Continuous Ammonia Recovery from Wastewaters Using an

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Continuous Ammonia Recovery from Wastewaters Using an Integrated Capacitive Flow Electrode Membrane Stripping System Changyong Zhang, Jinxing Ma, Jingke Song, Calvin He, and T. David Waite Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02743 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 28, 2018

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Environmental Science & Technology

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Continuous Ammonia Recovery from Wastewaters Using an Integrated

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Capacitive Flow Electrode Membrane Stripping System

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Changyong Zhang,a Jinxing Ma,a Jingke Song,a,b Calvin He,a and T. David Waite*a

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a UNSW

Water Research Centre, School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW, 2052, Australia

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bCollege

of Environmental Science and Engineering, State Key Laboratory of Pollution

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Control and Resource Reuse, Tongji University, Shanghai, 200092, P.R. China; Shanghai

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Institute of Pollution Control and Ecological Security, Shanghai, 200092, P.R. China

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Email

addresses:

[email protected]

(Changyong

Zhang);

10

[email protected] (Jinxing Ma); jingke.song@ student.unsw.edu.au (Jingke Song);

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[email protected] (Calvin He); [email protected] (T. David Waite)

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*Corresponding authors: Professor T. David Waite; [email protected]

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ABSTRACT

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We have previously described a novel flow-electrode capacitive deionization (FCDI) unit

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combined with a hydrophobic gas-permeable hollow fiber membrane contactor (designated

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“CapAmm”) and presented results showing efficient recovery of ammonia from

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synthetic wastewaters (Zhang et al., ES&T Letters 2018, 5, 43–49). We extend this earlier

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study here with description of an FCDI system with integrated flat sheet gas permeable

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membrane with comprehensive assessment of ammonia recovery performance from both

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dilute and concentrated wastewaters. The integrated CapAmm cell exhibited excellent

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ammonia removal and recovery efficiencies (up to ~90% and ~80% respectively). The energy

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consumptions for ammonia recovery from low-strength (i.e., domestic) and high-strength

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(i.e., synthetic urine) wastewaters were 20.4 kWh kg−1 N and 7.8 kWh kg−1 N, respectively,

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with these values comparable to those of more conventional alternatives. Stable ammonia

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recovery and salt removal performance was achieved over more than two days of continuous

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operation with ammonia concentrated by ~80 times that of the feed stream. These results

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demonstrate that the integrated CapAmm system described here could be a cost-effective

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technology capable of treating wastewaters and realizing both nutrient recovery and water

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reclamation in a sustainable manner.

dilute

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KEYWORDS

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Ammonia recovery, flow electrode capacitive deionization, flat-sheet membrane, nitrogen

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fertilizer, CapAmm

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1. INTRODUCTION

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While nitrogen gas constitutes about 78% of the terrestrial atmosphere, the availability of its

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active forms (i.e., ammonium, nitrite and nitrate) as essential nutrients for agricultural plant

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growth and other anthropogenic uses is limited.1 The invention of the Haber-Bosch process in

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the early twentieth century enabled the manufacture of ammonia (an important raw material

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for N-based fertilizers) on an industrial scale with the ready availability of this essential

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nutrient dramatically enhancing global agricultural productivity and consequently economic

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growth.1-3 However, the Haber-Bosch process involves the chemical combination of nitrogen

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and hydrogen gas under high temperature (~500 ℃) and high pressure (200-300 atm)

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conditions, accounting for about 2% of the world energy consumption and 1.6% of global

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greenhouse gases emissions.4,

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improvement in living standards, the demand for nitrogen fertilizer is projected to increase by

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50% by 2050.6 Therefore, the development of more sustainable, efficient, and economic

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routes for the production of ammonium-based fertlizers is of great significance.7-9

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With the continuous increase in world population and

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From a lifecycle perspective, most food proteins are degraded to ammonium ions and/or

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urea in metabolic processes and end up in sewage. It is reported that the waste stream

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accounts for 18-21% of the N-fertilizers that are produced by industrial nitrogen fixation

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with the discharge of sewage containing even dilute concentrations of nitrogen into

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waterbodies creating various negative environmental impacts including the deterioration of

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water quality and loss of biodiversity.3, 11-13 While conventional wastewater treatment plants

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might take advantage of nitrification/denitrification processes to convert reactive nitrogen

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back to N2, this practice essentially attempts to reverse the ammonia production process from

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which most of the sewage nitrogen is derived.14 Therefore, processes that enable the direct

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recovery of ammonia from sewage/urine and effectively close the loop between fertilizer

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production and wastewater treatment rather than using N2 as an intermediate have attracted 3 ACS Paragon Plus Environment

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increasing attention in view of the potential enormous economic and environmental benefits.

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Of the technologies available, air-stripping followed by acid adsorption is a well-

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established process that has been intensively studied over the last decade. However, air-

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stripping requires the addition of extra base (to raise the pH to 9.5-12) and/or heating to 25-

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85 ℃ to covert ammonium (NH4+) to its volatile form (NH3), resulting in high energy

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consumption (3.9-28.2 kWh kg−1 N).15,

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ammonium ions first being selectively adsorbed (by materials such as zeolites, resins and

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biochar) followed by chemical desorption to generate a concentrated ammonium stream.17, 18

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The drawbacks of this technology relate to the need for a large amount of chemicals (such as

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NaCl and NaOH) to regenerate the adsorbents. Recently, electrochemical ammonia removal

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(and recovery) technologies have gained in popularity as a result of advantages such as

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efficient ammonium enrichment driven by the electrical field and concomitant pH increase

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that enables ammonia recovery (e.g., by methods such as membrane stripping). In a dual-

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chamber electrochemical system, NH4+ migrates to the negatively charged chamber where it

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is then transformed to NH3 and recovered as value added products, with an average energy

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consumption ranging from 10 to 30 kWh kg−1 N.5, 19-23 Bio-electrochemical systems (BES),

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which use bacteria at the anode to oxidize organic matter and produce electricity, can recover

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ammonia at a lower energy demand (< 5 kWh kg−1 N) 24-28 though the recovery rate requires

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further improvement.

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Ion exchange is another recovery method with

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Capacitive membrane stripping for ammonia recovery (CapAmm), first proposed by

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Waite’s group,29 involves use of a flow-electrode capacitive deionization unit and a gas-

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permeable membrane contactor. This technology allows simultaneous salt removal and

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ammonia recovery from dilute wastewaters. While the mechanism involving NH4+ migration

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and transformation are similar to those reported in conventional electrochemical systems,5 the

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capacitive ion immobilisation and extraction from wastewaters as a result of the formation of 4 ACS Paragon Plus Environment

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electrical double layers on the charged flow-electrode sustains high coulombic efficiency in

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the CapAmm system.29,

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hollow-fibre membrane contactor, the CapAmm process could achieve an ammonium

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recovery rate of ~60% with relatively low energy requirement.29 Although the CapAmm

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process is, potentially, highly competitive with other technologies for the recovery of ammonia

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from dilute wastewaters in view of its moderate operating conditions (e.g., low voltage and

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temperature independence) and high energy efficiency, consideration should be given to (i)

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simplification of the CapAmm configuration, (ii) optimisation of operating conditions, particularly

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in relation to minimization of energy consumption, (iii) potential application of the CapAmm

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process for ammonia recovery from high-strength wastewaters (e.g., urine, digestate and

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landfill leachate) and (iv) the possibility of continuous (long-term) operation before this

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method could be recommended as a viable choice for large-scale ammonia recovery from

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wastewaters.

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Results of our previous study showed that with the use of a

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To facilitate achievement of the goals mentioned above, an integrated CapAmm system was

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developed in this study (Figure 1). Compared to the previous configuration that requires ex-

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situ ammonia stripping,29 a flat-sheet gas-permeable membrane was inserted

into an

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electrochemical unit in the new design, thus providing a more compact and cost-effective

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means of ammonia recovery. Impacts of different operating conditions on the performance of

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this modified CapAmm process were evaluated in order to ascertain the effect of particular

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parameters on ammonia recovery and energy demand. Continuous ammonia recovery (and

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water desalination) was then carried out using the optimal operating conditions identified

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with detailed analysis of the opportunities and challenges likely to be encountered in scaling

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up the CapAmm process. The overarching goal of this study was to ascertain how to best

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implement the CapAmm process such that ammonia and water recovery were maximized and

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energy usage minimized. 5 ACS Paragon Plus Environment


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2. MATERIAL AND METHODS

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2.1. Experiment Setup. The structure of the CapAmm apparatus is shown in Figure 1. It is a

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four-chambered parallel plate reactor in which four square silicon gasket frames and three

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membranes (i.e., anion exchange membrane (AEM), cation exchange membrane (CEM) and

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flat-sheet gas membrane (FGM)) were bolted together between two acrylic end plates to

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create the anode, desalination, cathode and acid chambers respectively (Figures 1a and b).

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AEM-Type I and CEM-Type I were provided by FUJIFILM Europe and the FGM was

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obtained from Ningbo Changqi Porous Membrane Technology Co., Ltd (Ningbo, China).

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The effective contact area (Aeff) between flow electrode and the membrane was 58.0 cm2. The

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desalination chamber (~2.5 mL) consisted of a nylon spacer sheet (100-mesh, 160 mm × 70

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mm) residing within the silicone gasket (500 μm thickness) sandwiched between the AEM

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and CEM.31,

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channels and the circulation reservoirs (Figures 1a and c). Similar to the desalination chamber,

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the acid chamber also consisted of a nylon spacer sheet (100-mesh, 160 mm × 70 mm) placed

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within a silicone gasket (500 μm thickness) between the FGM and the acrylic end plate (Figure

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1a).

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2.2. Operating Conditions. In this study, aqueous solutions for the saline wastewater and

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flow-electrode electrolyte were prepared using 18.2 MΩ cm Milli-Q water (Millipore) and

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NaCl and NH4Cl solutes (>99.0%, ACS grade) from Sigma Aldrich. The effects of

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operational parameters (carbon contents, current densities, HRTs and operational modes)

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were first investigated with the CapAmm operated in single-pass mode with synthetic dilute

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wastewater containing ~1000 mg L−1 NaCl and ~43 mg L−1 NH4+-N (similar to that of real

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AnMBR effluent) running through the desalination chamber at different flow rates with

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corresponding hydraulic retention times (HRT) of 0.98, 1.48, 1.96, and 2.94 min. The

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electrical conductivity of the treated wastewater was continuously monitored with the use of

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The flow electrodes were continuously cycled between the graphitic flow


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a conductivity meter (CON-BTA, Vernier) connected to a data acquisition system

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(SensorDAQ, Vernier). To evaluate the effect of carbon content on CapAmm performance,

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three carbon mass loadings (of 2, 5, and 10 wt%) were chosen and tested. Before all

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experiments, the flowable electrodes were continually mixed using a magnetic stirrer to

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achieve homogeneous carbon suspensions.32 A 40 mL receiving solution (0.5 M sulfuric acid)

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was continuously recirculated at 50 mL min−1 between the acid chamber and a stirred acid

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reservoir (Figure 1a).

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In this study, ammonia removal and recovery in CapAmm were carried out at a constant

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current density (1.8-10.4 A m−2) using a DC power supply. The performance in two

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operational modes, i.e., isolated closed-cycle (ICC) and short-circuited closed-cycle (SCC)

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modes (Figure 1c) were compared as reported previously.33-35 In ICC mode, 40 mL positively

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and negatively charged flow electrodes were individually recirculated between the CapAmm

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cell and two stirred reservoirs. A 2-hour charging stage was followed by reverse-current

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discharging (~0.3 h) to regenerate the electrodes and recover part of the energy. In SCC mode,

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80 mL carbon suspension was pumped in parallel through the anodic and cathodic flow

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channels and then returned and mixed in the reservoir with this approach resulting in charge

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neutralization and regeneration of the electrodes (Figure 1c). However, the acid and base

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produced by Faradaic reactions in the flow-electrode chamber will also be neutralized,

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resulting in a stable solution pH of around 7. Therefore, when operated in SCC mode, 0.5 mL

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2 M NaOH solution was periodically added into the flow electrodes to maintain an average

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pH value around 11 (a similar pH to that achieved in ICC mode). Control experiments were

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conducted under similar conditions but without addition of NaOH solutions.

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Real wastewater collected from a local AnMBR wastewater treatment plant was then

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treated by CapAmm under the optimal conditions obtained from the studies on synthetic

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wastewater. The water quality of the real wastewater used is provided in Table S1. 7 ACS Paragon Plus Environment


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Meanwhile, the possibility of ammonia recovery from high-strength wastewaters (in this case

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synthetic urine) using CapAmm was also tested. In order to achieve better removal/recovery

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performance, lower current density (27.2 A m−2) and longer HRT (5.88 min) were applied.

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Finally, continuous operation of the CapAmm was conducted in ICC mode over 50

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hours (25 cycles) of operation. Each cycle involved the sequence of (i) 2-h charging for

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electrosorption at a constant current density, (ii) regeneration of the electrodes by mixing the

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charged slurry and replacement of part of the electrolyte after sedimentation, and (iii) equal

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division of the mixed electrodes into the reservoirs. The analytical methods and calculations

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are presented in Supporting Information.

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3. RESULTS AND DISCUSSION

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3.1. Integrated Design of CapAmm. In this study, we developed an innovative and compact

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electrochemical system for ammonia recovery (and N-based fertilizer production) from

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wastewaters. Graphite plates carved with cut-through serpentine flow path channels (3 mm

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wide and 3 mm deep) (Figure 1d) were used as current collectors. This design allows efficient

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electron transfer between the flow electrodes and current collectors on the side walls of the

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flow channels. Meanwhile, in the cathode chamber, the flow electrode can readily contact

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with both a CEM and flat-sheet gas membrane (FGM) in the flow channels with the

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arrangement providing an effective pathway for ammonium migration from the desalination

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chamber to the cathode chamber followed by transport into the acid chamber during in-situ

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membrane stripping (Figure 1b). The FGM used in this study was comprised of a thin

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expanded polytetrafluoroethylene (ePTFE) active layer and a non-woven fabric support layer with

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a total membrane thickness of 80 μm. The average pore size and porosity of the ePTFE active

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layer were 0.45 µM and 90% (Figure 1e), respectively. The FGM was highly hydrophobic

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with a contact angle of approximate 127.8 degrees (Figures 1f and g), indicating the ability of

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allowing the migration of ammonia across the membrane whilst preventing the permeation of 8 ACS Paragon Plus Environment

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water, dissolved ions and carbon particles, thus guaranteeing the production of high purity

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ammonia in the receiving solution.

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With regard to the essential transformation of ammonium ions to volatile ammonia

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(NH3) in the cathode chamber prior to the recovery process (Figure 1b), at room temperature

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(i.e., 293 K), pH is the key factor determining the fraction of NH3 in the total ammonia

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nitrogen (TAN) present ([NH3]/[TAN] = 10pH/(e21.3+10pH)).36 While elevated pHs (pH >11)

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have been reported in the recirculation tank of the cathode in CapAmm as a result of

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concomitant Faradaic reactions,37 it should be noted that the local pH near the current

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collector (and flow channels) can be much higher than that of the bulk carbon suspension in

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the recirculation tank. Therefore, it is expected that the integrated design will favour the

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transformation and subsequent recovery of ammonia in-situ.

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3.2. Optimisation of CapAmm System

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3.2.1. The Effects of Carbon Content in Flow Electrode. Figure 2 and Figures S1-5

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summarise the ion removal and ammonia recovery performance using flow electrodes of

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different carbon contents. Under a constant charging current density of 6.8 A m−2 and HRT of

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1.48 min, an increase in the carbon content has a positive effect on ion removal with much

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lower steady-state effluent conductivity (1) in the influent stream.45 In contrast, our

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CapAmm cell only required 20.6 kWh kg−1 N of electrical energy under optimal operating

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conditions. The average electrical energy consumption for desalination of the wastewater was

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only 0.54 kWh m−3, much lower than other similar technologies in treating dilute streams.

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When it comes to handling high-strength wastewaters such as urine, digestate, landfill

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leachate and animal waste, there are a number of competitive approaches. These methods

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generally include an electrochemical unit (i.e., bioelectrochemical systems (BESs),

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electrodialysis or capacitive deionization) to transport ammonium ions towards the cathode

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(where pH automatically increases without base addition) and an ammonia separation unit

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(i.e., air stripping system, gas membrane contactor or air cathode) to selectively recover

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ammonia. BES-based processes (i.e., MFC, MEC and MDC), which produce electricity from

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the microbial oxidation of organic matter, concentrate ammonia in the cathode enabling

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ammonia recovery at extremely low energy consumption (< 5 kWh kg−1 N).24, 15 ACS Paragon Plus Environment

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These


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technologies have emerged as a promising pathway for simultaneously treating wastewater

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(COD removal), harvesting electricity and recovering resources. The low ammonia nitrogen

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flux obtained in MFCs can be increased by converting the system to a MEC as a result of the

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higher bio-current generated.26 Other non-biological electrochemical systems (ESs) are capable of

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providing higher current densities (10-100 A m−2) and enabling larger nitrogen fluxes (>100 g N

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m−2 d−1).49, 50 Electricity energy demand and nitrogen flux for these ESs can range from 6 to 25

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kWh kg−1 N, depending on the applied current densities (Figure S12) and wastewater type. It

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should be noted that the ammonia separation unit will also require either extra chemicals

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(such as sulfuric acid) or additional electricity consumption. Ammonia separation via a gas

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membrane contactor is expected to be a better option due to the fact that only small amounts

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of additional chemicals are required, whereas air stripping not only needs sulfuric acid for

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ammonia adsorption but requires extra electrical energy for aeration. Our CapAmm system,

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which combines FCDI with a gas membrane, exhibited a competitive performance with other

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similar alternatives; at a current density of 27.2 A m−2, an electrical energy demand of 7.8

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kWh kg−1 N was required with an ammonia flux of 264 g N m−2 d−1 and recovery efficiency

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of 58%.

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4. ENVIRONMENTAL IMPLICATIONS

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Domestic wastewater is increasingly being considered as a potential resource (i.e., a source of

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energy, fertilizing nutrients and reclaimed water) rather than as a waste. One approach that

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appears feasible involves the use of AnMBR for COD removal and biogas production

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(energy recovery) followed by the use of CapAmm for nitrogen and phosphate capture

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(fertilizing nutrients recovery) and salt removal (water recovery). While this approach is

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relatively complex and the electric energy input high, it does enable maximum resource

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recovery

from


wastewaters.

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An alternative approach we consider worthy of further investigation involves the

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recovery of nitrogen and phosphate directly from high-strength streams (e.g. source separated

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urine) using CapAmm followed by biological treatment (e.g. AnMBR, A/A/O) for organic

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matter degradation. This approach appears more attractive because the high ammonia

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concentrations and low concentrations of competing ions (Na+, K+ and Ca2+) in urine result in

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much lower electrical energy consumption.21, 27, 53 In this scenario, as most of the nutrients from

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urine (accounting for ~80% of the nutrient input in municipal WWTP) have been recovered before

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the waste stream enters the WWTP, it may beneficially impact the cost of domestic wastewater

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treatment as there will be little need for further N removal.

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One major limitation of the FCDI approach is the poor electrical conductivity of the

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carbon flow electrodes (several orders of magnitude lower than conventional static

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electrodes)54, 55 with this restricting the ion removal/recovery efficiency and increasing the

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internal resistance and, ultimately, resulting in higher energy consumption. In addition,

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pumping of the flow carbon particles consumes energy, which must be taken into account in

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the overall energy budget of the technology.56 Scale-up from small bench size units to full-

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sized modules will also obviously be required. It is envisaged that CapAmm cells could be

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stacked together to provide the required throughput but challenges will need to be overcome,

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particularly with regard to optimizing flow electrode composition (i.e., electrolyte, carbon

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loading and active electrode materials), flow cell design (i.e., channel geometry and current

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collector materials) and operating strategy (i.e., slurry flow rate, charging/discharging time

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control). Of particular attraction however is the low voltage, direct current requirements of

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the technology rendering it suitable for direct use of photovoltaic power supply with

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associated reduction in energy costs.

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ASSOCIATED CONTENT

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Supporting Information

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The Supporting Information is available: Details on the analytical methods and calculations,

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and further experimental data such as temporal variation of effluent conductivity using

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different carbon content, temporal variation of cathode pH in different operation scenario,

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variation of NH3-N (and NH4+) concentration in cathode chamber and acid chamber,

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distribution of ammonia, treatment of real domestic wastewater, urine and long-term

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operation results of CapAmm (Figure S1-12 and Table S1).

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AUTHOR INFORMATION

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Corresponding Author

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*Phone/fax: +61-2-9385-5060. E-mail: [email protected]

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ORCID:

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Changyong Zhang: 0000-0002-9288-627X

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Jinxing Ma: 0000-0002-5087-3972

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T. David Waite: 0000-0002-5411-3233

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGEMENT

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We gratefully acknowledge funding support from the Australian Research Council through

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ARC Linkage Grant LP170101180. Support for Dr Jinxing Ma through a UNSW Vice

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Chancellor’s Postdoctoral Fellowship is gratefully acknowledged.


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25. Zamora, P.; Georgieva, T.; Ter Heijne, A.; Sleutels, T. H. J. A.; Jeremiasse, A. W.; Saakes, M.; Buisman, C.

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bio-electroconcentration for recovery of nutrients from urine. Environ. Sci. Technol. Lett. 2017, 4, 119-124.

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wastewater using a novel photosynthetic microbial fuel cell (PMFC). Water Res. 2017, 108, 356-364.

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30. Zhang, C.; He, D.; Ma, J.; Tang, W.; Waite, T. D., Faradaic reactions in capacitive deionization (CDI) -

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problems and possibilities: A review. Water Res. 2018, 128, 314-330.

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batch-mode capacitive deionization. Water Res. 2015, 84, 342-9.

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electrodes for high-performance capacitive deionization. Environ. Sci. Technol. 2016, 50, 13495-13501.

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loading for water desalination by capacitive deionization. J. Mater. Chem. A 2016, 4, 3642-3647.

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CDI for brackish water softening. Environ. Sci. Technol. 2018.

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membrane contactor. Water Sci. Technol. 2011, 63, 2944.

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electricity production from anaerobic reactors containing high levels of ammonia. Bioresour. Technol. 2015,

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via bipolar bioelectrodialysis. Water Res. 2015, 85, 177-84.

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54. Xu, P.; Drewes, J. E.; Heil, D.; Wang, G., Treatment of brackish produced water using carbon aerogel-based

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capacitive deionization technology. Water Res. 2008, 42, 2605-17.

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Mater. Chem. A 2014, 2, 9313.

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556

Gogotsi, Y., Effect of oxidation of carbon material on suspension electrodes for flow electrode capacitive

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deionization. Environ. Sci. Technol. 2015, 49, 3040-7.

558



559

Figure 1. Schematic representations of (a) the integrated capacitive membrane stripping

560

process for ammonia recovery (CapAmm), (b) ammonia migration, transformation and

561

recovery in the CapAmm system, and (c) isolated closed-cycle (ICC) and short-circuited

562

closed-cycle (SCC) operation of the flow-electrode. (d) A photograph of the graphite current

563

collector carved with cut-through serpentine flow channels. (e) SEM image of the active layer

564

surface of the FGM. (f) A photograph of small water droplets on the surface of the FGM with

565

the contact angle shown in (g). AEM, CEM and FGM represent anion exchange membrane,

566

cation exchange membrane and flat-sheet gas membrane respectively.


Page 24 of 31


+

Na

+

80 60 40 20

(b)

100

Current efficiency (%)

Ion removal efficiency (%)

(a)

NH4 -N

100

90

40

80 30 70 20

60

10

50 40

0 0

2

5

10

0 0

Carbon content (wt%)

50

2

4

6

8

10

Electricity consumption (kwh kg-1 N)

Page 25 of 31

Carbon content (wt%)

567

Figure 2. The variations of (a) average removal efficiencies of NH4+ and Na+ and (b)

568

dynamic current efficiency (Λ) and electric energy consumption for ammonia recovery as a

569

function of carbon content in the flow electrode (i.e., 0 wt%, 2 wt%, 5 wt% and 10 wt%). All

570

experiments were performed in ICC mode at a constant current density of 6.8 A m−2 and HRT

571

of 1.48 min.


80

(c)

70 60 50 40 30 20 10 0 1.8

3.4

6.8

10.4

Electricity consumption (kWh kg-1 N)



100

Page 26 of 31

(d)

80

60

40

20

0 0.98

-2

1.48

1.96

2.94

HRT (min)

Current density (A m )

572

Figure 3. Fate and distribution of ammonia in the CapAmm system at different (a) current

573

densities with a constant HRT of 1.48 min and (b) HRTs with a constant current of 6.8 A m−2.

574

Variations of electric energy consumption as a function of (c) current densities at constant

575

HRT of 1.48 min and (d) HRTs at a constant current density of 6.8 A m−2.


Page 27 of 31


13

(a) ICC

12

pH

11 SCC_pH adjustment

10 9 8

SCC_no pH adjustment

7 6 0

20

40

60

80

100

120

(c)

ICC SCC_pH adjustment SCC_no pH adjustment

2.5

Voltage (V)

2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 0

20

40

60

80

100 120 140


Time (min) 3.0

80

(d)

60

40

20

0

Time (min)

ICC

SCC_pH adjustment

SCC_no pH adjustment

576

Figure 4. The variations of (a) pH in the cathode (or pH in the combined flow electrode in

577

SCC mode), (b) ammonia distribution, (c) cell voltage and (d) electrical energy consumption

578

in three different operation modse (i.e., ICC mode and SCC operation with/without pH

579

adjustment of the flow electrode). Experimental conditions: carbon content = 5 wt%, current

580

density = 6.8 A m−2 at the charging stage (current density = -6.8 A m−2 at the discharging stage,

581

if applicable) and HRT = 1.48 min.



(a)

(b)

582 583

Figure 5. A comparison of (a) proportion of major cation ions transported across the CEM

584

and (b) CEM nitrogen flux, FGM nitrogen flux, energy consumption and current efficiency

585

when treating three different wastewaters (i.e., synthetic domestic wastewater, real domestic

586

wastewater and synthetic urine) using CapAmm system.

587


Page 28 of 31

Page 29 of 31


2.0

(a)

Voltage (V)

1.6 1.2 0.8

Cathode pH

0.4

(b)

12 10 8

NH3-N (and NH4+-N) (mg L-1)

6

588

4000

(c)

3000 2000 1000 0 0

400

800

1200

1600

2000

2400

2800

3200

Time (min)

589

Figure 6. The variation of (a) cell voltage, (b) cathode pH and (c) ammonia concentration in

590

the acid chamber during the 25 cycles (50 hours in total) operation. The shade area represents

591

the pH range >9.3, indicating the deprotonation of NH4+ to NH3. Experimental conditions:

592

carbon content = 5 wt%, current density = 6.8 A m−2 at the charging stage and HRT = 1.48

593

min.



594 595

Table 1 Comparison of energy consumption and ammonia removal/recovery performance using different methods aimed at ammonia removal/recovery from wastewaters. Methods Electrochemical oxidation Electrochemical oxidation CDI-like electrochemical reactor CapAmm CapAmm CapAmm CapAmm (with energy recovery)

Synthetic domestic Synthetic domestic

Current density (A m−2) 250 30

Nitrogen flux across CEM (g N m−2 d−1) n.a. n.a.

NH4+-N removal/recovery efficiency (%) ~100 95

Electricity consumption for ammonia recovery (kWh kg−1 N) 383.0 39.9

Electricity consumption for water treatment (kWh m−3) a 64.3 1.9

Synthetic domestic

2.1

7.8

Continuous Ammonia Recovery from Wastewaters Using an

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