Abstract
Brine evaporation and crystallization are energy- and equipment-intensive technologies commonly used in industries. Alternatively, membrane distillation (MD) has shown ability for concentrating different saline solutions. However, there is a limited understanding of the impact of these solutions on the scaling and wetting behavior. To address this knowledge gap and reduce costs and energy consumption, we investigated a novel method called ‘seeded near-zero liquid discharge membrane crystallization’ (NZLDMC). This approach combines MD and crystallization in a single apparatus, reducing capital and operating costs while improving sustainability. Our study focused on applying seeding processes at NZLDMC for concentrating synthetic mono- and multi-salt solutions with a salinity of 300 g/L and a real salt solution. We investigated the effects of salt crystals and seeds on membrane performance under different operating conditions. Our findings indicate that seeding crystal technology leads to high yield and long-term stability, thereby offering the potential in industries and municipal water treatment to fractionate salt in multi-salt solutions and obtain purified water simultaneously.
HIGHLIGHTS
The seeded membrane distillation crystallization process allows the treatment of highly concentrated salt solutions.
The absence of membrane scaling is a benefit of seed crystals for long-term stability.
Using a membrane distillation and crystallization process in a single unit reduces capital and operating costs while improving sustainability.
INTRODUCTION
Severe water shortages affect over 4 billion people worldwide for at least 1 month each year, and the scarcity is driven by climate change and industrial expansion (Mekonnen & Hoekstra 2016; Dehghani et al. 2022). In particular, arid areas in Europe and America face persistent challenges in obtaining clean water, leading to economic and environmental consequences (Hristov et al. 2021). Desalination improves tap water supplies in arid regions and coastal cities (Al-Agha & Mortaja 2005; Shomar & Hawari 2017). Desalination and industrial processes generate considerable brine waste. The world produces more than 100 million m3/d of water from desalination, leaving an even higher volume of concentrated brine (141.5 million m3/d) (Jones et al. 2019). The disposal of brine poses a fundamental challenge. Common practices include direct disposals into the environment, such as surface water discharge, evaporation ponds, deep-well injection, and direct land applications, which can increase the salinity of surface water and harm the environment (Drioli et al. 2012; Creusen et al. 2013; Panagopoulos et al. 2019). The brine has thereby typical salinities of 70–85 g L−1, resulting in large amounts of contaminated water that could be recovered.
Addressing these challenges, zero liquid discharge (ZLD) processes have emerged as effective approaches in recovering a large amount of water from the rejected brine, reducing environmental impact and promoting sustainability (Almasri et al. 2015). Wastewater-free production has become a focal point in recent decades, partly driven by concerns about regulatory fines associated with improper wastewater management (Raff & Earnhart 2018). The ZLD technology employs a closed resource and water cycle to prevent saltwater release into the environment, which requires the crystallization process (Panagopoulos 2021; Date et al. 2022). In the context of high salt content and brine waste, ZLD is integrated with membrane-based water desalination processes (Conidi et al. 2018; Damtie et al. 2019; Zhao et al. 2020). Solar evaporation and wind-assisted intensified evaporation technology (WAIV) are commonly used methods for handling brine waste (Gilron et al. 2003; Bello et al. 2021). Solar evaporation consists of leaving the rejected water in a pond, where the water evaporates due to the sun. WAIV is an alternative to solar evaporation ponds and uses wind energy to evaporate the water (Ahmad & Baddour 2014; Morillo et al. 2014; Bello et al. 2021; Shah et al. 2022).
An innovative process to solve this problem is membrane distillation (MD) crystallization, which allows the recovery of water and raw materials from the brine solution. This process minimizes the brine volume considerably and results in a near-ZLD. MDC is a combination of MD and crystallization. MD represents an innovative membrane-based process that handles highly saline solutions, making it ideal for water production (Peters & Hankins 2021; Julian et al. 2022). MD is a separation process that utilizes hydrophobic membranes to allow water vapor molecules to pass due to the vapor pressure difference across the membrane (Rao et al. 2014; Gustafson et al. 2018). The main product of MD is water, while the remaining feed solution is highly concentrated. This creates a favorable situation for crystallizing the salts in the solution. Obtaining supersaturation allows crystals to be produced from the feed solution.
Previous studies have shown several challenges to overcome when developing a feasible MDC process for the treatment of high-saline solutions, such as unwanted crystallization in the membrane module and crystal blockages in the connecting tubes between the membrane module and the crystallizer (Tun et al. 2005; Guan et al. 2012). Therefore, the selection of the correct operating parameters is essential to avoid crystal deposits and enable the long-term operation of the process to obtain high water and salt recovery rates (Chen et al. 2014). For example, typical operating times without cleaning are less than 6–8 h until now. The addition of seeding crystals can have a positive effect on the stability of the MDC process. The vapor flow decreased during the experiment by only 9% with the gypsum seeding crystals and by 29% without seeding. When more than 5 g L−1 seeding crystals are added, the seeding, however, harms scaling (Yan et al. 2021).
In this study, a novel MDC process with seed crystallization technology and a newly introduced classifier for treating hypersaline solutions were developed and experimentally investigated. The work investigates the potential of long-term stability, high water recovery, and salt recovery without decreasing permeate quality from synthetic hypersaline solutions and the possibility of concentrating real wastewater in a long-term experiment.
METHODS AND MATERIALS
Mini NZLDMC plant
Schematic drawing of the mini NZLDMC plant (left) and the plant (right).
The used membrane is a polypropylene (PP) tubular membrane. The hydrophobic membrane is made from 3M® Accurel PP V8/2 HF. The mini-plant has an effective surface area of 0.017 m2, a nominal pore size radius of 0.2 μm, a thickness of 1,550 μm, and a porosity of 73%. The inner and outer diameters of the membrane are 5.5 and 8.6 mm, respectively. The membrane selection is based on hydrophobicity and previous experiments using MD (Rezaei et al. 2018, 2020). The tubular membrane provides, in general, a better hydrodynamic condition compared to the flat sheet membrane and is generally regarded as providing less fouling. The heat exchanger section has a polypropylene graphite (PP-GR) tube with a length of 1 m and a surface area of 0.024 m2, as well as a sedimentation section (Figure 1).
The feed is heated to a constant temperature in a co-current heat exchanger using a thermostat (Haake, NB 22) and then pumped into the lumen side of the tubular membrane at the top of the module. A magnetic flowmeter (Kobold, MIK 5N15AL343) indicates the flow rate, which is controlled by the speed of the peristaltic pump (Ismatec ISM 1080, Ecoline). The cold medium flows co-currently in the module at a temperature of 20 °C. The condensed permeate is collected in the permeate vessel and weighed by a balance (Kern, DE60K2N). The inlet and outlet temperatures of the membrane module, the permeate temperature in the permeate vessel, and the heat exchanger temperatures are measured with resistance thermometers (Kobold, MWE PT100). Conductivity meters are used to measure the electrical conductivity of the concentration loop (Kobold, LCI SG40MPF) and the permeate conductivity (Kobold, ACS-Z2T1G). An inline pH meter (Kobold, gel-filled pH electrode) is used to measure the pH of the concentrating loop. The process conditions are adjusted so that the temperature of the loop in the membrane module remains constant to ensure continuous crystallization. The loop flow rate is 70–100 L h− 1, which was selected to prevent the crystals from growing in the mini-plant and blocking the pipelines. After the flow becomes constant, the permeate flux is calculated. The process is conducted continuously in a fed-batch experiment.
Design of the membrane and heat exchanger modules



























Parameters for the design of the membrane module and the heat exchanger module
Properties . | . | Membrane module . | Heat exchanger module . |
---|---|---|---|
n | Amount | 1 | 1 |
Aeff (m2) | Effective area | 0.017 | 0.024 |
L (m) | Module length | 1 | 1 |
V (mL) | Volume | 24 | 45 |
Din (m) | Diameter | 0.0055 | 0.008 |
Re number (70 L h−1) | Reynolds number | 7,400 | 5,430 |
k (W m−2 K−1) | Overall heat transfer coefficient | – | 783 |
Properties . | . | Membrane module . | Heat exchanger module . |
---|---|---|---|
n | Amount | 1 | 1 |
Aeff (m2) | Effective area | 0.017 | 0.024 |
L (m) | Module length | 1 | 1 |
V (mL) | Volume | 24 | 45 |
Din (m) | Diameter | 0.0055 | 0.008 |
Re number (70 L h−1) | Reynolds number | 7,400 | 5,430 |
k (W m−2 K−1) | Overall heat transfer coefficient | – | 783 |
Performance of the MDC process
The permeate flux, water recovery, and salt rejection can evaluate the performance and effectiveness of the membrane process in water treatment.














Influence of the temperature on the process
Particle sedimentation and separation




Impact of the minimum separable particle diameter of particles with kg m− 3 and the temperature on the particle size. The dotted line shows the minimum separable particle diameter of the particle with
kg m− 3, corresponding to the crystals from wastewater.
Impact of the minimum separable particle diameter of particles with kg m− 3 and the temperature on the particle size. The dotted line shows the minimum separable particle diameter of the particle with
kg m− 3, corresponding to the crystals from wastewater.
The velocity in the tube slows down by a factor of 10 from 0.34 to 0.034 m s−1 when the tube diameter is increased from 12 to 32 mm at a feed flow of 78 L h−1. In the specific temperature range, the NaCl crystals settle earlier and only grow to 260 μm before the sediment due to the reduced velocity in the 32 mm settling tube (Figure 1, continuous line). By keeping the feed flow at 78 L h−1, at a settling tube diameter of 12 mm, the crystals grow to 879 μm before they are separated by settling due to a higher flow velocity. Calculations show that temperature affects crystal size, with crystals being 25–40% smaller at 338 compared to 318 K.
The ability to control crystal growth via the settling tube allows the process to be adapted directly to the size of the salt that is required. With the given tube diameters and given feed velocity, the separated crystal size range is from 829 to 205 μm for a 22.5% NaCl solution.
The tube velocity for the wastewater experiment was 0.046 m s−1with the 25 mm tube diameter. The crystals based on the theory should begin to settle with a diameter of 174 μm at 318 K and 148 μm at 338 K (Figure 1, dotted line).
Salt recovery


Crystal characterization
The precipitated crystals were observed with an optical microscope (Keyence VHX-7000), which provides exceptional depth of field for flexible crystal viewing, and the crystal sizes (2D) were characterized with the microscope software and Image J 1.54h software.
MATERIAL
Several factors were considered in the selection of materials, including corrosion resistance, weight, and the ability to expand the modules easily. Metals such as iron are susceptible to corrosion problems because most wastewater contains chlorides. These materials will rust through in a brief period and will not be used for extended periods. In addition, metals tend to be heavier and more expensive than plastics. Alloys, which are considered for the process because they are more resistant to salts, are even more expensive. The temperature range and maximum temperature at which a metal can be used are much higher than for plastic. The maximum temperature for a short time at which unplasticized polyvinyl chloride (PVC-U) piping can be used is approximately 70 °C. The driving force of MD is the vapor pressure difference. Hence, the boiling point of the solution does not need to be reached. An advantage is that the feed solution can be heated by solar energy. The energy consumption of this process can be covered by renewable energy. A major advantage of plastic is that it is inexpensive. Investment costs for plastic equipment are lower than for metallic materials. For the material of the condenser plate, copper was chosen so that the water vapor could condense on the cooled surface of the air gap. These were among the reasons for the choice of materials for the plant (Table 2).
Used material for the main body of the mini NZLDMC plant
Heat exchanger . | Tube in shell type . |
---|---|
Tube | PP-GR – thermally conductive (Technoform Germany) |
Shell | Standard PVC-U pipe |
Crystal separator . | Settling tube . |
Tube | Standard PVC-U pipe |
Collector | Transparent PVC-U pipe |
Membrane module . | Tube in shell type . |
Membrane tube | ACCUREL® PP V8/2HF 0.2 μm (3M Germany) |
Condensator tube | Copper |
Shell | Standard PVC-U pipe |
Heat exchanger . | Tube in shell type . |
---|---|
Tube | PP-GR – thermally conductive (Technoform Germany) |
Shell | Standard PVC-U pipe |
Crystal separator . | Settling tube . |
Tube | Standard PVC-U pipe |
Collector | Transparent PVC-U pipe |
Membrane module . | Tube in shell type . |
Membrane tube | ACCUREL® PP V8/2HF 0.2 μm (3M Germany) |
Condensator tube | Copper |
Shell | Standard PVC-U pipe |
Feed solutions
Properties of the synthetic solutions
Different feed solutions were prepared (Table 3). For experiment 1, a 1.2 L mono-salt solution with NaCl (Roth, >99%) and 0.05 wt% of silica sand was prepared. Considering the concentration, a higher concentration than the one that typically results from desalination brine solution was selected to be closer to the crystallization point and to speed up the initial phase. The NaCl solution was concentrated above the solubility limit until the salt began to crystallize and the crystals settled down in the collector tube. For experiment 2, a 1 L multi-salt solution containing NaCl, CaCl2 · 6H2O (Acros Organics, 98 + %), and 0.05 wt% of silica sand is used. These applied salts are commonly found in wastewater samples from desalination.
Experiments and initial salt concentrations for experiment 1 (mono-salt) and experiment 2 (multi-salt)
. | Experiment 1: Mono-salt . | Experiment 2: Multi-salt . |
---|---|---|
NaCl (g L−1) | 290 | 255 |
CaCl2 · 6 H2O (g L−1) | – | 255 |
. | Experiment 1: Mono-salt . | Experiment 2: Multi-salt . |
---|---|---|
NaCl (g L−1) | 290 | 255 |
CaCl2 · 6 H2O (g L−1) | – | 255 |
For the multi-salt fractional crystallization experiment, a large gap was chosen between solubilities in water so that they do not precipitate simultaneously or shortly after each other by adjusting the temperature. The feed temperature was optimized by systematically screening a range of operating temperatures, ensuring optimal permeate flux and crystallization occurrence. CaCl2 shows a temperature-dependent behavior; as the temperature increases, the solubility of the salts improves, which positively influences the permeate flux. When solubility increases, more solute particles are dissolved in the solvent, resulting in a higher concentration of solute in the solution. This higher concentration can create a greater concentration gradient across the membrane, which can drive a higher permeation rate. Specifically, at a temperature of 55 °C, the solubility of NaCl is 369 g L−1, while that of CaCl2 is significantly higher at 1,300 g L−1.
Properties of the real wastewater
Highway wastewater of 26 L was taken from a storage tank in Upper Austria. The water was light green, with organic impurities. In the first step, the wastewater solution was filtered with filter paper (Schleicher and Schuell 5983, diameter of 185 mm, mesh size of 2.2 μm). The highway wastewater components after filtration are shown in Table 4 and were determined by Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES) (Thermo Fisher, iCAP Pro).
Water characteristics of the saline highway wastewater used as feed solution
. | Experiment 3: highway wastewater . |
---|---|
Conductivity (mS cm−1) | 2 |
pH | 7.9 |
Na (mg L−1) | 3,540 |
K (mg L−1) | 56 |
Mg (mg L−1) | 60 |
Ca (mg L−1) | 17 |
Tl (mg L−1) | 5 |
. | Experiment 3: highway wastewater . |
---|---|
Conductivity (mS cm−1) | 2 |
pH | 7.9 |
Na (mg L−1) | 3,540 |
K (mg L−1) | 56 |
Mg (mg L−1) | 60 |
Ca (mg L−1) | 17 |
Tl (mg L−1) | 5 |
The physical parameters of the water quality are summarized as follows. The conductivity of the highway wastewater was maintained at 2 mS cm−1. The pH of the wastewater was slightly basic at 7.9. The main component of the wastewater is Na; it also contains K, Mg, Ca, and Tl.
Seeding crystals
The seed crystals prevent unwanted nucleation on the membrane surface and reduce the wetting behavior on the membrane surface (Yan et al. 2021). Commercially available SiO2 was used as the seed crystal, as it hardly dissolves in water. The suggested amount of seed crystals should fall between 0.05 and 1 wt% to minimize contamination in the produced salt. At least 90% of the silica sand is between 0.21 and 0.3 mm in size.
RESULTS AND DISCUSSION
The design and construction of the NZLDMC mini-plant were successfully completed, considering all the specified details. Experiments were conducted at the NZLDMC mini-plant, focusing on three different types of investigations. In the first experiment, a mono-salt solution was used to investigate the performance of the mini-plant. Subsequently, a multi-salt solution was prepared to test the performance of the plant with multiple salts and to investigate the separation performance of the salts. Finally, real highway wastewater was used to achieve high water recovery, concentrate the solution, and evaluate the behavior of the wastewater on the membrane.
Mono-salt experiment
The NZLDMC experiments were conducted using a 290 g L−1 NaCl mono-salt solution. The feed solution was subsequently heated to 55 ± 1 °C. Throughout the experiment, the temperature was maintained at a constant level.
Permeate flux (T = 55 ± 1 °C) under different feed flow over the experiment time.
Permeate flux (T = 55 ± 1 °C) under different feed flow over the experiment time.
Crystal size characterization
The measured size distribution of NaCl crystals. The key sizes of the NaCl crystals are d16 = 596 μm, d50 = 666 μm, and d84 = 808 μm. CSD shows that the largest number of crystals is between 500 and 850 μm.
The measured size distribution of NaCl crystals. The key sizes of the NaCl crystals are d16 = 596 μm, d50 = 666 μm, and d84 = 808 μm. CSD shows that the largest number of crystals is between 500 and 850 μm.
Multi-salt experiment
The permeate flux (black dots) and the water recovery of the multi-salt solution experiment using of 255 g L−1 NaCl and 255 g −1 CaCl2 · 6 H2O at a temperature of T = 55 ± 1 °C.
The permeate flux (black dots) and the water recovery of the multi-salt solution experiment using of 255 g L−1 NaCl and 255 g −1 CaCl2 · 6 H2O at a temperature of T = 55 ± 1 °C.
The multi-salt solution experiment results in an Rcon of 89% (Figure 5, red dots). A salt recovery of 60% in total is achieved without compromising the quantity and quality of the permeate. The permeate conductivity is on average 1.5 μS cm−1.
Crystal characterization
First fraction of salt: cubic NaCl-like crystal in different sizes (left) and second fraction: colorless and unstructured grown crystals CaCl2 (right).
First fraction of salt: cubic NaCl-like crystal in different sizes (left) and second fraction: colorless and unstructured grown crystals CaCl2 (right).
Highway wastewater
In the final phase, highway wastewater was concentrated and treated to demonstrate the feasibility of the process with real wastewater. During the experiment, the temperature was changed four times to observe the permeate flux at different temperatures (Table 5) (El-Bourawi et al. 2006).
Operating temperature of the highway wastewater during the experiment and the permeate fluxes of the MDC process
Day . | Temperature (°C) . | Permeate flux (kg m−2 h−1) . |
---|---|---|
1–4 | 61 | 1.1 |
5–8 | 67 | 2.3 |
9–19 | 70 | 4.1 |
19–22 | 65 | 2.4 |
Day . | Temperature (°C) . | Permeate flux (kg m−2 h−1) . |
---|---|---|
1–4 | 61 | 1.1 |
5–8 | 67 | 2.3 |
9–19 | 70 | 4.1 |
19–22 | 65 | 2.4 |
Permeate flux and water recovery of the real highway wastewater with varying temperature over 22 days.
Permeate flux and water recovery of the real highway wastewater with varying temperature over 22 days.
At the start of the experiment, the wastewater exhibited low salinity, with a conductivity of 2 mS cm−1. After 22 days, the salinity increased and reached a conductivity of 13 mS cm−1. During the experiment, it was possible to recover 3 g of a hardly soluble salt mixture. The permeate conductivity of the experiment was on average 25 μS cm−1.
Crystal size characterization of the real wastewater
The size distribution of the NaCl crystals is between 0 and 400 μm for the wastewater experiment. The key crystal sizes are d16= 42 μm, d50 = 95 μm, and d84 = 178 μm.
The size distribution of the NaCl crystals is between 0 and 400 μm for the wastewater experiment. The key crystal sizes are d16= 42 μm, d50 = 95 μm, and d84 = 178 μm.
CONCLUSIONS
The study demonstrated the feasibility and advantages of using seeded near-ZLD membrane crystallization as an alternative to conventional brine evaporation and crystallization processes; due to the needed temperature in the process, it is possible to use solar energy to heat the feed solution. The investigation successfully utilized seeding processes in the NZLDMC to concentrate synthetic mono- and multi-salt solutions. Long-term stability over 18 days of permeate flux could be achieved by implementing a classifier and avoiding the settling of crystals on the membrane. The effect of salt crystals and seeds on membrane performance was investigated under various operating conditions. For the mono-salt experiment, a recovery of 93% was achieved. The multi-salt experiment revealed that crystallization fractionation is possible and during these experiments, a recovery of 89% was achieved. For the real wastewater experiment coming from highway water, a recovery of even 99% could be monitored. A longer-term stability of the membrane could be achieved by the seeding crystallization. These results demonstrate the potential of NZLDMC for continuous operation and efficient water reclamation by increasing, in addition, the operational time further compared to the existing literature.
Future research endeavors will delve deeper into the effects of different salts, salt concentrations, seeding concentrations, and process parameters for mono-salt solutions, multi-salt solutions, and wastewater treatment. The objective is to achieve a consistent permeate flux over long-term experiments without compromising the quantity or quality of the permeate. This will involve optimizing operational parameters and exploring different seeding concentrations, for example, cooperation with renewable energy sources, particularly solar energy, to reach sustainability goals. Subsequently, the aim is to scale up the mini NZLDMC plant to a pilot plant, incorporating multiple membranes and heat exchangers within the membrane module and heat exchanger module, respectively. This scale-up process will facilitate the integration and optimization of seeding crystallization and MD processes, paving the way for their application in various industrial processes and municipal wastewater treatment.
Overall, this study provides valuable insights into the integration of seeding crystallization and MD processes, serving as a guide for scaling up and implementing this promising technology in practical applications.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.