Chemical Engineering Journal 472 (2023) 144855 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Review Bioelectrochemical ammonium recovery from wastewater: A review Mariella Belén Galeano, Mira Sulonen, Zainab Ul, Mireia Baeza, Juan Antonio Baeza, Albert Guisasola * GENOCOV, Departament d’Enginyeria Química, Biològica i Ambiental, Escola d’Enginyeria, Universitat Autònoma de Barcelona, 08193 Bellaterra (Cerdanyola del Vallès), Barcelona, Spain A R T I C L E I N F O A B S T R A C T Keywords: Due to the anticipated rise in demand for ammonia, the search for viable methods for its recovery has intensified Ammonia in recent years, as traditional ammonia production is a high energy intensive process. Bioelectrochemical systems Ammonium (BESs) offer an alternative solution for ammonia recovery since they have shown lower energy demand (in terms Bioelectrochemical systems of kJ g− 1N recovered) at lab-scale than other methodologies. In BESs, the bioelectrochemically generated current Nitrogen recovery Total ammoniacal nitrogen (TAN) drives the transport of NH+ 4 from the wastewater to a concentration chamber through a cation exchange membrane before its subsequent recovery. This paper describes the fundamentals and opportunities for bio­ electrochemical ammonia recovery (either by stripping, absorption, or precipitation) in different BES devices such as microbial fuel cells, microbial electrolysis cells, microbial desalination cells and bioelectroconcentration cells and compares the performance of all the reported experimental works so far. Moreover, the most critical challenges (low current density, nature and quantity of the carbon source, inlet ammonium concentration, use of membranes, energy yield and recovery efficiency) have been detailed and discussed in view of better under­ standing the current bottlenecks for its scale-up and, thus, for its prompt industrial adoption. 1. Introduction been appointed as a zero-carbon molecule that can provide the required energy storage medium for renewable sources [4]. NH3 can be stored The fundamental transformation of wastewater treatment towards a under mild temperature and pressure conditions, similarly to other circular economy scenario creates a demand for efficient recovery of current commercial fuels; it is easier to liquefy than hydrogen, the costs energy and nutrients such as nitrogen and phosphorus. Ammoniacal for its transportation and storage are lower, and liquid NH3 has a greater nitrogen in water causes eutrophication and is toxic in its non-ionized volumetric hydrogen density than liquid hydrogen itself [5,6]. NH3 form (ammonia, NH3). The combined costs related to its removal, already has a market of nearly 200 million tons/yr, making it the second health issues and climate change impact could amount to a total of €320 most commercialized chemical in the world. Therefore, infrastructure at billion/y [1]. Ammoniacal nitrogen is paradigmatic, since its recovery is every scale is available for immediate implementation of an “NH3 promising from both an energetic and material point of view. First, economy”. ammonia-based fertilizers increase crop yields and ensure efficient food In urban and industrial wastewater, inorganic nitrogen appears production. In the context of global population growth, the demand for mainly in two forms, NH3 and NH+ 4 , the sum of which constitutes total fertilizers is also expected to grow. Nowadays, most of the NH3 is still ammoniacal nitrogen (TAN). The different approaches used to remove produced through the energy-intensive Haber-Bosch process that uses and recover TAN from wastewater can be classified into chemical, harsh conditions. High temperature (typically 400–650 ◦ C) and pressure physical and biological processes. TAN can be removed chemically e.g. (100–400 bar) are required to activate the reaction between hydrogen via chlorination [7], electrochemical oxidation [8] and photo­ and nitrogen to produce NH3 in the presence of an iron catalyst, enabling electrocatalysis [9]. Struvite precipitation has also been proposed for it to occur at a rate that is economical for its industrial production [2]. In nitrogen removal and recovery [10], since it has an added-value as a 2020, NH3 production reached 235 million tones and accounted for 8.6 slow-release fertilizer. However, its technoeconomic feasibility is often EJ (8.6⋅1018 J) of energy consumption, equivalent to 2% of the total limited by the high reagent costs due to the unbalanced presence of 3− 2+ energy consumed globally [2,3]. On the other hand, NH3 has recently required ions (NH+ 4 , PO4 and Mg ) in the wastewater. * Corresponding author. E-mail address: albert.guisasola@uab.cat (A. Guisasola). https://doi.org/10.1016/j.cej.2023.144855 Received 23 May 2023; Received in revised form 11 July 2023; Accepted 15 July 2023 Available online 20 July 2023 1385-8947/© 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). M.B. Galeano et al. Chemical Engineering Journal 472 (2023) 144855 The physical processes for nitrogen removal include ammonia through an external circuit and are received by the cathode. At the stripping [11], reverse osmosis [12], ion exchange [13], electrodialysis cathode, a reduction reaction occurs (usually a chemical reaction), and [14] and the treatment using gas-permeable membranes [15,16]. These the electrons are donated to a terminal electron acceptor. physical processes allow efficient ammonia recovery but are resource BESs used for TAN recovery can be divided into four categories based and energy intensive. For instance, the optimum boiling point for NH3 on their operating principle (Fig. 1). If the oxidation and reduction re­ stripping in the case of vacuum thermal stripping has been determined actions result in a negative Gibbs energy variation, the flow of electrons to be 65 ◦ C at 25.1 kPa, and the liquid needs to be adjusted to a pH of becomes spontaneous and electrical energy is produced. This system is 9.23 to stripping out more than 95% [17]. called Microbial Fuel Cell (MFC). Oxygen is the most used cathodic Biological removal methods are sustainable and allow TAN removal electron acceptor in MFCs due to its high reduction potential and with lower capital and operational costs than physical or chemical availability. methods. The main biological processes for TAN removal include nitri­ In Microbial Electrolysis Cells (MECs), the occurring semi-reactions fication–denitrification [18], nitritation/denitritation or shortcut ni­ lead to a positive Gibbs energy variation and hence an external energy trogen removal [19], and partial nitritation [20] plus anammox [21,22]. is required to drive the oxidation and reduction reactions. The appli­ Finally, the use of bioelectrochemical systems (BES) [23,24] allows TAN cation of external energy allows the boosting of the electric current and removal and recovery without high energy demand or dosage of enables e.g. the generation of hydrogen at the cathode. Therefore, some chemicals to reach a favorable pH. of the external energy applied can be recovered as hydrogen. BES are biologically-catalyzed electrochemical systems that contain Similarly to MFCs, in microbial desalination cells (MDCs) the current two connected electrodes (an anode and a cathode). BESs have gained generation is spontaneous. In these systems, a third desalination importance in the fields of bioenergy and biorefinery as they can employ chamber placed between the anodic and cathodic chambers, which is microorganisms as biocatalysts and use solid electrodes in electro­ separated from them with a pair of ion exchange membranes (IEMs), chemical cells to supply or extract electrons driving versatile biochem­ usually with a cation exchange membrane (CEM) facing the cathode side ical reactions producing chemicals and fuels from CO2 or consuming and an anion exchange membrane (AEM) facing the anode side. The organic waste as carbon sources [25]. The biological catalysts are electric potential gradient created by the electroactive bacteria drives electroactive microorganisms capable of exchanging (i.e. transfer or ion transport through these membranes. uptake) electrons with a solid electrode. For instance, exoelectrogenic Bioelectroconcentration cells (BECs) also incorporate a third cham­ microorganisms (i.e., those able to transfer electrons extracellularly to a ber separated with IEMs and the electric field drives charged species solid anode) act as biocatalysts in the oxidation of readily biodegradable through these ion-selective membranes, but in contrast to MDCs, in BECs organic compounds. The electrons released in the oxidation travel external energy is applied and the ions are concentrated in the middle Fig. 1. Different bioelectrochemical systems for ammonium recovery: A) Microbial Fuel Cell; B) Microbial Electrolysis Cell; C) Microbial Desalination Cell, and D) Bioelectroconcentration cell. 2 M.B. Galeano et al. Chemical Engineering Journal 472 (2023) 144855 chamber [26,27]. charge balance as the driving force. Anodic organic matter oxidation In BESs, the energy stored in the biodegradable organic contami­ also releases protons that acidify the anolyte (Eq. (1)) whereas the nants of wastewaters can be converted into electrical energy that can cathodic chamber tends to become alkaline because of the cathodic drive the transport of TAN, with simultaneous production of other hydroxyl-producing reduction reactions (Eqs. (2)–(3)). The pH gradient valuable resources such as electricity or hydrogen [28,29]. Moreover, over the membrane is beneficial for TAN recovery since once NH+ 4 is the process can run with minimal dosage of chemicals for pH control transported to the cathode, it is partially displaced to NH3 (pKa = 9.246 [30]. The combination of both factors allows for more energy-efficient at T = 25 ◦ C) due to the high pH and the NH+ 4 concentration gradient is TAN recovery compared to conventional recovery methods [29]. Since maintained. However, the pH gradient also increases anodic and previous reviews focusing on bioelectrochemical TAN removal and re­ cathodic overpotentials and decreases the system performance from an covery were published [31–33] advances have been made with the energetic point of view [36]. Thus, there are three fundamental steps for reactor configurations, scaling up and recovery efficiencies and new bioelectrochemical TAN recovery: 1) NH+ 4 transfer from anodic to challenges have been encountered. This review provides an overview of cathodic chamber, 2) NH+ 4 displacement to NH3 due to alkaline condi­ the current state of bioelectrochemical TAN recovery, and the funda­ tions and 3) NH3 extraction for its recovery (Fig. 2). mental mechanisms involved. Afterwards, the section Challenges and Perspectives describes the aspects that need to be addressed in this field 2.1. NH+ 4 transfer from anodic to cathodic chamber to boost bioelectrochemical TAN recovery and proposes innovative guidelines for the design and operation of these systems. The first step is the transfer from the anodic to the cathodic chamber through the CEM. The CEM consists of dense crosslinked polymer chains 2. Fundamentals of bioelectrochemical TAN recovery from fixed with negatively charged groups, which, due to the relatively small wastewater hydrated ionic size and fast diffusivity, allows NH+ 4 to be easily trans­ ported across it [35]. In this sense, Volkov et al [38] reported the hy­ In BES, the anodic chamber usually receives a feed containing TAN drated radius for different cations, with NH+ 4 (0.331 nm) being similar to and organic matter. Organic matter is oxidized and electrons are K+ and smaller than other cations such as Na+ (0.358 nm), Ca2+ (0.412 released (i.e., acetate oxidation, Eq. (1)) and transported through an nm) or Mg2+ (0.430 nm). The main mechanisms driving the trans­ external circuit to the cathode, where they are used for processes such as portation of NH+ 4 through the CEM are: i) Donnan exclusion (the fixed oxygen reduction in MFC (Eq. (2)) or hydrogen production in MECs or negatively charged groups in CEMs create a Donnan potential across the BECs (Eq. (3)). membrane which prevents anion diffusion), ii) electromigration ANODE : CH 3 COO− + 4H2 O→2HCO−3 + 9H + + 8e− (1) (ammonium ions are attracted towards the cathode due to the potential difference applied across the membrane), iii) concentration gradient CATHODE MFC : O2 + 2H2 O + 4e− →4OH − (2) (ammonium diffuses from a higher concentration chamber, i.e. the anode, to an area of lower concentration, i.e. the cathode, until equi­ CATHODE MECs/BECs : 2H2 O + 2e− →H2 + 2OH − (3) librium is reached), iv) ion exchange (CEM have mobile cations asso­ ciated with the fixed negatively charged groups that can undergo ion The electron flow induces the transport of positively charged species exchange with ammonium ions enabling, thus, their diffusion through (cations) across the CEM to maintain the charge neutrality [34]. Thus, the membrane) and v) water-mediated transport (water molecules may this process, also known as electromigration, transports the NH+ 4 con­ form hydration shells around the ammonium ions, allowing them to tained in the wastewater from the anodic to the cathodic chamber using cross the membrane via a solvation/desolvation process). The specific Fig. 2. Schematics of bioelectrochemical TAN recovery in BES. 3 M.B. Galeano et al. Chemical Engineering Journal 472 (2023) 144855 mechanism occurring depends on factors such as ammonium concen­ lower values) when NH+ 4 ions are transported across the CEM due to tration, membrane properties, pH (see section 2.2), temperature, and the their reaction with hydroxyls. presence of other ions in the solution and the efficiency of the NH3 extraction from the catholyte [31,37]. 2.3. NH3 extraction for its recovery Yang and Qin [35] reviewed the applicability of widely used com­ mercial CEMs for TAN recovery (i.e. Nafion N117, CMI-7000, CMH-PP Efficient NH3 extraction can boost the performance of the cathodic Ralex, CEM Type I and II). However, the link between NH+ 4 transport and equilibrium-based steps and, thus, TAN recovery. The more common current density is not fully understood yet. While some studies indicate techniques for NH3 extraction are stripping, transmembrane chemi­ that NH+ 4 transport dominates the current across the CEM and one sorption (TMCS) and forward osmosis [29]. An emergent alternative is NH+ 4 ion is transported per each electron passing through the external the integration of an electrode to a hydrophobic membrane forming a 4 ions may transport only 40% circuit [39], other studies suggest that NH+ gas diffusion electrode (GDE). Then, TAN can be recovered on the other of the total current [40]. Na+, K+, Mg2+, Ca2+, or NH+ 4 are the cations side of the membrane as a dissolved salt with an additional recovery that are most frequently transported, based on their concentration. So, chamber containing an acidic solution. Afterwards, NH3 can be recov­ the presence of cations other than NH+ 4 in a high concentration can ered either using acidic absorption, concentration, or precipitation. hinder TAN recovery [40]. This competition can be quantified through Stripping is achieved by sparging a gas through the TAN- the transport number (Eq. (4)), a dimensionless parameter used to concentrated solution. High performance requires high gas flow rates, directly describe the number of ions that move across the membrane. elevated temperature and high pH and, thus, considerable energy input The transport number of an ion species i is defined as the proportion of (30–90 kJ gN− 1) in addition to the need for base dosage for pH adjust­ the charge transported by i with respect to the total charge transported ment [29,47]. Therefore, the integration of a hydrophobic membrane [41,42]. with gas-permeable microporous (i.e., PTFE- or PP-based membranes) has gained attention to decrease the energy requirements. The driving ci (0) − ci (t) ti = V⋅F⋅zi ⋅ ∫ t (4) force for TAN recovery is the ammonia concentration gradient across the I dt 0 tot membrane, which is promoted by the pH difference between the cath­ olyte and the solution in the recovery chamber. The least used meth­ where ti represents the transport number of i, V is the volume of the odology in BES is forward osmosis, which is based on the application of catholyte, F is the Faraday constant, zi is the charge carried by ion i, ci (0) an osmotic pressure through a semipermeable membrane to separate and ci(t) are the concentrations at time of zero and time of t, respec­ ∫t water from dissolved ammonium. tively, and 0 Itot dt is the sum of the charge transferred by the electrons at the external circuit within the time period t. Ideally, bioelectrochemical 3. TAN recovery using MFCs TAN recovery systems should aim at a NH4+ transport number close to one [35]. TAN removal from wastewater has been studied using MFCs Furthermore, since the migration of the ions is current driven, it is [45,48,49]. NH+ 4 oxidation linked to electricity generation has been important to consider the ratio between current and TAN loading rate. already reported at lab scale [50]. However, this review focuses on TAN The load radio (LN) [43], is the ratio of the electrical current generated recovery, which has been under explored. Table 1 summarizes all the from the fed wastewater over NH+ 4 loading [28,44] (Eq. (6)). research conducted on TAN recovery using MFCs. This technology is still i⋅MTAN emerging and most experiments have been conducted at lab-scale and LN = (6) more than 80 % have used synthetic wastewater mimicking urine, pig QTAN ⋅zNH4 + ⋅F slurry or digestate. The electrical energy output, the lack of need for where i is the electrical current generated (A), MTAN the molar mass of chemicals and the low energy requirements (no voltage applied) makes TAN (14 gN/molTAN), QTAN the TAN loading into the receiving MFCs interesting for TAN recovery. However, the low solubility of ox­ compartment in gTAN⋅d− 1, Z+ NH4 the charge of NH4 being 1, and F the + ygen in water can limit its availability for electrochemical reduction Faraday constant (96485 C mol− 1). A LN lower than 1 means that the when an external stripping unit is used. To avoid this limitation, air current is too low to transport all the TAN and, thus, TAN will accu­ diffusion cathodes where oxygen diffuses through the cathode electrode mulate in the anodic chamber. A LN equal to 1 describes a situation have been used [51]. There is no difference in the materials utilized for where the TAN loading and the applied current are balanced. Finally, LN TAN recovery in MFCs (Fig. 1A) compared to those employed in other values higher than 1 mean that all the TAN and additional cations could types of MFCs. The most common anodic materials are carbon felt and be transported by the current produced. graphite felt due to their high conductivity, mechanical stability, rela­ tively low cost, surface area and porosity for providing abundant redox reaction sites [52,53]. Cathodic materials are diverse, Pt being the most 2.2. NH+ 4 transformation to NH3 used catalyst for the reduction reaction and stainless steel the alternative cheaper solution. Ammonium is a weak acid (pKa = 9.246 at 25 ◦ C) and thus partially Regarding the configuration, more than 90% of the MFC-based TAN dissociates in aqueous solution. The relative amounts of NH+ 4 and NH3 recovery studies use double-chamber configuration and the current are determined by the pH of the medium. Therefore, a pH higher than drives the NH+ 4 migration from the anode chamber to the cathode the pKa is necessary to drive NH+ 4 displacement to NH3 [45,46]. Ac­ chamber through the CEM. Taking advantage of the high cathodic pH cording to Le Chatelieur’s law, a continuous NH3 extraction would boost and of the air inlet (in an MFC, oxygen reduction is the targeted cathodic NH+ 4 displacement to NH3, which would in turn favor NH4 transfer + reaction), TAN is recovered by stripping, volatilization or by recircu­ through the CEM due to a higher concentration gradient. Hence, most of lating the highly N-loaded catholyte through an absorption column studies aim at an electrochemically driven pH that is significantly higher [47,50]. The best results in double-chamber systems are those reported than the pKa of NH+ 4 . If the applied current enables a faradaic production by Kuntke et al. [54], that achieved a TAN recovery rate (TAN RR) of of hydroxyls in the cathode (Eqs. (2) and (3)), the pH along the electrode 9.57 gN m− 2 d− 1 at a current density of 2.6 A m− 2 using undiluted urine. surface can be calculated as a function of the applied current density, They also reported an energy net yield for ammonia recovery of − 10 kJ and the diffusion of H+ and OH− . For example, the pH along the cathode gN− 1 and, resulting in a net energy yield of − 2.8 kJ gN− 1. This result at 30 µm from the surface was calculated as 13.8 for current densities of highlights the advantages of the MFC facilitated TAN recovery over the approximately 17 A m− 2 (under certain operational conditions) [37]. conventional NH3 stripping that used around 32 kJ gN− 1 [55]. The more Liu et al [40] demonstrated that the pH can be regulated (i.e. it can reach recent reports on double-chamber MFCs have not led to higher TAN RRs. 4 M.B. Galeano et al. Chemical Engineering Journal 472 (2023) 144855 Yang et al. [56] reported an integration of MFC and TAN recovery as N- Reference source for microbial protein production. This two-step process reached recovery efficiency values up to 53–61% (at initial concentrations [58] [34] [54] [62] [63] [56] [51] [59] [50] [57] around 2 g/L) depending on the initial ammonium content of the wastes. Zhang et al. [50] reported a TAN recovery efficiency (TAN RE) of 90.8% with a TAN RR of 6.9 gN m− 2 d− 1 at a current density of 1.2 A m− 2 with the use of a soluble electron mediator in the catholyte. Han et al. [57] Energy consumption (+) or reported a high TAN RE (91%) at a current of 0.416 A m− 2 and a TAN RR production (-) [kJ gN− 1] of 2.6 gN m− 2 d− 1. Similarly, Losantos et al. [51] reported a triple- chamber MFC set-up where the cathode chamber was delimited by an air diffusion cathode setup which allowed an airflow to be forced − 3 3 111.6 kJ m− 154.8 kJ m through the cathode surface, providing oxygen while simultaneously enabling ammonia stripping. This system had a TAN RE of 31.2 % with a − 3.46 7.92 − 10 n.r. n.r. n.r. n.r. n.r. n.r. TAN RR of 6.8 gN m− 2 d− 1 at a current density of 1.6 A m− 2. On the other hand, fewer studies reported the use of single-chamber TAN RR [gN MFCs for TAN recovery. Besides volatilization/stripping [34,54] or ab­ m− 2 d− 1] sorption, precipitation in a single-chamber MFC has been proposed [58]. 15.1a 9.57 12.7 3.3b For instance, struvite (MgNH4PO4) precipitation has shown successful n.r. 0.9 6.8 6.8 6.9 2.7 n.r results at lab-scale: TAN RE around 50% in addition to the possibility of TAN RE recovering other nutrients, such as phosphorus [59,60]. Zang et al. [58] a 25.37 73.94 71.1 38.4 60.5 53.6 31.2 46.0 90.8 91.6 reached current densities close to 1 A m− 2 with TAN RE higher than 70% [%] n.r and TAN RR ≥ 10 gN m− 2 d− 1 at initial concentrations greater than 1 g/L Current density when coupling a struvite precipitation reactor with a single-chamber MFC [58]. The major drawback of these systems is that, in general, [A m− 2] there is an imbalance between phosphorus and nitrogen, which leads to b 0.97a 0.50 0.15 0.02 0.07 0.05 1.60 0.02 1.20 0.42 low N recovery. To avoid this problem, the addition of P-rich material 2.6 has been proposed [61]. Power density 4. TAN recovery using MECs [W m− 2] 0.36b 0.22 0.67 0.05 0.23 0.10 0.01 0.10 0.06 n.r. n.r. The switch from an MFC to an MEC for TAN recovery changes the perspective in terms of energy consumption. In MECs (Fig. 1B), external electrical energy is applied to run the current-generating oxidation and Ammonium Ammonium Ammonium Ammonium Ammonium Ammonium Ammonia Microbial production Microbial reduction reactions and, despite this increased energy demand, MECs Struvite Struvite Product protein sulfate sulfate sulfate sulfate sulfate sulfate have several advantages: 1) higher current densities can be achieved, which increases electromigration, 2) higher catholyte pH values, pro­ moted by alkalinity generation during the reduction reaction (Eq. (3)), stimulate the transformation of NH+ 4 in NH3, 3) hydrogen production Volatilization + can assist in stripping NH3 from the catholyte and maintaining a con­ Carbon fiber brush Volatilization Methodology Precipitation Precipitation Stripping + Stainless steel mesh Stripping + Stripping + Stripping + Absorption Absorption Absorption Absorption Absorption centration gradient over the CEM and 4) the produced hydrogen gas Recovery Stripping Summary of the experimental reports on bioelectrochemical TAN recovery using MFCs. holds a higher economic value than electricity. Table 2 summarizes the performance of the MECs for TAN recovery (current density, TAN RR and TAN RE). While MECs often aim at Stainless steel mesh Pt coated stainless hydrogen production, in some of the reports from Table 2, the cell is Pt coated carbon Pt coated carbon Pt coated carbon Pt coated Ti felt Pt coated Ti felt operated as an MEC (i.e., energy is applied) to boost current density Carbon brush steel mesh despite the cathodic reaction is mainly oxygen reduction. The two most Cathode usual configurations used in MEC-based TAN recovery are double- paper fibers cloth maximum value obtained, n.r. = not reported. Electrode materials chamber MECs and triple-chamber MECs. TAN has been also recov­ ered in single chamber MECs by precipitation as struvite [64], but most Stainless steel Carbon brush Carbon brush Carbon brush Graphite felt Graphite felt Graphite felt of the studies aiming at struvite precipitation have focused on P recovery Carbon felt Carbon felt Carbon felt optimization instead of TAN recovery [65,66] due to the stoichiometric Anode mesh mesh excess of N in real wastewaters. In double-chamber MECs, NH3 is usually extracted from the cath­ TAN [g olyte via stripping using the air inlet or the produced gas, and then 1.12 2.50 0.04 1.01 1.78 0.44 0.45 Synthetic urine 0.60 L− 1 ] recovered in a separate acidic absorption column. Either the cathodic Synthetic urine 4 4 1 chamber can be aerated [40] or the catholyte can be aerated in a Human urine separate stripping column [67]. The oxygen from air can serve as the Reject and Real urine Pig slurry Synthetic Synthetic Synthetic cathodic electron acceptor allowing operation with high cathode po­ digestate domestic Influent Urine tential that leads to low external energy demand. Stripping, however, is Type energy intensive as an important amount of gas needs to be sparged through the catholyte solution. Qin et al. [68] concluded that more than 50% of the total energy consumed was for active aeration in their b Configuration/ calculated, tubular double-chamber MEC. While TAN removal rates have been re­ Chambers ported to increase up to 173 gN m− 2 d− 1 [24], the highest TAN RR re­ Double Double Double Double Double Double Double Double Table 1 Single Triple ported in MEC has been 70 gN m− 2 d− 1 obtained by Carucci et al. [69] in a double-chamber MEC with TAN recovered via stripping and a 5 M.B. Galeano et al. Table 2 Summary of the experimental reports on bioelectrochemical TAN recovery using MECs. Configuration/ Influent Electrode materials Recovery Current TAN RE TAN RR [gN Energy Hydrogen Reference Chambers density [A [%] m− 2 d− 1] consumption [kJ production [m3 H2 Type TAN [g Anode Cathode Methodology Product m− 2] gN− 1] m− 3 d− 1] L− 1] Double Synthetic 1 Carbon felt Steel wire Stripping + Ammonium 1.6 94.0 45.9 35.6 2.48 [73] Absorption chloride Triple Synthetic 1 Carbon felt Air diffusion Stripping + Ammonium 2.5 45.3 9.6 7.92 n.p. [51] electrode Absorption nitrate Double Diluted urine 0.70 Graphite felt Platinum Stripping + Ammonium 23.1 n.r. n.r. n.r. 48.6 [24] Absorption borate complex a Double Pig slurry 2.12 Carbon felt Stainless steel Stripping + Ammonium 0.06 94.3 24 n.r. n.p. [62] mesh mesh Absorption sulfate Double Synthetic 5.1 Carbon felt Stainless steel Stripping + Ammonium 27 n.r. n.r. 21.7 n.r. [67] mesh Absorption sulfate Double Synthetic 0.39 Carbon brush Pt/C coated Stripping + Ammonium 1.89b 66.2 10.2 n.r. n.p. [40] carbon cloth Absorption sulfate Triple Digestate 1.9 Carbon felt Stainless steel TMCS Ammonium 1.4 89 66 n.r. n.r. [72] mesh sulfate Double Digestate 1.9 Carbon felt Stainless steel TMCS Ammonium 0.61 n.r. 16.8 n.r. n.r. mesh sulfate Double Effluent of MAP 3.40 Ti plate with Ti plate with TMCS Ammonium 1.7 31 n.r. 1.4 n.r. [74] 6 reactor MMO coating MMO coating sulfate Double Synthetic 0.78 Carbon brush Pt/C coated Stripping + Ammonium 1 83 7.1 16.2 n.p. [75] carbon cloth Absorption sulfate − 3 Double Synthetic 1 Granular Granular Concentration Ammonium in 19 A m 92 10.38 8.3 n.p. [76] graphite graphite catholyte Double Synthetic 0.78 Carbon brush A/C coated Stripping + Ammonium 0.01 Ab 90.1 n.r. 4.68 n.r. [68] Carbon cloth Absorption sulfate Double Digestate 1.50 Carbon felt Granular TMCS Ammonium 3.6 n.r. 6.72 18 n.p. [77] graphite sulfate Triple Synthetic 1 Carbon brush Ni-based GDE TMCS Ammonium 25.5 n.r 36.2 5.79 0.2 [16] sulfate Double Synthetic 2.50 Graphite felt Stainless steel Stripping + Ammonium 9.4b n.r. 70 12.9 n.p. [69] mesh Absorption sulfate Single Synthetic 0.06 Graphite brush Stainless steel Precipitation Struvite 12 n.r. n.r. n.r. 2.3 [64] mesh Chemical Engineering Journal 472 (2023) 144855 Triple Mineral Medium 0.09 Granular Granular Concentration Ammonium 145 mA n.r n.r n.d n.r [78] graphite graphite sulfate − 1 Triple Synthetic dark 0.03 Granular Granular Concentration Ammonium 300 mA 81.0 0.7 gN d 98.85 n.r [79] fermentation graphite graphite concentrate 1 Five Synthetic 20 mM Graphite fiber Stainless steel Concentration Ammonium 1.9 mA 68 0.47 gN L− 10.44 n.r [80] brush mesh sulfate a b calculated, maximum value obtained, n.r. = not reported, n.p. = not produced, A/C = Activated carbon, MMO = Mixed metal oxide, TMCS = Transmembrane chemiabsorption. M.B. Galeano et al. Chemical Engineering Journal 472 (2023) 144855 adsorption. Triple-chamber systems with membrane contactors have 5. TAN recovery using MDCs and BECs been studied as a less energy-intensive and simple alternative. In such systems, TAN is concentrated in a recovery chamber that is separated MDCs generate electrical energy like MFCs, but a distinguishing from the catholyte solution with a membrane. The recovery chamber feature is the incorporation of an additional chamber. In the conven­ needs to be separated in a way that allows efficient transfer of TAN from tional MDC configuration, the desalination chamber is placed between the cathodic chamber to the recovery chamber without this being the the anodic and cathodic chambers and is separated from the anodic limiting step and without affecting the cell performance. Two possibil­ chamber with an AEM and from the cathodic chamber with a CEM. The ities arise: the use of hydrophobic membranes and the use of GDEs. In charge imbalance between the anolyte and the catholyte solution the first case, the small pore size and the hydrophobic nature of the resulting from the current generated facilitates the transport of cations membrane prevent the liquid phase from entering the pores due to the and anions through the CEM and AEM, respectively [81]. The intro­ surface tension effect [70]. Gonzalez-Salgado et al. [71] reviewed the duction of additional membranes and chambers between the anode and fundamentals and pilot- and full-scale applications of these membranes the cathode increases the internal resistance of the system. However, the and concluded that the pre-treatment of the wastewater and the mem­ high concentration gradients between the anolyte and the desalination brane fouling/wetting are the major challenges of this technology [71]. solution, as well as between the catholyte and the desalination solution, Cerrillo et al.[72] demonstrated that a triple chamber MEC with generate junction potentials over the membranes, which can assist in hydrophobic membrane (e.g. PTFE) produced twice as much current overcoming the potential losses created by the addition of the desali­ intensity than a double chamber MEC (1.40 A m− 2 and 0.61 A m− 2, nation chamber [82]. Furthermore, since in MDCs TAN is transferred respectively). Similarly, TAN RR was significantly higher in triple from the desalination chamber to the cathodic chamber, anodic micro­ chamber MECs (36 gN m− 2 d− 1) than in double chamber (17 gN m− 2 organisms are not exposed to high concentrations of TAN, which mini­ d− 1), mainly due to a higher cathodic pH in the former scenario [72]. mizes the risk of anodic inhibition by ammonia and enables TAN Hou et al. [16] studied the use of a Ni-based GDE and obtained 40% recovery from extremely concentrated water streams [83]. In fact, the higher TAN RR (36.2 ± 1.2 gN m− 2 d− 1) and higher current density current densities obtained in MDCs for TAN recovery have been gener­ (25.5 A m− 2) than what was obtained in a control reactor with separated ally one order of magnitude higher than what has been obtained in MFCs electrode and hydrophobic membrane. Operation under applied for TAN recovery. external energy allows for increased current densities and thus the sys­ In MDCs (Fig. 1C), TAN is usually recovered from the catholyte via tems can be operated under high LN even when using wastewater with stripping followed by absorption as the cathodic pH increases and the high TAN concentration [16]. catholyte is aerated to provide oxygen as an electron acceptor. Yang TAN recovery performance in MECs can be affected by factors et al. [84] switched the position of AEM and CEM and obtained TAN RR limiting current generation (e.g., substrate composition, conductivity, of 11.5 g m− 2 d− 2 while concentrating TAN from the anolyte to the pH, inhibiting compounds) or ion transfer (e.g., competing cations, middle chamber. In addition to the traditional triple-chamber configu­ solids/compounds causing membrane fouling). Most MEC studies have ration, TAN recovery has been studied with submersible double- still been conducted using synthetic feed solutions due to the complex chamber MDCs, in which TAN is recovered from the surrounding solu­ matrix of most real streams. Operation under applied voltage or current tion [83,85]. In such systems, the AEM and CEM membranes are placed allows easy control of the system, as the amount of energy applied can be outside the anodic and cathodic chambers and no additional inner easily adjusted. However, due to the involvement of microorganisms, chamber is needed. TAN RRs in such submersible MDCs have increased BES are more subject to current fluctuations than electrochemical sys­ up to 86 gN m− 2 d− 1 [85]. The TAN RR reported in MDCS are the highest tems, which also affects the TAN recovery due to the current-driven among those BESs that do not require applied potential. electromigration. There are three different strategies for applying A BEC (Fig. 1D) is a hybrid microbial electrolysis/electrodialysis cell external electrical energy: 1) controlling electrode potential (potentio­ specifically designed to recover TAN and other ions. Like conventional static), 2) controlling cell voltage and 3) controlling the current (gal­ MDCs, BECs contain an additional chamber between the anodic and vanostatic). The external applied voltage required depends on the cathodic chambers, which is separated from the anodic chamber by a desired oxidation and reduction reactions as well as on the internal CEM and from the cathodic chamber by an AEM. External electrical resistance of the system. Values have ranged from 0.2 V with an oxygen energy is applied to generate current that drives the transport of TAN reducing cathode [51] to above 2 V with a hydrogen producing cathode and other cations from the anolyte to the concentration chamber, while [67]. Importantly, poor performance on one electrode may cause the simultaneously transferring anions (e.g. PO3−4 , HCO3 ) from the cathodic − other electrode potential to change accordingly, which could bring it out chamber to the same concentration chamber. Due to the alternation of of the range required for the desired oxidation or reduction reaction, AEM and CEM, configurations with up to six chambers have been re­ thus limiting the current intensity generated. ported. The outlet of these configurations is a TAN-concentrated solu­ In operation under constant current, electrode potentials can change tion and a TAN-free solution. The use of external energy allows the rapidly due to a variety of factors (e.g., changes in substrate concen­ operation at high current densities, which in turn makes it possible to trations) and, in the case of BES, extreme electrode potentials can reach high TAN RRs. The highest TAN RR in BESs reported so far was damage the electroactive biofilm. For example, a significant increase in reached by Ledezma et al. [27], who reported an RR of 430 gN m− 2 d− 1 the anode potential can bring the electrode potential into a range where obtained in BEC with 50 A m− 2. They demonstrated the recovery of a oxidation of water to oxygen occurs in addition to the oxidation of nitrogen-rich solid from synthetic urine (in the form of pure ammonium organic matter. This oxygen evolution can decrease the coulombic ef­ bicarbonate crystals with 17 % N content) without any chemical addi­ ficiency, trigger the anodic biofilm detachment, and enhance the growth tion. The use of external energy, however, increases the energy con­ of undesired aerobic microbial cultures, thus hindering the utilization of sumption of the system. the chemical energy of the organic matter. Undesired diffusion of TAN (as NH3) through the AEM has been re­ Similarly to MFCs, carbon-based materials are the most common as ported in both MDCs and BECs [27,86]. Such diffusion can not only anode electrodes in MECs. Cathode electrode materials, however, have decrease the TAN RE, but also lead to inhibition of the anodic biofilm by varied as the external energy addition allows the realization of different free ammonia. This transport usually results from the high concentration cathodic reduction reactions. The most commonly used catalyst for ox­ gradient over the membrane and insufficient permselectivity of the ygen reduction is Pt, while steel-based materials have been used for both membranes [27,87]. TAN can be transported through the AEM via oxygen reduction and hydrogen evolution as a cheaper alternative. facilitated diffusion in which a part of the NH+4 is converted to NH3 due Studies aiming at cathodic CO2 reduction have been conducted with to the high pH value inside the AEM. The NH3 then diffuses through the carbon-based cathodes (carbon granules). AEM and is converted back to NH+ 4 upon entering the acidic solution 7 M.B. Galeano et al. Chemical Engineering Journal 472 (2023) 144855 [87]. The transport of desired ions and the prevention of the transport of from synthetic solutions [47,98,99]. Most of electrochemical-based TAN undesired ions can be enhanced by using selective membranes that only recovery studies have a physical separation between the cathode and the allow the transport of specific ions. The use of selective CEMs can hydrophobic membrane, which can limit the diffusion of gaseous NH3 significantly increase the RE of TAN recovery from wastewaters that [100]. For instance, high TAN transport rates have been reported in a often contain multiple cations [88]. Membranes are also susceptible to purely electrochemical system such as 335 gN m− 2 d− 1 at a current fouling over time, so it is desirable to use membranes with better anti- density of 50 A m− 2 and an energy demand of 56.3 kJ gN− 1 [98]. Lee fouling characteristics. et al. [101] reported a novel electrochemical approach using GDEs with a TAN RR of 890 gN m− 2 d− 1 with synthetic livestock wastewater at a 6. Challenges and perspectives current density of 100 A m− 2 and a TAN RE close to 100%. As observed, the current produced in BES is at much lower than that produced in 6.1. Current density: Electrochemical systems vs bioelectrochemical electrochemical systems but, in return, BESs consume less energy and systems can use the electrons from a waste (wastewater treatment) rather than from a conventional anodic oxidation reaction. NH+ 4 transport through the CEM, i.e. electromigration, is driven by The higher energy requirements of electrochemical systems are their current and, thus, high current densities result in high TAN RRs. In MFCs main drawback, whereas the lower current densities are the main and MDCs, the current is generated spontaneously, but in MECs and shortcoming of BESs. In fact, the low amount of energy required per BECs, external energy is applied and significantly higher current den­ gram of N recovered currently justifies the present focus on BES. Beck­ sities (and, thus, higher TAN RRs) have been obtained (Fig. 3A) at the inghausen et al. [61] reviewed the current research regarding nitrogen expense of lower energy yield. Moreover, high current densities result in recovery and reported that MDC had the lowest ratio of the energy input a higher catholyte pH, which favors TAN recovery. In a different sce­ vs the energy output: 0.1. The authors mentioned that BES, membrane nario, migration of other cations from the cathode to the anode has also technologies, and stripping appear to be the best options for recovering been used to improve TAN recovery. The Donnan dialysis, which means TAN from reject water from digestate balancing energy requirements the exchange of ions of the same charge through a membrane, will take and efficiency [61]. Therefore, further research should focus on place when there is a concentration gradient over the membrane [96]. If increasing current densities without a substantial rise in energy there is no current intensity, then cations other than NH+ 4 that have requirement. Even though external energy is required in MECs and accumulated on the catholyte side may move back to the anolyte side if a BECs, the energy demand remains comparable to that of many other positively charged ion moves simultaneously from the anolyte to the conventional nitrogen removal processes (note that TAN removal is less catholyte. During TAN recovery, it is possible to utilize the accumulated beneficial than TAN recovery from a circular economy point of view). K+ and Na+ cations to facilitate the transport of NH+ 4 from the anolyte to Several MEC studies have reported energy consumption of less than the catholyte, provided that most NH3 has been effectively removed 21.6 kJ gN− 1, which is in the same range as nitrification/denitrification from the catholyte. (46.8 kJ gN− 1), air stripping (32.4 kJ gN− 1) and anammox (18 kJ gN− 1) Thus, bioelectrochemical TAN recovery needs high current densities. [30]. Cerrillo et al. [30] reported that membrane-based BESs can lead to For instance, the highest current densities reported so far are 37.6 A m− 2 high TAN RR (up to 119 gN m− 2d− 1) with energy consumption in the (average of 29.3 A m− 2) applying 1.46 V by Ledezma et al. [27] in a BEC. range of 4.5–10 kJ gN− 1. Overall, bioelectrochemical TAN recovery is This reactor was operating with synthetic urine (pH 9.2) and adapted certainly more competitive than the Haber–Bosch process. In MECs, the biomass. CE was 94.43%, meaning that the BECs transformed most of increased energy demand resulting from the applied electrical energy the chemical energy of the COD into electric charge. It has been reported can be partially or fully compensated by the production of energy car­ that high anodic pH results in high CE and high current densities since riers at the cathode (i.e. hydrogen [24] or methane [77]) and the mi­ microbial diversity is lower due to the more restrictive conditions and, crobial oxidation of waste organic compounds at the anode. Wu et al. thus, exoelectrogens face less competition for substrate [97]. [73] reported that the energy content of the hydrogen generated during In any case, current densities obtained using BES are still much lower TAN recovery in MECs was up to 142% of the electrical energy than those reported for purely electrochemical systems (Fig. 3B). TAN consumed. Kuntke et al. [24] reported high average volumetric RRs in electrochemical separation processes are usually 120–1010 gN hydrogen production rate of 30–50 m3 H2 m− 3 d− 1, while the system still m− 2 d− 1 across membranes with high selectivity (greater than 90%) required an average energy input between 7.2–9 MJ m− 3 H2. Fig. 3. (A) Current densities for different bioelectrochemical systems reported in Table 1, 2 and 3 and (B) Average energy yield (red) and current density (blue) between electrochemical and bioelectrochemical systems [31]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 8 M.B. Galeano et al. Chemical Engineering Journal 472 (2023) 144855 6.2. BESs are limited by current density; how can we improve the current 6.3. The importance of ammonium concentration in the feed density produced? TAN recovery is designed for wastewaters with high NH+ 4 concen­ Research aims at BESs not limited by current density. The load ratio tration since high anolyte concentrations boost NH+ 4 transport through (LN) is a parameter that describes the ratio between current density and the CEM [51–56]. NH+ 4 is expected to be the main charge carrier because influent TAN concentration. Rodriguez-Arredondo et al. [28] demon­ of a higher concentration gradient than other cations, however, multi­ strated that current density and TAN loading rate cannot be controlled valent ions, even at low concentrations, are able to migrate. Lee et al. independently. The magnitude of the current density depends on COD [42] demonstrated that electromigration is proportional to the initial oxidation, which in turn is influenced by several factors such as anode concentration of each ion using real livestock wastewater. Nevertheless, potential and pH. The LN for MFC tends to be less than one, whereas for high TAN concentrations can also hinder the biological process due to MEC it is greater than one. A LN less than 1 means that the current the potential inhibitory effect of free ammonia [105], which can density is not high enough to transfer all the NH+ 4 from the anolyte to the decrease the performance and increase the start-up time in MFCs [106]. catholyte. Thus, in such systems, TAN load must be decreased to achieve The formation of a mature biofilm on the working electrode able to high TAN RE. Even for values close to 1, cations other than NH+ 4 must be tolerate high free ammonia concentration is crucial. Thus, the microbial transported. Moreover, as operation at higher LNs did not significantly distribution in these systems may differ from the commonly reported improve TAN RE, 1.3 has been suggested to be an optimal LN [28,43]. non-NH3-acclimated anodic biofilms. So far, the best method for The most important parameters for current generation are electrode enriching an ammonium tolerant exoelectrogenic microbial community materials, biofilm development, and the nature of the organic source. is unknown, but a common practice is to switch from an ammonium The organic loading rate is essential, since exoelectrogenesis should not exposed MFC to MEC rather than starting directly in MEC mode [62]. be the limiting step for obtaining high current density. Thus, the Wang et al. [107] studied the effects of free ammonia on electrochem­ preferred real scenarios for ammonia recovery are highly organic ically active biofilms in MECs and the authors reported that the concentrated streams such as pig slurry, urine or digestate. Since this is threshold of electrochemical activity of anode biofilm was 1 gN L− 1, an emerging technology studied at lab-scale, most of the reviewed works whereas concentrations up to 4 gN L− 1 have been reported in acclimated use synthetic wastewaters mimicking these effluents, which contain a biofilms (Tables 1–3). Inhibition by free ammonia in biofilms has been wide range of organic substrates with different chemical composition extensively studied [108–110]. Inhibition by free ammonia could be and biodegradability. The nature of the organic matter affects microbial mitigated by lowering the pH to maintain most TAN as NH+ 4 [109,111]. activity: readily biodegradable substrates are desired when aiming for High TAN concentrations may also cause undesired/uncontrolled high intensities. Complex organic compounds present in real waste precipitation of ammonium salts, so partial removal/precipitation of the streams could hinder bioelectrochemical performance [102]. other anions can be necessary as a pretreatment step to avoid scaling Therefore, urine is a paradigmatic substrate for BES (high amount of inside the BES. Alternatively, controlled precipitation of these minerals biodegradable COD) and for TAN recovery (high ammonium concen­ can be seen as a chemical TAN recovery method. Simultaneous N and P tration) and has received much attention [24,32,34,54,103]. However, recovery as struvite has been described in BESs in Cerrillo et al [30]. The some compounds like urea or uric acid could not be directly used as major drawback with bioelectrochemical struvite precipitation has been carbon source by exoelectrogens and a previous hydrolysis/fermenta­ the scaling of the cathode: the high pH close to the cathode electrode tion step is required [104]. Yang et al. [56] compared reject water and leads to the formation of struvite precipitates on the electrode, which digestate (around 2 gN L− 1) and obtained much higher performance can block mass transfer close to the cathode and thus hinder the elec­ with digestate reaching TAN RE of about 40% and TAN RR rates 2.5 trochemical performance [112,113]. Simultaneous recovery of N and P times higher than that with reject water. by polyphosphate-accumulating organisms on the anodic side of Regarding electrode materials, BESs for TAN recovery must follow electricity-driven BES for TAN recovery has also been demonstrated the same trends as conventional BESs. Novel anodes should have low [114]. In any case, full TAN recovery via struvite precipitation is often overpotentials, as well as high conductivity, surface area, biocompati­ unfeasible with TAN-rich wastewaters due to the lack of sufficient bility, and cost-effectiveness. Novel cathodes (in BEC or MEC) should amounts of P and Mg and external dosage of these elements would aim at decreasing the overpotentials of hydrogen evolution at the significantly increase the cost of TAN recovery. cathode, while using cheaper and more sustainable materials than Pt. If TAN recovery is the main objective of the MEC, hydrogen production at 6.4. Scaling-up BES for ammonia recovery the cathode may not be essential and more efficient alternatives (i.e. the use of biocathodes) can be found. Biocathodes may reduce the cathode The most studied methodologies for TAN extraction from the cath­ overpotential and thus the energy requirements. For instance, Cerrillo ode chamber include precipitation, stripping + absorption and mem­ et al. [77] coupled an electromethanogenic MEC to TAN recovery when brane based NH3 extraction. These methods have already been tested at they reduced organic matter content and upgrading biogas from dige­ full scale without integrating them into a BES. When aiming at scaling- stated pig slurry. The catholyte was pumped into a recovery cell with a up, stripping + absorption is simpler than the other methods, but a high hydrophobic membrane for TAN recovery and this led to a lower amount of energy is required for aeration. For instance, Boehler et al. cathodic pH (7.6 with the TAN recovery system and 8.88 without) and [70] showed the performance of an ammonia stripping plant treating boosted methane production (73 L m− 3 d− 1)[77]. 5–7 m3/h sludge water at the Kloten-Opfikon WWTP. A CO2-stripper Kuntke et al. [98] and Georg et al. [95] have proposed a novel effi­ was integrated to reduce NaOH requirements. cient way for using the hydrogen produced in the cathode as an electron When stripping is conducted with external air addition, oxygen donor to decrease the anode potential. They named this configuration as intrusion can affect the bioanode performance. In MECs, NH3 can be hydrogen recycling electrochemical system: the effluent from the MEC stripped with the gas produced (i.e., H2), but efficient stripping requires was fed to the anode of a parallel MEC that shared a cathode with the high gas formation rates. In both cases, a separate absorption process is MEC for electrochemical TAN recovery. The recovery efficiency ob­ required after the stripping. For recovering TAN directly from the liquid, tained after the MEC was 94%, while the additional treatment with the gas permeable membranes can be used. As observed, hydrophobic electrochemical hydrogen recycling system increased it to 99.8% [95]. membranes are nowadays more widely adopted industrially than pilot No hydrogen is recovered, but energy requirements and the potential scale GDEs. risk of chlorine evolution are reduced. Membrane-based electrodes are more complex (fouling, pressure- build up, selective NH3 separation). However, higher efficiency and lower energy requirements are expected. Boehler et al. [70] also report 9 M.B. Galeano et al. Chemical Engineering Journal 472 (2023) 144855 the performance of hydrophobic hollow fiber membranes at the Neugut Reference WWTP to recover free ammonia gas at pH greater than 9.3 into concentrated sulfuric acid, forming ammonium sulfate. Molinuevo- [85] [89] [90] [84] [91] [83] [27] [26] [92] [93] [94] [86] [95] Salces et al. [115] reported the results of a pilot-scale demonstration plant on a farm using gas-permeable membranes to recover nitrogen from raw manure. They reported TAN RRs of 38.20 g NH3-N m− 2 d− 1 [gN m− 2 d− 1] consumption [115]. Other examples of pilot or full-scale applications of membrane- [kJ gN− 1] based NH3 recovery can be found in the literature [116–118]. The use Energy 31.52 34.29 60.48 34.23 339.8 21.96 6.48 12.0 of membrane electrodes for gas extraction is complicated, for example, n.r n.r n.r n.r n.r n.r by membrane permeability, material conductivity, and membrane cost and scalability [46]. Membrane permeability is critical because, for a 0.04 gNH+ 4 TAN RR 430.0 desired recovery target, it directly determines the amount of materials 86.0 80.0 0.41 11.5 23.0 needed and the cost associated with the process. The conductivity of d− 1 5.1 1.4 n.r n.r n.r n.r n.r membrane cathode is also critical, as low conductivity leads to large potential drops (due to resistive losses) across the surface, demanding TAN RE higher energy and leading to undesired electrochemical reactions [72]. 40.8 87.9 64.3 70.0 75.8 90.1 49.5 12.0 42.0 75.5 82.5 [%] n.r n.r n.r The development of conductive polymeric membranes, including those containing carbon nanotubes (CNTs), graphene, or carbon black, has 3 3 improved the performance, but conductivity is still considered low [A m-2] 45 A m− 80 A m− Current 2.8 mA density [30,72]. 2 mA 29.3 0.26 26.0 2.3a a 4.3 2.9 1.4 2.0 3.0 9.8 Despite the high TAN recovery rates obtained in lab-scale electro­ chemical systems, only a few studies have focused on TAN recovery in Ammonium sulfate Ammonium sulfate Ammonium sulfate Ammonium sulfate pilot scale. Ward et al. [119] recovered TAN from wastewater using a 30-cell pair electrodialysis system under applied current intensity of 20 Concentration Concentrate Concentrate Concentrate Concentrate Concentrate bicarbonate bicarbonate Ammonium Ammonium A m− 2 with a total membrane area of 7.2 m2 treating a pre-treated Ammonia Product centrate of 75 L h− 1. The TAN RR was 100 gN m− 2 d− 1 with energy consumption of 17.6 kJ gN− 1. Ferrari et al. [120] coupled a 65 cell-pair pilot scale bipolar electrodialysis unit (total membrane area 3.15 m2) to a system of two liquid/liquid membrane contactor modules. This pilot plant treated 150 L h− 1 of anaerobic digestate with a TAN RR of 193.3 Concentration Concentration Concentration Concentration Concentration Concentration Concentration Volatilization Methodology Concentrate Stripping + Stripping + gN m− 2 d− 1 consuming 129 kJ gN− 1 under continuously applied current Absorption Absorption Recovery Stripping of 75 A m− 2 and with TAN RR of 74.9 gN m− 2 d− 1 consuming 22.7 kJ TMCS gN− 1 under intermittent current (Donnan mode). Recently, Rodrigues et al. [30] used the same 65 cell-pair pilot scale unit for pH-controlled Ruthenium-iridium coated Ti bipolar electrodialysis treating diluted urine 41.7 L h− 1 and obtained Carbon cloth coated with Pt Synthetic graphite granules TAN RR of 223 gN m− 2 d− 1 under current density of 100 A m− 2 with an energy consumption of 46.8 kJ gN− 1. Stainless steel mesh Stainless steel mesh Stainless steel mesh Stainless steel mesh Summary of the experimental reports on bioelectrochemical TAN recovery using MDCs and BECs. Graphite granules Pilot-scale TAN recovery using BES has been even less studied. Zamora et al. [74] reported for the first time the application of a scaled Carbon cloth Carbon cloth Carbon cloth Carbon cloth up MEC for nutrient and energy recovery from urine. In the first step, the Cathode Carbon system recovered phosphorus as a struvite reactor. In the second step, a mesh MEC was coupled with a permeable hydrophobic membrane in which Synthetic graphite granules TAN was recovered as ammonium sulfate solution. The system was graphite granules/graphite stable during the six-month operation when fed with diluted (2 times) Graphite fiber brush and undiluted urine at an applied voltage of 0.5 V with an average Electrode materials Carbon fiber brush Graphite granules current density of 1.7 ± 0.2 A m− 2. During stable current production, Graphite brush Carbon paper Carbon brush Carbon brush Carbon brush Carbon brush Graphite felt the TAN transport efficiency over the CEM was 92 ± 25% and the energy Carbon felt maximum value obtained, n.r. = not reported. consumption for TAN recovery was 4.9 kJ gN− 1, which is lower than Anode competing electrochemical nitrogen removal/recovery technologies. rods 7. Conclusions TAN [g Concentrated leachate 0.06 0.70 0.07 5.88 0.06 0.10 1.02 1.80 0.06 0.12 L− 1] Due to increasing demand, finding alternative sources for NH3 is 1 4 7 1 critical. TAN recovery from wastewater using BESs offers an interesting Digested pig slurry low-energy alternative to conventional systems. Nowadays, electro­ Synthetic urine Synthetic urine Synthetic urine chemical TAN recovery is more applied than BESs at higher scale due to Real domestic Soil washing wastewater the high TAN RR reported. However, the fact that BESs shows lower Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Influent energy requirements per gram of nitrogen recovered has centered the Type focus in BES TAN recovery. This work has compared the performance of all the reported experimental works on TAN recovery so far and has Double chamber BEC Triple chamber MDC Triple chamber MDC Triple chamber MDC Triple chamber BEC Triple chamber BEC Triple chamber BEC Triple chamber BEC discussed on the essential factors/bottlenecks that are currently hin­ Four Chamber BEC Four chamber BEC Six chambers BEC Submersible MDC dering its industrial adoption. TAN RR highly depends on the current b Configuration/ calculated, Tubular MDC density produced and the highest TAN RRs have been obtained in BECs. Chambers The nature and concentration of the organic matter is essential, as the Table 3 COD has to be sufficient to generate enough current to drive the trans­ port of ammonium. 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