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

The development and applications of a micro-gap perforated electrode flow through cell

Electrochemical techniques provide convenient and environmentally compatible ways of bringing about chemical transformations. However they generally lose their economic viability when used with low conducting electrolyte systems. This has limited their usefulness in the treatment of water and wastewater. Increasing the electrolyte concentration of these systems is not an option as it is with industrial processes such as the chlor-alkali process. Cell resistance is the major limiting factor. Cell resistance can be reduced by reducing the inter-electrode gap. A novel micro gap perforated electrode flow through (PEFT) cell has been developed for efficient and cost effective treatment of aqueous systems of low ionic strength. The PEFT cell is an undivided flow through design which encompasses both parallel plate and porous electrode features. It consists of plate electrodes and flow is both through the electrodes and parallel to the surfaces of the two electrodes. The perforations in the electrodes and the short flow distance between the electrodes allow the inter-electrode gap to be reduced to 50 microns and less without causing excessive resistance to hydraulic flow. With reduced electrical resistance, effective electrochemical treatment of natural water and other low electrolyte systems is possible. The PEFT cell was first applied to overcome a local water supply problem, the Waikato region’s iron and manganese contaminated bore waters. These waters form stable colloidal suspensions during slow air oxidation. The problem can be overcome by rapid electrochemical oxidation using the PEFT cell. Electrochemical oxidation was found to be more effective and efficient than chemical oxidation allowing removal of iron and manganese to meet drinking water standards with minimal formation of disinfection by-products (DBP). Electrochemical oxidation of water and wastewater systems is brought about principally by chlorine mediated indirect oxidation processes. A 240 µm gap PEFT cell, with a graphite anode was used for chlorine generation. It produced chlorine at current efficiencies above 60% with an energy consumption of 4.83 kWh/kg of chlorine from a 0.5 mol/L NaCl solution. This result compares well with industrial hypochlorite production using an undivided cell. Chlorine mediated electro-oxidation of effluents was successfully demonstrated by the degradation of textile dyes in water. Complete single pass electrochemical decolourisation of indigo carmine (IC) dye effluent containing 0.35 mol/L NaCl was achieved using a graphite anode PEFT cell. Energy consumption was 0.8 kWh/m3 or 8.3 kWh/kg of dye. This is an order of magnitude less than the energy consumption reported for colour removal using graphite anodes. It is comparable or lower than most colour removal work carried out using metal oxide coated dimensionally stable anodes (DSAs) and boron doped diamond (BDD) anodes. Reduction of pH from 7 to 3 reduced the energy consumption for decolourisation of IC dye by 50% and also increased the TOC removal by 20%. When NaSO4 was used as the electrolyte rather than sodium chloride, colour removal was much less effective. A single pass through a 50 µm gap PEFT cell with a stainless steel cathode and a graphite anode operated at 5.5 V achieved a 6 log inactivation of Escherichia coli bacteria in a water sample containing only 1.7 mmol/L of chloride ions. The power consumption was 0.5 kWh/m3 of water. The narrow inter-electrode gap allows high electric fields to be produced from low applied voltages. When the cell was operated at above 5.0 volts, a synergistic electric field effect was observed. Specific lethality of the chlorine was increased to at least 50 L/(mgmin), approximately two orders of magnitude higher than in the absence of the field. Increased specific lethality means that disinfection can be achieved at much lower free available chlorine levels than previously possible. This reduces the risk of DBP formation. Improved current efficiencies and reduced energy consumption for electrolysis at low electrolyte concentrations were achieved by partial insulation of the active anode surface of a 50 µm gap PEFT cell. This electro-catalytic effect was consistent with enhanced transport of the electroactive species to the active part of the electrode, reducing concentration and resistance overpotentials. In the electrochemical production of chlorine from 0.85 mmol/L NaCl at a current density of 2 mA/cm2, current efficiency was tripled and power consumption was reduced by a factor of two, relative to the cell without the anode modification. The reduction in the inter-electrode gap to 50 µm and less has allowed the production of electric field strengths greater than 10 kV/cm from applied voltages of less than a 100V. Field strengths between 1and 10 kV/cm are known to cause reversible electroporation whereas irreversible electroporation occurs above 10 kV/cm. Evidence for irreversible electroporation was provided by the 6 log inactivation of Escherichia coli (in the absence of chlorine) at an applied electric field of 22.5 kV/cm generated in a 40 µm gap PEFT cell by a 90 V DC supply. The energy consumption was 430 J/mL and without cooling, the temperature remained below 42oC. Inactivation was achieved by 20 hydrodynamically generated DC pulses. The low applied voltage, the elimination of the need for pulsed electric fields, avoidance of external cooling and the simplicity of the experiment bring commercial non thermal electro-pasteurisation one step closer

Treatment of mining waters by electrocoagulation

The mining industry is known to be one of the major contributors to the pollution of aquatic systems. The presence, in mining water, of various contaminants in relatively high concentrations (the most common cations being Fen+, Al3+, Si4+, Ca2+, Mg2+, Cu2+, Zn2+, Ni2+, Na+, K+ and anions Cl-, SO42-, NO3-, CO32-, HCO3-) can cause severe health problems, as well as hindering the reuse and recycling of process water. Currently, there are two approaches used in mining waters management. The first concept aims to reduce the concentrations of harmful dissolved contaminants to acceptable discharge levels. The basis for the second concept is the reuse and recycling of water within the process. Among treatment methods, electrocoagulation (EC) is receiving more and more attention as an alternative technology to treat mining waters. Eventually, electrocoagulation may replace conventional technologies that require an addition of chemicals and an even higher energy consumption. To ensure the suitability of electrocoagulation for the treatment of mining waters, the prime objective of this thesis was to investigate the removal mechanisms of sulfate and cyanide involved in EC-processes. Along with the removal mechanisms, parameters affecting the electrocoagulation process were investigated. 33-full factorial design and response surface methodology were implemented in order to systematically study the significance of process parameters (applied current, initial sulfate concentration and initial pH) and their combination on sulfate removal by electrocoagulation. In addition, EC performance using iron and aluminum electrodes was compared. Laboratory tests with both synthetic and real mining waters were performed. In terms of cyanide removal, the electrodes materials and electric charge were the main studied parameters. Another research question was devoted to the development of a novel treatment concept based on continuous EC operation and solids recirculation. Established process resulted in a more efficient sulfate and metals removal as well as allowed operation at desired initial pH conditions comparing to the batch operation. The performance of this novel continuous treatment concept was compared with batch electrocoagulation and conventional chemical coagulation. According to the results of this study, removal mechanisms of sulfate are different at neutral/base and acidic conditions, while the removal mechanisms of cyanide vary greatly, depending upon the material of the electrodes. Iron electrodes are more suitable for the EC-treatment of sulfate and cyanide rich waters. With iron electrodes, partial removal of sulfate and almost complete removal of cyanide were obtained. Under the studied conditions the iron species produced were positively charged favoring the removal of negatively charged ions. Most probably, the lower removal rates of studied contaminants with aluminum electrodes are due to the negatively charged species formed hindering the particle charge neutralization of anions. In batch operation, the applied current and initial concentration of sulfate are the most critical parameters affecting the removal of sulfate by electrocoagulation, while the effect of initial pH is insignificant and mainly affects the formation of metal species. No matter what the initial pH of the solution, the EC-treatment took place at base conditions with final pH over 11. On the contrary, the initial pH of the solution had a pronounced effect for continuous EC tests, the initial pH remained constant and operation at acidic conditions improved the removal of sulfate. In this study, received knowledge on sulfate and cyanide removal prove the suitability of electrocoagulation to treat mining waters. Awareness of the removal mechanisms makes the scale-up of the electrocoagulation process more robust and exact. For further process implementation, at industrial scale, there is a clear need to characterize the solids formed during the EC-process, to develop methods for sludge dewatering and recovery of valuable components from the sludge formed. In addition, there is a need to establish scale-up rules and validate the process at pilot and industrial scales. Having covered these considerations, it will be possible to conclude that electrocoagulation is a sustainable water treatment technology that meets all circular economy principles

Landfill Leachate Management

Landfill leachate is a complex mix of organics, inorganics and heavy metals produced from the conventional and engineering landfilling practices. The adverse effects of landfill leachate on the human and environmental health have forced the relevant authorities to stipulate stringent disposal requirements, producing the requirement for ground-breaking technological solutions for effective management of landfill leachate. The researchers and field engineers are still looking for robust options for leachate management. This timely book on landfill leachate management is a valued addition into this domain. The key features of the book include: broad range of treatment techniques covered, conventional to advanced technological options discussed, along with the inclusion of successful case studies

Energy: A continuing bibliography with indexes, issue 32

This bibliography lists 1316 reports, articles, and other documents introduced into the NASA scientific and technical information system from October 1, 1981 through December 31, 1981