New regulations in many developed countries call for a signi cant reduction in phosphorus concentration for euents released to the environment. At the same time, recovery of phosphorus - a non-renewable resource used mainly as fertilizer - from anthropogenic waste is extensively studied and bolstered as a crucial component in maintaining future food security. Thus far studies on phosphorus recovery mainly focused on concentrated streams, although di- luted euents such as treated wastewater often contain a signi cant portion of the phosphorus mass. Here we propose a new approach for the simultane- ous removal and recovery of phosphorus from diluted euents using a mem- brane characterized by high phosphate rejection and acid resistance. High P rejection allows for the concentration of phosphorus in the retentate until recoverable calcium-phosphate precipitants are formed, while acid resistance Corresponding author Email addresses: odni@bgu.ac.il (O. Nir), manuscripts.cvt@avt.rwth-aachen.de (M. Wessling) Preprint submitted to Elsevier December 18, 2017 enables a simple and e ective chemical cleaning of the membrane. Factors a ecting the removal and recovery of phosphorus during ltration are stud- ied here experimentally and through thermochemical modeling. Avoiding CaCO 3precipitation in the retentate was found critical as it results in severe scaling. In contrast, calcium-phosphate precipitates mostly in the bulk, re- sulting in colloidal fouling which is manageable by maintaining sub-critical permeate ux. Using thermochemical modeling we show that selecting Ca-P precipitation over CaCO 3is feasible by operating at neutral pH, requiring none or very little acid addition. Calcium-phosphate deposits were easily removed from the feed channel through acid-cleaning, and the membrane performance was completely restored. Furthermore, phosphorus removal by nano ltration was shown to require less operating expenses compared to the conventional ferric chloride addition. Our results therefore demonstrate the potential of this new approach as a step forward towards closing the anthro- pogenic phosphorus cycle. Keywords: Nutrients recovery; Phosphate; Nano ltration; Secondary Euent; Tertiary Treatment; Wastewater 1. Introduction The element phosphorus (P) is at the center of two major global issues, i.e. food security and environmental conservation [1]. P is one of the primary macro-nutrients (together with nitrogen (N) and potassium (K)) required for plant growth and is therefore widely applied as fertilizer. Currently, P is supplied to the fertilizer industry predominantly via mining of calcium- phosphate (Ca-P) rocks, which are composed mainly of the thermodynami- 2 cally stable hydroxyapatite (Ca 5(PO 4)3(OH)) and uorapatite (Ca 5(PO 4)3F) minerals. Extraction of P from these ores is typically performed through their dissolution in sulfuric acid, producing concentrated phosphoric acid as the main product and solid gypsum (CaSO 4) as a byproduct [2]. Phosphoric acid could be then used for producing a range of soluble N-P-K fertilizers suitable for fertigation commonly applied in modern agriculture. Despite be- ing relatively economic, P production from Ca-P rocks cannot be considered sustainable, since it is based on a non-renewable resource. Estimations of the timescale required for depletion of P ores range from few decades to several centuries [3], however rising prices and the limited spread of large Ca-P de- posits to only a few countries may pose a threat to global food security even in the near future. Following its application and consumption, the disposal of excess P into the environment impairs aqueous ecosystems. Since P is the limiting nutrient in many freshwater ecosystems its introduction results in increased growth of microorganisms (eutrophication), which deplete dissolved oxygen creating anoxic conditions. Water quality is signi cantly reduced in terms of turbid- ity, odor and toxicity, and services provided by P enriched water bodies in terms of both water-supply and recreation are severely hindered. Moreover, native aqueous organisms often cannot survive this transformation, thus the biodiversity signi cantly drops. Consequently, there is a strong incentive to further decrease P discharge limitations required for treated wastewater (euent). In a recent life-cycle analysis study [4] found that enforcing the P<0.1 mg/l limit instead of the widely used P <1 mg/l would signi cantly 3 reduce eutrophication. P concentration in domestic wastewater is in the range of 1-20 mg-P/l and varies considerably between di erent regions and seasons. Conventional bio- logical treatment typically removes 15-25% of the P concentration [5], leaving most of the P mass in the secondary euent. To further reduce P euent concentration, chemical precipitation of P by Al and Fe salts is commonly ap- plied. The disadvantages of this method are high chemical consumption and increased sludge volume. When properly designed, the enhanced biological P removal (EBPR) operational approach could achieve the P <1mg/l target with minimal increase in sludge volume. Even P <0.1 mg/l is attainable un- der certain conditions [5] and with the addition of carbon source. However, like any biological process, EBPR su ers from large (relatively to physico- chemical processes) uctuations in euent quality. Therefore, further pol- ishing step is usually required for meeting low P discharge requirements [6]. The chemical precipitation method is commonly applied for this purpose. Adsorption by metal hydroxides, most commonly Fe and Al, is another ap- proach enabling very low P euent concentration [7]. Though extensively studied, P adsorption is not widely used mainly due to costs associated with the adsorbing material and / or the regeneration process [7, 8]. Combining P removal with P recovery was widely adopted as a sustain- able approach to simultaneously address the negative environmental impacts and the impending P global shortage [9]. Works on phosphate recovery from wastewater have thus far almost exclusively focused on concentrated streams, e.g. sewage sludge [10], 'black water' [11], or the supernatant origi- nating from sludge dewatering following anaerobic biological treatment [12]. 4 Direct P extraction from wastewater was also suggested [13]. However, re- covery of P from the secondary euent was scarcely studied, even though secondary euent often consists a signi cant portion of the P mass enter- ing the wastewater treatment plant. Studies on this topic thus far focused almost exclusively on sorption processes [8, 14], i.e. adsorption of P, its sub- sequent desorption and separation from the regeneration solution. Although feasibility was demonstrated in these studies, long term functionality of the adsorbing materials which may be hindered by irreversible adsorption or ero- sion is yet to be proven [7]. Here we suggest an alternative approach based on membrane ltration for the removal and recovery of P from euents having low P concentration. The approach o ers increased robustness compared to biological processes and signi cantly reduced chemicals demand compared to sorption and chemical precipitation. Unlike these methods which mainly aims to remove P, the ap- proach suggested here simultaneously removes and recovers P from diluted solutions such as secondary euents. A proof of concept is demonstrated through theoretical and experimental results. 2. Description and rationale of the suggested approach The approach suggested here comprises of a membrane ltration step in which Ca2+and phosphate ions are concentrated, thus inducing Ca-P crystals formation in the retentate stream. Ca-P crystals remaining on the membrane surface or in the feed ow channel are removed by applying acid-cleaning. To facilitate this scheme, a non-porous membrane characterized by high phos- phate rejection and high stability in strongly acidic solutions is required. 5 secondary eﬄuent permeateRO/NF acid durable NF Ca-P crystalsFurther treatment /WWTP crystallizeroptionalpH controlacid durable NF cleaning solution recycledsolutionstart of cycle end of cycleHNO3 HNO3 Ca2+ H3PO4directuse as fertilizera) b) further processing at phosphate ore factoryFigure 1: Schematic description of the new treatment concept. (a) Continuous, high- recovery membrane ltration coupled to a crystallizer for calcium-phosphate recovery. (b) Batch cleaning cycle utilizing nitric acid and generating N-P-Ca fertilizer solution, which can be used locally. Currently, to the best of our knowledge, only nano ltration (NF) non-porous acid-durable membranes are commercially available [15], although increased acid stability was demonstrated for surface modi ed reverse osmosis (RO) membranes [16]. RO is increasingly studied and already applied on a large scale for potable water reclamation from wastewater (in e.g. Singapore and California), while NF was suggested as an advanced tertiary treatment [17] for removing contaminants such as heavy metals [18] and emerging organics [19]. 'Tight' NF membranes also provide partial NaCl removal, thus prevent- ing soil salinization when the euent is reused for irrigation [20]. Therefore, although P removal and recovery is the focus of this work, the suggested operational approach should also be viewed as a potential solution for Ca-P scaling within RO/NF wastewater treatment at high permeate recovery. As seen in Fig. 1a, tertiary wastewater is continuously ltered through a 6 two stage nano ltration cascade with an acid-durable non-porous membrane in the second stage. The two stage approach enables local pH control to con- trol inorganic salt precipitation. Further, the membrane for the rst stage is not restricted to acid-resistant membranes providing a higher degree of freedom in the system design. Pretreatment is required for removing colloids from the feed, however it may be omitted in case membrane bioreactor is applied for the secondary treatment [19]. The process is aimed at achieving high recovery ratio (i.e. the ratio of permeate ow to feed ow) which is both economically and environmentally bene cial as it increases the output of reclaimed water, while at the same time minimizes the volume of con- centrate, allowing for more ecient treatment and resource recovery. With the increase of recovery ratio, P Tand Ca2+concentrations increase until su- persaturation with respect to a certain Ca-P solid phase is suciently high for overcoming the kinetic barrier and inducing its chemical precipitation. According to [21], this phenomenon is currently the main limitation on per- meate recovery in NF/RO of secondary euents. Chemical precipitation during ltration could occur either in the retentate solution (homogeneous or seeded heterogeneous precipitation), forming small suspended colloids and particles, or on the membrane surface / feed channel spacer (heterogeneous surface precipitation), forming a scaling layer. In both cases the water ux (productivity) and rejection (functionality) are hindered, however particulate fouling is more in uenced by the hydrodynamic condi- tions and is thus easier to regulate. Evidence from previous studies suggests that particulate and colloidal deposition are the principal mechanisms for 7 the case of Ca-P fouling [22, 23, 24]. As a result, below a particular perme- ate ux, which may be termed the `threshold ux' [25], most of the Ca-P particles will not accumulate rapidly on the surface, but rather conveyed out of the feed channel by the tangential ow. This enabled a 32 days of stable operation in a real wastewater RO plant study [26]. Typically, the retention time in the membrane is not sucient for ap- proaching solid-liquid equilibrium (saturation). Therefore, in order to recover phosphorus as Ca-P solids further precipitation and crystallization are car- ried out in a seeded crystallizer. This step could also be used for decreasing supersaturation between consecutive membrane stages [27]. Increased phos- phorus recovery could be achieved by adjusting the pH in the crystallizer. The resulting Ca-P solid minerals are transferred to further processing at a phosphate ore bene ciation factory as illustrated in Fig. 1a. The feasibility of recovering Ca-P from supersaturated solutions using a pellet reactor was demonstrated by [28]. Further in this text we discuss the e ects of NF reten- tate pH and recovery ratio on potential P recovery from a thermochemical point of view. Even when operating below the threshold ux, long term decline in per- formance is still expected due to Ca-P microcrystals retained by the feed spacer or randomly deposited on the membrane surface. Moreover, hetero- geneous precipitation on the membrane surface could not be ruled out com- pletely. As previously shown, membrane performance could be completely restored by acid-cleaning [26]. Nevertheless, frequent acid cleaning of stan- 8 dard polyamide RO/NF membranes is generally undesirable since it shortens the e ective lifespan of the membrane. Furthermore, the acid concentration in the cleaning solution is limited by the minimal pH value permitted by the manufacturer (typically pH >1-2). Since both the kinetics of dissolution and the thermodynamic dissolution potential decrease with increasing pH, this limitation a ects the cleaning duration, the volume of cleaning solution needed and the phosphorus concentration in the exhausted cleaning solution. However by placing acid-durable membrane elements in the stages suscepti- ble to Ca-P fouling, frequent and ecient cleaning with concentrated acids is permitted. Using membranes with high acid resistance allows for the application of nitric-acid (HNO 3) which was found to be more aggressive towards polyamide NF membranes compared to sulfuric acid [29]. As seen in Fig. 1b, when HNO 3is used in the acid cleaning step for dissolving the Ca-P solid foulants, it generates a solution containing the macro-nutrients N&P, which could be applied locally as fertilizer by mixing it with irrigation water. The presence of Ca2+ions in the solution will result in the additional bene t of reducing the SAR (sodium adsorption ratio) thus maintaining the aggregated structure of the soil. Using hydrochloric acid (HCl) for the purpose of cleaning is also possible, however this practice will enrich groundwater with unwanted Cl ions. Sulfuric acid is not suitable for this purpose since CaSO 4will precipitate on the membrane upon Ca-P dissolution. 9 3. Materials and Methods 3.1. Experimental A cross- ow nano ltration setup, supporting a ow cell housing a 70 sqcm at sheet membrane, was used in the ltration experiments. The setup in- cludes a high pressure pump and a booster pump for controlling the pressure and ow independently (Schematics of the experimental system are given in the supporting information). The Membrane used was Duracid by GE Osmonics, the membrane was compacted prior to experiments by ltering distilled water at 40 bars for 5 hrs. The rejection of sodium-phosphate salts at di erent pH values was determined by electric conductivity. Measurements were repeated until steady-state conditions were observed. The pH was ad- justed by the ratio of NaHPO 4to NaH 2PO4added to the solutions while the total concentration of phosphate was 0.1M. These rejection experiments were performed at low recovery ratio, with the permeate stream constantly directed back to the feed tank. For the scaling experiments, three di erent salt mixtures were used as speci ed in 1. In these experiments the recovery ratio was slowly increased by continuously extracting permeate out of the system. Ion concentrations in the feed and permeate were determined by ion chromatography (cations: Agilent 1100, Metrosep C4 column (Methrom), eluent: 1 mM oxalic acid, 1.2 mM nitric acid, owrate = 1.4 ml/min, temperature = 35 C, conductivity detector ; anions: Agilent 100, PRP-X100 column (Chromatographie Service), eluent: 4-hydroxybenzoic acid pH 8.5, owrate = 2 ml/min, temperature = 35 C, conductivity detector). The pH was measured using an Orion Star A211 pH 10 meter by ThermoFisher with ROSS Triode pH/ATC electrode. Standard 3- point calibration was performed at the beginning of each day. Acid-cleaning was performed at TMP of 8 bars for both scaling experiments. 3.2. Theormochemical modeling All thermochemical calculations were performed with the computer pro- gram PHREEQC[30], using the minteq.v4 database, which is the thermo- chemical database maintained by the USEPA. Both speciation calculations (see Fig. 2b) and precipitation potentials (see Figs. 6-7) were performed using Python scripts running the IPhreeqc com module. Secondary euent composition was obtained from [24]. For calculating the ion composition C at a certain recovery ratio S, the following mass-balance based equation was used, C(S) =Cf(1S)R(1) Where Cfis the feed concentration and Ris the rejection. Eq. 1 is valid when the local rejection over the relevant recovery ratio is constant and was previously validated for NF of groundwater [31]. In the current case, only Na+and Clexhibited a signi cant change in rejection when concentrated synthetic euents were used as feed (see Table 1). In that case, Eq. 1 was used separately in two di erent recovery ranges (0-80% and 80-90%) and the average rejection (rejections are shown in Table 1) of two ends of each range was used. The Python scripts, including the solution compositions used are included as supplementary material. 11 4. Results and discussion 4.1. Experimental results High rejection of P by the acid durable membrane at all operation con- ditions relevant for the continuous ltration step (Fig. 1a) is a perquisite for the approach suggested here. Since P rejection by the Duracid membrane used was previously tested only with highly acidic solutions [10], ltration experiments of sodium-phosphate salts at the applicable pH range (6-8) were performed. The high P rejections seen in Fig. 2a indicate that the Duracid membrane could be employed in the phosphate polishing step. At pH 7-8, rejections exceeded 97% even at low permeate ux, while at pH 6 P rejections in the range 92-95% were measured. The e ect of pH on P rejection also seen in Fig. 2a is a direct result of the phosphate species distribution for the 0.1M solutions plotted in Fig. 2b. Increasing the pH from pH 6 to pH 7 resulted in a signi cant increase in rejection, while only an incremental increase was seen when the pH was further increased to 8. This is in accordance with the signi cant absolute decrease (from 0.1M to 0.03M) in the concentration of the more permeable monovalent ion H 2PO4when the pH is increased from 6 to 7, compared to the relatively low absolute decrease (from 0.03M to 0.003M) when pH is further increased from 7 to 8. High P rejection at neutral and mildly acidic pH is highly bene cial in the context of the suggested approach since formation of CaCO 3crystals in the retentate could be avoided by means of pH adjustments. Preventing CaCO 3 crystallization is critical with respect to both P recovery and membrane per- formance. At basic pH, CaCO 3solids hinder Ca-P crystallization through competition over Ca2+ions. In case both Ca-P and CaCO 3solids are formed 12 0102030405075808590951004 567891011-5-4-3-2-1 pH6 pH7 pH8Rejection (%)F lux (Lm-2h-1)(a)P O3-4 H2PO-4 H PO2-4 Log Cp HH3PO4(b)Figure 2: (a) Rejection of sodium phosphate salts by the acid durable NF membrane at varying permeate ux and pH values. (b) Distribution of the dominant phosphate species in a 0.1M Na-P solution as a function of pH. 13 or co-precipitate, the weight fraction of P in the resulting sediment decreases, making P recovery less economically attractive. In addition, the e ect of CaCO 3scaling on membrane performance at low ux is markedly more se- vere compared to Ca-P precipitation as demonstrated by the experimental results shown on Fig. 3 and Fig. 4 respectively and further discussed below. The desired rejections of other ions apart from calcium and phosphate de- pends on the purpose of the tertiary membrane- ltration treatment per- formed. In case the purpose is partial or complete desalination, high re- jection rate for monovalent ions is required. As seen in table 1, at ionic concentrations representing secondary euent, the commercial acid-durable membrane used here exhibit high rejections for both multi and monovalent ions. Increasing ionic concentrations to levels expected at higher recovery ratios resulted in higher selectivity as multivalent ions rejection was only slightly a ected, while Na+and Clrejections dropped. P rejection was al- most una ected by the increased ion concentrations. Since at the pH values used (6.8-7.1)50% of P is in the monovalent form, the signi cantly higher rejection for this ion over the smaller monovalent ions indicates the domi- nance of steric exclusion mechanism. These rejection properties along with low-pH compatibility makes this membrane suitable to be used within the approach depicted in Fig. 1 when partial desalination is desired. In case complete desalination is not the purpose, low rejection of Na+and Clis preferred in order to minimize the trans-membrane osmotic pressure di er- ence and decrease energy consumption. This latter is addressed in a study currently underway. In order to examine the di erent e ects on membrane performance result- 14 Table 1: Total P and ion rejections obtained with the Duracid NF membrane. Feed ionic composition were chosen to simulate a. secondary euent (pH = 7.8-8.0; C T= 175 mg/l as CaCO 3); and b. secondary euent concentrated by nano ltration at approx. 80% (P T & Ca2+x5; pH = 6.8-7.1; C T= 25 mg/l as CaCO 3) and 90% (P T& Ca2+x10; pH = 6.8-7.1; C T= 25 mg/l as CaCO 3) recovery ratios. Inorganic carbon content was reduced in the concentrated solutions to avoid rapid CaCO 3scaling. PT Ca2+Na+Cl SO2 4Mg2+ Sec. euent Conc. (mg/l) 5 2 3811 11528 12230 123 2-3 Rejection (%) 99 1 990.5 912 923 50.2>95 80%recovery Conc. (mg/l) 23 220 160 470 160 19 Rejection (%) >99 96 72 82 >99>94 90%recovery Conc. (mg/l) 44 455 220 895 198 19 Rejection (%) 98 97 50 77 >99>94 15 ing from CaCO 3vs. Ca-P scaling, ltration and cleaning experiments were conducted at conditions highly favouring the precipitation of either CaCO 3 or Ca-P. The synthetic feed solutions simulated concentrated secondary ef- uents such as obtained in the retentate originating from a high recovery NF/RO step. As seen in Fig. 3 when operating at high supersaturation with respect to calcite, the ux rapidly decreased to third of its original value after an induction period of 20 minutes. This rapid ux decline and the lack of visible precipitants in the feed/concentrate tank indicate that CaCO 3 precipitation occurred heterogeneously on the membrane surface, forming a scale layer which signi cantly increased the hydraulic resistance of the mem- brane. This result agrees with membrane autopsy performed by [32] which revealed CaCO 3surface scale formation on RO and NF membranes 16 02 55 02 252502750510152025Permeate Flux (Lm-2h-1)T ime (min)H2OHCl 1M CleaningH 2O a fterf lushingCaCO3 precipitationFigure 3: Permeate ux development during ltration of synthetic secondary euent concentrate (Applied pressure: 8 bars. pH = 8), which is highly supersaturated with respect to calcite (feed solution compositions are given in table 1), followed by a distilled water permeation experiment, an acid cleaning step (1M HCl) and an additional distilled water permeation experiment. Flushing the membrane with pure water only partly restored the ux, pointing at surface scaling which is hard to remove by hydrodynamic shear. At least 33% of the ux increase after ushing can be explained by a 2 bars reduction in osmotic pressure di erence between the feed and permeate solu- tions (even without considering concentration-polarization), while the rest of the ux increase is due to removing colloidal or loose CaCO 3crystals. Acid- 17 cleaning with 1 M HCl restored the pure water permeability to the value measured previously for the clean membrane (21 lm2hr1bar1), indicat- ing that the scaling layer was completely removed. Flux during acid cleaning was slightly higher The right, probably due to changes in the membrane structure at this extreme pH. The development of permeate ux during the ltration of a solution hav- ing high precipitation potential of Ca-P, shown in Fig. 4, was markedly di erent from the case of CaCO 3. When operating at TMP of 8 bars the permeate ux only slightly decreased due to the increase of feed concentra- tion and the osmotic pressure di erence. After 140 minutes, in which no signi cant ux reduction occurred, the solution supersaturation was arti - cially increased by adding CaCl 2and NaH 2PO4, resulting in the appearance of a white colloidal precipitant in the feed/concentrate tank. Notably, in spite of the high feed turbidity the permeate ux increased back to its origi- nal value, which is explained by the accelerated Ca-P crystallization resulted in lower ion concentration and decreased the osmotic pressure di erence. Subsequently, the pressure was increased to 20 bars resulting in the initial increase of permeate ux, followed by a sharp 50% decrease after 50 minutes of ltration. This response to fouling and to applied pressure variation seen in Fig. 4 is typical to the case of colloidal suspensions ltration, where short term deviation from the pure water ux only occur above the threshold ux. Thus our results suggest that Ca-p fouling was mostly of colloidal nature, in accordance with [23]. Following the Ca-P deposition experiment, membrane cleaning was per- 18 05 01 001502 0001020304050W hite precipitatei n the feed tankP T & Ca2+ addedTMP = 20 barsPermeate Flux (Lm-2h-1)T ime (min)TMP = 8 barsFigure 4: Permeate ux development during ltration of synthetic secondary euent concentrate, which is highly supersaturated with respect to various Ca-P phases including ACP and HAP. Supersaturation was arti cially increased after 2 hrs by adding calcium and phosphate salts. TMP was raised from 8 to 20 after 2.5 hrs. Feed solution compositions are given in table 1. pH was held constant at 7. 19 formed by rst circulating pure water through the feed channel and subse- quently applying acid cleaning with 0.1 M nitric acid. As seen in Fig. 5, prior to acid cleaning the ux at 8 bars (15 m/s) was similar to the ux during Ca-P deposition (Fig. 4), although the osmotic pressure di erence was lower, indicating that part of the Ca-P solids remained on the membrane surface. On the other hand, the non-linear response of permeate ux to the increase in TMP suggested the occurrence of Ca-P colloids in the feed-side solution. During acid cleaning, the permeate ux increased close to the level of pure water at low TMP's, while at the highest TMP the permeate ux increased sub-linearly due to ion concentration-polarization. After the acid cleaning, the pure water ux showed a linear response to TMP and the membrane permeability was restored to its original value. Ca2+and P concentrations in the cleaning solution following acid cleaning were found to be 3.87 and 2.74 mM respectively. The Ca/P ratio was 1.41, which is between the value usu- ally assigned to ACP (1.5) and the one assigned to octacalcium phosphate (1.33), indicating that full transformation to HAP (Ca/P = 1.66) has not occurred. 20 6810121416182001020304050P ure water after cleaningD uring acid cleaningFlux (Lm-2h-1)A pplied Pressure (bar)Pure water before cleaningFigure 5: Permeate ux as a function of applied trans-membrane pressure following the ltration of supersaturated Ca-P solution. Acid-cleaning for several minutes with 0.1M HNO 3fully restored the water permeability of the membrane The results shown in Fig. 3 and Fig. 4 indicate that by avoiding CaCO 3 and operating at suciently low permeate ux, sustainable ltration is achiev- able. Fortunately, CaCO 3precipitation could be eciently avoided by re- 21 ducing the pH via strong acid dosing, a solution which is both simple and relatively cheap. The target pH depends on the thermodynamics and ki- netics controlling the formation of Ca-P and CaCO 3phases. These in turn are strongly in uenced by the solution composition (e.g. pH, C T, PT, ionic strength, speci c interactions with other ions [33, 34], organic matter[35] and temperature. Thermochemical analysis, such as presented in Fig. 6 and Fig. 7 provide useful guidelines regarding the pH required for selective precipita- tion of Ca-P crystals for known rejections and feed compositions. 4.2. Theoretical results Theoretical phosphorous precipitation potential (PPP) for two Ca-P phases at di erent recovery ratios are shown in Fig. 6 along with CaCO 3pre- cipitation potential (CCPP) with respect to calcite. The rst Ca-P phase considered is hydroxylapatite (HAP, Ca 5(PO 4)3OH), which is the most ther- modynamically stable Ca-P phase [33]. However, HAP usually does not precipitate directly from supersaturated solutions due to unfavorable kinet- ics. Instead, HAP occurs through a serious of phase transformations, starting from the phase which precipitates directly from solution. Here we consider amorphous calcium phosphate (ACP) as the latter based on previous work [36, 13, 37], however we do note that other Ca-P phases e.g. di-calcium phos- phate (CaHPO 4) and octa-calcium phosphate (Ca 8H2(PO 4)65H2O) may also precipitate rst, depending on conditions and composition [33]. Precipita- tion potential is chosen here over the more common SI (saturation index), due to its clearer quantitative meaning, i.e. the theoretical amount of a solid phase which will precipitate from a supersaturated solution until solid-liquid 22 6.06.57.07.58.08.59.0010203040p H (-)PPP (mg/l as P)0 50100150200H APA CPC CPP (mg/l as CaCO3)C alcite6.06.57.07.58.08.59.005101520p H (-)PPP (mg/l as P)0 255075100H APA CPC CPP (mg/l as CaCO3)C alcite6 .06.57.07.58.08.59.00.02.55.07.510.09 0% recovery80% recoveryp H (-)PPP (mg/l as P)60% recovery0 10203040H APA CPC CPP (mg/l as CaCO3)C alcite6 .06.57.07.58.08.59.002040608095% recoveryp H (-)PPP (mg/l as P)0 100200300400500H APA CPC CPP (mg/l as CaCO3)C alciteFigure 6: Precipitation potential (the theoretical amount of solid phase which would pre- cipitate from 1L of solution until solid-liquid equilibrium is reached) of hydroxyapatite (HAP), amorphous calcium phosphate (ACP), and calcite as a function of pH, at di erent recovery ratios. Solution composition at each recovery ratio was calculated using feed com- position from [24] and current experimental rejections (table 1).For the Ca-P phases the phosphate precipitation potential (PPP) is shown while for calcite the calcium carbonate precipitation potential (CCPP) is shown. Here precipitation potentials were calculated separately for each phase assuming only one solid phase is formed. 23 equilibrium (saturation) is reached. Precipitation potential results shown in Fig. 6 provide useful information regarding two important aspects of the process, i.e. scaling propensity and the amount of P which could be recovered. In Fig. 6, precipitation potential of each phase is calculated independently, without considering competition over the common Ca2+. In this manner, the conditions where precipita- tion is thermodynamically impossible, are obtained for the potentially scal- ing phases (ACP and calcite). As expected, the limiting pH decreases with recovery ratio, while precipitation potentials at higher pH values increase due to increased concentrations of scaling ions. The results demonstrates the challenge of mitigating Ca-P scale formation at high permeate recovery where the limiting pH <6.5 and large amounts of acid is needed due to the carbonate bu er system. In contrast, controlling calcite scaling through pH modi cation is feasible even at high permeate recovery. This di erence in limiting pH values between ACP and calcite throughout the permeate recov- ery range is critical to the suggested operational scheme, since it provides a simple handle for controlling the precipitating phase. It is noted that the pH of the retentate stream could change signi cantly during the ltration process [38], which may require inter-stage pH control as illustrated in Fig. 1. Inter-stage acid injection may be used to tackle the trade-o between P rejection and CaCO 3scaling. While low pH is required for preventing CaCO 3scaling, operation at ex- cessively low pH values hinders both P removal, due to lower P rejection 24 (see Fig. 2, and recovery. For the water composition analyzed here, at pH <6.5, full recovery of P, marked by the HAP PPP asymptote on Fig. 6, is not obtainable even for 95% permeate recovery. At pH >7, almost complete recovery of P through HAP precipitation is thermodynamically obtainable, however at high permeate recovery, the high CCPP is likely to induce calcite formation. On top of the thermodynamic considerations, precipitation and phase transition kinetics also plays a major role in determining the extent of scaling and the practical amount of recoverable P. As indicated by the gap be- tween the ACP and HAP PPP curves depicted in Fig. 6, the transformation rate of ACP (or other Ca-P phase) to HAP (or to other meta-stable phase) could have a considerable e ect on the mass of P which would precipitate from the concentrate after a given retention time. This has implications on the crystallization reactor at the concentrate outlet, which has to be designed according to the P recovery goal. In case NaCl removal is not required, the retentate could be recycled back to the wastewater treatment plant, which leaves only the permeate stream and precipitated Ca-P solids as P out ows and increases P recovery. 25 6.06 .57 .07 .58 .08 .5020406080PP (mg/l)p H (-)ACPC alciteFigure 7: Precipitation potential of amorphous calcium phosphate (ACP) and calcite at 95% recovery ratio. Here precipitation potentials were calculated simultaneously for both phases which were allowed to compete over the common ion Ca2+. Other details are as in Fig. 6. While the analysis shown in Fig. 6 shows the limiting pH below which calcite precipitation is thermodynamically restricted, nite nucleation and crystallization kinetics allow operation at higher pH without calcite scaling. 26 Higher retentate pH results in increase in both P removal (higher rejection) and recovery (decrease Ca-P solubility). From that perspective, it is useful to examine the competitive solid-liquid equilibrium between calcite and ACP as presented in Fig. 7. At 95% recovery, when both phases compete on the common Ca2+ions, calcite precipitation starts only at pH 7.5, whereas for the non-competitive equilibrium analysis calcite precipitation starts already at pH<6.5. This pH di erence translates to additional 20 mg/l of P which could be potentially recovered from the retentate i.e increase from 60% to >90% of P recovery as ACP. In addition to being thermodynamically favor- able at high recovery ratio, ACP crystallization should be also accelerated due to seeding e ect. Since at lower recovery (e.g. 60% pH = 7.5) ACP precipitation is thermodynamically permitted and calcite is not, ACP nucle- ation will occur earlier in the retentate, promoting formation of ACP over calcite formation at higher recovery. 4.3. Estimation of operating expenses A preliminary assessment of the major operating expenses was conducted for the new nano ltration process and compared with the costs of chemical precipitation using ferric chloride (table 2). An average phosphorus con- centration of 10 mgP/l was considered with a typical Fe/P molar ratio of 2 [39, 40]. The costs of ferric-chloride (635 USD/dry ton) and sulfuric acid (131 USD/dry ton) are taken from actual contracts made by US based towns or counties in 2017 for purchasing chemicals for wastewater treatment plants. The speci c energy consumption (SEC) for the nano ltration step was cal- 27 culated using SEC (kWh m3) =P(bar)1(kWh m3) 36(bar)1 r(2) , where P is the applied pressure (8 bars) and r is the recovery ratio (0.95). The energy expense was then calculated using the average cost of electricity to US industry, 0.07 USD/kWh. The sulfuric acid dose needed to decrease the pH from the original value (7.5) to 6.7 for preventing calcite formation was calculated using a PHREEQC [30] model of a high alkalinity (4.9 meq/l) secondary euent [41]. Table 2: Main components of the operating expenses required for reducing phosphorus concentration in wastewater by >90%. Comparison between chemical precipitation (10 mgP/l and Fe/P molar ratio = 2) and nano ltration (applied pressure = 8 bars and 95% recovery). Assumptions and calculations are explained in more detail in the main text. Chemical Precipitaion Costs Nano ltration Costs FeCl 3 0.067 $/m3H2SO40.0067 $/m3 Energy 0.019 $/m3 Total 0.067 $/m3Total 0.029 $/m3 As seen in table 2, the energy consumption was the main operational expense for the wastewater nano ltration step. Nevertheless, the applied pressure and thus the energy consumption can be potentially reduced by 50% or more. This can be achieved with "loose" high- ux NF membranes, as opposed to the "tight" NF membrane used in this study. Pressure reduc- tion is practical since the osmotic pressure of the euent is low (0.1 bars) and the salt rejection is only partial, thus the trans-membrane osmotic pres- 28 sure is of little signi cance in the current case. Such loose NF membranes were previously developed by the authors [42] and are currently tested for phosphorus rejection. Expenses on acid for pH modi cations were found to be insigni cant, despite the fact that high alkalinity water was used for the calculation. This is due to the low bu er capacity of the carbonate system at the relevant pH range (7-8). Overall, the operational expenses were sig- ni cantly lower for the NF scheme in the considered case, demonstrating its economical feasibility. It should be noted that for P <4 mg/l, the operational expenses for ferric chloride treatment are lower when using the same euent and assumptions as above (8 bars, high alkalinity). However, the NF process has the additional bene ts of removing pollutants (e.g. organic pollutants and heavy metals), decreasing salinity and enabling phosphorus recovery. 5. Conclusions The analysis shown above introduces a new approach for the removal and recovery of phosphorus from diluted euents. It comprises a low-pressure nano ltration step followed by a Ca-P crystallization step applied on the re- tained solution. Frequent acid cleaning, enabled by the use of acid-durable membrane, is applied for restoring membrane performance and recover the Ca-P deposited on the membrane or the feed channel spacer. A commer- cial acid-durable membrane was found to provide high P rejection ( >97%) even at neutral pH. NaCl rejection by this membrane was also high (50-90%) making it ideal when salinity reduction of the euent is desirable. In case salinity reduction is not required, low NaCl rejection is favoured for reducing energy consumption. A promising approach for achieving high H 2PO 4/Cl 29 selectivity, was previously demonstrated by [43] for nano ltration membranes produced via layer by layer deposition of polyelectrolytes on a porous sup- port. We currently test this type of membranes for their applicability in treating synthetic and real secondary euents. Avoiding CaCO 3precipi- tation during ltration was found to be critical for the technical feasibility of the suggested process. CaCO 3competes with CA-P for Ca2+ions, thus limits P recovery potential. Moreover, CaCO 3forms a scaling layer on the membrane which rapidly reduces permeate ux, while Ca-P tends to precip- itate in the bulk, allowing for more sustainable operation. A comprehensive thermochemical theoretical analysis, performed on model euent, indicated that mitigating CaCO 3precipitation, while maintaining high Ca-P supersat- uration is achievable via pH adjustments. Acid cleaning was shown to re- cover membrane performance completely. A preliminary economical analysis demonstrated that in many cases the operational expenses of the suggested ltration scheme are lower than those for chemical precipitation with ferric- chloride, a widely used method for P removal. Furthermore, the ltration method signi cantly improves other water quality parameters and enables P recovery. Overall our results suggest that low-pressure nano ltration is a techno-economically viable alternative for P removal from dilute solutions. Moreover when acid-resistant membranes are used, Ca-P precipitation can turn from a limitation to an advantage. Acknowledgement This project has received funding from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation 30 program (grant agreement no. 694946). R.S. is supported by the Cluster of Excellence "Tailor-made Fuels from Biomass" funded by the Excellence Initiative of the German federal and state governments. M.W. acknowledges the support through an Alexander-von-Humboldt Professorship.