A systematic study of ammonia recovery from anaerobic digestate using membrane-based separation

  Citation: Rivera, F.; Muñoz, R.; Prádanos, P.; Hernández, A.; Palacio, L. A Systematic Study of Ammonia Recovery from Anaerobic Digestate Using Membrane-Based Separation. Membranes 2022,12, 19. https://doi.org/10.3390/ membranes12010019 Academic Editors: Heitor Bento and Cristiano E. Rodrigues Reis Received: 9 December 2021 Accepted: 23 December 2021 Published: 24 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). membranes Article A Systematic Study of Ammonia Recovery from Anaerobic Digestate Using Membrane-Based Separation Fanny Rivera 1,2, Raúl Muñoz 1,3 , Pedro Prádanos 1,2 , Antonio Hernández 1,2 and Laura Palacio 1,2,* 1Institute of Sustainable Processes, University of Valladolid, 47011 Valladolid, Spain; [email protected] (F.R.); [email protected] (R.M.); [email protected] (P.P.); [email protected] (A.H.) 2Department of Applied Physics, Science Faculty, University of Valladolid, 47011 Valladolid, Spain 3Department of Chemical Engineering and Environmental Technology, University of Valladolid, 47011 Valladolid, Spain *Correspondence: [email protected] Abstract: Ammonia recovery from synthetic and real anaerobic digestates was accomplished using hydrophobic flat sheet membranes operated with H 2 SO 4 solutions to convert ammonia into ammonium sulphate. The influence of the membrane material, flow rate (0.007, 0.015, 0.030 and 0.045 m3h−1 ) and pH (7.6, 8.9, 10 and 11) of the digestate on ammonia recovery was investigated. The process was carried out with a flat sheet configuration at a temperature of 35 ◦ C and with a 1 M, or 0.005 M, H 2 SO 4 solution on the other side of the membrane. Polytetrafluoroethylene membranes with a nominal pore radius of 0.22 µ m provided ammonia recoveries from synthetic and real digestates of 84.6% ± 1.0% and 71.6% ± 0.3%, respectively, for a membrane area of 8.6 × 10 −4 m 2 and a reservoir volume of 0.5 L, in 3.5 h with a 1 M H 2 SO 4 solution and a recirculation flow on the feed side of the membrane of 0.030 m 3 h −1 . NH 3 recovery followed first order kinetics and was faster at higher pHs of the H 2 SO 4 solution and recirculation flow rate on the membrane feed side. Fouling resulted in changes in membrane surface morphology and pore size, which were confirmed by Atomic Force Microscopy and Air Displacement Porometry. Keywords: ammonia recovery; anaerobic digestate; flat sheet membranes; mass transfer; membrane fouling 1. Introduction A change in the perception of the uses of anaerobic digestion has occurred over past decades. Thus, anaerobic digestion was initially considered as a cost-competitive technology for organic matter stabilization in wastewaters and solid waste, then a sustainable platform for renewable electricity and heat generation via biogas production, and more recently, it has been regarded as the potential core of a multiproduct biorefinery. Energy and carbon in the form of biogas, along with nutrients dissolved in anaerobic effluents (typically called digestates), represent nowadays the main by-products from anaerobic waste treatment. NH 3 recovery from digestates will prevent the pollution of natural water bodies, which is desirable as this nitrogenous compound is toxic to fish, increases oxygen demand and induces eutrophication [ 1 ]. NH 3 is also harmful to humans as it causes respiratory problems and is considered a precursor of N 2 O, a potent greenhouse gas [ 2 – 4 ]. Therefore, NH 3 recovery from digestates is crucial to improve the environmental sustainability of anaerobic digestion processes and could bring additional economic benefits. Unfortunately, only 10% of the nitrogen present in wastewater is recovered by conventional wastewater treatment plants (WWTPs) [ 5 – 7 ]. Today, there is a wide range of commercial technologies to remove ammonia from wastewater, but most of them entail high operating costs [ 8 ] and environmental impacts resulting from nitrogen conversion and release to the atmosphere. Thus, conventional physical/chemical technologies are based on selective ion exchange, air stripping, chemical precipitation, adsorption, advanced oxidation processes, breakpoint Membranes 2022,12, 19. https://doi.org/10.3390/membranes12010019 https://www.mdpi.com/journal/membranes Membranes 2022,12, 19 2 of 14 chlorination, etc. [ 9 – 12 ]. For instance, NH 3 stripping ranks among the most popular, but energy demanding, technologies available to remove nitrogen from high-strength wastewater, with an average energy consumption of 4 kwh per kg of nitrogen [ 13 ]. On the other hand, conventional nitrogen removal in domestic WWTPs relies on nitrification, which is the conversion of ammoniacal nitrogen (NH 3 -N) to NO− 3 , and denitrification, which is the conversion of NO− 3 to N 2 gas [ 14 – 16 ]. In this context, 1.5 kWh of electricity are typically required to remove 1 kg of N in denitrification/nitrification or anammox processes [ 17 ]. In addition, conventional biological nitrogen removal in WWTPs entails 0.9 kg CO 2 eq per m 3 of treated water [ 18 , 19 ]. Therefore, there is an urgent need to develop and implement cost-competitive and sustainable NH 3 recovery technologies in high strength wastewater such as digestates [20–22]. Recently, the use of membranes for NH 3 recovery from high strength wastewater has been proposed as a cost-competitive and environmentally friendly approach to upgrade residual nitrogen. For example, Brennan et al. [ 23 ] made an extensive analysis of costs demonstrating the feasibility of NH 3 recovery by membranes over conventional methods at a pilot plant scale. Membrane contactors require less energy because of their high specific surface area, which provides a faster NH 3 separation. In this context, membrane contactors have been successfully implemented for NH 3 recovery from gaseous emissions [ 24 – 26 ]. Membrane contactors have also been implemented to support a direct NH 3 recovery from wastewater [ 27 – 29 ]. This approach is based on hydrophobic membranes made of polymers highly permeable to NH 3 , which is transported through the membrane pores into the receiving liquid phase recirculating through the permeate side [ 20 , 21 , 30 ]. This permeating NH3reacts with an acid contained in the permeate side of the membrane. For the process to be efficient, ammoniacal nitrogen has to be present in its volatile form (NH 3 ), which can be ensured by increasing pH and temperature [ 22 ]. Ammonium sulphate, a commercial chemical fertilizer, can be generated during membrane-based NH 3 recovery when using H 2 SO 4 to boost NH 3 diffusion (Equation (1)) [ 31 ]. The use of H 3 PO 4 and HNO 3 on the permeate side also results in the generation of chemical fertilizers such as ammonium phosphate and ammonium nitrate, respectively [ 27 – 29 ]. In this regard, Damtie and coworkers concluded that H 2 SO 4 mediates a more effective ammonia capture than H 3 PO 4 and HNO3[32]. 2NH3+H2SO4→(NH4)2SO4(1) In this work, the performance of commercial flat sheet membranes for NH 3 recovery from synthetic and real digestates was systematically evaluated. To optimize the operation conditions, the influence of digestate pH, within the range of coexisting NH 4+ , NH 3 [ 33 ], and recirculation flow rate, was investigated using different concentrations of sulfuric acid on the other side of the membrane. To make this new ammonia process accessible, our proposal involves the use of commercial membranes that were previously employed for other applications. By scanning the variables that can be controlled at the place of installation, we reached the optimal conditions, without using the latest generation and non-commercial materials. Ultimately, our objective was to propose a solution within the reach of the farmer who intends to implement it in their facilities. Finally, membrane fouling was studied using Atomic Force Microscopy (AFM) to examine the surface morphology of the membranes and Air Displacement Porometry (ADP) to determine the corresponding changes of the pore sizes of the membranes. 2. Materials and Methods 2.1. Real and Synthetic Digestate Characterization The synthetic digestate (SD) composition mimicked a real digestate and consisted of 5.0 g NaHCO 3 , 0.85 g C 8 H 5 KO 4 , 0.73 g peptone from casein, 1.70 g NH 4 Cl, 0.90 g CO(NH 2 ) 2 , 0.224 g K 2 HPO 4 , 0.0175 g NaCl, 0.01 Ca 2 Cl and 0.005 g MgSO 4 (per litre of distilled water) [ 34 ]. All reagents were purchased from PANREAC (Panreac, Química SAU, Barcelona, Spain). The real digestate (RD) used in this study was supplied by Valladolid WWTP (Spain). This digestate was obtained from the anaerobic digestion of mixed sludge Membranes 2022,12, 19 3 of 14 and on-site centrifuged prior to use to eliminate suspended solids. The concentration of NH 3 in SD and RD averaged 605.8 ± 0.1 and 678.9 ± 0.6 ppm, respectively, while pHs averaged 7.57 ±0.05 and 8.99 ±0.08, respectively. 2.2. Experimental Setup A schematic representation of the experimental setup used for ammonia recovery from digestates is shown in Figure 1. The target digestate (SD or RD) was continuously circulated using a peristaltic pump (Watson–Marlow Sci-Q 323, Spirax–Sarco Engineering plc, Cheltenham, England, UK) over the active layer of the membrane in a customized cell module [ 35 ]. A sulfuric acid solution was recirculated using a similar peristaltic pump on the support layer of the membrane. Both digestate and sulfuric acid solutions were maintained at 35 ◦ C in a thermostatic bath (HAAKE type E12, Thermo Fisher Scientific, Waltham, MA, USA) in 0.5 L enclosed Erlenmeyer bottles. Membranes 2022, 11, x FOR PEER REVIEW 3 of 14 0.224 g K2HPO4, 0.0175 g NaCl, 0.01 Ca2Cl and 0.005 g MgSO4 (per litre of distilled water) [34]. All reagents were purchased from PANREAC (Panreac, Química SAU, Barcelona, Spain). The real digestate (RD) used in this study was supplied by Valladolid WWTP (Spain). This digestate was obtained from the anaerobic digestion of mixed sludge and on-site centrifuged prior to use to eliminate suspended solids. The concentration of NH3 in SD and RD averaged 605.8 ± 0.1 and 678.9 ± 0.6 ppm, respectively, while pHs averaged 7.57 ± 0.05 and 8.99 ± 0.08, respectively. 2.2. Experimental Setup A schematic representation of the experimental setup used for ammonia recovery from digestates is shown in Figure 1. The target digestate (SD or RD) was continuously circulated using a peristaltic pump (Watson–Marlow Sci-Q 323, Spirax–Sarco Engineering plc, Cheltenham, England, UK) over the active layer of the membrane in a customized cell module [35]. A sulfuric acid solution was recirculated using a similar peristaltic pump on the support layer of the membrane. Both digestate and sulfuric acid solutions were maintained at 35 °C in a thermostatic bath (HAAKE type E12, Thermo Fisher Scientific, Waltham, MA, USA) in 0.5 L enclosed Erlenmeyer bottles. Figure 1. Schematic representation of the lab scale ammonia recovery system. The membranes used and their main characteristics are shown in Table 1. Polytetrafluoroethylene (PTFE) and polyvinylidene difluoride (PVDF) were selected as model commercial membranes based on their ideal properties for the separation of light molecules from water in a gaseous phase. Table 1. Description of the flat sheet membranes studied. Membrane Material Pore Size Nominal Thickness (µm) Contact Angle (θ) Porosity (%) Wettability Manufacturer PVDF-100 PVDF 100 kDa 160 130–135 * hydrophobic KOCH PVDF-0.10 PVDF 0.10 µm 130 130–135 * hydrophobic Sterlitech PTFE-0.20 PTFE 0.20 µm 139 142 * hydrophobic Pall Gelman PTFE-0.22 PTFE 0.22 µm 175 150 70 hydrophobic Millipore PTFE-0.45 PTFE 0.45 µm 135 155 * hydrophobic Pall Gelman * Information not supplied by manufacturers. Figure 1. Schematic representation of the lab scale ammonia recovery system. The membranes used and their main characteristics are shown in Table 1. Polytetrafluoroethylene (PTFE) and polyvinylidene difluoride (PVDF) were selected as model commercial membranes based on their ideal properties for the separation of light molecules from water in a gaseous phase. Table 1. Description of the flat sheet membranes studied. Membrane Material Pore Size Nominal Thickness (µm) Contact Angle (θ)Porosity (%) Wettability Manufacturer PVDF-100 PVDF 100 kDa 160 130–135 * hydrophobic KOCH PVDF-0.10 PVDF 0.10 µm 130 130–135 * hydrophobic Sterlitech PTFE-0.20 PTFE 0.20 µm 139 142 * hydrophobic Pall Gelman PTFE-0.22 PTFE 0.22 µm 175 150 70 hydrophobic Millipore PTFE-0.45 PTFE 0.45 µm 135 155 * hydrophobic Pall Gelman * Information not supplied by manufacturers. 2.3. Operational Conditions and Process Evaluation An initial test series was carried out with all the membranes initially selected (Table 1) using synthetic digestate at two different pHs (7.6 and 10) and two sulfuric acid concentrations (0.005 and 1 M) at 35 ◦ C with recirculation flow rates of 0.030 m 3 h −1 . Trials were carried out with intermediate sulfuric acid concentration, but with no relevance in the results. A second series of experiments was conducted with PTFE-0.22 and real digestate at pHs of 8.9 and 10 using recirculation flow rates of 0.030 m 3 h −1 and a sulfuric acid concentration of 1 M. Finally, a third series of experiments was carried out with both RD Membranes 2022,12, 19 4 of 14 and SD and the PTFE-0.22 membrane, at a pH of 10, with a sulfuric acid concentration of 1 M and recirculation flow rates of 0.007, 0.015, 0.030 and 0.045 m 3 h −1 . The membrane system was washed twice for 1 h after each experiment with distilled water when using SD, and with tap water (which contains sodium hypochlorite) when using RD. Samples of the digestate solution were drawn every 30 min over 3.5 h to analyse the concentration of NH 3 and obtain pH values. Finally, a systematic evaluation of membrane fouling was performed by determining the pore size distribution of PTFE-0.22 membranes after their use with SD and RD, at a pH of 10 and with a sulfuric acid concentration of 1 M at different recirculation flow rates (0.007, 0.015, 0.030 and 0.045 m 3 h −1 ). All experiments were carried out in duplicate. 2.4. Analytical Methods The values of pH and temperature were monitored using a HI5522 pH meter Hanna Instruments (Woonsocket, RI, USA) and a Basic 20 pH meter (Crison Instruments, S.A., Alella, Barcelona, Spain) in the liquids on both sides of the membrane. NH 3 was measured using the Nessler analytical method in a Shimadzu UV-160 A spectrophotometer at 425 nm (Shimadzu, Kyoto, Japan). 2.5. Membrane Characterization Techniques Surface morphology was analysed by using Atomic Force Microscopy. Images were obtained with a Nanoscope IIIA microscope (Digital Instruments, Veeco Metrology Group, Chadds Ford, PA, USA) using the Tapping mode. Pore size distribution was analysed by the extended bubble point method, or Air Displacement Porometry (ADP), using a Coulter ® Porometer-II manufactured by Coulter Electronics (Porometer, Aptco Invest, Dulles, VA, USA) [ 36 ]. Samples were first wetted with an electronic liquid FC-43 (Fluorinert TM , 3 M, St. Paul, MN, USA) of low surface tension ( γ 1.6 × 10 −2 N m −1 ), low vapour pressure (192 Pa) and low reactivity, that can be assumed to fill all the pores given a zero contact-angle with the membrane material. The wetted sample was subjected to increasing pressure, applied by a compressed clean and dry air source. The pressure range for PTFE 0.22 membrane was 0.09–0.60 µm. 2.6. Theoretical Methods The overall mass transfer coefficient of NH 3 was calculated with Equation (2). according to [21]. 1 kov =1 ks +1 km(2) where k s and k m are the mass transfer coefficients in the digestate side and within pores, respectively. The resistance on the acid solution side can be considered negligible (i.e., mass transfer coefficient on the acid side is much larger than k s and k m ). In this context, the mass transfer coefficient on the digestate side can be estimated using Equation (3): ks=DAWSh DH (3) where D AW ,Sh and D H are: diffusion coefficient of NH 3 in water (calculated using the software ASPEN (AspenTech, Bedford, MA, USA) at 35 ◦ C), the Sherwood dimensionless number [ 37 ], and the hydraulic diameter, respectively. Similarly, the mass transfer coefficient within pores can be calculated by Equation (4) [26,38]: ks= eDij τδ (4) Membranes 2022,12, 19 5 of 14 where τ , δ ,D ij and e are: the tortuosity, the wall thickness of the membrane, the diffusion coefficient of NH 3 in the air gap within pores, and the porosity of the membrane, respectively. Accepting Knudsen regime [21]: Dij =dp 3r8RT πm(5) in terms of the pore diameter and NH3molar mass m. The molar theoretical flux of NH 3 across the membrane can be estimated using Equation (6) [39]: JT=kov∆C(6) here, ∆C is the difference in NH 3 concentrations between the feed and permeate sides. Finally, NH 3 recovery seems to follow first-order kinetics, which can be estimated using Equation (7) [40,41]: ln C0 Ct =kov Am Vt t(7) where k ov ,A m /V t and tare the overall mass transfer coefficient, the effective ratio of membrane area to feed volume of the digestate, and the elapsed time, respectively. C 0 and C t are the concentration of ammonia at time zero and time tin the digestate, respectively. NH3recovery (R) ranges between 0 and 1 (Equation (8)): R=1−Ct C0 =1−e−kov Am Vtt≡1−e−αt(8) here α=kov Am Vt is the rate of concentration decrease and recovery increase. Therefore, the time needed to reach a given Rcan be estimated using Equation (9): t(R)=−ln(1−R) kov Am Vt =−ln(1−R) α(9) 3. Results and Discussion 3.1. Influence of Membrane Material, pH and H2SO4Concentration Five flat sheet membranes (Table 1) were investigated at a recirculation flow rate of 0.030 m 3 h −1 , a pH of 10 and a concentration of H 2 SO 4 of 1 M, to elucidate the optimum material, pore size and wettability for NH 3 recovery from a synthetic digestate. PTFE membranes exhibited a superior performance to PVDF membranes in terms of NH 3 recovery at a pH of 10 (Figure 2). In this case, the highest NH 3 removal recovery after 3.5 h ( 77.7% ±0.4% ) was provided by PTFE-0.22. This removal efficiency was 3.6-fold higher than those obtained for the two PVDF membranes tested. As shown in Table 1, PTFE-0.22 has a contact angle ( θ ) of 150 degrees [ 42 ], which corresponds to a rather hydrophobic surface [ 43 ]. In fact, a membrane with θ > 140 ◦ could be considered super-hydrophobic according to Tylkowski et al. [ 43 ]. The superior performance of the PTFE-0.22 membrane could be also explained by its asymmetric structure, as membranes with symmetric structures, with similar thickness, have been consistently shown to be less permeable [ 20 ]. Therefore, the following experiments were also conducted using PTFE-0.22 membranes. Membranes 2022,12, 19 6 of 14 Membranes 2022, 11, x FOR PEER REVIEW 6 of 14 Figure 2. Influence of the type of membrane on ammonia recovery, after 3.5 h, in SD with a recirculation flow rate of 0.030 m3 h−1, a pH of 10 and a concentration of H2SO4 of 1 M. Vertical bars represent the standard deviation obtained from duplicate measurements. The influence of the digestate pH was also studied. This parameter determines NH3 mass transfer as it shifts the equilibrium NH  ↔NH +H. In our study, higher ammonia recoveries were achieved by increasing the pH of the synthetic digestate because of the increase in the NH3 concentration gradient (Figure S1 in the Supplementary material). Nevertheless, an increase in the pH of the digestate above 10 does not entail large improvement in the ammonia removal [20]. Operating the process at a pH of 11 with a PTFE- 0.22 membrane, a recirculation flow rate of 0.030 m3 h−1 and a concentration of H2SO4 of 1 M, resulted in a NH3 recovery of 82.8% ± 0.4%, only 10% higher than the recoveries recorded at a pH of 10. NH3 recovery was lower in real digestate than in synthetic digestate using a PTFE-0.22 membrane at pHs of 10 and 7.6, the latter pH supporting lower NH3 recoveries (Figure S1 in the Supplementary material). Thus, the times needed to reach 95% NH3 recovery significantly increased by a factor of 6.5 and 2.3 in SD and RD, respectively, when digestate pH was increased from 7.6 to 10 (Table 2). Table 2. Time needed to reach 95% NH3 recovery for the PTFE-0.22 membrane operated with a concentration of H2SO4 of 1 M and a flow rate of 0.030 m3 h−1. Digestate SD RD pH t (95%)/h 7.6 36.03 16.88 10 5.49 7.31 H2SO4 concentration was identified, also, as a key operational parameter for NH3 recovery. The performance of a PTFE-0.22 membrane at two concentrations of H2SO4 (0.005 and 1 M) was studied at pHs of 7.6 and 10, under a flow rate of 0.030 m3 h−1 using synthetic digestate. Acid concentrations of 0.005 M resulted in NH3 recoveries of 12.2% ± 0.2% and 55.5% ± 0.1% at a pH of 7.6 and 10, respectively, after 3.5 h of operation at the same flow rate. Similarly, under the same conditions, NH3 recoveries of 22.9% ± 0.4% and 77.7% ± 0.4% were achieved using H2SO4 concentrations of 1 M (Figure 3). Therefore, it was concluded that the best operating conditions correspond to a pH of 10 and H2SO4 concentrations of 1 M. 0 25 50 75 100 PVDF 100 kDa PVDF 0.10 µm PTFE 0.20 µm PTFE 0.22 µm PTFE 0.45 µm NH 3 Recovery (%) Membrane Type Figure 2. Influence of the type of membrane on ammonia recovery, after 3.5 h, in SD with a recirculation flow rate of 0.030 m 3 h −1 , a pH of 10 and a concentration of H 2 SO 4 of 1 M. Vertical bars represent the standard deviation obtained from duplicate measurements. The influence of the digestate pH was also studied. This parameter determines NH 3 mass transfer as it shifts the equilibrium NH+ 4↔NH3+H+ . In our study, higher ammonia recoveries were achieved by increasing the pH of the synthetic digestate because of the increase in the NH 3 concentration gradient (Figure S1 in the Supplementary material). Nevertheless, an increase in the pH of the digestate above 10 does not entail large improvement in the ammonia removal [ 20 ]. Operating the process at a pH of 11 with a PTFE-0.22 membrane, a recirculation flow rate of 0.030 m 3 h −1 and a concentration of H 2 SO 4 of 1 M, resulted in a NH 3 recovery of 82.8% ± 0.4%, only 10% higher than the recoveries recorded at a pH of 10. NH 3 recovery was lower in real digestate than in synthetic digestate using a PTFE-0.22 membrane at pHs of 10 and 7.6, the latter pH supporting lower NH 3 recoveries (Figure S1 in the Supplementary material). Thus, the times needed to reach 95% NH 3 recovery significantly increased by a factor of 6.5 and 2.3 in SD and RD, respectively, when digestate pH was increased from 7.6 to 10 (Table 2). Table 2. Time needed to reach 95% NH 3 recovery for the PTFE-0.22 membrane operated with a concentration of H2SO4of 1 M and a flow rate of 0.030 m3h−1. Digestate SD RD pH t(95%)/h 7.6 36.03 16.88 10 5.49 7.31 H 2 SO 4 concentration was identified, also, as a key operational parameter for NH 3 recovery. The performance of a PTFE-0.22 membrane at two concentrations of H 2 SO 4 (0.005 and 1 M) was studied at pHs of 7.6 and 10, under a flow rate of 0.030 m 3 h −1 using synthetic digestate. Acid concentrations of 0.005 M resulted in NH 3 recoveries of 12.2% ±0.2% and 55.5% ±0.1% at a pH of 7.6 and 10, respectively, after 3.5 h of operation at the same flow rate. Similarly, under the same conditions, NH 3 recoveries of 22.9% ± 0.4% and 77.7% ± 0.4% were achieved using H 2 SO 4 concentrations of 1 M (Figure 3). Therefore, it was concluded that the best operating conditions correspond to a pH of 10 and H 2 SO 4 concentrations of 1 M. Membranes 2022,12, 19 7 of 14 Membranes 2022, 11, x FOR PEER REVIEW 7 of 14 Figure 3. Influence of the sulfuric acid concentration on NH3 recovery from SD at pHs of 7.6 and 10 and a flow rate of 0.030 m3 h−1 using the PTFE-0.22 membrane. Vertical bars (very short here) represent the standard deviation from replicate measurements. 3.2. Influence of the Digestate Recirculation Flow Rate Ammonia removal efficiencies of 84.6% ± 1.0% were achieved using the PTFE-0.22 membrane after 3.5 h of experiment in synthetic digestate, which were higher than the removal efficiencies in real digestate (71.6% ± 0.3%), at 0.045 m3 h−1 and a pH of 10. The lower NH3 recoveries in real digestate were likely due to membrane fouling. This fouling affected the NH3 permeate flux mainly due to adsorption of colloids, or solutes including ammonia, within the membrane pores, with a subsequent reduction in mean pore sizes. According to Meng et al. [44], membrane fouling typically causes cake layer formation, deposition of sludge flocs and their deposition on the membrane. The influence of the velocity of the acid solution was not herein assessed as previous literature has consistently shown that the velocity of the acid solutions does not impact significantly on NH3 mass transfer [20,31]. The increase in the digestate recirculation flow rate brought about a rise in the pace of the NH3 recovery across the PTFE-0.22 membrane regardless of the type of digestate (real or synthetic). Figure S2a,b (in Supplementary Material) show the exponential decrease of the ammonia concentration, which resulted in a rapid increase of NH3 recovery (Equation (7)). In Figure 4a, the corresponding rate of concentration decrease and recovery increase α, is shown as a function of the recirculation flow rate of SD and RD (Equation (8)). The value of α was higher for the synthetic digestate, while the increase in digestate recirculation flow rate was positively correlated with NH3 recovery rate regardless of the type of digestate. Figure 4b shows the evolution of the time needed to reach 95% recovery (Equation (9)). This time is higher for the real digestate than for the synthetic one. It is worth noting that an increase of the recirculation flow of the digestate from 0.007 to 0.045 m3 h−1 brought about a substantial decrease of the time needed to reach 95% recovery of NH3. In all cases, a pH of 10 was maintained. The duration of the experiment conducted at a liquid recirculation rate of 0.03 m3 h−1 was extended to 6 h, in order to check how a longer time improved recovery. This experiment resulted in a final NH3 recovery of 94.3% ± 0.3% in synthetic digestate in accordance with Figure 4b. Therefore, we concluded that over long time intervals, the evolution of recovery follows the pace assumed here. H 2 SO 4 0.005 M H 2 SO 4 0.005 M H 2 SO 4 1 M H 2 SO 4 1 M 0 25 50 75 100 pH ~ 7.6 pH ~10 NH 3 Recovery (%) Digestate pH Figure 3. Influence of the sulfuric acid concentration on NH 3 recovery from SD at pHs of 7.6 and 10 and a flow rate of 0.030 m 3 h −1 using the PTFE-0.22 membrane. Vertical bars (very short here) represent the standard deviation from replicate measurements. 3.2. Influence of the Digestate Recirculation Flow Rate Ammonia removal efficiencies of 84.6% ± 1.0% were achieved using the PTFE-0.22 membrane after 3.5 h of experiment in synthetic digestate, which were higher than the removal efficiencies in real digestate (71.6% ± 0.3%), at 0.045 m 3 h −1 and a pH of 10. The lower NH 3 recoveries in real digestate were likely due to membrane fouling. This fouling affected the NH3permeate flux mainly due to adsorption of colloids, or solutes including ammonia, within the membrane pores, with a subsequent reduction in mean pore sizes. According to Meng et al. [ 44 ], membrane fouling typically causes cake layer formation, deposition of sludge flocs and their deposition on the membrane. The influence of the velocity of the acid solution was not herein assessed as previous literature has consistently shown that the velocity of the acid solutions does not impact significantly on NH 3 mass transfer [20,31]. The increase in the digestate recirculation flow rate brought about a rise in the pace of the NH 3 recovery across the PTFE-0.22 membrane regardless of the type of digestate (real or synthetic). Figure S2a,b (in Supplementary Material) show the exponential decrease of the ammonia concentration, which resulted in a rapid increase of NH 3 recovery (Equation (7)). In Figure 4a, the corresponding rate of concentration decrease and recovery increase α , is shown as a function of the recirculation flow rate of SD and RD (Equation (8)). The value of α was higher for the synthetic digestate, while the increase in digestate recirculation flow rate was positively correlated with NH 3 recovery rate regardless of the type of digestate. Figure 4b shows the evolution of the time needed to reach 95% recovery (Equation (9)). This time is higher for the real digestate than for the synthetic one. It is worth noting that an increase of the recirculation flow of the digestate from 0.007 to 0.045 m 3 h −1 brought about a substantial decrease of the time needed to reach 95% recovery of NH 3 . In all cases, a pH of 10 was maintained. The duration of the experiment conducted at a liquid recirculation rate of 0.03 m 3 h −1 was extended to 6 h, in order to check how a longer time improved recovery. This experiment resulted in a final NH 3 recovery of 94.3% ± 0.3% in synthetic digestate in accordance with Figure 4b. Therefore, we concluded that over long time intervals, the evolution of recovery follows the pace assumed here. Membranes 2022,12, 19 8 of 14 Membranes 2022, 11, x FOR PEER REVIEW 8 of 14 Figure 4. Influence of the recirculation flow rate on (a) α, and (b) the time to reach 95% NH3 recovery using a PTFE-0.22 membrane at different recirculation flow rates at a pH of 10 and a concentration of H2SO4 of 1M for both digestates. The NH3 fluxes across the PTFE-0.22 membrane in both SD and RD are shown in Figure 5. They decreased as the recirculation flow rate increased because of fouling, probably induced by shear. In addition, NH3 fluxes were slightly higher in SD than in RD according to its lower induction of membrane fouling. By fitting theoretical to experimental fluxes and using Equations (2)–(6), the corresponding effective pore diameters were evaluated at the end of each experiment, as a function of the recirculation flow Q, which was 0.25 µm. Figure 4. Influence of the recirculation flow rate on ( a ) α , and ( b ) the time to reach 95% NH 3 recovery using a PTFE-0.22 membrane at different recirculation flow rates at a pH of 10 and a concentration of H2SO4of 1M for both digestates. The NH 3 fluxes across the PTFE-0.22 membrane in both SD and RD are shown in Figure 5. They decreased as the recirculation flow rate increased because of fouling, probably induced by shear. In addition, NH 3 fluxes were slightly higher in SD than in RD according to its lower induction of membrane fouling. By fitting theoretical to experimental fluxes and using Equations (2)–(6), the corresponding effective pore diameters Membranes 2022,12, 19 9 of 14 were evaluated at the end of each experiment, as a function of the recirculation flow Q, which was 0.25 µm. Membranes 2022, 11, x FOR PEER REVIEW 9 of 14 Figure 5. Experimental molar fluxes JE for recirculation rate 0.030 m3 h−1 for PTFE-0.22 at a pH of 10 and 1 M H2SO4. The experimental NH3 fluxes herein obtained with flat plate membranes were comparable to, but clearly higher than, those reported in the literature when using similar pHs with hollow fibre or tubular membranes (Table 3). Table 3. Summary of fluxes using hydrophobic membranes. Matrix Co (ppm) pH T (°C) H2SO4 (mol L−1) Flux (mol m−2 h−1) Membrane Configuration Reference Water containing NH3 400 9–10 40 0.3 0.11 Hollow fibre (PP) [21] Simulated wastewater 120 10 25 - 0.18 Hollow fibre (PVDF) [20] Digested effluents 1554 8 25 1 0.06 Hollow fibre (PP) [45] Raw swine manure 2390 9 25 1 0.33 Tubular (PE) [46] Landfill leachate 1300 10 25 0.1 1.27 Hollow fibre (PP) [37] Synthetic Digestate 679 10 35 1 4.52 Flat sheet (PTFE) This work In contrast, the overall mass transfer coefficient increased at increasing digestate recirculation flow rates. An analysis of the relative importance of the membrane mass transfer coefficient km, and that on the digestate side ks, compared to the overall mass transfer coefficient kov, show that the largest restriction to mass transfer occurred on the digestate side of the membrane, while through the pores, there was a very high NH3 mass transfer. NH3 mass transfer through the pores decreased steeply when the recirculation flow rate was increased, but with a concomitant increase of the mass transfer on the digestate side of the membrane. Values of Table 4 correspond to synthetic digestate. Figure 5. Experimental molar fluxes J E for recirculation rate 0.030 m 3 h −1 for PTFE-0.22 at a pH of 10 and 1 M H2SO4. The experimental NH 3 fluxes herein obtained with flat plate membranes were comparable to, but clearly higher than, those reported in the literature when using similar pHs with hollow fibre or tubular membranes (Table 3). Table 3. Summary of fluxes using hydrophobic membranes. Matrix C0 (ppm) pH T (◦C) H2SO4 (mol L−1) Flux (mol m−2h−1)Membrane Configuration Reference Water containing NH3 400 9–10 40 0.3 0.11 Hollow fibre (PP) [21] Simulated wastewater 120 10 25 - 0.18 Hollow fibre (PVDF) [20] Digested effluents 1554 8 25 1 0.06 Hollow fibre (PP) [45] Raw swine manure 2390 9 25 1 0.33 Tubular (PE) [46] Landfill leachate 1300 10 25 0.1 1.27 Hollow fibre (PP) [37] Synthetic Digestate 679 10 35 1 4.52 Flat sheet (PTFE) This work In contrast, the overall mass transfer coefficient increased at increasing digestate recirculation flow rates. An analysis of the relative importance of the membrane mass transfer coefficient k m , and that on the digestate side k s , compared to the overall mass transfer coefficient k ov , show that the largest restriction to mass transfer occurred on the