Article pubs.acs.org/est
Cite This: Environ. Sci. Technol. 2018, 52, 14275−14285
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. 2018.52:14275-14285. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 01/12/19. For personal use only.
UNSW Water Research Centre, School of Civil and Environmental Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia ‡ College of Environmental Science and Engineering, State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Shanghai, 200092, P. R. China § Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, P. R. China S Supporting Information *
ABSTRACT: We have previously described a novel flowelectrode capacitive deionization (FCDI) unit combined with a hydrophobic gas-permeable hollow fiber membrane contactor (designated “CapAmm”) and presented results showing efficient recovery of ammonia from dilute synthetic wastewaters (Zhang et al., Environ. Sci. Technol. Lett. 2018, 5, 43−49). We extend this earlier study here with description of an FCDI system with integrated flat sheet gas permeable membrane with comprehensive assessment of ammonia recovery performance from both dilute and concentrated wastewaters. The integrated CapAmm cell exhibited excellent ammonia removal and recovery efficiencies (up to ∼90% and ∼80% respectively). The energy consumptions for ammonia recovery from low-strength (i.e., domestic) and high-strength (i.e., synthetic urine) wastewaters were 20.4 kWh kg−1 N and 7.8 kWh kg−1 N, respectively, with these values comparable to those of more conventional alternatives. Stable ammonia recovery and salt removal performance was achieved over more than two days of continuous operation with ammonia concentrated by ∼80 times that of the feed stream. These results demonstrate that the integrated CapAmm system described here could be a cost-effective technology capable of treating wastewaters and realizing both nutrient recovery and water reclamation in a sustainable manner.
1. INTRODUCTION While nitrogen gas constitutes about 78% of the terrestrial atmosphere, the availability of its active forms (i.e., ammonium, nitrite, and nitrate) as essential nutrients for agricultural plant growth and other anthropogenic uses is limited.1 The invention of the Haber−Bosch process in the early twentieth century enabled the manufacture of ammonia (an important raw material for N-based fertilizers) on an industrial scale with the ready availability of this essential nutrient dramatically enhancing global agricultural productivity and consequently economic growth.1−3 However, the Haber−Bosch process involves the chemical combination of nitrogen and hydrogen gas under high temperature (∼500 °C) and high pressure (200−300 atm) conditions, accounting for about 2% of the world energy consumption and 1.6% of global greenhouse gases emissions.4,5 With the continuous increase in world population and improvement in living standards, the demand for nitrogen fertilizer is projected to increase by 50% by 2050.6 Therefore, the development of more sustainable, efficient, and economic routes for the production of ammonium-based fertlizers is of great significance.7−9 From a lifecycle perspective, most food proteins are degraded to ammonium ions and/or urea in metabolic © 2018 American Chemical Society
processes and end up in sewage. It is reported that the waste stream accounts for 18−21% of the N-fertilizers that are produced by industrial nitrogen fixation10 with the discharge of sewage containing even dilute concentrations of nitrogen into waterbodies creating various negative environmental impacts including the deterioration of water quality and loss of biodiversity.3,11−13 While conventional wastewater treatment plants might take advantage of nitrification/denitrification processes to convert reactive nitrogen back to N2, this practice essentially attempts to reverse the ammonia production process from which most of the sewage nitrogen is derived.14 Therefore, processes that enable the direct recovery of ammonia from sewage/urine and effectively close the loop between fertilizer production and wastewater treatment rather than using N2 as an intermediate have attracted increasing attention in view of the potential enormous economic and environmental benefits. Received: Revised: Accepted: Published: 14275
May 22, 2018 November 17, 2018 November 21, 2018 November 21, 2018 DOI: 10.1021/acs.est.8b02743 Environ. Sci. Technol. 2018, 52, 14275−14285
Article
Environmental Science & Technology
Figure 1. Schematic representations of (a) the integrated capacitive membrane stripping process for ammonia recovery (CapAmm), (b) ammonia migration, transformation, and recovery in the CapAmm system, and (c) isolated closed-cycle (ICC) and short-circuited closed-cycle (SCC) operation of the flow-electrode. (d) A photograph of the graphite current collector carved with cut-through serpentine flow channels. (e) SEM image of the active layer surface of the FGM. (f) A photograph of small water droplets on the surface of the FGM with the contact angle shown in (g). AEM, CEM, and FGM represent anion exchange membrane, cation exchange membrane, and flat-sheet gas membrane, respectively.
consumption ranging from 10 to 30 kWh kg−1 N.5,19−23 Bioelectrochemical systems (BES), which use bacteria at the anode to oxidize organic matter and produce electricity, can recover ammonia at a lower energy demand (99.0%, ACS grade) from Sigma Aldrich. The effects of operational parameters (carbon contents, current densities, HRTs, and operational modes) were first investigated with the CapAmm operated in single-pass mode with synthetic dilute wastewater
3. RESULTS AND DISCUSSION 3.1. Integrated Design of CapAmm. In this study, we developed an innovative and compact electrochemical system for ammonia recovery (and N-based fertilizer production) from wastewaters. Graphite plates carved with cut-through serpentine flow path channels (3 mm wide and 3 mm deep) 14277
DOI: 10.1021/acs.est.8b02743 Environ. Sci. Technol. 2018, 52, 14275−14285
Article
Environmental Science & Technology
Figure 2. Variations of (a) average removal efficiencies of NH4+ and Na+ and (b) dynamic current efficiency (Λ) and electric energy consumption for ammonia recovery as a function of carbon content in the flow electrode (i.e., 0 wt%, 2 wt%, 5 wt%, and 10 wt%). All experiments were performed in ICC mode at a constant current density of 6.8 A m−2 and HRT of 1.48 min.
efficiencies of the CapAmm system were relatively low (63.7% and 71.9%) at carbon contents of 0 wt% and 2 wt% but rose to higher values (∼90%) at 5 wt% and 10 wt% (Figure 2b). Meanwhile, the cathode pH can increase to higher values than the pKa of NH4+ (9.3) in all cases (Figure S2), leading to the successful recovery of ammonia in the acid chamber (Figures S3 and S4). Although our investigation into the fate of ammonia revealed that the majority was recovered as ammonium sulfate in the acid chamber in all cases, the use of higher carbon contents led to a slight decrease in recovery efficiency due to the adsorption/trapping of ammonia on the carbon particles (Figure S4). In addition, an increase in the carbon content significantly reduced the cell voltage and electrical energy consumption (Figure S5). Specifically, the control experiment at 0 wt% carbon (similar to the electrodialysis process) exhibited a high electrical energy consumption of 45.2 kWh kg−1 N but decreased to 35.8, 24.8, and 21.2 kWh kg−1 N at carbon contents of 2 wt%, 5 wt%, and 10 wt%, respectively (Figure 2b). Overall, it can be concluded that increasing the carbon content enhances the ion removal and ammonia recovery efficiency in the integrated CapAmm system with this effect largely attributed to the increased extent of direct contact between carbon particles that is achieved at the higher carbon loadings. Indeed, it has been reported that the formation of a conductive 3D interconnected particle network at high carbon densities in flow electrodes can facilitate electron transfer and decrease the internal resistance of the whole cell.38,39 Note that using 5 wt% carbon loading achieved very similar performance to that of 10 wt% carbon (Figure 2). Therefore, in view of the low residual NH4+−N retained on the carbon particles, superior current efficiency, and lower risk of pipe and flow channel blocking, a flow-electrode with a 5 wt% carbon content was used in following experiments unless otherwise stated. 3.2.2. Effects of Current Density and HRT. Figure S6a and b clearly show that both current density and HRT have a positive effect on ion removal, with higher NH4+ removal efficiency achieved than Na+ due to the fact that NH4+ has a slightly smaller hydrated radius (3.31 Å) than Na+ (3.58 Å).40,41 More importantly, Faradaic reactions such as O2 reduction occurring in the cathode flow electrode resulted in the cathodic pH increasing to values higher than the pKa of NH4+ thus
(Figure 1d) were used as current collectors. This design allows efficient electron transfer between the flow electrodes and current collectors on the side walls of the flow channels. Meanwhile, in the cathode chamber, the flow electrode can readily contact with both a CEM and FGM in the flow channels with the arrangement providing an effective pathway for ammonium migration from the desalination chamber to the cathode chamber followed by transport into the acid chamber during in situ membrane stripping (Figure 1b). The FGM used in this study was composed of a thin expanded polytetrafluoroethylene (ePTFE) active layer and a nonwoven fabric support layer with a total membrane thickness of 80 μm. The average pore size and porosity of the ePTFE active layer were 0.45 μM and 90% (Figure 1e), respectively. The FGM was highly hydrophobic with a contact angle of approximate 127.8° (Figure 1f and g), indicating the ability of allowing the migration of ammonia across the membrane while preventing the permeation of water, dissolved ions, and carbon particles, thus guaranteeing the production of high purity ammonium salts in the receiving solution. With regard to the essential transformation of ammonium ions to volatile ammonia (NH3) in the cathode chamber prior to the recovery process (Figure 1b), at room temperature (i.e., 293 K), pH is the key factor determining the fraction of NH3 in the total ammonia nitrogen (TAN) present ([NH3]/[TAN] = 10pH/(e21.3 + 10pH)).36 While elevated pHs (pH > 11) have been reported in the recirculation tank of the cathode in CapAmm as a result of concomitant Faradaic reactions,37 it should be noted that the local pH near the current collector (and flow channels) can be much higher than that of the bulk carbon suspension in the recirculation tank. Therefore, it is expected that the integrated design will favor the transformation and subsequent recovery of ammonia in situ. 3.2. Optimization of CapAmm System. 3.2.1. Effects of Carbon Content in Flow Electrode. Figures 2 and S1−S5 summarize the ion removal and ammonia recovery performance using flow electrodes of different carbon contents. Under a constant charging current density of 6.8 A m−2 and HRT of 1.48 min, an increase in the carbon content has a positive effect on ion removal with much lower steady-state effluent conductivity (
