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.

Microbial fuel cells are a promising technology for future wastewater treatment, as it allows cleaning and power generation simultaneously. The bottleneck of microbial fuel cells is often its cathodes because they determine the power output. Gas diffusion electrodes might overcome this bottleneck due to their low production costs and high oxygen reduction rates. However, biofilm formation on the gas diffusion electrodes reduces their performance over time. In this work, a new reactor design of the microbial fuel cell using rotating gas diffusion electrodes is presented. The biofilm growth on the electrode during operation was observed and its effect on the performance of the microbial fuel cell was examined. In addition, different antifouling strategies were investigated over a period of 80 days. It was found that already after 7 days of operation a complete biofilm had grown on an untreated gas diffusion electrode. However, this does not seem to affect the performance of the cells in the beginning. Differences in the performance of the reactors with and without an antifouling strategy only become apparent from day 15 onwards. The use of UV radiation and antibacterial membranes leads to the best results with maximum power densities of approx. 200 mW m⁻² while the untreated microbial fuel cell only achieves a maximum power density of approx. 20 mW m⁻².

A system of boron-doped diamond (BDD) anode combined with a gas diffusion electrode (GDE) as a cathode is an attractive kind of electrolysis system to treat wastewater to remove organic pollutants. Depending on the operating parameters and water matrix, the kinetics of the electrochemical reaction must be defined to calculate the reaction rate constant, which enables designing the treatment reactor in a continuous process. In this work, synthetic wastewater simulating the vacuum toilet sewage on trains was treated via a BDD-GDE reactor, where the kinetics was presented as the abatement of chemical oxygen demand (COD) over time. By investigating three different initial COD concentrations (C_0,1 ≈ 2 × C_0,2 ≈ 4 × C_0,3), the kinetics was presented and the observed reaction rate constant k_obs. was derived at different current densities (20, 50, 100 mA/cm²). Accordingly, a mathematical model has derived k_obs. as a function of the cell potential $E_{cell}$. Ranging from 1 × 10⁻⁵ to 7.4 × 10⁻⁵ s⁻¹, the k_obs. is readily calculated when $E_{cell}$ varies in a range of 2.5–21 V. Furthermore, it was experimentally stated that the highest economic removal of COD was achieved at 20 mA/cm² demanding the lowest specific charge (~7 Ah/g_COD) and acquiring the highest current efficiency (up to ~48%).

To enable the evaluation of water treatment efficiency of electrochemical advanced oxidation processes (EAOPs), an approach to remove ${\mathrm{H}}_2{\mathrm{O}}_2$ as a catalytic pretreatment was investigated to avoid interference in chemical oxygen demand (COD) measurements. Four wastewater types with COD and ${\mathrm{H}}_2{\mathrm{O}}_2$ concentrations up to $\mathrm{1,300}\mathrm{mg}/\mathrm{L}$ and $90\mathrm{mmol}/\mathrm{L}$ , respectively, were investigated with a novel method. The method requires the addition of sodium bicarbonate (${\mathrm{NaHCO}}_3$ ), which decomposes ${\mathrm{H}}_2{\mathrm{O}}_2$ at ambient temperature within 24 h without changing COD. For synthetic wastewater (SWW), this time is reduced to 1 h by heating at 70°C. A side-by-side comparison of ${\mathrm{NaHCO}}_3$ and ${\mathrm{Na}}_2{\mathrm{CO}}_3$ experiments confirmed ${\mathrm{H}}_2{\mathrm{O}}_2$ removal without changes in COD via ${\mathrm{NaHCO}}_3$ , whereas a $\sim 20\%$ decrease in original COD was observed using ${\mathrm{Na}}_2{\mathrm{CO}}_3$ . The change in COD during catalytical ${\mathrm{H}}_2{\mathrm{O}}_2$ decomposition in ${\mathrm{Na}}_2{\mathrm{CO}}_3$ solution was highly correlated with the high pH value.

Using electrolysis systems to degrade organics in wastewater encourages utilizing this technique to remove micropollutants (MPs) in different types of water. In this work, a cell consisting of an anode as a boron-doped diamond (BDD) electrode combined with a gas diffusion (GDE) cathode without a separator showed that MPs degradation can be effectively achieved. Investigating different operating parameters, it was stated that applying a low current density (2 mA/cm2) and setting the Reynolds number of the electrolyte flow through the cell at the laminar range raised the treatment time by 3 folds at the same energy demand. This arrangement increased the MPs removal. Some substances like diclofenac were removed up to 84% at a longer treatment time of 180 min coupled with an increase in energy demand. The results at the mentioned parameters indicated an adequate generation rate of radicals needed to remove MPs and the oxidation reactions were promoted. The results show high practicability to the investigated electrolysis system in removing MPs in wastewater under considering the need to further reduction of the energy demand.