Evaluation of a New Electrochemical Concept for Vacuum Toilet Wastewater Treatment - Comparison with Ozonation and Perozone Processes
Authors: Haupt Dennis René , Sievers Michael , Muddemann Thorben , Kunz Ulrich 
Published: February 2020

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.