Novel decentralized greywater treatment system based on combined adsorption and electrochemical oxidation

Abstract

Electrochemical technologies for wastewater treatment present several promising characteristics to be used as decentralized water treatment systems to provide a sustainable and cost effective growing water supply complementing centralized water supply systems. However, their application and scaling up has been hindered by the high energy input required, and the mass transfer limitations.To address these limitations, the aim of this thesis is to develop a novel decentralized water treatment system to treat greywater produced at household scale. The system, a three-dimensional (3D) electrochemical reactor, comprises a flow-through electrochemical reactor, in which the contaminated solution will pass through the boron-doped diamond (BDD) anode mesh, maximizing the total contact surface of the electrode. In addition, granular activated carbon (GAC) has been added as a bed material in the reactor, which has been recognized that can enhance the performance of the conventional two-dimensional (2D) system due to its adsorption properties and by acting as a third electrode within the system.The main goals of this thesis are: (i) to elucidate how the presence of a bed material can enhance the performance of the electrochemical treatment; (ii) to develop a proof-of-concepts on the performance of a 3D electrochemical oxidation system for the treatment of greywater; (iii) to determine the effect of current density and volume of greywater loaded, and the bed material on the performance of the 3D system; (iv) to explore the viability of the electrochemical regeneration of the bed material when operating the 3D system for the treatment of greywater.The reactor has worked under three different configurations: (i) adsorption onto activated carbon (GAC system), (ii) electrochemical oxidation on BDD anode (2D system), and (iii) combined adsorption and electrochemical oxidation (3D system) for the treatment of simulated (SGW) and real greywater (RGW). Electrochemical experiments were conducted at a fixed current density of 15 A m-1. Disinfection experiments have been performed using Escherichia Coli (e. coli), spores of Clostridium Perfringens (SCP) and somatic coliphages (SOMCPH) as model microorganisms for pathogenic bacteria, protozoa and viruses respectively in SGW (Chapter 5). The three reactor configurations have operated at two different current densities (i.e., 15 and 20 A m-2) and two different volume loads (2 L and 6 L) to study the effect of these two parameters in the performance of the 3D system (Chapter 6). To minimise the adsorption and catalytic effect of the GAC and determine the electrochemical oxidation capacity of the bed material acting as a third electrode, it has been substituted granular graphite (GG) (Chapter 7). Electrochemical regeneration of the GAC in the 3D system has been studied over the time by loading the reactor with saturated GAC (Chapter 8). Finally, an economic analysis has been conducted for the implementation of a 3D electrochemical system at household scale (Chapter 9).The results showed that the efficiency of a conventional 2D electrochemical system can be enhanced by up to 86% for the removal of chemical oxygen demand (COD) and total organic carbon (TOC) in the 3D system. Obtained positive synergy values suggest that the combination of these two processes may trigger other electrochemical reactions in the 3D system while enabling the electrochemical regeneration of the GAC and lowering the energy consumption of the process. In addition, despite the 3D system achieved the same removal of SOMCPH than the 2D system, the removal of e. coli and SCP was slightly higher in the 2D configuration due to the accumulation of electrochemically produced chlorine.Saturation of the bed material in a 3D system can occur using high volumes of SGW or low values of applied current density. While increasing the applied current density or decreasing the volume of SGW treated, regeneration of the GAC is favoured, however the energy efficiency of the treatment process decreases.When substituting GAC by GG, its adsorption capacity decreases by up to 30% for the removal of COD and TOC from SGW. However, the performance of the 3D system using GG is enhanced by up to 50% and 30% for the removal of COD and TOC respectively compared to adsorption and electrochemical oxidation operating separately. In addition, electrochemical regeneration of saturated GAC has been demonstrated in the 3D electrochemical configuration. Calculated regeneration efficiency (RE) of the GAC in the 3D system increases to 64.8% after 31 consecutive runs (2 L of SGW treated in every 7-hour run).Finally, an economic evaluation showed that 70% of the capital investment corresponds to the BDD anode which can hinder its applicability as other more economic (but less effective) materials may be preferred. In addition, the operational costs of treating RGW in a 2D system can be reduced by 20% in a 3D system using GAC bed. In order to outweigh the operational costs, modular electrochemical systems can be operated by a photovoltaic power supply which makes it more sustainable process.