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  Contents lists available at ScienceDirect Journal of Environmental Chemical Engineering  journal homepage: www.elsevier.com/locate/jece Engineering of struvite crystals by regulating supersaturation  –  Correlationwith phosphorus recovery, crystal morphology and process e ffi ciency Sina Shaddel a, ⁎ , Seniz Ucar b , Jens-Petter Andreassen b , Stein W. Østerhus a a  Department of Civil and Environmental Engineering, Norwegian University of Science and Technology (NTNU) Trondheim, Norway  b  Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU) Trondheim, Norway  A R T I C L E I N F O  Keywords: StruvitePhosphorus recoverySupersaturationCrystal morphologySettling velocity A B S T R A C T Struvite crystallization is widely applied for nutrient recovery from wastewater streams. The better under-standing of the e ff  ects of reaction conditions on  󿬁 nal crystal properties will contribute to improve both therecovery e ffi ciency and product quality of struvite as a fertilizer. In this study, batch crystallization experimentswere performed in laboratory scale to reveal the e ff  ect of supersaturation on the phosphorus recovery and crystalproperties. For this purpose, supersaturation is regulated through varying the pH, magnesium and ammoniumconcentrations in solution. The e ff  ects of these parameters on controlling crystal properties such as size andmorphology are highlighted through their role as supersaturation regulators.The potential implications of di ff  erent crystal morphologies on settling velocity and aggregation of crystalsare also discussed. This improved understanding could aid in improved struvite crystallization processes forwastewater treatment. 1. Introduction Phosphorus (P) is one of the essential elements in living organismsand an irreplaceable ingredient for agricultural fertilizers that are usedin crop and livestock production. However, phosphate rocks, which arethe main source of P, are non-renewable and in danger of depletion [1].In addition, beside diminishing phosphate rocks, P content has in-creased notably in the hydrosphere because of human activity throughdomestic and industrial waste, which can cause eutrophication [2].Therefore, its e ffi cient use, recovery and recycling are important stepstowards environmental safety and a sustainable development.P-recovery from wastewater is an e ff  ective strategy to address bothproblems and has gained great attention [1]. The most commonly usedapproach has been the precipitation of P-bearing minerals from was-tewater, such as struvite (magnesium ammonium phosphate hexahy-drate, MgNH 4 PO 4 ·6H 2 O). Struvite has the advantages of being com-posed of primary macronutrient (nitrogen and phosphorus) andsecondary macronutrient (magnesium), and being a slow-release ferti-lizer that can be used directly as precipitated [3,4]. In addition, con- trolled precipitation of struvite in wastewater treatment plants aids inavoiding pipe clogging and extra costs of equipment cleaning [5,6]. E ffi ciency of a P-recovery process and struvite product quality canbe enhanced by controlling the precipitation reaction to optimize thereaction time as well as the size and morphology of the crystals. Theshape and size of crystals have strong impact on the properties of   󿬁 nalproduct as well as the e ffi ciency of downstream processes like settle-ability,  󿬁 ltering and drying. Therefore, it is important to be able totailor process conditions for optimization of these properties. It couldbe misleading to measure the process e ffi ciency in terms of soluble-Premoval as the  󿬁 ne particles can be washed-out and redissolve indownstream processes, which a ff  ect the reliability of modeling andeconomic predictions for the whole process [7]. Therefore, improvingthe e ffi ciency of downstream processes via regulating reaction para-meters enhances the phosphorus recovery e ffi ciency and ensures thequality of e ffl uent. The consistency in the quality and properties of  󿬁 nalproduct is one of the determinative factors for the success of struviteproduction. The particular processes in wastewater treatment plants,feed and seasonal variations often create changes in the composition of the input material to the crystallizer. Therefore, the operational con-ditions should be properly adjusted to minimize the risk of fail to meetthe product quality and process requirements.Supersaturation is the main parameter that governs size and mor-phology of the precipitated struvite and the phosphorus recovery e ffi -ciency [8,9]. Crystal size is determined dominantly by supersaturation since the nucleation and growth kinetics are correlated with the ther-modynamic driving force in the crystallizing system. The application of the kinetic models for nucleation and growth can be used to control thesize related settling characteristics of struvite crystals [10]. The crystal https://doi.org/10.1016/j.jece.2019.102918Received 29 October 2018; Received in revised form 14 January 2019; Accepted 19 January 2019 ⁎ Corresponding author at: S. P. Andersens veg 5, 7031 Trondheim, Norway.  E-mail address:  sina.shaddel@ntnu.no (S. Shaddel). Journal of Environmental Chemical Engineering 7 (2019) 102918Available online 23 January 20192213-3437/ © 2019 Elsevier Ltd. All rights reserved.    morphology is an outcome of the internal (crystal lattice structure andcrystal defects) and external factors (supersaturation, the presence of impurities, temperature, etc.). The modi 󿬁 cation of crystal morphologyhas been receiving growing attention due to both theoretical interestand industrial needs [11,12]. However, a systematic correlation of  struvite morphology with supersaturation and transition boundary be-tween di ff  erent morphologies for wastewater application is not yet fullyexplored in the literature. The dependency of struvite morphology onsupersaturation, mixing energy and retention time has already beenmentioned by previous studies [8,13,14]. Although it is well established that crystal morphology is highly e ff  ective on the settling velocity, thisaspect has not been reported for struvite previously.Previous work also implicitly reported the dominant e ff  ect of su-persaturation on precipitation kinetics and aggregative properties of crystals [15,16]. The liquid from dewatering of anaerobically digested sludge is the main feed for the majority of struvite reactors [17]. Al-though, the reported studies are not directly applicable for wastewaterapplications due to distinct compositions and characteristics related towastewater. The equimolar and low concentration of struvite con-stituents ions or the high phosphate concentrations in swine wastewaterare not representative for wastewater applications.High initial supersaturation in the vicinity of inlet ports of struvitereactor can favor nucleation over crystal growth at the onset of crys-tallization. Improving aggregation of the generated crystals in this stageis important; otherwise distribution of them in the space and time willreduce the chance of collision and aggregation in later stages.Moreover, improved aggregation contributes to better granulation andwill further reduce the chance of product loss from reactor by wash out.While the struvite aggregation is mainly explained based on zeta po-tential in previous studies [18,19], the e ff  ect of initial supersaturationon crystal aggregation is mainly disregarded.Numerous studies have investigated the feasibility of P-recovery viastruvite precipitation from laboratory to pilot scale and the existing full-scale struvite crystallization techniques all advertise high recovery ef- 󿬁 ciency (80 – 90%) [7,19 – 21]. However, the impacts of operationalconditions on the di ff  erent characteristics of obtained products remainunclear. Thus, large-scale processes are yet to be optimized in terms of product quality and e ffi ciency, where variations can be expected due toindividual processes employed at di ff  erent plants. The individual opti-mization of reaction parameters such as pH and struvite constituentions (Mg 2+ , NH 4+ ) is the most common approach to enhance thephosphorus recovery. However, this approach without a clear under-standing of bene 󿬁 ts and disadvantages on product properties will notimprove the overall e ffi ciency of the process. Further, owing to thecomplexity of crystallization processes, considering a comprehensiveparameter is necessary to consistently achieve the desired recoverye ffi ciency and product properties. Supersaturation is an inclusiveparameter independent of reactor type which takes into account thee ff  ect of several parameters (struvite constituent ions, ionic strength,pH and temperature). Although supersaturation regulation is crucial toimprove the overall process performance, lack of a fundamental un-derstanding on lumped recovery e ffi ciency and struvite propertieshindered the utilization of supersaturation as a design parameter.Therefore, coupling the recovery e ffi ciency and product properties re-quires a better understanding of the role of supersaturation from afundamental viewpoint.The aim of this study is to explore the central role of supersaturationon phosphorus recovery e ffi ciency and shaping product properties.Revealing the correlation between supersaturation and the size andmorphology of the  󿬁 nal precipitation products, and further evaluatetheir consequent e ff  ects on downstream processes is the main focus of this work.One of the objectives of this study was to utilize the supersaturationas an inclusive parameter to couple the recovery e ffi ciency and productproperties. Therefore, the e ff  ects of impurities were eliminated byconducting the experiments with synthetic reject water that enablesaccurate calculation of supersaturation. Both reaction conditions andcrystal properties are evaluated for discussing the  󿬁 nal crystal mor-phology where supersaturation was followed as the main parameter.In order to de 󿬁 ne a proper operational strategy for supersaturationregulation, a systematic approach was employed to investigate the ef-fects of pH and molar ratios of constituent ions on the P-removal e ffi -ciency and product quality. By scanning through a relevant range of supersaturation values, e ffi cient control on crystal size and morphologywas attained and further correlation of these properties with settlingand aggregation characteristics were evaluated. This fundamental un-derstanding is crucial for improving full-scale applications of phos-phorus recovery by struvite crystallization via optimization of opera-tional conditions and their e ff  ects on crystal properties. 2. Materials and methods  2.1. Materials Magnesium chloride hexahydrate (MgCl 2 ·6H 2 O), sodium dihy-drogen phosphate (NaH 2 PO 4 ·2H 2 O), ammonium chloride (NH 4 Cl) andsodium hydroxide (NaOH) were used for the synthesis of struvite. Allchemical reagents were purchased from Merck with analytical grade.Milli-Q water (18.2 M Ω .cm) was used for all purposes.  2.2. Methods All experiments were carried out using a lab-scale crystallizationsystem, composed of a 1L glass reactor, stirred with Te 󿬂 on two-bladepropeller controlled by a mechanical stirrer operated at 200rpm.Temperature was regulated by a water bath and maintained at20 ± 0.5  ͦ  C for all experiments. The pH was constantly measured andrecorded by a combined glass electrode with KCl reference electrolyteconnected to EasyDirect ™  pH Software (Metrohm), and calibrationswere carried out daily. The precipitation reaction was initiated by ad-dition of MgCl 2  solution to a synthetic reject solution and the reactionswere let to proceed for 60min. The pH was kept constant during ex-periments by addition of 1M NaOH. In the case of terminating crys-tallization reactions for further studies, the pH of reaction was loweredto pH=7 by adding appropriate amounts of HCl followed by quick 󿬁 ltration to prevent dissolution. Nitrogen atmosphere was constantlypreserved on top of the solutions throughout the crystallization reac-tions to prevent intrusion of atmospheric carbon dioxide. The chemicalspeciation and activity based supersaturation were determined bythermodynamic calculation program Visual MINTEQ 3.1. The pre-cipitates were collected at the end of each experiment by vacuum  󿬁 l-tration through a 0.2 μ m pore size  󿬁 lter (PP membranes). The ionconcentrations in the  󿬁 ltrate were determined via spectrophotometry(Hach DR Lange 1900). The particle size distribution was performedand analyzed with Beckman Coulter LS230 laser di ff  raction particle sizeanalyzer. The presented particle size distributions are based on dynamiclight scattering technique and derived based on sphericity. Thus, thepresented results are nominal size of crystals for the comparison of theresults.Solid phases were characterized via powder X-ray di ff  raction (XRD)(D8 Advance DaVinci, Bruker AXS GmBH) in the range of 5 − 75° witha step size of 0.013° and a step time of 0.67s. The analysis of XRD datawas performed by DIFRACC.SUITE EVA software (Bruker) and theInternational Centre for Di ff  raction Data database (ICDD PDF-4+2018) was used to characterize the precipitates. SEM analyses(Hitachi S-3400N) were performed where samples were placed oncarbon tape and sputter coated with gold. The zeta potential mea-surements were conducted by Malvern Zetasizer Nano ZS in a dip cell at20°C.  S. Shaddel et al.  Journal of Environmental Chemical Engineering 7 (2019) 102918 2   2.3. Design of experiments The solution concentrations in this study were selected based on adewatering reject of anaerobically digested sludge at a municipalwastewater treatment plant. The feed to the digester is a mixture of thethickened primary sludge and dewatered sludge that resulted from anenhanced biological phosphorus removal (EBPR) process. The opera-tion strategy for supersaturation regulation is important in order tokeep high recovery e ffi ciency and product quality. The Mg:N:P=1:1:1is the theoretic requirement for struvite precipitation, while there is animbalance between molar ratios of these ions in practical implications.The reagent cost for magnesium and alkali may a ff  ect the economicfeasibility of struvite production [22]. In this work, di ff  erent pH andMg:N:P molar ratios were evaluated for struvite crystallization. Thereaction kinetics at pH < 7.5 were very slow and pH > 9.5 may resultin precipitation of other phases than struvite [23], thus the reaction pHwere selected as pH=7.5 (low), pH=8.5 (medium) and pH=9.5(high). By combining di ff  erent molar ratios with di ff  erent pH values,struvite crystallization was investigated under a broad range of super-saturation and growth kinetics. The addition of magnesium beyond theconsumption potential by struvite crystallization increases the oper-ating cost, may result in formation of various magnesium phosphateprecipitates and increases the chance of unintentional struvite pre-cipitation in other processes [24]. Therefore, a series of preliminaryexperiments were performed to maximize P-recovery with minimumincrement of Mg:P molar ratio beyond the stoichiometric value of 1:1.The Mg:P=1.67:1 was selected based on the result of these experi-ments. The dewatering sidestreams of EBPR sludge both before andafter anaerobic digestion are rich in phosphorus, while the reject  󿬂 owsafter anaerobic digestion have signi 󿬁 cant surplus of ammonium withrespect to phosphate and magnesium [25]. Therefore, the ammoniumconcentration to the crystallizer (N:P molar ratio) can be controlled byregulating the contribution of phosphate rich and ammonium richstreams or by control of recycle ratio of the reactor. According totechnology providers N:P>6 is optimal to maximize the recovery ef- 󿬁 ciency and purity of struvite and struvite precipitation would be of interest if N:P>1 [19,26]. The molar ratio of N:P=12 is the srcinal N:P in the selected reject water and the other N:P molar ratios wereselected based on these criteria. All experiments were performed induplicates and Table 1 shows the experimental conditions and thethermodynamically calculated activity based supersaturation, S a , foreach experiment by using Eq. (1) [27]: ⎜ ⎟ = ⎛⎝⎞⎠ S IAPK asp(13) (1)IAP=ion activity product=a Mg2+ · a NH4+ · a PO43 − K sp = thermodynamic solubility productThe thermodynamic equilibrium calculations for struviteprecipitation were performed by Visual MINTEQ 3.1 software to cal-culate the theoretical yield for each solution and the thermodynamicevaluation was followed by lab-scale batch experiments with a reactionduration of 60min. The presented theoretical and experimental resultsof percent phosphorus recovery were calculated by using Eq. (2). ⎜ ⎟ − = ⎛⎝− ⎞⎠ × P recovery% P PP100% initial finalinitial  (2)  2.4. Preparation of solutions Stock solutions of magnesium chloride hexahydrate (MgCl 2 ·6H 2 O),sodium dihydrogen phosphate (NaH 2 PO 4 ·2H 2 O) and ammoniumchloride (NH 4 Cl) were prepared from their corresponding crystallinesolids (Merck, reagent-grade) using MQ-water. Synthetic reject waterwas then prepared from the stock solutions following the originalcomposition of the reject water with total ammonium nitrogen (NH 4 -N=745mg/L) and total phosphate (PO 4 -P=137mg/L) conformingN:P=12:1, and the  󿬁 nal composition in each experiment was adjustedto achieve the compositions in Table 1. The supersaturated solutionswith respect to struvite were prepared by addition of the magnesium-containing solution to synthetic reject water under constant stirring.  2.5. Measurements of settling velocity  Sedimentation tests were conducted on an experimental set-upwhich was a modi 󿬁 ed form of the Andreasen pipette method [28]. Theset-up consisted of a cylindrical glass burette with 8mm internal dia-meter  󿬁 lled with water pre-saturated with respect to struvite to avoiddissolution. The sieved crystals were dispersed in 1mL of the same li-quid and allowed to make a distinct front before measurements. Themeasuring system was based on visual space-time registration andtravel time of the particles measured by digital-display stopwatch. 3. Results and discussion 3.1. E   ffi ciency of phosphorus recovery  The initial phosphorus concentration is equal in all conditions andthe  󿬁 nal phosphorus concentration is determined by the solubility of struvite, which is constant at constant temperature, for the calculationof theoretical yield. Fig. 1 presents the experimentally measured andtheoretically percent P-recovery values for all determined conditions.The increasing values of P-recovery shown with thermodynamic cal-culations re 󿬂 ect the e ff  ect of higher supersaturation in the reactionmedium that is achieved either by increasing the pH or molar ratio of the constituent ions with respect to P. The results for struvite yield Table 1 Experimental conditions and calculated activity based supersaturation values(P=137mgL − 1 ). ExP. Mg:N:P pH Sa1 1:2:1 7.5 1.52 1:2:1 8.5 3.33 1:2:1 9.5 5.84 1:6:1 7.5 2.15 1:6:1 8.5 4.56 1:6:1 9.5 7.97 1:12:1 7.5 2.48 1:12:1 8.5 5.39 1:12:1 9.5 9.510 1.67:12:1 7.5 2.711 1.67:12:1 8.5 5.912 1.67:12:1 9.5 10.6  Fig. 1.  The P-recovery measured in the experiments and equilibrium theoreticalP-recovery calculated from Visual MINTEQ.  S. Shaddel et al.  Journal of Environmental Chemical Engineering 7 (2019) 102918 3  based on thermodynamic equilibrium calculation are presented inFigure S1.The strong in 󿬂 uence of pH on supersaturation results from its de-terministic e ff  ect on phosphate and ammonia speciation in solution(Table S1) [60]. Increasing pH shifts the equilibrium reactions of struvite constituents, which consequently increases the supersaturationwith respect to struvite. A satisfactory  󿬁 t was observed between theo-retical and experimental results within the reaction time of 60mindetermined for this study, except for two sets of experiments atpH=7.5 with lowest supersaturation values due to slower reactionkinetics (Mg:N:P=1:6:1 and Mg:N:P=1:2:1). Higher supersaturationmeans a higher chemical potential for the crystallization of struvite andconsequently a higher phosphorus recovery as observed in the ther-modynamic calculations. Increasing Mg:P and N:P molar ratio at lowpH (7.5) caused a greater e ff  ect on P-recovery e ffi ciency than that inhigher pH values.Moreover, experimental results also showed the strong e ff  ect of supersaturation on the kinetics of the precipitation reaction and in-dicated that supersaturation is a key parameter for e ffi cient P-recoveryby precipitation. Both nucleation and crystal growth rates scale withsupersaturation, thus, the reaction times for completing the precipita-tion shorten under high supersaturation. The rate of NaOH addition formaintaining the solution pH during the precipitation reactions was anindication of the reaction progression [22] and it was found that amongthe studied factors, pH showed the major e ff  ect on the reaction time.Increasing the reaction pH from 7.5 to 8.5 and from 8.5 to 9.5, reducedthe time for completing the reactions by a factor of   ≈ 5 and ≈ 3, re-spectively (data not shown). According to the results of ion and com-plex speciation by Visual MINTEQ 3.1 (Figure S2) the increase of pHchanges the ion speciation and ion complexations towards a higherPO 43 − and MgPO 4 − content while decreasing free Mg 2+ and NH 4+ .This implies that higher rates of the precipitation reactions at higher pHcan mainly be explained based on higher amounts of available phos-phate precursors. This hypothesis is in line with the  󿬁 ndings of Abbonaet al. deducted from molecular modeling based on periodic bond chain(PBC) theory [8].The slow reaction kinetics observed at low supersaturation levelscan be disadvantageous in the case of full-scale application specially forbig treatment plants with high  󿬂 ows of reject water. Therefore, super-saturation adjustment via pH and precursor concentration is of vitalimportance for enhanced recovery e ffi ciency. The struvite rectors mustbe optimized to not only convert the soluble phosphorus to struvite butalso to maximize the quality and collection of the product [29]. How-ever, the dissimilarities associated with reactor type (i.e.  󿬂 uidized bed,air-agitated and stirred) make this optimization challenging. Aeration ismainly practiced for pH increase by CO 2  stripping, while process reg-ulation by aeration adjustment could be challenging due to complexdependency of turbulence on bubble size and reactor dimensions.Further, it has been reported that adjustment of aeration intensity didnot improved the struvite recovery and particle settling [30]. In stirredreactors, the mixing improvement enhances the struvite recovery bymore homogenous distribution of supersaturation and preventing localpeaks in supersaturation, otherwise mixing by itself can not be con-sidered as the main parameter for optimization. Therefore, processoptimization requires selection of a proper control parameter. Super-saturation incorporates the impact of essential parameters (constituentions, pH and temperature) and has the privilege to uncouple the opti-mization to a great extent from reactor type. Moreover, supersaturationregulation approach can be further utilized for crystal growth and ki-netics studies as a well-established theoretical and practical knowledgeis available on the supersaturation role in di ff  erent aspects of crystal-lization process [10,27,31]. 3.2. Product characterization: purity and crystal morphology  In the crystallization systems with su ffi cient concentrations of Mg 2+ , NH 4+ and PO 43 − ions, struvite and various magnesium phos-phate minerals can potentially precipitate. However, the  󿬁 nal pre-cipitate is the result of both thermodynamic and reactions kinetics. Thethermodynamic calculations showed that at the de 󿬁 ned experimentalconditions, the supersaturated phases were limited to struvite, new-beryite (MgHPO 4 ·3H 2 O) and trimagnesium phosphate (Mg 3 (PO 4 ) 2 ).The intensity and positions of XRD patterns matched with the re-ference powder di ff  raction  󿬁 le for struvite (PDF 00-015-0762) thatfurther con 󿬁 rmed presence of pure struvite under all experimentalconditions (Figure S3). Further, based on de 󿬁 ned experimental condi-tions, obtaining pure struvite is in accordance with previous studies. Itis reported that nucleation rate of struvite is greater than that of new-beryite and newbryite is the stable form at low pH (pH < 6) and highmagnesium concentrations [32]. Trimagnesium phosphate has neverbeen observed in the pH range of 6 – 9.5 and is reported to have lowprecipitation rate. Therefore, in the pH range of this work, neithernewberyite nor trimagnesium phosphate are kinetically favorablephases and struvite precipitation is more abundant [23,32,33]. The morphology (external shape) of a crystalline particle is de-termined by the intrinsic characteristics of the crystal structure and theexternal factors of growth conditions such as solution composition andtemperature. The  󿬁 nal crystal morphology arises as a result of the re-lative growth rates of each of its faces which are a ff  ected by both in-ternal and external elements of growth [24,34]. Fig. 2 shows the wedge shape morphology of struvite crystals with corresponding miller indicesfor di ff  erent faces.The e ff  ects of reaction conditions on the  󿬁 nal crystal morphologyare investigated in detail and Fig. 3 summarizes the di ff  erentmorphologies observed at varying conditions of supersaturation andammonium concentration. The represented morphologies in Fig. 3 areselected after thorough inspection by SEM in order to report thedominant crystal morphology in each experiment. The observedmorphologies are categorized in three groups as polyhedral (well-fa-ceted), hopper and rough (dendritic) morphologies with a possibletransition boundary between them. 3.2.1. E   ff  ects of supersaturation on struvite morphology  Struvite morphologies observed at low supersaturations (S a =1 – 3)showed a well-faceted structure with a bipyramidal appearance andgenerally free of major defects (zone 1). The crystals at S a =1.5(Fig. 3a) re 󿬂 ected the most basic morphology of struvite crystal that ishemimorphic with unequally developed [001] and [00 1¯ ] faces. In thiszone, increasing the ammonium concentration triggered the growth of [001] face which appeared as a sharp and narrow face on the top of thecrystal (Fig. 3b, c and d). It can be seen that in zone 1 increasing pH byone unit caused longitudinal elongation of the crystals (Fig. 3e). In-creasing the supersaturation beyond S a =3, either by increasing the pHor the concentration of constituent ions, initiated the transition be-tween zone 1 with well-faceted crystals and zone 2 with hopper crys-tals. The crystals observed in zone 2 seemed to be twin crystals with the Fig. 2.  The wedge shape morphology of struvite crystals. The facets are de 󿬁 nedwith corresponding Miller indices.  S. Shaddel et al.  Journal of Environmental Chemical Engineering 7 (2019) 102918 4
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