This study designed and tested a novel type of solar-energy-integrated vacuum membrane distillation (VMD) system for seawater desalination under actual environmental conditions in Wuhan, China. The system consists of eight parts: a seawater tank, solar collector, solar cooker, inclined VMD evaporator, circulating water vacuum pump, heat exchanger, fresh water tank, and brine tank. Natural seawater was used as feed and a hydrophobic hollow-fiber membrane module was used to improve seawater desalination. The experiment was conducted during a typical summer day. Results showed that when the highest ambient temperature was 33 °C, the maximum value of the average solar intensity was 1,080 W/m2. The system was able to generate 36 kg (per m2 membrane module) distilled fresh water during 1 day (7:00 am until 6:00 pm), the retention rate was between 99.67 and 99.987%, and electrical conductivity was between 0.00276 and 0.0673 mS/cm. The average salt rejection was over 90%. The proposed VMD system shows favorable potential application in desalination of brackish waters or high-salt wastewater treatment, as well.
INTRODUCTION
Currently, there is an urgent need for pure, clean drinking water in many countries across the globe. Water shortages have become a major environmental issue that is further impacted by global warming. Brackish water sources are not potable due to the content of dissolved salts and harmful bacteria. Similarly, many coastal areas have abundant seawater, but no safe drinking water. Distillation can be used to purify water supply, and is one of many techniques for desalinating seawater (Aybar et al. 2005). With the rapid increase of the world population, desalination is increasingly considered to be necessary and feasible. By 2025, about 70% of the world's population will face water shortage problems (Li et al. 2013). Seawater desalination is recognized as one of mankind's earliest ways of water treatment, and it provides fresh water for many communities and manufacturers. It plays an important role in economic development in many developing countries, especially in water shortage countries such as Pacific Asia, Africa and Middle East countries (Shatat et al. 2013).
There have been several recent developments in water desalination techniques, including membrane distillation (MD). MD is considered a valid alternative to traditional desalination techniques such as coupling to reverse osmosis (RO), also called ‘integrated membrane systems’, or multi-stage flash vaporization (MSFV). MD is less influenced by osmotic pressure than RO, and consumes less energy than MSFV. According to which pattern is used to condense volatile components in the permeate side of the system, MD can be classified into the following four structures: (i) direct contact MD (Gryta & Barancewicz 2010; Teoh et al. 2011; Yu et al. 2011); (ii) air gap MD (Banat et al. 1999; Yao et al. 2013); (iii) sweeping gas MD (Rivier et al. 2002; Cojocaru & Khayet 2011); and (iv) vacuum MD (VMD) (Porter 1972; Bandini et al. 1992, 1997; Sarti et al. 1993; Bandini & Sarti 1999; Zhao et al. 2011). MD can be applied in many fields. A previous study confirmed that benzene and heavy metals can be removed from water by MD for environmental applications (El-Bourawi et al. 2006; Khayet 2011; Susanto 2011; Alkhudhiri et al. 2012), for example, and another suggested that MD can be applied successfully in the food industry, where concentrated fruit juices and sugar solutions can be prepared with better flavor and color using MD with a low operating temperature (Calabro et al. 1994). Research has also shown that high-temperature MD can be applied successfully in the medical field to sterilize biological fluids (Sakai et al. 1988).
Although MD has many attractive features, such as the possibility of coupling to low-grade sources of energy, it has not yet been commercialized for large-scale desalination plants due to technical problems involving low flux and membrane wetting. These, and other design drawbacks, are expected to be overcome because a wealth of research has gone into developing MD components and processes, including membranes (Sakai et al. 1988; Calabro et al. 1994; Lawson & Lloyd 1997; Susanto 2011). Researchers have highlighted the possibility that MD can be integrated with renewable and low-grade energy sources such as solar and wind, which offers promising techniques (Cabassud & Wirth 2003; Xu et al. 2006). Solar energy, low-grade waste heat, and geothermal energy represent favorable alternative sources. Koschikowski et al. (2003) showed a solar thermal-driven spiral wound polytetrafluoroethylene MD module was used to obtain potable water from brackish water and seawater in another study. Aybar (2006) reported an inclined solar water distillation system can generate 3.5–5.4 kg (per m2 absorber plate area) distilled water during a normal summer day in North Cyprus. Asadi et al. (2013) have shown that solar still systems can remove inorganic, organic, and bacteriological contaminants quite effectively; a test system proved extremely successful in removing such contaminants from wastewater. Kaya et al. (2015) developed a single NF (nanofiltration) and SWRO (seawater reverse osmosis) membranes, as well as an NF + SWRO integrated system, which were tested in terms of permeate quality and quantity using natural seawater. The NF + SWRO integrated system proved an effective pre-treatment for seawater desalination.
There have been relatively few studies on solar-energy-integrated VMD systems for seawater desalination. The present study designed and built a new type of solar-energy-integrated VMD system for seawater desalination. The objective of this study was to investigate the feasibility of the proposed system to produce fresh water from natural seawater during a typical summer day in Wuhan, China. Changes in ambient temperature and solar intensity with time were measured, as well as temperature changes of the feed seawater, seawater in the solar collector, seawater in the solar cooker, glass lid, evaporator, vapor, fresh water, and strong brine with time. The quality and quantity of fresh water output from the system were also tested. The results altogether confirmed the feasibility of producing fresh water with solar-energy-integrated VMD systems.
MATERIALS AND METHODS
Materials
Shade type hydrophobic hollow-fiber membrane module was from China Hangzhou Haotian Membrane Separation Technology Co., Ltd. The membrane component parameters are reported in Table 1. Seawater was taken from the South China Sea. The characteristics of natural seawater are listed in Table 2.
Parameter . | Value or definition . |
---|---|
Membrane material | Polypropylene |
Membrane inner diameter (μm) | 250–300 |
Membrane outer diameter (μm) | 350–400 |
Membrane pore size (μm) | 0.1–0.2 |
Wall thickness (μm) | 40–50 |
Porosity (%) | 40–50 |
Tensile strength (Mpa) | 120 |
pH | 0–14 |
Membrane area (m2) | 2 |
Dimensions of membrane module (mm): (length × width) | 810 × 520 |
Parameter . | Value or definition . |
---|---|
Membrane material | Polypropylene |
Membrane inner diameter (μm) | 250–300 |
Membrane outer diameter (μm) | 350–400 |
Membrane pore size (μm) | 0.1–0.2 |
Wall thickness (μm) | 40–50 |
Porosity (%) | 40–50 |
Tensile strength (Mpa) | 120 |
pH | 0–14 |
Membrane area (m2) | 2 |
Dimensions of membrane module (mm): (length × width) | 810 × 520 |
Parameter . | Unit . | Value . |
---|---|---|
pH | – | 7.88 |
Temperature | °C | 20 |
Salinity | psu | 15 |
Turbidity | NTU | 10.8 |
EC | mS/cm | 20.7 |
SS | (mg/L) | 4.0 |
TDS | (mg/L) | 10,310 |
TOC | (mg/L) | 4.38 |
Total bacterial count | CFU/mL | 24,100 |
COD | (mg/L) | 16.8 |
Na+ | (mg/L) | 4,316.5 |
Mg2+ | (mg/L) | 504.3 |
Ca2+ | (mg/L) | 204.1 |
K+ | (mg/L) | 175.6 |
Sr2+ | (mg/L) | 3.47 |
Cl− | (mg/L) | 7,795.5 |
SO42 | (mg/L) | 1,115.8 |
Br | (mg/L) | 38.1 |
F− | (mg/L) | 0.54 |
Parameter . | Unit . | Value . |
---|---|---|
pH | – | 7.88 |
Temperature | °C | 20 |
Salinity | psu | 15 |
Turbidity | NTU | 10.8 |
EC | mS/cm | 20.7 |
SS | (mg/L) | 4.0 |
TDS | (mg/L) | 10,310 |
TOC | (mg/L) | 4.38 |
Total bacterial count | CFU/mL | 24,100 |
COD | (mg/L) | 16.8 |
Na+ | (mg/L) | 4,316.5 |
Mg2+ | (mg/L) | 504.3 |
Ca2+ | (mg/L) | 204.1 |
K+ | (mg/L) | 175.6 |
Sr2+ | (mg/L) | 3.47 |
Cl− | (mg/L) | 7,795.5 |
SO42 | (mg/L) | 1,115.8 |
Br | (mg/L) | 38.1 |
F− | (mg/L) | 0.54 |
psu, practical salinity units; NTU, nephelometric turbidity unit; ms/cm, milli Siemens per cm; CFU, colony forming units.
Experimental system descriptions
Main device . | Parameter . | Value or definition . |
---|---|---|
Solar collector | ||
Material of solar collector | All-glass vacuum tube | |
Number of tubes | 12 | |
Heat absorption efficiency (W/cm2·h) | 850 | |
Effective heat absorbing area (m2) | 1.62 | |
Evaporator | ||
Material of evaporator | Stainless steel | |
Material of glass lid | Hollow toughened glass | |
Heat-absorbing aluminum | Magnetron sputter blue film | |
Glass angle (°) | 30 | |
Dimensions of evaporator | ||
Length (m) | 1.90 | |
Width (m) | 0.95 | |
Height (m) | 0.3 | |
Water inlet | Top of evaporator | |
Material of the support plate | Stainless steel | |
No. of the support plate | 4 | |
Height of the support plate (cm) | 5 | |
Diameter of the hole (cm) | 2.5 | |
No. of the hole (each support plate) | 3 | |
Solar cooker | ||
Material of solar cooker | Carbon steel plate | |
Stove diameter (m) | 1.80 | |
Focal length (m) | 0.68 | |
Focal spot temperature (°C) | 1,100 | |
Effective heat absorbing area (m2) | 2.20 | |
Concentration ratio | 8/45 | |
Sunny day power (W) | 2,000 | |
Seawater tank | ||
Material of the seawater tank | Stainless steel | |
Dimensions of seawater tank | ||
Length (m) | 0.60 | |
Width (m) | 0.30 | |
Height (m) | 0.30 | |
Solar hot water tank | ||
Material of solar hot water tank | Stainless steel cylinder, black | |
Dimensions of solar hot water tank | ||
Diameter (m) | 0.32 | |
Height (m) | 0.15 | |
Heat exchanger | ||
Dimensions of heat exchanger | ||
Length (m) | 0.20 | |
Width (m) | 0.20 | |
Height (m) | 0.40 | |
S-type heat exchanger coil (m) | 9.42 | |
Brine tank | ||
Dimensions of brine tank | ||
Length (m) | 0.20 | |
Width (m) | 0.20 | |
Height (m) | 0.40 | |
Vacuum pump | ||
Power (W) | 180 | |
Voltage/frequency (V/Hz) | 220/5 | |
Flow (L/min) | 60 | |
Maximum vacuum degree (Mpa) | 0.098 | |
Peristaltic pump | ||
Power (W) | 200 | |
Range of speed | 60–600 rpm | |
Rotate base | ||
Dimensions of rotating base | ||
Length (m) | 5 | |
Width (m) | 1.2 | |
Material of rotating base | Steel |
Main device . | Parameter . | Value or definition . |
---|---|---|
Solar collector | ||
Material of solar collector | All-glass vacuum tube | |
Number of tubes | 12 | |
Heat absorption efficiency (W/cm2·h) | 850 | |
Effective heat absorbing area (m2) | 1.62 | |
Evaporator | ||
Material of evaporator | Stainless steel | |
Material of glass lid | Hollow toughened glass | |
Heat-absorbing aluminum | Magnetron sputter blue film | |
Glass angle (°) | 30 | |
Dimensions of evaporator | ||
Length (m) | 1.90 | |
Width (m) | 0.95 | |
Height (m) | 0.3 | |
Water inlet | Top of evaporator | |
Material of the support plate | Stainless steel | |
No. of the support plate | 4 | |
Height of the support plate (cm) | 5 | |
Diameter of the hole (cm) | 2.5 | |
No. of the hole (each support plate) | 3 | |
Solar cooker | ||
Material of solar cooker | Carbon steel plate | |
Stove diameter (m) | 1.80 | |
Focal length (m) | 0.68 | |
Focal spot temperature (°C) | 1,100 | |
Effective heat absorbing area (m2) | 2.20 | |
Concentration ratio | 8/45 | |
Sunny day power (W) | 2,000 | |
Seawater tank | ||
Material of the seawater tank | Stainless steel | |
Dimensions of seawater tank | ||
Length (m) | 0.60 | |
Width (m) | 0.30 | |
Height (m) | 0.30 | |
Solar hot water tank | ||
Material of solar hot water tank | Stainless steel cylinder, black | |
Dimensions of solar hot water tank | ||
Diameter (m) | 0.32 | |
Height (m) | 0.15 | |
Heat exchanger | ||
Dimensions of heat exchanger | ||
Length (m) | 0.20 | |
Width (m) | 0.20 | |
Height (m) | 0.40 | |
S-type heat exchanger coil (m) | 9.42 | |
Brine tank | ||
Dimensions of brine tank | ||
Length (m) | 0.20 | |
Width (m) | 0.20 | |
Height (m) | 0.40 | |
Vacuum pump | ||
Power (W) | 180 | |
Voltage/frequency (V/Hz) | 220/5 | |
Flow (L/min) | 60 | |
Maximum vacuum degree (Mpa) | 0.098 | |
Peristaltic pump | ||
Power (W) | 200 | |
Range of speed | 60–600 rpm | |
Rotate base | ||
Dimensions of rotating base | ||
Length (m) | 5 | |
Width (m) | 1.2 | |
Material of rotating base | Steel |
As shown in Figures 1 and 2, the operation process of the system includes the following steps. First, natural seawater is put into the seawater tank and enters the heat exchanger via the first peristaltic pump, where natural seawater is preheated by vapor. Next, seawater enters the solar collector via the second peristaltic pump, where it is heated. Seawater then flows into the solar hot water tank and is heated again in the solar cooker. After being heated a total of three times, the seawater enters the VMD evaporator and flows down through the hole in the support plate; at this time, the membrane module is hung above the support plate, and then the hot seawater evaporates slowly into vapor. The vapor passes through the membrane hole from the hot side of the membrane module into the cold side of the membrane module, where the circulating water vacuum pump reduces the pressure. The vapor is then pumped into the heat exchanger where heat exchange with natural seawater takes place, thus condensing the vapor as fresh water. The strong brine left after desalination is recycled into the seawater tank and mixed with natural seawater to obtain a higher feed temperature, but in a practical application, the strong brine goes directly into the ocean.
Analytical methods
System performance was mainly tested in terms of fresh water quality and quantity. The quantity of fresh water was checked according to membrane flux and retention rate, and the quality of fresh water (and seawater) was analyzed according to salinity, conductivity, temperature measurements, and total dissolved solids (TDS) content, determined using a portable HACH Sension5 conductivity meter. The pH values were measured with a digital pH meter (Sartorius PB-10). Suspended solids (SS) content was measured according to the weight method, and chemical oxygen demand (COD) was determined by the basic potassium permanganate method. Total organic carbon (TOC) was determined using a TOC analyzer (Multi N/C2100) and the total number of bacteria was counted by plate count method. The Na+, Mg2+, Ca2+, K+, and Sr2+ ion concentrations were each measured with an atomic absorption spectrophotometer (GBC AVANTA M Model). The Cl−, SO42−, Br−, and F− ion levels were determined with ion chromatography equipment (ICS 900 model).
Calculations
RESULTS AND DISCUSSION
The system operated from 7:00 am to 6:00 pm on August 22nd, 2015, a day which had normal weather and good air quality. When the system is running, the quantity of feed seawater was set to 220 L, seawater inlet flow was set to 20 L/h, and the vacuum degree of the cold side was set to 0.095 Mpa. All tests were performed and repeated in the same environment.
Changes of ambient temperature and solar radiation intensity
Temperature changes of feed seawater, solar collector seawater, solar cooker seawater, glass lid, evaporator, vapor, fresh water, and strong brine
Change of membrane flux
Changes of fresh water electrical conductivity and retention rate
Characteristics of fresh water obtained by the system
Table 4 lists the characteristics of fresh water obtained by the test system. The average removal rates of SS, TDS, TOC, COD, Na+, Mg2+, Ca2+, K+, Sr2+, Cl−, SO42− and F− were 50–70%, 99.68–99.99%, 68–94%, 97.56–97.86%, 99.942–99.996%, 99.677–99.996%, 97.09–99.86%, 99.47–99.91%, over 96.83%, 99.95–99.99%, 99.68–99.95%, and above 93%, respectively. In the literature, Kaya et al. (2015) also showed a similar rejection value by NF90 (30 bar) + SW30–RO (40 bar) combination; their average removal rates of TDS, Na+, Mg2+, Ca2+, K+, Cl− and SO42− were 98.89%, 98.94%, over 99.991%, 99.43%, 99.02%, 99.03%, and over 99.994%. These results altogether confirm that the test system desalinated seawater very effectively.
Parameter . | Unit . | Value (min–max) . |
---|---|---|
pH | – | 7.57–7.64 |
Temperature | °C | 27.4–27.5 |
Salinity | Psu | 0 |
Turbidity | NTU | 0.17–0.23 |
EC | mS/cm | 0.00276–0.0673 |
SS | (mg/L) | 1.2–2.0 |
TDS | (mg/L) | 1.2–33.4 |
TOC | (mg/L) | 0.28–1.39 |
Total bacterial count | CFU/mL | 1–16 |
COD | (mg/L) | 0.36–0.41 |
Na+ | (mg/L) | 0.19–2.50 |
Mg2+ | (mg/L) | 0.02–1.63 |
Ca2+ | (mg/L) | 0.28–5.94 |
K+ | (mg/L) | 0.15–0.93 |
Sr2+ | (mg/L) | <0.11 |
Cl− | (mg/L) | 0.612–4.053 |
SO42− | (mg/L) | 0.608–3.598 |
Br− | (mg/L) | Not detected |
F− | (mg/L) | <0.039 |
Parameter . | Unit . | Value (min–max) . |
---|---|---|
pH | – | 7.57–7.64 |
Temperature | °C | 27.4–27.5 |
Salinity | Psu | 0 |
Turbidity | NTU | 0.17–0.23 |
EC | mS/cm | 0.00276–0.0673 |
SS | (mg/L) | 1.2–2.0 |
TDS | (mg/L) | 1.2–33.4 |
TOC | (mg/L) | 0.28–1.39 |
Total bacterial count | CFU/mL | 1–16 |
COD | (mg/L) | 0.36–0.41 |
Na+ | (mg/L) | 0.19–2.50 |
Mg2+ | (mg/L) | 0.02–1.63 |
Ca2+ | (mg/L) | 0.28–5.94 |
K+ | (mg/L) | 0.15–0.93 |
Sr2+ | (mg/L) | <0.11 |
Cl− | (mg/L) | 0.612–4.053 |
SO42− | (mg/L) | 0.608–3.598 |
Br− | (mg/L) | Not detected |
F− | (mg/L) | <0.039 |
The design advantages of this device can be summarized according to three major aspects: (i) the rotating base, which can be manually adjusted to suit the sun's position and fully exploit all available solar energy radiation; (ii) the solar collector coupled with the solar cooker to build the solar energy heat system, in which seawater can be rapidly heated to obtain very high water flux; and (iii) the hydrophobic, polypropylene hollow-fiber membrane module used for water vapor separation, which ensures high-quality fresh water.
CONCLUSIONS
In this study, a new type of solar-energy-integrated VMD system for seawater desalination was designed and tested under actual environmental conditions in Wuhan, China. The aim of this study was to determine the feasibility of using solar-energy-integrated VMD to obtain fresh water from natural seawater during a typical day, and to test the performance of the system in terms of both the quality and quantity of fresh water produced. According to our experimental results, the solar collector and solar cooker showed favorable performance related to temperature, membrane flux increased as seawater temperature increased, and the system was able to generate 36 kg (per m2 membrane module) distilled fresh water during the test day (7:00 am until 6:00 pm). The retention rate was between 99.67 and 99.987%, EC was between 0.00276 and 0.0673 mS/cm, and the average salt rejection was above 90%. These results altogether confirm that solar energy integrated with VMD is an appropriate and effective combination for seawater desalination systems.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the financial support provided by the National ‘Twelfth Five-Year’ Plan for Science & Technology Support of China (No. 2014BAC13B02) and the National Science Foundation of China (NSFC) (No. 21377023).