Water treatment and reclamation by implementing electrochemical systems with constructed wetlands

Abstract

Seasonal or permanent water scarcity in off-grid communities can be alleviated by recycling water in decentralized wastewater treatment systems. Nature-based solutions, such as constructed wetlands (CWs), have become popular solutions for sanitation in remote locations. Although typical CWs can efficiently remove solids and organics to meet water reuse standards, polishing remains necessary for other parameters, such as pathogens, nutrients, and recalcitrant pollutants. Different CW designs and CWs coupled with electrochemical technologies have been proposed to improve treatment efficiency. Electrochemical systems (ECs) have been either implemented within the CW bed (ECin-CW) or as a stage in a sequential treatment (CW + EC). A large body of literature has focused on ECin-CW, and multiple scaled-up systems have recently been successfully implemented, primarily to remove recalcitrant organics. Conversely, only a few reports have explored the opportunity to polish CW effluents in a downstream electrochemical module for the electro-oxidation of micropollutants or electro-disinfection of pathogens to meet more stringent water reuse standards. This paper aims to critically review the opportunities, challenges, and future research directions of the different couplings of CW with EC as a decentralized technology for water treatment and recovery.

Keywords: Advanced oxidation; Decentralized systems; Disinfection; Electrification; Sanitation and reuse.

Graphical abstract

Fig. 1

Variations in constructed wetland configurations applied for water treatment and reclamation. a, A floating constructed wetland as an example of surface flow (SF) systems. b–c, The sub-surface flow constructed wetlands (SSF) in their vertical (b) and horizontal (c) feeding patterns. d, An example of intensified CWs: the aerated horizontal sub-surface flow CW (HSSF-CW).

Fig. 2

A wetland can integrate an electrochemical cell (ECin-CW) functioning either as a power source (MFC), in short-circuit where electrons flow across a conductive bed (METland®), or in electrolysis mode for wastewater treatment.

Fig. 3

Scheme of a typical METland® illustrating the electron flow within the conductive bed, enhancing the removal rate of organics and nitrogen compounds. Reprinted (adapted) with permission from Ref. [33]. Copyright 2020 Elsevier.

Fig. 4

The two main operational modes of METland® systems: a, flooded upflow; b, non-flooded downflow. Reprinted with permission from Ref. [87]. Copyrights 2022 Creative Commons.

Fig. 5

Different implementations of METland® treating real urban wastewater. a, Modular METland® treating 25 m³ d⁻¹ at Otos municipality (Spain). b, Modular single-house system treating 1 m³ d⁻¹c, Constructed METland® treating 20 m³ d⁻¹ at Orby (Denmark). d, Constructed METland® treating 5 m³ d⁻¹ at Carrion de los Céspedes (Spain). e, Containerized modular METland® operating in series for treating municipal wastewater at Seville (Spain).

Fig. 6

Various electrochemical systems (EC) configurations are applied for the disinfection and enhanced organic degradation of secondary treated wastewater. a, The EC can be designed to operate in a single compartment (membrane-less system) or two-compartment by implementing a separator such as ion exchange membranes (e.g., cation exchange membrane, CEM). b, Bioelectrochemical systems oxidize biodegradable organics at the anode and can reduce oxygen to hydrogen peroxide at the cathode. c–d, The batch (c) and continuous (d) operation of a two-chamber system.

Fig. 7

Electrochemical systems produce oxidants to polish constructed wetland effluents. Naturally present chloride is typically oxidized to chlorine on an anode. Oxygen can be reduced to hydrogen peroxide on carbon-based cathodes. Reprinted and adapted according to Copyright 2021 Royal Society of Chemistry [32].