Forward Osmosis As Concentration Process: Review of Opportunities and Challenges
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membranes Review Forward Osmosis as Concentration Process: Review of Opportunities and Challenges Gaetan Blandin 1,2,* , Federico Ferrari 3, Geoffroy Lesage 2 , Pierre Le-Clech 4, Marc Héran 2 and Xavier Martinez-Lladó 1 1 Eurecat, Centre Tecnològic de Catalunya, Water, Air and Soil Unit, 08242 Manresa, Spain; [email protected] 2 Institut Européen des Membranes, IEM, Université de Montpellier, CNRS, ENSCM, 34090 Montpellier, France; geoff[email protected] (G.L.); [email protected] (M.H.) 3 Catalan Institute for Water Research (ICRA), 17003 Girona, Spain; ff[email protected] 4 UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, University of New South Wales (UNSW), Sydney, NSW 2052, Australia; [email protected] * Correspondence: [email protected] Received: 11 September 2020; Accepted: 9 October 2020; Published: 14 October 2020 Abstract: In the past few years, osmotic membrane systems, such as forward osmosis (FO), have gained popularity as “soft” concentration processes. FO has unique properties by combining high rejection rate and low fouling propensity and can be operated without significant pressure or temperature gradient, and therefore can be considered as a potential candidate for a broad range of concentration applications where current technologies still suffer from critical limitations. This review extensively compiles and critically assesses recent considerations of FO as a concentration process for applications, including food and beverages, organics value added compounds, water reuse and nutrients recovery, treatment of waste streams and brine management. Specific requirements for the concentration process regarding the evaluation of concentration factor, modules and design and process operation, draw selection and fouling aspects are also described. Encouraging potential is demonstrated to concentrate streams more than 20-fold with high rejection rate of most compounds and preservation of added value products. For applications dealing with highly concentrated or complex streams, FO still features lower propensity to fouling compared to other membranes technologies along with good versatility and robustness. However, further assessments on lab and pilot scales are expected to better define the achievable concentration factor, rejection and effective concentration of valuable compounds and to clearly demonstrate process limitations (such as fouling or clogging) when reaching high concentration rate. Another important consideration is the draw solution selection and its recovery that should be in line with application needs (i.e., food compatible draw for food and beverage applications, high osmotic pressure for brine management, etc.) and be economically competitive. Keywords: cold concentration; food concentration; nutrients recovery; water reuse; brine concentration; osmotic process 1. Introduction 1.1. State of the Art Concentration Processes Concentration processes are part of the main operation units in many industrial sectors, such as food processing, mineral extraction, chemicals processing or environmental applications. Various processes exist and their selection depends both on the properties of the stream to be Membranes 2020, 10, 284; doi:10.3390/membranes10100284 www.mdpi.com/journal/membranes Membranes 2020, 10, 284 2 of 40 concentrated, requirements of the applications and final products. Those processes are classified in two main categories—membrane and thermal concentration processes. Among membrane processes, microfiltration (MF) and ultrafiltration (UF) are very broadly used and considered state of the art separation technologies in various applications (food industry, chemical industries, water treatment, etc.) [1]. These technologies are of interest for all concentration processes that require the concentration of molecules and particles with particle sizes above 0.01 and up to 1 µm. Typically, pathogens (bacteria, viruses, etc.), organic macromolecules (proteins, carbohydrates, etc.), and minerals (clays, latex, etc.) can be well rejected (and extracted or concentrated) by microporous membranes. Its main advantage is the relatively low operating pressure (from 0.1 to 5 bar) and therefore limited energy costs. High pressure membrane processes, such as reverse osmosis (RO) retain the smallest organics (i.e., tannins, organic acids, polyphenols, pesticides, pharmaceuticals, etc.), heavy metals and salts. In comparison with thermal processes, RO proves to be less energy intensive, requires less investment costs, has a lower footprint and avoids the thermal degradation of compounds (cooking taste) in food industry [2]. One main disadvantage of RO is its inability to reach high concentration levels due to the hydraulic pressure limitation. Current practices limit RO operational pressure to 80 bar due to mechanical resistance and required energy. Another limitation of RO is the fouling propensity when treating complex stream and given the high pressure operation [3]. As an alternative to UF/MF and RO, nanofiltration (NF) features intermediate properties (operating pressure, molecular weight cut-off) that allow for the rejection of compounds down to divalent ions and with a variety of membrane characteristics that can make it a more adequate choice [4]. Membrane distillation (MD) presents many attractive features, such as lower operating temperatures (<70 ◦C) compared to thermal processes, compact process, high rejection, not limited by osmotic pressure and possibility to use waste heat [5]. However, MD, so far, remains at the pilot scale, as it is not energetically and economically outperforming existing technologies (even when waste heat is used) and there are no commercially available modules for large scale applications [6]. Osmotic distillation (or osmotic evaporation) is particularly suited for the processing of heat-sensitive aqueous solutions, such as fruit juices and pharmaceutical products. The solution to be concentrated is separated by a porous hydrophobic membrane from a high osmotic pressure solution. OD was mostly tested for food concentration [2,7] but, as for MD so far, it remains on pilot scale. Among the thermal processes, evaporation is an established treatment used in a broad range of applications. The main advantages of evaporation are the relatively low technology grade, low maintenance costs and the possibility to remove high content of water. As main drawbacks, temperature sensitive compounds are altered by the high temperature and evaporation requires high amount of energy to produce heat. As alternatives, solar evaporation allows for operation at lower temperature but needs an extensive surface (lagoons). Cryoconcentration is a natural phenomenon which occurs during ice thawing. A more concentrated phase is then separated from the initial solution. By avoiding high temperature treatment, cryoconcentration technology can produce highly nutritive compounds, with biological and organoleptic value [8]. However, the commercial solution still requires remarkable energy consumption and achievable concentration remains below that of evaporation [9]. Thus, to date, none of the existing technologies have offered a fully satisfying solution allowing for high concentration rate at low energy costs and without affecting the properties of the concentrated products. 1.2. The Emergence of Forward Osmosis In forward osmosis (FO), the solute concentration gradient (also called osmotic pressure differential, Dπ) acts as the driving force between two liquids separated by a selectively permeable membrane. As a result, the permeation of water occurs through the membrane from the lowest (feed solution) to the highest solute concentration solutions (so-called draw solution) while suspended solids, most of them dissolved molecules or ions, do not pass through the membrane and are concentrated in the feed (Figure1). First work of Loeb in the late 1970s [ 10–12] on osmotic processes remained relatively unexplored until new semi-permeable membranes, tailor-made for osmosis applications, Membranes 2020, 10, 284 3 of 40 were developed in the early 2000s [13–16]. The two main osmotically driven processes that were initiallyMembranes considered 2020, 10, x FOR were PEER FO REVIEW and pressure retarded osmosis (PRO). Both FO and PRO sparked3 of 42 intense research, being seen as novel and highly promising technologies for seawater desalination sparked intense research, being seen as novel and highly promising technologies for seawater and energy production, respectively [17,18]. However, following progress in the technology and desalination and energy production, respectively [17,18]. However, following progress in the development from initial laboratory proof of concept towards full scale implementation in those technology and development from initial laboratory proof of concept towards full scale targeted applications, both FO and PRO faced challenges both in terms of technical limitations (low implementation in those targeted applications, both FO and PRO faced challenges both in terms of flux, membrane resistance at high pressure for PRO) and clear competitive advantages with regard technical limitations (low flux, membrane resistance at high pressure for PRO) and clear competitive to initial applications forecasted [19,20]. Thus, and despite significative advances in membrane advantages with regard to initial applications forecasted [19,20]. Thus, and despite significative and modules designs and operation, so far those applications of osmotic