Liquid fertilizer production by ammonia recovery from treated ammonia-rich regenerated streams using liquid-liquid membrane contactors

Chemical Engineering Journal 360 (2019) 890–899

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Liquid fertilizer production by ammonia recovery from treated ammoniarich regenerated streams using liquid-liquid membrane contactors X. Vecinoa,b,

⁎,1

T

, M. Reiga,b,1, B. Bhushana,b, O. Giberta,b, C. Valderramaa,b, J.L. Cortinaa,b,c

a

Chemical Engineering Department, Escola d’Enginyeria de Barcelona Est (EEBE), Universitat Politècnica de Catalunya (UPC)-BarcelonaTECH, C/ Eduard Maristany 1014, Campus Diagonal-Besòs, 08930 Barcelona, Spain b Barcelona Research Center for Multiscale Science and Engineering, Campus Diagonal-Besòs, 08930 Barcelona, Spain c CETaqua, Carretera d'Esplugues, 75, 08940 Cornellà de Llobregat, Spain

HIGHLIGHTS

GRAPHICAL ABSTRACT

carbon was used to remove • Activated DOM and reduce fouling before HFLLMC process.

streams, from a zeolites • Ammonia-rich regeneration step, were treated by HFLLMC.

HF-LLMC configura• One/two-steps tion were tested using different stripping solutions.

PO was the best acid stripping so• Hlution by two-step HF-LLMC removing 3

4

94% ammonia.

was recovered as a multi• Ammonia nutrient liquid fertilizer (7.8% N and 21.6% P2O5).

ARTICLE INFO

ABSTRACT

Keywords: Urban wastewater Ammonium salts Hollow fibre Hydrophobic membrane Nutrient recovery Water transport

The nitrogen load on urban wastewater should be considered as a secondary resource for nitrogen-based fertilizers. From this point of view, ammonia-rich streams obtained from a regeneration step with zeolites using 70–80 g NaOH/L could contain up to 3.5–4.5 g NH3/L as well as other ionic species (Na+, K+, Ca2+, Mg2+) and dissolved organic matter (DOM). Thus, they could be treated to obtain a valuable product composed by ammonia for fertilizing applications. A sorption pre-treatment process was carried out to remove the residual amount of DOM and to reduce membrane fouling before processing with hollow fibres liquid-liquid membrane contactors (HF-LLMC). Polypropylene HF-LLMC was used to selectively extract ammonia in single or two-step configurations using different acid stripping solutions (H3PO4, HNO3 or a mixture of HNO3/H3PO4). For ammonia recovery, the ammonia mass transfer coefficient (Km(NH3)) and water transport by HF-LLMC were determined. H3PO4 was found to be the best acid stripping solution by one-step HF-LLMC; the ammonia removal was 76% with a Km(NH3) of 8.8 × 10−7 m/s. Additionally, the ammonia was concentrated 26 times, and it was recovered as multi-nutrient liquid fertilizer (NP) composed of 7.8% N and 21.6% P2O5. Furthermore, ammonia recovery was increased, reaching values up to 94%, through two-step HF-LLMC, and again H3PO4 was used as stripping solution. Finally, water transport from the feed to the stripping phase was estimated to be 0.022 L/m2·h for one-step HF-LLMC with H3PO4. Finally,

Corresponding author at: Chemical Engineering Department, Escola d’Enginyeria de Barcelona Est (EEBE), Universitat Politècnica de Catalunya (UPC)BarcelonaTECH, C/ Eduard Maristany 10-14, Campus Diagonal-Besòs, 08930 Barcelona, Spain. E-mail address: [email protected] (X. Vecino). 1 These authors contributed equally to the work. ⁎

https://doi.org/10.1016/j.cej.2018.12.004 Received 14 September 2018; Received in revised form 8 November 2018; Accepted 3 December 2018 Available online 05 December 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.

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the use of UV–Vis and 2D-Fluorescence was shown to be a successful approach for monitoring pore wetting events.

1. Introduction

of exhausted zeolite beds with basic solutions of NaOH or NaOH/NaCl brines generates ammonia-rich streams with ammonium concentration values up to 5 g NH4+/L [17,23,24]. Thus, researchers have proposed hollow fibre liquid-liquid membrane contactors (HF-LLMCs) as a novel technology for nutrient recovery [23–30]. This technology allows one to recover ammonia from caustic streams (e.g., pH > 12) using strong inorganic acids (HNO3, H3PO4) as stripping solutions. This transforms ammonia into ammonium salts such as NH4NO3, (NH4)2SO4, (NH4)2HPO4 or (NH4)H2PO4, and others which can be used as liquid fertilizers [23,24,31]. HF-LLMCs are based on membranes made of hydrophobic polymers like polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), or polytetrafluoroethylene (PTFE). The driving force of the process is the vapour pressure or concentration difference on both sides (named lumen and shell) of the membrane [28]. For membrane contactors, the membrane chemical composition as well as the geometry of the pores are critical parameters that control the wettability phenomena of the membrane. Moreover, surface tension of the absorption liquid and the interactions between it and the membrane (e.g., contact angle, fouling) can cause membrane wetting [32,33]. However, when a liquid pressure surpasses the breakthrough pressure, the pores of the membrane get wet and the membrane loses its hydrophobicity [34]. The aim of this work is to evaluate different operational parameters for HF-LLMCs (types of acid used as stripping solution or the configuration of the HF-LLMC modules) that are used for ammonia recovery and concentration in order to produce ammonium salt solutions that are useful as liquid fertilizers. Additionally, water transport between the feed and stripping sides was quantified as a function of the strong acid used (H3PO4, HNO3 or a mixture of both). Finally, UV–Vis and 2DFluorescence techniques were used to monitor pore wetting.

Ammonia and ammonium (NH3/NH4+) are common species that present harmful environmental problems because their overload causes excessive plant growth in the ecosystem. This generates the well-known eutrophication phenomena and strongly reduces water quality [1,2]. Furthermore, NH3 is an important air pollutant (through the formation of secondary particulate aerosols) because of its adverse impact on human health as well as climate change [3]. In this sense, ammonia production is considered an industrial source of CO2 emissions causing the greenhouse effect [4,5]. From the total ammonia production in the world, around 85% is consumed as fertilizer, while 15% is used in other industrial applications, such as in fibres, plastics, explosives, and others. Therefore, agriculture is the major source of ammonia in the environmental scenario [6,7]. Nitrogen (N), phosphorus (P), and potassium (K) are primary nutrients used in fertilizer production for soils or crop plants [8]. In regard to primary nutrient content, they can be classified as follows: (i) singlenutrient fertilizers, composed of only one primary nutrient and (ii) multi-nutrient fertilizers, which comprise two or more primary nutrients. Single-nutrient fertilizers can be composed of N, P, K, or magnesium; whereas mixtures of different primary nutrients such as NPK, NP, NK and PK can be considered to be multi-nutrient fertilizers [9]. The global growth of nitrogen and phosphate demand for fertilizer production was 1.4% and 2.2%, respectively, between 2014 and 2018 [10]. For this reason, the global nitrogen and phosphate demand in 2018 is expected to be around 119.4 and 46.6 millions of tonnes, respectively [10]. Additionally, N can be assimilated by plants as NO3− and NH4+, depending on the soil pH. If soil has acid pH, the plants adapt to the NH4+ form, while the NO3− form is preferred by plants when the pH of the soil is basic [8]. On the other hand, urban wastewater effluents are generally composed of a variety of inorganic and organic substances such as organic matter, nitrogen, phosphorus, and others which are considered secondary resources for water, energy, and organic and inorganic fertilizers [11]. Previous efforts have been centred on the recovery of phosphorous as struvite (MgNH4PO4·6H2O). Only 10% of the total N is recovered [12] to produce energy from the organic matter fraction by using anaerobic digestion [13] or to produce organic fertilizers after an appropriate composting process [14]. For this reason, one of the major challenges is to give an added value to nitrogen in wastewater treatment plants (WWTPs). Traditionally, various forms of nitrogen are removed by biological methods [15,16] whereby they are converted into biomass or transformed into N2 (g). However, when biomass is anaerobically digested to produce biogas, N is re-mineralized to NH4+. New technologies are focused on the recovery of ammonium in order to give it an added value. One of these approaches is to collect concentrates from anaerobic digestion where only a marginal fraction of the ammonium ion present is recovered as struvite. Another method is to by recover ammonium ion from main-streams generated when using high rate activated sludge (HRAS) configurations which have higher ammonium loads (up to 100 mg/L) when compared to conventional activated sludge (CAS) [17]. EU regulations have targeted N discharge values for treated wastewater to be lower than 1 mg NH4+/L [18]. Among the new adding value strategies, the use of sorption and membrane based processes (e.g., membrane distillation, forward osmosis, membrane contactors) are postulated solutions [19–22]. Ion-exchange processes by means of zeolites as a novel stage of nitrogen removal in HRAS can be used to reduce ammonium levels (e.g., from 150−100 mg/L to 1–5 mg/L NH4+). However, regeneration

2. Materials and methods 2.1. Reagents Nitric acid (65%, HNO3), phosphoric acid (85%, H3PO4), methanesulfonic acid (CH3SO3H, 99%), sodium hydrogen carbonate (NaHCO3, 99%), and anhydrous sodium carbonate (Na2CO3, 99%) were supplied by Sigma-Aldrich. All chemicals were analytical grade reagents. 2.2. Ammonia-rich streams generated on the regeneration of zeolite beds A new stage for ammonium recovery, based on the use of granular natural zeolites, has been previously used to eliminate the residual ammonium levels after a HRAS and ultrafiltration (UF) processes at a pilot-plant located at the Vilanova i la Geltrú WWTP (Barcelona, Spain) [23,24]. This treatment was able to diminish the ammonia concentration, but a regeneration step was required to recover the adsorbed ammonia inside the zeolites, and to obtain a stream with even higher ammonia concentration values. The feed solution treated in this work was an urban wastewater side-stream (ammonia-rich regenerated stream) containing around 4000 mg NH3/L, other species (Na+, K+, Cl−, Zn2+ and Cu2+), and dissolved organic matter (DOM). 2.2.1. Pre-treatment for dissolved organic matter reduction The ammonia-rich regenerated stream was pre-treated with granular activated carbon (GAC) F400 (Chemviron, Belgium). Batch sorption experiments using different amounts (0.5, 1.25 and 2.5 g) of GAC and 50 mL of the ammonia-rich regenerated stream were carried out to determine the sorbent mass effect on DOM removal. The batch 891

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experiments were evaluated during 2 h at 22 ± 1 °C and qualitatively analysed by UV–Vis at λ = 254 nm (UV–visible, HP8453, Hewlett Packard) as well as by 2D-Fluorescence with an λexcitation range of 200–375 nm and an λemission range of 325–600 nm (Cary eclipse fluorescence spectrophotometer G9800A, Agilent Technologies) to detect DOM removal. Then, a 10 cm internal diameter and 60 cm length column was used in a fixed bed configuration to treat 60 L of the ammonia-rich regenerated stream (with the best solid/liquid ratio determined) maintaining the flow rate at 83.3 mL/min and room temperature at 22 ± 1 °C. The composition changes on samples from batch and column experiments were determined by Inductively Coupling Plasma (ICP) (Optima 8300 and Elan 6000, Perkin Elmer) (P, Zn, Cu, S) and ionic chromatography (Dionex ICS-1000 and ICS-1100 Thermo-Fisher Scientific, USA) (NH4+, Na+, K+, Cl−, Mg2+, Ca2+, PO43−, SO42−). Also, pH (pH GLP 22, Crison) and conductivity (EC-Metro GLP 31, Crison) were measured.

6–7 when using phosphoric acid as the stripping solution in the shell side. On the other hand, HNO3 (65%) was added when using nitric acid or a mixture of both (phosphoric and nitric) as stripping solution in order to fix the pH value around 2–3. The flow rates of both sides of the HF-LLMC were 450 mL/min. As exhibited in Fig. 1, these experiments were conducted in one single-step (Fig. 1a) and two-step (Fig. 1b) HF-LLMC configurations where both streams were re-circulated into the initial tanks in order to increase the ammonia removal. The two-step configuration was carried out with the treated feed water from the single configuration. At regular time intervals, samples were collected from the feed tank to measure the pH, ammonia and other elements concentration, while samples were taken from the stripping tank in order to measure the ammonium salt concentration. All of the assays were carried out in duplicate at room temperature (22 ± 1 °C).

2.3. Experimental HF-LLMC set-up

2.4. Analytical methodologies

The experimental set-up consisted of a HF-LLMC (2.5 × 8 Liqui-Cel® Membrane Contactor X-50 PP fibre) provided from 3 M Company (USA) with an active membrane area of 1.4 m2. The HF-LLMC module worked in a closed loop and a counter-current mode. PVC flexible tubes connected the two tanks used as feed and acid stripping solutions, which are named the lumen and shell side, respectively. This lab mode was prepared following the methodology proposed previously by Licon et al. [23]. Feed solution (ammonia-rich regenerated stream treated by sorbent) volume was 60 L (lumen side), while acid stripping solutions were 0.5 L of 0.4 mol/L H3PO4, 0.4 mol/L HNO3 and a mixture of both (0.3 mol/L HNO3 and 0.1 mol/L H3PO4), depending on the experiment requirements. The pH on the stripping side (shell) was kept constant during experiments by adding concentrated acid. On the one hand, H3PO4 (85%) was added in order to keep the pH at a constant value of

Inductively coupling plasma-optical emission spectrometry (ICPOES) was used to determine mg/L element concentrations (Optima 8300, Perkin Elmer) and ICP mass spectrometry (ICP-MS) was used to determine elements in the μg/L concentration range (Elan 6000, Perkin Elmer). Moreover, experimental samples were analysed by an ionic chromatograph system (Dionex ICS-1000 and ICS-1100 Thermo-Fisher Scientific, USA), equipped with ICS-1000 and ICS-1100 cationic and anionic detectors, respectively, and controlled by using Chromeleon® chromatographic software. For the ion chromatographic quantification, a CS16 column (4 × 250 mm) and an AS23 column (4 × 250 mm) (Phenomenex, Barcelona, Spain) were used for cation and anion determination, respectively. The mobile phase was a 0.03 mol/L CH3SO3H solution for cations and a mixture of 0.8 mmol/L NaHCO3 and 4.5 mmol/L Na2CO3 for anions.

a)

b)

Fig. 1. HF-LLMC scheme (a) one-step and (b) two-step configuration. 892

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2.5. Ammonia mass transfer rate

[32]. However, water vapour transport occurs during the HF-LLMC operation. The water flux (Jw (L/m2·h)) through the membrane is controlled by the water partial pressure difference between the feed and the stripping streams (Δpw) and by the membrane permeability of water vapour (Pw), as described by Eq. (5) [36]:

Ammonia transport across the HF-LLMC (PP membrane) was evaluated as a function of the nature and composition of the stripping phase (H3PO4, HNO3 and a mixture of both). The ammonia flux ( JNH3 (kmol/ m2·s) through the membrane is controlled by the NH3 partial pressure difference between feed and stripping streams (ΔpNH3) and by the overall ammonia mass transfer coefficient (Km(NH3)), as described by Eq. (1):

JNH 3 =

Km (NH3)

pNH 3

RT

=

Km (NH3) (p NH3, f RT

p NH3, s )

JW = PW pw = PW (pWo , f a

W ,f

C0(NH3) K m (NH3)· Am = ·t Ct (NH3) V

C0(NH3)

Cf (NH3)

C0(NH3)

(1)

JW = PW (pWo , f

Jw =

(3)

Cf (NH3, shell) C0(NH3)

W ,f

x

W ,f

pWo , s

W ,s x W ,s )

(6)

Vw Am · t

(7)

where Vw is the water transported from the feed tank to the ammonium salt solution produced in the stripping side (L), Am is the membrane area (m2), and t is the running time of the experiment (h). In order to evaluate the water flux, the water transport was calculated by a mass balance in the shell tank, taking into account the ammonia transported from the feed solution, the concentrated acid added, and its initial and final volume. During the liquid-liquid membrane operation, the membrane can lose its hydrophobicity and therefore pore wetting occurred inside the HF-LLMC pores. Since the initial feed solution contained DOM (the presence of DOM is explained by sorption mechanisms on the zeolite surfaces along the ammonium recovery and concentration stage), its transport to the stripping solution could be explained if pore wetting occurred. In this way, a monitoring program was used to analyse stripping side samples during the experiments by UV–Vis and 2DFluorescence in order to detect if there was DOM transport from the feed to the stripping side. Ion (Na+, Cl−, etc…) concentrations were also determined in the feed and stripping tanks to corroborate that they were not transported.

where Cf (NH3) is the final ammonium concentration in the lumen tank. The concentration factor (CF) of ammonia from lumen to shell side was also determined by Eq. (4):

CF =

(5)

where γw and xw are the water activity coefficient and the mole fraction of water respectively in the feed (f) and stripping side (s). During any operation cycle performed at constant temperature and pressure, for both streams (feed and stripping), water activity (e.g., water transport driving force) will be controlled by the changes in the solution composition (e.g., γw and xw). Then, during the ammonia recovery process, the water activity of the feed side will be higher than its activity on the stripping side due to the increase in salinity because ammonia salts are produced. Moreover, since the ammonia is transported from the lumen to the stripping side and concentrated acid is also added to the stripping solution, the increase of salinity will reduce the water activity of the stripping side and water will be transported from the feed to the stripping side. The water flux Jw (L/m2·h) through HF-LLMC can be calculated following Eq. (7):

(2)

·100

)

where is the water vapour pressure at a given T, and aw is the activity of water in the feed (f) and stripping side (s). Water activity of both streams could be calculated taking into account the ionic composition as it is described by Eq. (6) following Raoult’s law,

where C0(NH3) is the initial NH3 concentration and Ct (NH3) is the NH3 concentration at time “t” both at the feed tank solution (lumen side), Am is the membrane area (m2), V is the feed solution volume (m3), and t is the running time of the experiment (s). Ammonia removal from feed solution was studied by representing ln(C0(NH3) /Ct (NH3) ) over time, obtaining a linear relationship. Then, by means of the slope of the plotted curve it was possible to determine the ammonia mass transfer coefficient (K m (NH3) , m/s) taking into account Eq. (2). Furthermore, the percentage of NH3 removal in the feed side was calculated following Eq. (3):

%NH3 removal =

W ,s

poW

where K m (NH3) is the ammonia mass transfer coefficient (m/s), pNH3,f and pNH3,s are the ammonia partial pressures (atm) of the feed and the acid stripping stream, respectively. R is the ideal gas constant (0.082 atm·m3/kmol·K) and T is the temperature of the system (K). Assuming that the pNH3 is directly proportional to the ammonia concentration, according to the Henry Law and taking into account that the pH of the feed stream (pHf > 13) remained constant during the experimental tests, the ammonia concentration transferred to the acid solution is proportional to the ammonia concentration (Ct(NH3)) in the feed tank. In the stripping stream, the acidity was maintained constant below given pH values to ensure that the total ammonia content will be in the NH4+ form (> 99%). The ammonia partial pressure of the stripping stream (pNH3,s) can be considered to be a very small value when compared with the ammonia partial pressure of the feed (pNH3,f) (pNH3,f ≫ pNH3,s). Under this hypothesis and taking into account the total ammonium/ammonia mass balance and Eq. (1), Km(NH3) (m/s) can be experimentally determined using Eq. (2) [35]:

ln

pWo , s a

(4)

where Cf (NH3, shell) indicates the final NH3 concentration at the shell tank. On the other hand, the final ammonium salt concentrations obtained in the shell tank were determined as %N and %P2O5 content in order to determine the type (single or multi-nutrient) and the composition of the liquid fertilizer obtained by the HF-LLMC process.

3. Results and discussion 3.1. Characterization of the ammonia-rich streams from zeolite sorption stages The ammonia-rich regenerated stream from a new stage for ammonium removal (based on the use of granular natural zeolites after a HRAS stage in a pilot-plant located at the Vilanova i la Geltrú WWTP) is characterized by a strong basicity and a high content of ammonia. The main components of this solution are NaOH with values ranging from 70−80 g NaOH/L and NH3 with values ranging from 3.5−4.5 g/L depending on the sorption/desorption operation cycles of the pilot plant. With such strong basic conditions during the regeneration process,

2.6. Water transport phenomenon The hollow fibres of LLMC are hydrophobic porous membranes (contact angle is 102.1° and pore diameter is 3·10−8 m) that prevent liquids from being transported into the pores. Both solutions (feed and acid stripping) are in contact with the hydrophobic membrane and the selected solute (NH3 (g)) is only transported by diffusion phenomena 893

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As can be seen in Table 1, four main groups of fluorescent compounds can be found associated with fluorescence peaks when treating wastewaters. In this work, only peak A and peak C were detected on the 2D-Fluorescence spectra of the ammonia-rich regenerated stream (Fig. 3a). This implies the presence of fulvic-like and humic-like substances, respectively [37,39,40]. Both groups of organic matter are typically associated with the natural organic matter present in natural water bodies. The matter originates from flora and is incorporated through the drinking water distribution networks. According to Table 1, two more peaks could be found, although they were not detected in this work. In our case, the protein-like substances were removed by the UF pre-treatment before the zeolites stage. On the other hand, as can be seen in Fig. 3b, the fluorescence spectra of the ammonia-rich regenerated stream treated by GAC, shows that the fluorescence intensity was reduced by around 70% indicating that both families of fluorescence compounds were efficiently removed. The remaining fraction of DOM with non-fluorescence response, determined by UV–Vis response, could be associated with high molecular weight matter that was not well absorbed by GAC. Then, it can be concluded that GAC showed a preferential removal of fluorescent organic matter that is associated with humic and fulvic-like compounds. Afterwards, GAC, which is a common sorbent used for DOM removal in wastewater treatments [41], was used in column operation to treat 60 L of the ammonia-rich regenerated stream before being introduced in the HF-LLMC and its characterization was done (Table 2). As can be seen in Table 2, after the GAC pre-treatment, the feed water for the HF-LLMC was still ammonia-rich and with a high pH. Besides, the presence of other cations such as potassium, magnesium, or calcium (from the WWTP) should be mentioned; these are also exchanged on the zeolite sites during the sorption cycles and desorbed under such high NaOH concentrations. Due to the strong basic conditions used for the regeneration step, it is expected that both divalent cations will precipitate as Mg(OH)2(s) and Ca(OH)2(s), respectively and values of these were below the detection limits of quantification. Moreover, other anions, such as Cl−, SO42− or PO43− are present in the ammonia-rich regenerated streams at low levels because they are partially retained on the zeolites by formation of mineral phases with Mg2+ and Ca2+ ions and their solubility in such basic solutions is very small. Moreover, minor trace inorganic elements as Zn2+ and Cu2+ can be also be present at low levels.

Fig. 2. Absorbance values of ammonia wastewater (WW) before (black column) and after adsorption process (grey columns) in a batch system varying the mass of GAC.

almost all of the sorbed ammonium is converted into ammonia. Residual levels of DOM are present in the ammonia-rich regenerated stream because zeolites show a low affinity for organic matter sorption [17]. To prevent organic fouling events on the HF-LLMC, a pre-treatment was carried out to reduce the DOM by using GAC. Fig. 2 shows UV–Vis absorbance values of the ammonia-rich regenerated streams before and after pre-treatment with GAC in batch tests. In this figure, no significant differences were observed in terms of the quantities of sorbent used, obtaining 42.5–54.1% of total DOM removal. Then, if a minimum DOM extraction of 50% is desired, it is necessary to a use minimum of 1.25 g of GAC for treating 50 mL of feed sample. Furthermore, 2D-Fluorescence analyses were carried out in order to study which fractions of the fluorescent DOM were removed during the sorption processes with GAC. Fig. 3 shows the presence of peaks associated with fluorescent compounds before (Fig. 3a) and after the sorption treatment with 1.25 g GAC (Fig. 3b). By means of the 2D-Fluorescence spectra of Fig. 3, it is possible to determine the nature of the DOM composition present on ammonia-rich regenerated streams. Fluorescence peaks can be characterized by excitation/emission zones with higher fluorescence intensity values. Table 1 summarizes the peaks detected in this work along with peaks detected by other authors when analysing conventional activated sludge samples including tertiary treatments.

nm

a)

2.190

b)

1.040

894

Fig. 3. 2D-Fluorescence spectra and UV–Vis values: (a) for ammonia-rich regenerated stream before pre-treatment and (b) ammonia stream treated with 1.25 g GAC in 50 mL of ammoniarich regenerated stream. The Z axis represents the excitation wavelength values from (5 by 5) 225 to 400 nm, whereas the X axis represents the emission wavelengths. The colour index indicates the emission intensity.

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Table 1 Spectral characteristics of the fluorescence peaks of wastewaters. Peak

Excitation/Emission range

Description

Excitation/Emission in this work

A

237–380/375–500

300/400

T B C

275–290/330–352 270–275/304–316 240–260/434–520 339–420/420–520

Fulvic-like [37] Microbial humic-like [38] Humic-like compounds [39] Tryptophan-like, protein-like [38,40] Tryrosine-like, protein-like [38,40] Humic-like compounds [40] Terrestrial humic-like fluorescence in high nutrient and wastewater impacted environments [38] Large molecular size, hydrophobic [39]

treating wastewaters with initial concentrations of 1500 mg/L and 500 mg NH3/L using HNO3 acid as stripping solution, respectively. The ammonia mass transfer coefficient was determined previously by Licon et al. [23] when using HNO3 acid for ammonia nitrate salt production by the same HF-LLMC than in this work, obtaining a Km(NH3) of 0.4 × 10−5 m/s. In the same work, the authors corroborated that the mass transfer coefficient did not vary substantially with the initial ammonia concentration. However, checking Eq. (2), the mass transfer coefficient depends on several parameters, some of which are different than those used in the work of Licon et al. [23], e.g., the ammonia concentration value (around 4 times higher in this work), the feed tank volume (6 times higher in this study), the membrane area (1.4 m2 in both works) and the operational time (24 times longer in this case). For this reason, the Km(NH3) was two orders of magnitude higher than the Km(NH3) value obtained in our work. On the other hand, it was possible to recover ammonia in the stripping solution and concentrate it as ammonium salts, such as (NH4)2HPO4/(NH4)H2PO4 or NH4NO3 if H3PO4 or HNO3 are used as stripping solutions, respectively. Since H3PO4 showed the highest

Table 2 Ammonia-rich regenerated stream characterization after sorbent pre-treatment. Ions

Concentration (mg/L)

NH4+ + Na K+ ClMg2+ Ca2+

3905 ± 177 44272 ± 118 621 ± 16 220 ± 7 < 0.05 < 0.05

Elements

Concentration (mg/L)

Zn Cu Fe S P

1.4 ± 0.1 0.2 ± 0.05 < 0.02 < 0.5 < 0.5

Other parameters pH Electrical conductivity UV–Vis absorbance (λ = 254 nm)

Not detected Not detected 225/450 and 325/450

13.5 ± 0.4 252 ± 0.5 mS/cm 1.0 ± 0.1

a) 4500

HNO3

4000

3.2. Ammonia recovery by HF-LLMC: effect of operational parameters

HNO3/H3PO4

3500

Ammonia concentration variation was studied for all experiments carried out (using H3PO4, HNO3, or a mixture of both acids as stripping solution) in the one-step HF-LLMC configuration as can be seen in Fig. 4a. As it is shown in this figure, ammonia concentration decreased gradually from 4000 mg/L to values around 1000 mg/L after 24 h of operation; this implies an average NH3 removal of 71.1 ± 3.8%. Besides, taking into account the initial ammonia concentration and the ammonia concentration at each experimental time in the feed tank, it was possible to plot its concentration profile (Ln C0/Ct) over time (data not shown). Then, considering the parameters obtained for linear correlation, the mass transfer coefficient was calculated by Eq. (2). The experimental results for the one single-step HF-LLMC configuration using different acid stripping solutions are summarized in Table 3. Table 3 exhibits initial and final ammonia concentrations, ammonia removal (in the feed tank), the calculated mass transfer coefficients, as well as other parameters obtained from the shell side, such as concentration factor, %N, and %P2O5 content. In one single-step HF-LLMC configuration, it was possible to achieve NH3 removal of 69.3 ± 5.5% and 69.8 ± 9.6% using HNO3 and a mixture of HNO3 and H3PO4 as stripping solutions (in shell side), respectively. Nevertheless, the highest NH3 removal was obtained using H3PO4 as acid stripping solution (76.1 ± 5.5%) as well as the highest ammonia mass transfer coefficient (8.8 × 10−7 ± 1.7 × 10−7) (Table 3). Although ammonia removal by HF-LLMC using H3PO4 and HNO3 as stripping acids has been poorly studied, the results herein achieved are comparable with those obtained by previous authors [23,24]. Sancho et al. [24] concluded that lower ammonia removal values were achieved when the initial ammonia concentration in the feed solution was higher. For example, removal percentages of 85% and 98% were obtained when

H3PO4

NH3 (mg/L)

3000 2500 2000 1500 1000 500 0

0

200

400

600

800

1000

1200

1400

1600

Time (min)

b) 4500 4000

Step 1 Step 2

NH3 (mg/L)

3500 3000 2500 2000 1500 1000 500 0

0

200

400

600

800 1000 1200 1400 1600 1800 2000 Time (min)

Fig. 4. Ammonia concentration over time in the feed tank (a) using different acid stripping solutions by one single-step HF-LLMC and (b) using H3PO4 as an acid stripping solution by two-step HF-LLMC. 895

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Table 3 HF-LLMC results using different acid stripping solutions working in one single-step.

One-step HFLLMC

Acid stripping

Feed side

Acid stripping side

NH3 concentration (mg/L)

NH3 removal (%)

Km(NH3) (m/s)

CF

% N (NH4)

% N (NO3)

% P2O5

H3PO4 HNO3 HNO3/ H3PO4

3710 to 890 4050 to 1250 3960 to 1200

76.1 ± 5.5 69.3 ± 5.5 69.8 ± 9.6

8.8 × 10−7 ± 1.7× 10−7 6.5 × 10−7 ± 5.9 × 10−8 6.6 × 10−7 ± 1.0× 10−7

26.3 ± 3.3 17.4 ± 2.4 18.8 ± 1.3

7.8 ± 1.2 7.3 ± 0.7 6.3 ± 1.1

– 4.1 ± 0.6 3.7 ± 0.7

21.6 ± 4.4 – 0.1 ± 0.04

Operational time (h)

23.5 24.0 24.0

CF = concentration factor. Table 4 HF-LLMC results working with H3PO4 as an acid stripping solution by two-steps. Acid stripping

Feed side NH3 concentration (mg/L)

Step 1 Step 2 Two-steps HF-LLMC (global)

H3PO4

3710 to 890 1040 to 240 3710 to 240

Acid stripping side NH3 removal (%) 76.1 ± 5.5 77.4 ± 4.5 93.6 ± 1.6

Km(NH3) (m/s) −7

−7

8.8 × 10 ± 1.7× 10 1.9 × 10−6 ± 4.1 × 10−7 8.2 × 10−7 ± 1.5 × 10−7

CF

% N (NH4)

% N (NO3)

% P2O5

26.3 ± 3.3 64.8 ± 3.9

7.8 ± 1.2 2.6 ± 0.3

– –

21.6 ± 4.4 8.2 ± 0.8

CF = concentration factor.

ammonia removal rate, the ammonia concentration factor value was also higher than that achieved when using other stripping acids. Thus, the ammonia concentration factor reached values up to 26.3 ± 3.3 when this acid was used; whereas similar ammonia concentration factors values around 18 were obtained using HNO3 or a mixture of HNO3/ H3PO4 in the stripping tank (Table 3). It is possible to confirm that HF-LLMC is a membrane technology that is able to concentrate ammonia streams. However, as exhibited in Fig. 4, it was not possible to totally recover the ammonia by only one single-step HF-LLMC. Moreover, since higher ammonia recovery was obtained when using phosphoric acid as stripping solution in one-step HF-LLMC, this acid was used to conduct experiments in two-step configurations in order to increase the ammonia removal from the ammonia-rich regenerated stream by HF-LLMC. Then, the resulting feed solution after the first HFLLMC step was fed in the lumen side (60 L with an ammonia concentration around 1000 mg/L), and 0.5 L of 0.4 mol/L H3PO4 was used as stripping solution in the shell side of the second HF-LLMC step. Thus, for industrial applications, it would be possible to use a two-step HFLLMC configuration where the feed solution passed through HF-LLMC in series, whereas the acid stripping solution could enter into the twostages HF-LLMC in parallel (Fig. 1b). The ammonia concentration profile in a two-step HF-LLMC configuration is depicted in Fig. 4b. As can be seen, by means of the second step, it was possible to decrease the ammonia concentration up to 240 mg/L. Moreover, it was possible to remove more than 93.6 ± 1.6% of the initial ammonia concentration by the two-steps process (Table 4). The ammonia mass transfer coefficient (8.2 × 10−7 ± 1.5 × 10−7) obtained using the two-step HFLLMC configuration was similar to that obtained with a single-step. For this reason, it can be concluded that same ammonia recovery efficiency was obtained in one-step and two-stage HF-LLMC configurations. As can be seen in Table 4, the low initial concentration of ammonia fed in step 2 implied that the ammonia concentration factor, calculated by Eq. (4), was higher (65 times by the second-step instead of 26 by the first-step). Furthermore, only ammonia transport occurred from the lumen to the shell side, since at the working pH (13.4 ± 0.1) ammonia is a gas and it is the only one that can be transferred through the HF-LLMC. The pH and concentration of all elements present in the ammonia-rich regenerated stream were monitored over time during the trials. The pH was constant in lumen and shell sides, and for all elements evaluated (Na+, K+, Cl−, Zn2+ and Cu2+), concentration values were also constant in the feed tank and they were not transferred into the acid

stripping solution (data not shown). In this sense, metal ions and other dissolved species could not be converted to gas forms, so they could not be transported through the membrane pores. Therefore, HF-LLMC avoids the transfer of toxic metal ions (e.g., Zn2+ and Cu2+) from the feed stream to the ammonium salt solution. 3.3. Ammonium salt production as liquid fertilizer In regard to liquid fertilizer composition, the N content (in %) was calculated taking into account the NH4+ and NO3− forms (from ammonia and nitric acid components, respectively) of the N carrier; while the phosphorous content (in %) was expressed as P2O5 carrier (from the phosphoric acid). The ammonium salt composition was determined in the stripping tank after the HF-LLMC process. From this point of view, depending on the acid used as stripping solution, it was possible to obtain different liquid fertilizer types; for instance, (i) a single-nutrient fertilizer composed only of N content (from NH4+ and NO3−) using HNO3 and (ii) multi-nutrient fertilizers, which are a mixture of N (from NH4+) and P2O5, named NP fertilizer, using H3PO4. The liquid fertilizer compositions after one single-step HF-LLMC are collected in Table 3, observing that N fertilizer was composed of around 7.3% NH4+ and 4.1% NO3−, while NP fertilizer consisted of about 7.8% N and 21.6% P2O5. Otherwise, when the HNO3/H3PO4 mixture was used as stripping solution, the liquid fertilizer composition was around 6.3%, 3.7% and 0.1% of NH4+, NO3− and P2O5, respectively. In this case, a smaller content of P2O5 was observed because phosphoric acid concentration (0.1 mol/L) was lower than nitric acid concentration (0.3 mol/L) in the mixture prepared in the stripping tank. Thus, when ammonia gas reacted with acid stripping solution, more N content was formed in comparison with P2O5. For this reason, it can be concluded that this product may also be considered a single-nutrient fertilizer. Using the two-step HF-LLMC configuration, the high initial concentration of ammonia that was observed in step 1 produced higher percentages of N and P2O5, because more ammonia was transferred to the stripping solution and more concentrated acid was used to stabilize the pH (Table 4). These results can be compared with the existing literature for ammonia salt production by HF-LLMC using phosphoric and nitric acid. One may conclude that higher N and P2O5 percentages were obtained in this work because a higher initial ammonia concentration was treated. For instance, Licon et al. [23] produced 9 g/L (NH4)2HPO4 and 9 g/L NH4NO3 which is equivalent to 0.2% N and 0.5% P2O5 using H3PO4 and 896

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0.3% N using HNO3, respectively. On the other hand, results obtained in this work, taking into account the liquid fertilizer compositions, are in agreement with the reference percentage of fertilizer production. That is, the nutrient composition as N and P2O5 were around 15–25% for both nutrients according to the Austria Environment Agency [9]. Moreover, the Fertiberia company, which is one of the major NPK producers in the EU [42], manufactures different types of liquid fertilizers in which the N content can be 20–32%. Consequently, HF-LLMC is a promising technology for recovering nitrogen from wastewaters, converting it into ammonium salts, and then using it as liquid fertilizer. On the one hand, HF-LLMC is an alternative method to biological (e.g., nitrification-denitrification) or anammox processes. It not only minimizes sludge production and energy consumption in WWTPs, but also to increases the nitrogen value. On the other hand, HFLLMCs opens an industrial application in the fertilizer industry, because this technology allows for different types of fertilizers to be created depending on the acid used as the stripping solution in the process.

As can be seen in Fig. 5, which compares the water flux through the HF-LLMC in a single-step, lower water flux over time was achieved when phosphoric acid was used, and the final value obtained was 0.022 ± 0.006 L/m2·h. Water transport values obtained are in concordance with the ammonia transport results, since the higher ammonia recovery was obtained using H3PO4 where the lower water transport fluxes were measured. Moreover, final water flux in the second-step was 0.036 ± 0.005 L/m2·h (profile not shown). This is even lower than the final water flux that occurred when nitric acid (0.041 ± 0.007 L/m2·h) or a mixture of HNO3/H3PO4 (0.042 ± 0.004 L/m2·h) were employed in one-step HF-LLMC. For this reason, it can be concluded that the twostep configuration is a viable technology for ammonia recovery, although water transport occurred in both steps. Therefore, water transport has to be considered when using HFLLMC, since it has been proven that it always occurs and it is necessary to close the mass balance of the produced liquid fertilizer solution as ammonium salts. Water transport has been widely studied for membrane distillation processes [43,44] and gas-liquid membrane contactors technology [34,45,46], but few studies were found in the literature that consider water transport through liquid-liquid membrane contactors [47]. On the other hand, pore wetting has been checked during the ammonium recovery processes. Samples from the stripping circuit were collected and UV–Vis and 2D-Fluorescence spectra were monitored (Fig. 6), apart from ion chromatography and ICP analysis. As was mentioned previously, no ion transport was observed from the feed to the stripping side. As can be seen in Fig. 6a after GAC treatment, organic matter was still present in the ammonia-rich stream regenerated from zeolites, which was used as the initial feed for the HF-LLMC process, although its emission fluorescence intensity was lower when compared to the initial wastewater without pre-treatment (Fig. 3a). Besides, 2D-Fluorescence spectra and UV–Vis data of the feed solution after the HF-LLMC process are not shown because the values were constant in this side. On the other hand, as shown in Fig. 6b, no organic matter was detected in the stripping side after the HF-LLMC process. Then, it is suggested that no organic matter transport occurred from the feed to the stripping side, and for this reason, it can be supposed that no pore wetting occurred during the HF-LLMC experiments. Besides, 2D-Fluorescence and UV–Vis have been proven to be valuable monitoring techniques to detect pore wetting events when using HF-LLMC.

3.4. Water transport in HF-LLMC: the effect of different acid stripping solutions Water transport was calculated for all the experiments taking into account Eq. (7) and the water flux values obtained using different acid stripping solutions (H3PO4, HNO3 and a mixture of both) in the one-step configuration are depicted in Fig. 5. 0.08 0.06

Jw (L/m2·h)

0.04 0.02 0.00 -0.02

0.0

5.0

10.0

15.0

20.0

25.0

HNO3 HNO3/H3PO4 H3PO4

-0.04 -0.06 -0.08 -0.10

Time (h)

Fig. 5. Water flux (Jw) through one-step HF-LLMC using different acid stripping solutions.

nm

a)

0.686

b)

0.000

897

Fig. 6. 2D-Fluorescence spectra and UV–Vis values: (a) in the feed solution after pre-treatment in column system with GAC at initial time and (b) after one-single HF-LLMC configuration (using H3PO4 as stripping solution) in the shell side. The Z axis represents the excitation wavelength values from (5 by 5) 225 to 400 nm, whereas the X axis represents the emission wavelengths. Colour index indicates the emission intensity.

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4. Conclusions

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The promising results obtained in this work suggest that an ammonia-rich regenerated stream could be a resource for ammonia recovery as liquid fertilizer by using hollow fibre liquid-liquid membrane contactors. Additionally, it can be stated that the fertilizer composition can change according to the type of acid used in the stripping solution. This makes possible to produce fertilizers “à la carte” and consequently to increase their potential in different industrial applications. In this case, using HNO3 as the acid in a stripping solution produces a N fertilizer (single-nutrient fertilizer); whereas when H3PO4 was used, a NP fertilizer (multi-nutrient fertilizer) was obtained. On the other hand, high ammonia removal values were obtained, 76% and 94%, through one-step and two-step HF-LLMC processes respectively, although water transport phenomenon in this technology was observed. Acknowledgments This research was supported by the Waste2Product project (CTM2014-57302-R) and by R2MIT project (CTM2017-85346-R) financed by the Spanish Ministry of Economy and Competitiveness (MINECO) and the Catalan Government (2017-SGR-312), Spain. As well, Xanel Vecino thanks MINECO for her Juan de la Cierva contracts (ref. FJCI-2014-19732 and ref. IJCI-2016-27445). Authors would also like to acknowledge J.L. Beltrán for his valuable comments on the 2DFluorescence analysis; and I. Sancho and A. Mayor for the supply of ammonia-rich regenerated stream from Vilanova i la Geltrú WWTP (Barcelona). References [1] M. Perez-Ameneiro, X. Vecino, L. Vega, R. Devesa-Rey, J.M. Cruz, A.B. Moldes, Elimination of micronutrients from winery wastewater using entrapped grape marc in alginate beads, CyTA J. Food 12 (2014) 73–79, https://doi.org/10.1080/ 19476337.2013.797923. [2] J. González-Martínez, A. Calderón, K. González-López, New concepts of microbial treatment processes for the nitrogen removal: effect of protein and amino acids degradation, Amino Acids 48 (2016) 1123–1130, https://doi.org/10.1007/s00726016-2185-4. [3] S.N. Behera, M. Sharma, V.P. Aneja, R. Balasubramanian, Ammonia in the atmosphere: a review on emission sources, atmospheric chemistry and deposition on terrestrial bodies, Environ. Sci. Pollut. Res. 20 (2013) 8092–8131, https://doi.org/ 10.1007/s11356-013-2051-9. [4] M. Sadowski, A. Romańczak, I. Kargulewicz, The role of industrial processes in the reduction of selected greenhouse gases emission, Environ. Prot. Nat. Resour. 27 (2016) 28–31 10.1515/OsZn-2016-0023 Maciej. [5] P. Bains, P. Psarras, J. Wilcox, CO2 capture from the industry sector, Prog. Energy Combust. Sci. 63 (2017) 146–172, https://doi.org/10.1016/j.pecs.2017.07.001. [6] A.N. Evolving, I. Essential, Overview of PotashCorp and Its Industry 2008, 2013. [7] M. Appl, Ammonia: principles and industrial practice, Ammon. Princ. Ind. Pract. Wiley-VCH Verlag GmbH, 2007, , https://doi.org/10.1002/9783527613885. fmatter. [8] H.W. Scherer, Fertilizers and fertilization, Encycl. Soils Environ. (2005) 20–26. [9] E.A.A. Umweltbundesamt, production of ammonia, nitric acid, urea and n - fertilizer – Industrial Processes used, 2017. [10] FAO, World fertilizer trends and outlook to 2018, 2015. [11] S. Bagchi, R. Biswas, T. Nandy, Autotrophic ammonia removal processes: ecology to technology, Crit. Rev. Environ. Sci. Technol. 42 (2012) 1353–1418, https://doi. org/10.1080/10643389.2011.556885. [12] M. Hermassi, C. Valderrama, J. Dosta, J.L. Cortina, N.H. Batis, Evaluation of hydroxyapatite crystallization in a batch reactor for the valorization of alkaline phosphate concentrates from wastewater treatment plants using calcium chloride, Chem. Eng. J. 267 (2015) 142–152, https://doi.org/10.1016/j.cej.2014.12.079. [13] P.M. Sutton, B.E. Rittmann, O.J. Schraa, J.E. Banaszak, A.P. Togna, Wastewater as a resource: a unique approach to achieving energy sustainability, Water Sci. Technol. 63 (2011) 2004–2009, https://doi.org/10.2166/wst.2011.462. [14] H. Ni, X.M. Fan, H.N. Guo, J.H. Liang, Q.R. Li, L. Yang, H. Li, H.H. Li, Comprehensive utilization of activated sludge for the preparation of hydrolytic enzymes, polyhydroxyalkanoates, and water-retaining organic fertilizer, Prep. Biochem. Biotechnol. 47 (2017) 611–618, https://doi.org/10.1080/10826068. 2017.1286599. [15] A. González-Martínez, J.M. Poyatos, E. Hontoria, J. Gonzalez-Lopez, F. Osorio, Treatment of effluents polluted by nitrogen with new biological technologies based on autotrophic nitrification-denitrification processes, Recent Pat. Biotechnol. 5 (2011), https://doi.org/10.2174/187220811796365671.

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