Evaluation and prospects of nanomaterial-enabled innovative processes and devices for water disinfection: A state-of-the-art review

Highlights

• Brief overview of NM-enabled processes and devices for water disinfection.

• Evaluation of NM-enabled processes/devices based on performance and energy efficiency.

• Applying contact time and electrical energy per order (E_EO) for assessment of processes and devices.

• Discussion of influencing factors on contact time and E_EO for NM-enabled disinfection.

• Summary of NMs recovery and electrode strengthening methods for practical application.

Abstract

This study provided an overview of established and emerging nanomaterial (NM)-enabled processes and devices for water disinfection for both centralized and decentralized systems. In addition to a discussion of major disinfection mechanisms, data on disinfection performance (shortest contact time for complete disinfection) and energy efficiency (electrical energy per order; E_EO) were collected enabling assessments firstly for disinfection processes and then for disinfection devices. The NM-enabled electro-based disinfection process gained the highest disinfection efficiency with the lowest energy consumption compared with physical-based, peroxy-based, and photo-based disinfection processes owing to the unique disinfection mechanism and the direct mean of translating energy input to microbes. Among the established disinfection devices (e.g., the stirred, the plug-flow, and the flow-through reactor), the flow-through reactor with mesh/membrane or 3-dimensional porous electrodes showed the highest disinfection performance and energy efficiency attributed to its highest mass transfer efficiency. Additionally, we also summarized recent knowledge about current and potential NMs separation and recovery methods as well as electrode strengthening and optimization strategies. Magnetic separation and robust immobilization (anchoring and coating) are feasible strategies to prompt the practical application of NM-enabled disinfection devices. Magnetic separation effectively solved the problem for the separation of evenly distributed particle-sized NMs from microbial solution and robust immobilization increased the stability of NM-modified electrodes and prevented these electrodes from degradation by hydraulic detachment and/or electrochemical dissolution. Furthermore, the study of computational fluid dynamics (CFD) was capable of simulating NM-enabled devices, which showed great potential for system optimization and reactor expansion. In this overview, we stressed the need to concern not only the treatment performance and energy efficiency of NM-enabled disinfection processes and devices but also the overall feasibility of system construction and operation for practical application.

Introduction

Water is a basic human need. During water utilization, disinfection is an essential process to protect people from pathogenic infections and has been broadly applied for more than 100 years (Rutala and Weber, 2016; Shannon et al., 2008). However, conventional water disinfection methods (e.g., chlorination and UV radiation) suffer from obvious drawbacks such as harmful disinfection byproducts (DBPs) formation and pathogen regrowth (Guo et al., 2015; Li and Mitch, 2018; Sedlak and von Gunten, 2011). Alternative methods such as membrane filtration and ozonation also suffer from low throughput and intensive energy consumption (Al-Karaghouli and Kazmerski, 2013; Miklos et al., 2018). Additionally, these current disinfection methods are commonly feasible for centralized systems, whereas they are not easily applicable in developing areas where sanitation facilities and power supplies are not available (Liu et al., 2019b). Thus, there is an urgent need for a highly efficient, reliable, low-energy-consuming, and easily-applicable disinfection method for both centralized and decentralized systems.

Nanomaterials (NMs, such as silver nanoparticles, carbon nanotubes, and titanium dioxide nanoparticles) with unique biological, electronic, and/or catalytic properties provide a viable and effective option for water disinfection (Alvarez et al., 2018; Mauter et al., 2018; Qu et al., 2013b). These NMs involve new cell inactivation mechanisms such as free radical generation and cell structure decomposition, which are capable of achieving microbial disinfection with low DBPs formation and without pathogen regrowth (Li et al., 2008; Qu et al., 2013a; Zhang et al., 2019a). The suitability of the NM-enabled disinfection technology has also been recognized for decentralized application attributed to the flexible reactor design, the low energy consumption, and the great performance at small scales (Hodges et al., 2018; Lee et al., 2010b).

To develop the high-performance NM-enabled disinfection system, one not only needs to exploit unique disinfection processes enabled by anti-microbial NMs, but also design novel NM-enabled disinfection devices to improve the treatment efficiency and lifetime. From this aspect, there are three prerequisite factors need to be deliberated: (1) rational developments of NM-enabled disinfection processes based on novel antimicrobial mechanisms, (2) feasible designs of NM-enabled disinfection devices, and (3) suitable strategies to recover NMs and/or improve the stability of NMs.

Various novel NM-enabled disinfection processes have already achieved high treatment performance, especially those involving photo-catalytic and electrochemistry (Martinez-Huitle and Brillas, 2008; Pelaez et al., 2012). Additionally, an increasing number of new NM-enabled disinfection processes (i.e., physical-, photoelectro-, photothermal-, and electroporation-based processes) are continuously being reported by various researchers (Hayden et al., 2010; Ivanova et al., 2013; Kotnik et al., 2015; Loeb et al., 2016). The huge amount of different studies and proposed processes pose an enormous challenge for a critical assessment of NM-enabled disinfection processes. Thus, it is essential to enable the comparison between different NM-enabled disinfection processes based on their disinfection performance, operational costs, and the general feasibility (i.e., resource use and eco-toxicity caused by NMs release). However, the study for such a comprehensive comparison of various NM-enabled disinfection processes is still limited.

Considering NMs are not dissolved in the solution during disinfection processes, commonly, NM-enabled disinfection devices are heterogeneous reactors instead of homogeneous reactors (Alvarez et al., 2018; Hodges et al., 2018). Due to the small particle size of NMs, microbial disinfection commonly happens near NMs surface with the distance from several nanometers to micrometers (Huo et al., 2018a; Liu et al., 2014b). Thus, the treatment efficiency of the NM-enabled disinfection device is significantly impacted by the microbial mass transfer efficiency from the bulk solution to the NMs surface. It is, therefore, critical to design feasible disinfection devices to promote treatment efficiency. However, despite the large volume of publications dedicated to achieving disinfection with novel NMs in a batch reactor at lab-scale, implementation in practical applications using feasible devices has been limited (Zhang et al., 2019a). To address this issue, researchers made efforts to develop feasible NM-enabled disinfection device for practical application and design various types of reactors (such as the stirred, the plug-flow, and the flow-through reactor) based on different disinfection processes (Gu et al., 2019; Loeb et al., 2019; Radjenovic and Sedlak, 2015). However, due to the different experimental setup in different studies, it is hard to make comprehensive assessments of those reported devices concerning the disinfection efficiency and energy consumption, and thus, studies for such assessments is severely limited.

After clarifying the necessity of assessments for both NM-enabled disinfection processes and devices, before practical application, another huge challenge is the lack of feasible strategies for separation and recovery the applied NMs or strengthening the insufficient stability of nanostructure. Due to the small particle size, NMs may release to effluents during the treatment process, leading to considerable eco-toxicity as well as a lowered disinfection performance and device lifetime (Dale et al., 2015; Lead et al., 2018; Sun et al., 2016). Although there are several strategies including magnetic recovery and facial coating have been proven effective for recovering or protecting NMs, these studies are limited and only applicable case by case (Huo et al., 2019b; Yang et al., 2016). Thus, it is critical to developing general and feasible strategies to recycle and/or protect NMs according to types of NM-enabled disinfection processes and/or devices. Such strategies can be general guidance for the future design of NM-enabled disinfection systems, whereas the related studies are severely limited.

With this in mind, we overviewed the established and emerging NM-enabled processes and devices for water disinfection as well as the current and potential NMs separation/recovery methods and electrode strengthening/optimization strategies. We first enabled critical assessments for different disinfection processes and devices based on the disinfection performance (the shortest contact time required for complete disinfection) and the operation cost (the amount of electrical energy needed to decrease by one order of magnitude of the microbial concentration; E_EO) from a technical standpoint. We also provided recommendations for disinfection systems development and operation from a practical standpoint by reviewing the emerging NMs recovery and electrode strengthening strategies.