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.

Electrochemical advanced oxidation processes (EAOP^®) are promising technologies for the decentralized treatment of water and will be important elements in achieving a circular economy. To overcome the drawback of the high operational expenses of EAOP^® systems, two novel reactors based on a next-generation boron-doped diamond (BDD) anode and a stainless steel cathode or a hydrogen-peroxide-generating gas diffusion electrode (GDE) are presented. This reactor design ensures the long-term stability of BDD anodes. The application potential of the novel reactors is evaluated with artificial wastewater containing phenol (COD of 2000 mg L⁻¹); the reactors are compared to each other and to ozone and peroxone systems. The investigations show that the BDD anode can be optimized for a service life of up to 18 years, reducing the costs for EAOP^® significantly. The process comparison shows a degradation efficiency for the BDD–GDE system of up to 135% in comparison to the BDD–stainless steel electrode combination, showing only 75%, 14%, and 8% of the energy consumption of the BDD–stainless steel, ozonation, and peroxonation systems, respectively. Treatment efficiencies of nearly 100% are achieved with both novel electrolysis reactors. Due to the current density adaptation and the GDE integration, which result in energy savings as well as the improvements that significantly extend the lifetime of the BDD electrode, less resources and raw materials are consumed for the power generation and electrode manufacturing processes.

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.

In the present contribution, the behavior of sodium chloride containing electrolytes at boron-doped diamond anodes are investigated. The study is focusing on industrial process water treatment and is based on half-cell measurements (active area of the working electrode: 3.14 cm²) and technical laboratory electrolysis measurements (active area: 100 cm²). It was found that the diamond electrode has a considerable overvoltage for chlorine formation (approx. 740 mV vs. RHE), regrettably not sufficient for hydroxyl radical formation before chlorine formation, regarding the electrochemical potential. Nevertheless, an operating point was identified through process parameter variation, which leads to wastewater purification at the diamond electrode without generating chlorine emissions. The application of the identified operating point is shown at the purification of real diaminodiphenylmethane-production wastewater (10 wt% sodium chloride) and artificial process water contaminated with N-(chloromethyl)-N-ethylpiperidinium chloride organic. Valuable component of both wastewaters are the high salt loads, which are reusable if the organic substances are successfully treated. The observations are confirmed by ultratrace analysis of organic products and do not show any chlorinated products at the desired operating point.

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.