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Article

Ammonium and Phosphate Recovery in a Three Chambered Microbial Electrolysis Cell: Towards Obtaining Struvite from Livestock Manure

by
Míriam Cerrillo
*,
Laura Burgos
,
Joan Noguerol
,
Victor Riau
and
August Bonmatí
*
IRTA, GIRO, ctra. C-59, km 12,1, Caldes de Montbui, E-08140 Barcelona, Spain
*
Authors to whom correspondence should be addressed.
Processes 2021, 9(11), 1916; https://doi.org/10.3390/pr9111916
Submission received: 28 September 2021 / Revised: 22 October 2021 / Accepted: 22 October 2021 / Published: 27 October 2021
(This article belongs to the Special Issue Advanced Applications, Processes, and Materials in Microbial Electrochemical Technologies)
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Versions Notes

Abstract

:
Ammonia and phosphate, which are present in large quantities in waste streams such as livestock manure, are key compounds in fertilization activities. Their recovery will help close natural cycles and take a step forward in the framework of a circular economy. In this work, a lab-scale three-chambered microbial electrolysis cell (MEC) has been operated in continuous mode for the recovery of ammonia and phosphate from digested pig slurry in order to obtain a nutrient concentrated solution as a potential source of fertilizer (struvite). The maximum average removal efficiencies for ammonium and phosphate were 20% ± 4% and 36% ± 10%, respectively. The pH of the recovered solution was below 7, avoiding salt precipitation in the reactor. According to Visual MINTEQ software modelling, an increase of pH value to 8 outside the reactor would be enough to recover most of the potential struvite (0.21 mmol L−1 d−1), while the addition of up to 0.2 mM of magnesium to the nutrient recovered solution would enhance struvite production from 5.6 to 17.7 mM. The application of three-chambered MECs to the recovery of nutrients from high strength wastewater is a promising technology to avoid ammonia production through industrial processes or phosphate mineral extraction and close nutrient natural cycles.

Graphical Abstract

1. Introduction

Intensive livestock farming is a strategic economic activity in different areas of Europe that generates large amounts of manure. Fertilizing crops with livestock manure is a common waste management practice. Another possibility is to digest the manure anaerobically, recovering energy from waste in the form of biogas [1], with the option of also using digestates as a soil fertilizer. Livestock manure can improve soil fertility by adding organic matter and nutrients to soil. However, manure or its digestates usually contain a high concentration of nutrients that hampers their direct application to soils. Uncontrolled applications of slurry to the soil could have negative effects on the environment, such as nitrate groundwater contamination or freshwater eutrophication [2]. Due to environmental concerns, there are legal limitations for the application of livestock manure to soil. These limitations may involve the exportation of livestock manure surplus to distant farmlands, which will increase transportation cost.
An alternative to traditional livestock manure management is the implementation of the circular agrosystems approach. This proposal implies the consideration of livestock manure as a nutrient resource [3], among others, that must be recovered and reintroduced to closed loop agriculture. There are several technologies that allow for closing the nutrients cycle, mainly recovering nitrogen and phosphorus to produce fertilizers, stripping and absorption [4], membrane distillation technologies [5,6], and vacuum evaporation [7], among others [8]. The combined nitrogen and phosphorus recovery can be achieved by struvite precipitation [9]. Struvite is a salt composed of ammonia, phosphate, and magnesium (heptahydrate), and has been described as a slow-release fertilizer. The struvite formation reaction is as follows:
Mg2+ + NH4+ + HnPO43−n + 6H2O → MgNH4PO4 + 6H2O + nH+
In addition to more conventional nutrient recovery technologies, bioelectrochemical systems (BES) are emerging as candidates for the recovery of multiple resources from wastes [10,11]. BES can be operated as an independent technology, or in combination with other technologies, such as anaerobic digestion [12], allowing for the concomitant recovery of energy or other compounds coupled with nutrient recovery. BES are devices where electrogenic microorganisms catalyse oxidation and/or reduction reactions at an electrode (anode and/or cathode, respectively). BES have been tested with different configurations for ammonia recovery. Ammonia recovery BES are generally based on the migration of ammonium through a cation exchange membrane (CEM) by two-chamber cells coupled to ammonia stripping or hydrophobic membranes [5,12,13]. On the other hand, the recovery of phosphate in BES is performed by precipitation or migration through anion exchange membranes (AEM) [14]. A triple-chamber microbial fuel cell (MFC) has been used for phosphate remobilization from iron phosphate contained in digested sewage sludge [15]. Recently, a four-chamber microbial electrolysis cell (MEC) has been reported to achieve an 80% recovery efficiency of phosphorus in the form of hydroxyapatite [16]. A submersed BES has also been used to recover nutrients from a synthetic solution [17].
Several studies have dealt with the recovery of struvite in BES [18]. Single chamber cells have been tested [19,20,21,22,23], showing that a pH buffering may limit the recovery of phosphate [24]. To overcome this limitation, multi-chamber cells have also been developed. The phosphate recovery process improves in double-chamber BES [25], due to the separation between the anode and the cathode. This separation creates an alkaline environment around the cathode, which favours the precipitation and recovery of phosphate [10]. To promote bulk phase struvite precipitation and minimize cathode scaling, a fluidized bed cathode MEC was developed and fed with domestic wastewater treatment plant digestate [26]. Using a magnesium anode, struvite has been recovered in the anode compartment of an MEC fed with digested swine wastewater [27].
These previous studies on BES struvite recovery have shown different drawbacks. On the one hand, struvite precipitates mainly on the cathode, which makes it difficult to recover the salt and reduces the cathode performance [21,24]. On the other hand, the presence of organic matter may reduce the purity of struvite, due to salt crystallisation around the particles [9]. Hence, the design of BES for nutrient recovery has evolved to three-chamber BES to treat synthetic wastewater [28,29], synthetic urine [30], urine [31,32], domestic wastewater [33], or rejected water from the anaerobic digesters of centralised wastewater treatment plants (WWTPs) [34]. A multiple chamber cell has also been operated with wastewater for the recovery of nutrients in order to obtain struvite [28].
However, issues such as the low solubility of phosphate in substrates with a high organic and solid content must be addressed [35,36]. Furthermore, complex substrates provide a variety of cations and anions that not only compete with ammonium and phosphate for migration across ion exchange membranes, but can also affect struvite recovery from the nutrient concentrate solution. In this study, a three-chamber MEC setup is proposed to recover ammonium and phosphate from a high strength organic and nitrogen wastewater (digested livestock manure), providing new insights into these relevant issues. With this configuration, struvite precipitation in the reactor will be avoided, minimising salt precipitation on the electrodes and/or membranes so that it can be carried out independently of the BES reactor [37]. Furthermore, the struvite obtained will be recovered in a clean solution, free of organic matter.
The aim of this study is to evaluate a three-chamber MEC for the recovery of ammonium and phosphate from livestock manure in order to obtain struvite. Continuous assays have been performed under different pH conditions and phosphate concentrations using digested livestock manure. Visual MINTEQ software has been used to model the struvite formation potential of the concentrated solutions obtained with each condition.

2. Materials and Methods

2.1. Experimental Set-Up

An H-type three-chamber MEC was constructed, which consisted of three 0.6 L glass bottles connected with side openings (Figure 1). A cation exchange membrane (CEM; dimensions: 20 cm2; Ultrex CMI-7000, Membranes International Inc., Ringwood, NJ, USA) was placed between the side openings of the first and second (or intermediate) bottle (anode and recovery compartments, respectively). An anion exchange membrane (AMI-7100, Membranes International Inc., Ringwood, NJ, USA) with the same dimensions as the CEM was inserted between the second and third bottle (cathode compartment). The recovery compartment was equipped with a magnetic stirrer. A piece of carbon felt (dimensions: 175 cm2; thickness: 3.18 mm; Alfa Aesar GmbH and Co KG, Karlsruhe, Germany) was used as the anode, and 304 stainless steel mesh was used as the cathode (dimensions: 156 cm2; mesh width: 150 μm; wire thickness: 112 μm; Feval Filtros, Spain).
The anode (working electrode) potential was poised to −300 mV by a potentiostat (VSP, BIO-LOGIC SCIENCE INSTRUMENTS ESPAÑA, S.L.U, Barcelona, Spain) in the three-electrode mode. An Ag/AgCl reference electrode (Bioanalytical Systems, Inc., West Lafayette, IN, USA; +197 mV vs. standard hydrogen electrode, SHE) was inserted into the anode compartment of the cell. All potential values in this paper refer to SHE. The potentiostat recorded the electrode potentials and current, every 5 min, using a computer with EC-Lab software (BIO-LOGIC SCIENCE INSTRUMENTS ESPAÑA, S.L.U, Barcelona, Spain).

2.2. Feeding Solutions

The digestate used to feed the anode compartment of the MEC was collected from a 5 L lab-scale thermophilic anaerobic digester, which was fed with pig slurry. The pig slurry was collected in a farm in Gurb (Catalonia, Spain), sieved (500 µm), and diluted before feeding the anaerobic digester. The digestate was stored at 6 °C until its use and was sieved (125 µm). The composition of the sieved digestate is summarised in Table 1.
The digestate was first fed to the anode compartment to recover the ammonium, and then the anodic effluent was circulated to feed the cathode compartment and recover the phosphate. The catholyte was modified in certain assays to achieve the different conditions tested during the experiment (Table 2). The pH was modified in order to assess the effect of this parameter over phosphate solubility. Phosphate is mainly present in the particulate matter of manure and can be released into the liquid fraction as soluble inorganic phosphate by lowering the pH [35]. When acidifying was performed, H2SO4 (95–97%) was added to the effluent of the anode compartment before using it as feed to the cathode compartment. As the phosphate concentration of the substrate that was used in the assays was relatively low (Table 1), KH2PO4 was added (16 g L−1), according to Table 2, either in the catholyte or in the anolyte, in order to test a substrate with a higher phosphate concentration in some of the assays.
The recovery compartment was filled with 600 mL of distillate water, which was replaced after each assay.

2.3. Reactors Operation

The anode carbon felt was inoculated with the same digested pig slurry described in Section 2.2, completely filling the anode compartment with digestate. The star-up consisted of operating the MEC in batch mode for one week and then in continuous mode for 3 weeks (data not shown).
After the start-up, the MEC was operated for 115 days in six different phases (Table 2), using the substrates amended or not, according to Section 2.2. After each change in feeding conditions, the MEC was operated for at least four hydraulic retention times (HRT) to ensure steady-state conditions.
The influent solutions from both the anode and the cathode compartments were fed in continuous mode with a pump at 12 mL h−1 and were mixed by recirculating them using an external pump. The anodic effluent was circulated to feed the cathode compartment and was modified when required (Table 2).
The HRT was 41 h and 36 h for the anode and the cathode compartments, respectively, while the recovery compartment was operated in batches. The organic and nitrogen loading rates (OLR and NLR, respectively) of the anode compartments were established at 8.5 kgCOD m−3 day−1 and 0.9 kgN m−3 day−1, respectively. Discrete samples were taken from the anode, cathode, and recovery compartments on weekdays. The MEC was operated at room temperature throughout the tests (23 ± 2 °C).

2.4. Analytical Methods and Calculations

The chemical oxygen demand (COD) was determined in the anolyte and effluent samples. The pH was determined in the influents and effluents of the anode and cathode compartments, and in the recovering compartment samples by a CRISON 2000 pH electrode. All of the analyses were performed following standard methods [38].
The anion (Cl−, NO3−, NO2−, PO43−, and SO42−) and cation (Na+, K+, NH4+, Ca2+, and Mg2+) concentrations were measured by ionic chromatography (IC) using an 861 Advanced Compact IC (Metrohm, Switzerland). A Metrosep A Supp 4 (Metrohm, Switzerland) column with a Metrospt A Supp 4/5 Guard pre-column and a CO2 suppressor were used for anion determination. A Metrohm C4 150/4.0 column (Metrohm, Switzerland) and a Metrosep C4 Guard pre-column were used for the cation determination. Prior to the IC analysis, the samples were diluted and filtrated with nylon (0.45 mm) and Bond Elut JR C18 microfilters (Agilent Technologies, Inc., Santa Clara, CA, USA).
The current density (A m−2) of the MEC was calculated as the quotient between the intensity recorded by the potentiostat (A) and the area of the anode (m2). The ammonium, phosphate, and COD removal efficiencies were calculated as the ratio of the difference between the anode compartment influent and cathode compartment effluent concentrations and the influent concentration (mg L−1). The recovery rate of the different ions was calculated as the ratio between the mass (mg) of each ion accumulated in the recovery compartment and the elapsed time (d).
A balance of charge was performed to evaluate the number of electrons that were used for ion migration through the CEM and AEM. When calculating the charge (Q), a distinction was made between the transport of negative charges in the form of electrons through the electric circuit, and the transport of positive (Q+) and negative (Q−) charges in the form of the dominantly present cations (Na+, K+, NH4+, Ca2+, and Mg2+) and anions (Cl−, PO43−, and SO42−), respectively. The total charge production (Q), expressed in coulombs (C), was determined by integrating current over time. Transport of charges in the form of ions in the system through the membrane, Q+ or Q−, expressed in coulombs (C), were determined as follows:
Q+ = ∑cat (xcat,t · V · zcat · F)
Q− = ∑an (xan,t · V · zan · F)
where xcat,t and xan,t are the molar cation or anion concentration, respectively, of each ion species in the recovery compartment at the end of an experimental run expressed in mol L−1 (M); V is the recovery compartment liquid volume expressed in litres (L); zcat and zan are the valence of the cation or anion species, respectively; and F is Faraday’s constant (96,485 C mol−1).

2.5. Struvite Potential Recovery

A theoretical calculation was carried out to evaluate the struvite recovery potential of the solutions obtained in each assay using Visual MINTEQ (KTH, Sweden, https://vminteq.lwr.kth.se/, accessed on 12 July 2021). Visual MINTEQ (ver. 3.1) is a chemical equilibrium software that allows for the calculation of the speciation, solubility, solid equilibrium, and the mineral dissolved phases in laboratory and natural aqueous systems.
Concentrations of the major interest ions present in the solution obtained in each assay (Na+, K+, NH4+, Ca2+, Mg2+, Cl−, PO43−, and SO42−) were introduced in the modelling program, with a temperature of 25 °C. Precipitation and aqueous reactions were provided by the standard databases in the chemical equilibrium program Visual MINTEQ. The sweep utility of the software was employed to calculate the mass of struvite that could be precipitated when increasing the pH value between 6 and 11. The same utility was employed to evaluate the amount of struvite obtained when increasing the amount of magnesium present in the solution.

3. Results and Discussion

3.1. Performance of the MEC

The current density produced by the MEC in the different phases of operation is shown in Figure S1. Phase 3 and Phase 4 were the periods with a higher average current density, reaching 0.26 A m−2 (Table 3), although the differences among the phases were not significantly different. The current densities obtained in these assays are slightly lower than those obtained in previous work performed with similar digested pig slurry and OLR, using H-type cells with adjacent anode and cathode compartments, where 0.35 A m−2 were achieved [5]. The increase in distance between the anode and the cathode in this assay, as the recovery compartment was placed between them, may have increased the electrical resistance of the system and in turn reduced the current density [39].
The average COD removal efficiency was in a range of 21–34%, with no significant differences among the different phases (Table 3). This COD removal efficiency is similar to the one reported by previous assays where a MEC was fed with a similar pig slurry digestate [5], and is typical of BES working with complex substrates such as food or agricultural wastes [40].
Ammonia removal efficiency during Phase 1, with no substrate modification, was 29%, slightly higher than the 23% obtained in the previous assays performed by Cerrillo et co-workers (2021) with similar conditions [5]. The phases with acidification of the catholyte (Phases 2, 3, and 4) reduced this removal efficiency to a range of 7–16%, while the amendment of the anolyte with phosphate with no pH adjust (Phase 5) increased the removal efficiency value to 20%. In Phases 3 to 5, the ammonium concentration of the cathode effluent was generally higher than in the anode effluent. This fact was previously reported by other authors when operating three-cambered cells with urine or domestic wastewater, indicating that the AEM allowed for the permeation of part of the nitrogen recovered in the intermediate compartment [31,41]. Dissolved ammonia gas can be transported through the AEM as an uncharged species by diffusional forces only, as described before [42].
Regarding the phosphate removal efficiency, a higher value with PO43− amendment (Phases 4 to 6) was achieved in Phase 6 (36%). In general, the reduction of pH by H2SO4 addition decreased PO43− removal due to competition with the sulphate anions. Previous electrodialysis studies have reported that the PO43− removal efficiency decreased due to the co-existence of accompanying ions in the feed solution, especially when competing with SO42−, reducing PO43− removal efficiency from 50.7% to 29.5% [43].
Other authors have reported the use of three-chambered cells for the recovery and concentration of nutrients. Koskue and co-workers operated a three-chambered cell for ammonia recovery and achieved higher recovery values of 75.5% with synthetic reject water and 53% with real reject water [34]. The use of a complex wastewater with other cations such as Na+, Ca2+, and Mg2+ present in the organic matrix may decrease NH4+ removal efficiency, as they would compete with NH4+ ions for current driven migration [37]. Furthermore, the back diffusion of NH4+ due to the high concentration in the recovery chamber can decrease the recovery efficiency. Li and co-workers operated an MEC in batch mode and achieved removal efficiencies from synthetic wastewater of 36 and 30% for NH4+ and PO43−, similar to the ones obtained in this assay, although increasing the pairs of ion exchange membranes exhibited a higher removal efficiency of 79% and 79%, respectively [29]). Ledezma and co-workers achieved 59.7% removal of the nitrogen from the anodic compartment, and 42.8% of the phosphorus in a MEC supplied with synthetic urine [31].

3.2. Ions Accumulation in the Recovery Compartment

The rate of accumulation of the main ions in the recovery compartment of the MEC system is shown in Figure 2. During the operating period of the system with unmodified digested slurry (Phase 1), no PO43− was detected in the recovery compartment, with Cl− being the dominant anion in the solution. With regards to the cations, K+ (12.6 mg d−1), followed by NH4+ (6.8 mg d−1), were the ones that accumulated the fastest. When acidifying the catholyte to pH 6 (Phase 2), to solubilize the phosphate that could be present in the slurry, the increase of SO42− in the recovery solution was observed, promoted by H2SO4 addition to the catholyte, as well as a slight increase in the Cl− transfer rate. Both NH4+ and K+ also increased their accumulation rate to 14.0 mg d−1 and 19.2 mg d−1, respectively. This increase in accumulation rates may be related to the increase in current density produced in this phase (Table 2). In contrast, PO43− remained undetected in the recovery compartment solution, probably due to the increase from 51 to 167 mg L−1 of PO43− concentration achieved by acidification to pH 6. The acidification of the catholyte to pH 5 (Phase 3) also failed to significantly solubilize PO43−, achieving a maximum value of 375 mg L−1 in the catholyte. Instead, the accumulation of Cl− and SO42− increased, also accompanied by a greater accumulation of NH4+ (39.9 mg d−1) and K+ (32.4 mg d−1). As PO43− is mainly adsorbed on the particulate matter of pig slurry, and the substrate was sieved before use, it is evident that little PO43− to be solubilised remained in the substrate.
The amendment of the catholyte with KH2PO4 to simulate a higher PO43− content of the substrate (Phase 4) while maintaining the acidification of the catholyte (pH 5) reduced the transfer rate of Cl− and SO42− compared to previous phases, in favour of PO43− (51.3 mg d−1), being the most favourable condition of those tested regarding PO43− migration. The PO43− transfer was slightly reduced (35.7 mg d−1) when the acidification of the catholyte was eliminated (Phase 5), probably due to the decrease in current density of this phase. Finally, the addition of PO43− to the anolyte in the form of KH2PO4 (Phase 6) maintained the PO43− migration achieved in Phase 5. On the other hand, the increase in K+ in the feeding to the anode compartment caused an increase in the migration of this (35.2 mg d−1), to the detriment of NH4+ (8.5 mg d−1).
As the current density produced in MEC affects the migration of ions across the CEM and AEM, these results must also be analysed in the framework of the electrical charge balance, as described in Section 3.2.

3.3. Electrical Charge Balance

Figure 3 shows the rate of charge transfer in the system, in order to compare the number of electrons transferred from the anode to the cathode, with the number of positive and negative charges migrated through the cation and anion exchange membranes, respectively. As can be seen in Figure S2, the phases with the highest charge transferred were the two in which the catholyte was acidified to pH 5. In general, the transport of charges in the form of cations is slightly higher than that of anions, and also higher than the amount of charge transferred in the form of electrons, except in Phases 2 and 6. This may be due to the diffusion of uncharged species (NH3), as described before. So, while the migration of anions consumed 95%, 79%, and 78% of the electrical charge transferred by electrons in Phases 4 to 6, with PO43− amended substrates, cations represented 104%, 133%, and 87%, respectively.
NH4+ represented 39%, 43%, 42%, and 41% of the cation charge content in the recovery solutions of Phases 2, 3, 4, and 5, respectively, with a high competition of K+, which represented 50%, 33%, 37%, and 43% in the same phases. This distribution was strongly affected in Phase 6, with the addition of KH2PO4 to the anolyte, causing a decrease of NH4+ charge in the recovery solution to 16%, while K+ increased to 65%. Although Na+ has been described as a strong competitor for NH4+ migration, due to a similar hydrated radius (0.358 nm and 0.331 nm, respectively) [37,44], in this study, Na+ is represented in only between 6% and 17% of the positive charges in the recovery solution. This lower participation in the cation migration of sodium may be due to its lower concentration compared to NH4+ or K+ in this substrate (Table 1).
Regarding PO43−, this anion accounted for 43%, 56%, and 42% of the negative charges that were present in the recovery solution in Phases 4 to 6, respectively—the KH2PO4 amended phases. SO42− competed strongly with phosphate when the catholyte was acidified in Phase 4, representing 30% of the negative charge of the recovery solution. This proportion decreased to 17% and 3% when the acidification step was supressed in Phase 5 and Phase 6, respectively. In the acidified catholyte in Phase 4, most of phosphate was a monovalent species (H2PO4-). SO42− has a smaller hydrated radius (rh = 0.23 nm) and a higher valence; thus, it moved across the membrane faster than H2PO4- (rh = 0.302 nm). These facts, added to the increase in SO42− concentration in the catholyte due to acidification with H2SO4, decreased PO43− removal. The increase in the pH of the catholyte in Phase 5 and Phase 6 caused a shift of the H2PO4- anions to their multivalent forms, HPO42− and PO4−3, which have a higher tendency to move across the AEM, reducing the competence of SO42− [43].
Thus, the use of a complex substrate, with a wide variety of anions and cations accompanying NH4+ and PO43−, limits the concentration of the main components of struvite in the recovery solution. However, it must be deciphered whether these companion ions may hamper obtaining struvite when the suitable pH conditions are provided, as will be discussed in Section 3.3.

3.4. Struvite Potential Recovery

The concentration of the main components of the recovery solution obtained in the intermediate compartment after each assay is shown in Figure 4. As these solutions were intended to be used to recover struvite, which precipitates at a basic pH [9], Visual MINTEQ software was used to estimate the amount of salt that could form in each condition if the pH was externally modified (Figure S3). The recovery solution obtained in Phases 1 to 3 would not be suitable for struvite precipitation due to their low content of PO43−. However, hydroxyapatite (Ca5(PO4)3(OH)) would be recovered in small amounts (<2 mmol L−1) in all pH ranges, and brucite (Mg(OH)2) formed at pH values of 10 and 11, thanks to the small amounts of Ca2+ and Mg2+ present in the solutions.
Hydroxyapatite would still form in the solutions recovered in Phases 4 to 6, although struvite would precipitate when increasing the pH to 7, being the main component of the recovered solid. The maximum amount of struvite would be obtained at pH 10 (5.4, 3.7, and 5.6 mmol L−1 in the solution recovered in Phases 4, 5, and 6, respectively), although at pH 8 the recovering efficiency would be similar, with a lower alkali consumption. The solution recovered in the intermediate compartment had a pH lower than 7, thus avoiding uncontrolled precipitation of struvite inside the reactor, as shown by the mathematical model.
The estimated struvite recovery rate in Phases 3 to 6 at pH 8 would be 0.24, 0.22, and 0.21 mmol L−1 d−1, respectively. Added to the hydroxyapatite formation, nearly 39% of the phosphate and 14% of the ammonium recovered in the intermediate compartment were precipitated in a salt that could be used as a fertiliser in Phase 6 (Table 4). Mg2+ is clearly the limiting component for increasing the amount of recovered struvite, as 99% of this cation forms precipitates. The external addition of a Mg2+ source would enhance struvite recovery, as shown in Figure S4. A concentration of 0.23 mmol L−1 of Mg2+ in the recovery solution of Phase 6, at pH 8, would increase the concentration of struvite to 17.7 mmol L−1, thus tripling the value obtained with no Mg2+ addition. In this case, 84% and 41% of the PO43− and NH4+ of the recovery solution would be precipitated.
Struvite recovery outside the MEC by increasing pH is proposed over inside precipitation to increase the practicality of the system. Salt collection in an independent tank is simpler, and scaling is avoided, especially on the ion exchange membranes, which will decrease BES performance [37].
The use of pig slurry or digestate with a high phosphate concentration, instead of amended with KH2PO4 and acidified, will reduce the amount of K+ or SO42− present in the substrate and their competition with NH4+ or PO43− in the migration through the CEM and AEM, respectively. This way, the ratio between the later cation and phosphate will equilibrate and enhance struvite recovery. To assure the presence of soluble PO43− in the substrate, a solubilisation step may be needed, which will preferably be applied to raw or digested pig slurry before any mechanical separation treatment.

4. Conclusions

The suitability of a three-chamber MEC for the recovery of ammonium and phosphate from a high organic and nitrogen strength wastewater (digested pig slurry) has been demonstrated. The maximum average ammonium and phosphate removal efficiencies were 20% ± 4% and 36% ± 10%, respectively, when the substrate was amended with KH2PO4 to simulate a high phosphate content in the pig slurry. In addition, the COD was reduced by 34% ± 12%. Ammonium represented a maximum of 43% of the positive charges of the recovered solution, due to the strong competition of potassium cations, while phosphate reached a maximum of 56% of the negative charges. The pH value of the recovered solution was kept under 7, avoiding struvite precipitation in the reactor, which is an advantage over other MEC configurations for struvite recovery that promote struvite precipitation inside the reactor. Visual MINTEQ software showed that increasing the pH value of the concentrate solution to 8 outside of the reactor would be enough to recover most of the potential struvite (0.21 mmol L−1 d−1). However, the addition of magnesium to the recovered nutrient solution of up to 0.2 mM is needed to enhance struvite production from 5.6 to 17.7 mM. The solubilisation of phosphate reveals as a key issue in struvite recovery from livestock manure in BES, as acidification with H2SO4 may interfere, on the one hand, with biomass growth in the anode compartment, and, on the other hand, with phosphate migration. Phosphate solubilization techniques compatible with BES performance should be evaluated to improve the recovery efficiency of the system. Furthermore, it would be feasible to apply this technology for the treatment of other complex substrates rich in nutrients, mainly those produced in the agro-industrial sector. In this way, the recovery of ammonia and phosphate in the form of struvite, a slow-release fertiliser, will help close the nutrient natural cycles.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/pr9111916/s1, Figure S1: Current density produced in the MEC in the different phases of operation. Phase 1: not amended feeding was used; Phase 2: catholyte acidified to pH 6; Phase 3; catholyte acidified to pH 5; Phase 4: catholyte acidified to pH 5 and phosphate amended; Phase 5: phosphate amended catholyte; and Phase 6: phosphate amended anolyte, Figure S2: Charge distribution of the solutions recovered in each phase in the intermediate compartment (a) cations and (b) anions, Figure S3: Visual Minteq output for the mathematical calculation of struvite and other salts precipitation when increasing pH from value 6 to 11 with each recovery solution obtained in the different operation phases. (a) Phase 1: not amended feeding was used; (b) Phase 2: catholyte acidified to pH 6; (c) Phase 3; catholyte acidified to pH 5; (d) Phase 4: catholyte acidified to pH 5 and phosphate amended; (e) Phase 5: phosphate amended catholyte; and (f) Phase 6: phosphate amended anolyte, Figure S4: Visual Minteq output for the mathematical calculation of struvite obtained at pH 8 with the recovery solution of Phase 6 amended with increasing concentrations of magnesium.

Author Contributions

Conceptualization, A.B. and M.C.; methodology, M.C.; validation, M.C.; formal analysis, M.C.; investigation, M.C. and L.B.; resources, A.B.; data curation, M.C.; writing—original draft preparation, M.C.; writing—review and editing, A.B., L.B., V.R., J.N. and M.C.; visualization, M.C.; supervision, A.B.; project administration, A.B.; funding acquisition, A.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Spanish Ministry of Economy and Competitiveness (INIA project RTA2015-00079-C02−01 and PID2020-118830RR-I00). The support of the CERCA Program and of the Consolidated Research Group TERRA (ref. 2017 SGR 1290), both from the Generalitat de Catalunya, is also acknowledged.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

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Processes 09 01916 g001 550
Figure 1. Scheme of the set-up of the three-chambered MEC.
Figure 1. Scheme of the set-up of the three-chambered MEC.
Processes 09 01916 g001
Processes 09 01916 g002 550
Figure 2. Accumulation rates of NH4+, K+, Na+, PO43−, SO24-, and Cl− in the recovery compartment in the different phases. Phase 1: not amended feeding was used; Phase 2: catholyte acidified to pH 6; Phase 3; catholyte acidified to pH 5; Phase 4: catholyte acidified to pH 5 and phosphate amended; Phase 5: phosphate amended catholyte; Phase 6: phosphate amended anolyte.
Figure 2. Accumulation rates of NH4+, K+, Na+, PO43−, SO24-, and Cl− in the recovery compartment in the different phases. Phase 1: not amended feeding was used; Phase 2: catholyte acidified to pH 6; Phase 3; catholyte acidified to pH 5; Phase 4: catholyte acidified to pH 5 and phosphate amended; Phase 5: phosphate amended catholyte; Phase 6: phosphate amended anolyte.
Processes 09 01916 g002
Processes 09 01916 g003 550
Figure 3. Amount of charge transferred through the electrical circuit (electrons) compared with the charge accumulated in the recovery compartment in the form of anions (C−) and cations (C+). Phase 1: not amended feeding was used; Phase 2: catholyte acidified to pH 6; Phase 3; catholyte acidified to pH 5; Phase 4: catholyte acidified to pH 5 and phosphate amended; Phase 5: phosphate amended catholyte; Phase 6: phosphate amended anolyte.
Figure 3. Amount of charge transferred through the electrical circuit (electrons) compared with the charge accumulated in the recovery compartment in the form of anions (C−) and cations (C+). Phase 1: not amended feeding was used; Phase 2: catholyte acidified to pH 6; Phase 3; catholyte acidified to pH 5; Phase 4: catholyte acidified to pH 5 and phosphate amended; Phase 5: phosphate amended catholyte; Phase 6: phosphate amended anolyte.
Processes 09 01916 g003
Processes 09 01916 g004 550
Figure 4. Composition of the main ions in the concentrated solution of the recovery compartment after each condition tested. Phase 1: not amended feeding was used; Phase 2: catholyte acidified to pH 6; Phase 3; catholyte acidified to pH 5; Phase 4: catholyte acidified to pH 5 and phosphate amended; Phase 5: phosphate amended catholyte; Phase 6: phosphate amended anolyte.
Figure 4. Composition of the main ions in the concentrated solution of the recovery compartment after each condition tested. Phase 1: not amended feeding was used; Phase 2: catholyte acidified to pH 6; Phase 3; catholyte acidified to pH 5; Phase 4: catholyte acidified to pH 5 and phosphate amended; Phase 5: phosphate amended catholyte; Phase 6: phosphate amended anolyte.
Processes 09 01916 g004
Table
Table 1. Composition of the digested pig slurry used as the substrate (average ± standard deviation). Number of samples, n = 8.
Table 1. Composition of the digested pig slurry used as the substrate (average ± standard deviation). Number of samples, n = 8.
ParameterUnitValue
pH-7.7 ± 0.2
CODmg L−114,473 ± 981
Na+mg L−1941 ± 72
NH4+mg L−11897 ± 395
K+mg L−12042 ± 152
Ca2+mg L−1407 ± 199
Mg2+mg L−1176 ± 77
PO43−mg L−1143 ± 120
SO42−mg L−1151 ± 115
Table
Table 2. Operational phases of the MEC and modification of the anolyte or catholyte, regarding the pH value or phosphate content.
Table 2. Operational phases of the MEC and modification of the anolyte or catholyte, regarding the pH value or phosphate content.
PhasePeriod (d)AnolyteCatholyte
10–16Not modifiedNot modified
216–25Not modifiedpH modified to 6
325–49Not modifiedpH modified to 5
449–73Not modifiedpH modified to 5
Addition of KH2PO4 *
573–86Not modifiedAddition of KH2PO4 *
685–115Addition of KH2PO4 *Not modified
* 16 g L−1.
Table
Table 3. Summary of the main operation parameters of the MEC in the different phases of digestate operation (average ± standard deviation). Phase 1: not amended feeding was used (n = 7); Phase 2: catholyte acidified to pH 6 (n = 4); Phase 3; catholyte acidified to pH 5 (n = 8); Phase 4: catholyte acidified to pH 5 and phosphate amended (n = 6); Phase 5: phosphate amended catholyte (n = 4); Phase 6: phosphate amended anolyte (n = 7).
Table 3. Summary of the main operation parameters of the MEC in the different phases of digestate operation (average ± standard deviation). Phase 1: not amended feeding was used (n = 7); Phase 2: catholyte acidified to pH 6 (n = 4); Phase 3; catholyte acidified to pH 5 (n = 8); Phase 4: catholyte acidified to pH 5 and phosphate amended (n = 6); Phase 5: phosphate amended catholyte (n = 4); Phase 6: phosphate amended anolyte (n = 7).
PhaseCurrent Density (A m−2)COD Removal Efficiency (%)NH4+ Removal Efficiency (%)PO43− Removal Efficiency (%)
10.10 ± 0.06-29 ± 1489 ± 3
20.17 ± 0.09-16 ± 655 ± 1
30.26 ± 0.1821 ± 711 ± 252 ± 23
40.25 ± 0.1224 ± 711 ± 717 ± 7
50.17 ± 0.0932 ± 87 ± 25 ± 5
60.21 ± 0.0634 ± 1220 ± 436 ± 10
- Not determined.
Table
Table 4. Estimation of the fraction of precipitated components on the recovery solution in each phase when adjusting the pH to 8. Phase 4: catholyte acidified to pH 5 and phosphate amended; Phase 5: phosphate amended catholyte; Phase 6: phosphate amended anolyte.
Table 4. Estimation of the fraction of precipitated components on the recovery solution in each phase when adjusting the pH to 8. Phase 4: catholyte acidified to pH 5 and phosphate amended; Phase 5: phosphate amended catholyte; Phase 6: phosphate amended anolyte.
PhaseNH4+
(%)		PO43−
(%)		Mg2+
(%)
44.533.099.5
54.823.699.1
613.638.998.9
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Cerrillo, M.; Burgos, L.; Noguerol, J.; Riau, V.; Bonmatí, A. Ammonium and Phosphate Recovery in a Three Chambered Microbial Electrolysis Cell: Towards Obtaining Struvite from Livestock Manure. Processes 2021, 9, 1916. https://doi.org/10.3390/pr9111916

AMA Style

Cerrillo M, Burgos L, Noguerol J, Riau V, Bonmatí A. Ammonium and Phosphate Recovery in a Three Chambered Microbial Electrolysis Cell: Towards Obtaining Struvite from Livestock Manure. Processes. 2021; 9(11):1916. https://doi.org/10.3390/pr9111916

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Cerrillo, Míriam, Laura Burgos, Joan Noguerol, Victor Riau, and August Bonmatí. 2021. "Ammonium and Phosphate Recovery in a Three Chambered Microbial Electrolysis Cell: Towards Obtaining Struvite from Livestock Manure" Processes 9, no. 11: 1916. https://doi.org/10.3390/pr9111916

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