A. Rodrigo and Ch. Comninellis, Journal of the Electrochemical Society(150 (3)), D79-D83 (2003). M. Sievers, Treatise on Water Science, 4 (2011). J. Kintrup, M. Millaruelo, V. Trieu, A. Bulan and E. S. Mojica, Electrochem. Soc. Interface, 26(2), 73–76 (2017). T. Muddemann, U. Kunz, D. R. Haupt and M. Sievers, ECS Trans., 86(4), 41–53 (2018). </jats:p

Evaluation of a New Electrochemical Concept for Vacuum Toilet Wastewater Treatment - Comparison with Ozonation and Perozone Processes

Over the last several years, the removal of more than 150 micropollutants was investigated. Many pharmaceuticals are persistent and pass the conventional wastewater treatment plants (WWTP). According to literature, the removal efficiency of various pharmaceuticals is between 0 and 50 % in activated sludge processes. For a standard analgesic such as Diclofenac, the removal efficiency is approx. 30 %. Therefore, additional treatment such as 4th treatment step is necessary, which is already implemented in some WWTPs in Europe, to save water bodies as much as possible. Options for the 4th treatment step are, for example, activated carbon adsorption, nanofiltration and advanced oxidation processes (AOP). By far one of the most cost efficient AOP is the ozonation. However, it is not able to remove X-ray contrast media sufficiently. For this purpose, electrochemical oxidation processes (EAOP) based on boron-doped diamond electrodes are much more effective. Keeping in mind that AOPs are still an emerging technology, especially for water reuse options and also might cause even toxic intermediates, one of the biggest disadvantages are the relatively high operational costs due to the electrical energy demand. Especially electrochemical reactors, which are based on two boron-doped diamond (BDD) electrodes are very cost-intensive, particularly in comparison to ozonation or perozone (ozone plus peroxide) processes. For that reason, a new electrochemical reactor concept, which promises lower CAPEX and OPEX costs, was investigated. This reactor concept consists mainly of one BDD electrode as anode and a gas diffusion electrode (GDE) as cathode. The innovative reactor concept combines the generation of highly reactive hydroxyl radicals on the anode surface and simultaneously hydrogen peroxide on the cathode surface, which is very efficient as there is a double use of the same applied power. Both electrodes are mounted in a membrane-less flow-through reactor in pilot-scale for direct treatment of wastewater. Due to the direct contact between the surfaces of both electrodes, the wastewater gets treated specifically, which leads to a high energy efficiency. First results already prove the more energy efficient removal of COD in an artificial wastewater with the new reactor concept. For a detailed evaluation, a defined artificial wastewater with compounds and concentration equal to vacuum toilet wastewater was treated by the new reactor concept as well as by ozonation and perozone process (ozone plus peroxide). COD removal as well as the associated energy demand were measured after defined intervals in each step of the treatment to determine the performance of the different processes. A comparison was made between the different kind of treatments (EAOP, ozonation, perozone) concerning COD degradation and the power demand. The evaluation is based on electrical energy per order (EEO) conception as well as degradation for given reaction kinetics. </jats:p