Seawater desalination with solar-energy-integrated vacuum membrane distillation system

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- ﬁ ber 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 W C, the maximum value of the average solar intensity was 1,080 W/m 2 . The system was able to generate 36 kg (per m 2 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. ). 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. ). 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. ).
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 multistage 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 &  SWRO (seawater reverse osmosis) membranes, as well as an NF þ SWRO integrated system, which were tested in terms of permeate quality and quantity using natural seawater.
The NF þ SWRO integrated system proved an effective pretreatment for seawater desalination.
There have been relatively few studies on solar-energyintegrated VMD systems for seawater desalination. The present study designed and built a new type of solar-energyintegrated 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
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.

Experimental system descriptions
A schematic diagram of the system is shown in Figure 1, a simplified schematic diagram is shown in Figure 2, and a photo of the system is shown in Figure 3. A miniature, pilot-scale seawater desalination system was installed on the Wuhan Textile University laboratory building roof (Wuhan City, Hubei Province, China). The system module characteristics are listed in Table 3. The complete system consists of a seawater tank, solar collector, solar cooker, inclined VMD evaporator, circulating water vacuum pump, heat exchanger, fresh water tank, and brine tank. The system is equipped with a rotating base that can be adjusted manually according to the position of the sun, allowing the system to fully receive the available solar radiation. Thermometers were installed on the inlet and outlet of all pipes in the system to gather data for subsequent analysis.
As shown in Figures 1 and 2 water 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 ana-

Calculations
Membrane flux (J ) was the amount of liquid produced per unit area of a membrane surface per unit of time. Retention rate (η) was the percentage of dissolved solids intercepted by the membrane that occupied the total quantity of the solute in the solution. Due to the ratio of NaCl concentration and its conductivity being approximate to constant, when the retention rate was calculated, conductivity was used instead of concentration. They were calculated using the following equations: where W, S, t, ρ h , and ρ c are fresh water quantity (kg), the effective area of the membrane (m 2 ), operation time (h), the conductivity of natural seawater (mS/cm), and the conductivity of fresh water (mS/cm), respectively.

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 Figure 4 shows the changes in ambient temperature and hourly average of solar radiation intensity with time. Over the course of the operation time, both ambient temperature and solar radiation intensity with time increased at first, and then decreased.
Meteorological data showed that the average ambient temperature was between 26 and 33 W C, and that the average solar radiation intensity was between 744 and 1,080 W/m 2 . Solar intensity increased as the ambient temperature rose; the two values form a parabola when plotted. The highest ambient temperature, 33 W C, was recorded at 2:00 pm; solar radiation intensity reached its peak value of 1,080 W/m 2 at the same time.
Temperature changes of feed seawater, solar collector seawater, solar cooker seawater, glass lid, evaporator, vapor, fresh water, and strong brine Temperature changes in the hourly average of the feed seawater, the seawater in the solar collector, and the seawater in  the solar cooker, as well as the glass lid, evaporator, vapor, fresh water, and strong brine with time were measured as shown in Figure 5. The temperature of feed seawater rose continually throughout the operation process as strong brine was recycled into the seawater tank and mixed with seawater, and as feed seawater was heated by the vapor in the heat exchanger. Throughout the test day, the seawater temperature in the solar collector and solar cooker first increased while solar radiation intensity increased, then decreased as solar radiation intensity decreased. The temperature of the evaporator was lower than that of the solar cooker, naturally, because seawater temperature increased in the cooker then lost heat during evaporation. Fresh water temperature was lower than vapor temperature, because vapor in the heat exchanger was condensed by feed seawater until becoming fresh water. The above temperature measurements altogether prove that it is feasible to heat seawater by using a solar collector coupled with a solar cooker.

Change of membrane flux
A hydrophobic, hollow-fiber membrane module was employed in this study. The performance of the system was evaluated in terms of both the quality and quantity of fresh water produced.
The quantity of fresh water was checked by membrane flux; changes in membrane flux over time are shown in Figure 6.
The hydrophobic, hollow-fiber membrane exhibited different fresh water production outputs and membrane flux values at different times during the test process. At 2:00 pm, at 0.095 Mpa vacuum degree on the cold side, the maximum value for membrane flux was 7.88 kg/m 2 h. As seawater temperature increased, steam partial pressure of the membrane on the hot side plus the driving force of the steam through the membrane increased, thus causing increased membrane flux.
As mentioned above, at 2:00 pm, the seawater temperature in the solar cooker also reached its maximum value.
Changes of fresh water electrical conductivity and retention rate   Downloaded from https://iwaponline.com/jwrd/article-pdf/7/1/16/376828/jwrd0070016.pdf" /><meta name="description" content="This study designed and tested a novel type of solar-energy-integrated vacuum membrane distillation by guest Characteristics of fresh water obtained by the system

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 m 2 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.