Turbidity Study of Solar Ponds Utilizing Seawater As Salt Source

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Solar Energy 84 (2010) 289–295 www.elsevier.com/locate/solener

Turbidity study of solar ponds utilizing seawater as salt source

Nan Li a, Fang Yin b, Wence Sun a,*, Caihong Zhang c, Yufeng Shi a

a School of Energy and Power Engineering, Dalian University of Technology, Dalian 116023, China b YLab, 358 South 700 East, Suit B-139, Salt Lake City, UT 84102, USA c Dalian Thermoelectric Group Co. Ltd., Dalian 116001, China

Received 12 January 2009; received in revised form 21 July 2009; accepted 23 November 2009 Available online 24 December 2009

Communicated by: Associate Editor Aliakbar Akbarzadeh

Abstract

A series of experiments were conducted to study the turbidity reduction in solar ponds utilizing seawater as salt source. The experiment on the turbidity reduction efficiency with chemicals indicates that alum (KAl(SO4)212H2O) has a better turbidity control property because of its strongly flocculating and also well depressing the growing of algae and bacteria in the seawater. In comparison with bittern and seawater, our experiment shows that the residual brine after desalination can keep limpidity for a long time even without any chemical in it. Experiments were also conducted on the diffusion of turbidity and salinity, which show that the turbidity did not diffuse upwards in the solution. In the experiment on subsidence of soil in the bittern and saline with the same salinity, it was found that soil subsided quite quickly in the pure saline water, but very slowly in the bittern. In this paper we also proposed an economical method to protect the solar pond from the damage of rain. Finally, thermal performance of a solar pond was simulated in the conditions of different turbidities using a thermal diffusion model. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Solar pond; Desalination; Turbidity; Rainwater

1. Introduction mance of solar ponds based on their experiments, and also conducted relevant thermal performance computations for A salinity gradient solar pond, an integral device for col- the solar ponds. Solar ponds can be ﬁlled up with seawater, lecting and storing solar energy, has been recognized as the residual brine after desalination and salt-work bittern, potential heat source equipment. It has advantages of sim- which not only saves resource and reduces investment cost ple design and low cost. The thermal behavior of solar but also prevents possible pollution to the coastal waters by ponds has been extensively studied and it has been believed the brine and bittern. However, seawater and bittern are that keeping a high transparency of pond water is very turbid, thus, reducing transfer of solar radiation in the important to maintain good thermal performance of solar water and decreasing heat performance of ponds. The ponds (Hassab et al., 1989; Hull, 1990; Lu et al., 2004). water turbidity control of pond water is still a challenging Wang and Seyed-Yagoobi (1995) introduced a turbidity problem during the operation period of solar ponds since concept to describe water limpidity and used nephelometric its transparency is easy to get worse in the open environ- turbidity units (ntu) to express the extent of turbidity. They ment, subjected to the disturbance of the environmental proposed a correlation of water turbidity and transmissiv- conditions, such as wind and rain (Atkinson and Harl- ity of solar radiation into pond water with thermal perfor- eman, 1983; Hull, 1980; Punyasena et al., 2003). In addition, when ponds are exposed to the nature, it is easy to * Corresponding author. Tel.: +86 411 84706302. breed algae and bacteria inside them (John et al., 1986). E-mail address: [email protected] (W. Sun). Both our previous and current experiments show water in

0038-092X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2009.11.010 转载中国科技论文在线 http://www.paper.edu.cn 290 N. Li et al. / Solar Energy 84 (2010) 289–295 the solar pond utilizing seawater as salt source becomes bidities with time for three samples, which are shown in turbid very easily during its operation (Sun et al., 2003). Fig. 1. As seen from Fig. 1, the turbidity of seawater with- So far few studies have been reported on water turbidity out chemicals increased dramatically. It was observed that though it plays an important role in heat performance in water turned green in this sample and our analysis indicates solar ponds. One report we found is from Folchitto who the existing of the algae. In the solution with sodium hypo- introduced a turbidity reduction technique for the bittern chlorite, we observed that its turbidity kept decreasing dur- used to fill up the solar pond (Folchitto, 1991). Based on ing the first 50 h, reflecting the good turbidity reduction Folchitto’s study, in this paper, we investigated the turbid- property of sodium hypochlorite. However, the solution ity of a lab-scale solar pond with an area of 6 m2 utilizing turned green afterwards. We think this might result from seawater as salt source, including turbidity measurements, the no-longer working of sodium hypochlorite as a turbid- turbidity characteristics, as well as its relevant issues such ity reduction chemical after 50 h. In the solution with alum, as turbidity control and limpidity maintenance. one layer of white floccules was formed at the bottom of the bucket, with a thickness less than 5% of the solution 2. Experimental study depth. The solution remained a relatively good degree of transparency and no green algae were observed through 2.1. Turbidity control of seawater and bittern the whole experiment period. As a correspondence, its turbidity kept decreasing with time, as seen from Fig. 1. From Seawater usually has a high turbidity because it contains the above study, we can conclude that alum has a better lots of silt, sand and other impurities, and also many algae turbidity control property since it can well flocculate and and bacteria (Incropera and Thomas, 1978), especially in also well depress the growing of algae and bacteria in the the summer. Even after simple sedimentation and filtration, seawater so that it is a better candidate to reduce the tur- turbidity in seawater is till around 5.0 ntu, which is higher bidity of the solar pond. than the filling requirement of upper convective zone Salt-work bittern usually has a high turbidity. An inor- (UCZ) and non-convective zone (NCZ). Therefore, the ganic macro-molecular coagulant, polyaluminium chloride process to reduce its turbidity is necessary to maintain a (PAC), with a molecular formula of [Al2(OH)nCl6n]m with 6 nice limpidity in a pond utilizing seawater before it is filled n = 1–5 and m 10, is more often used to reduce the tur- up. Physical methods can only remove partial silt, sand and bidity of bittern. This is a conventional and inexpensive algae, but they are not able to depress the growing of bac- chemical for water treatment, and can be used for a wide teria and algae. Instead, adding chemicals to seawater is the range of the pH value. Experiments show that in the case popular method to reduce its turbidity. Considering the of treating the bittern with this coagulant alone, the formed treatment cost of enormous water needed to fill up a large floc subsides rather slowly. Fig. 2 gives the experimental solar pond, we selected two economical and easily-obtain- result which shows the correlation between the dose and able chemicals, sodium hypochlorite (NaClO) (Alagao the turbidity for the subsidence times of 12 and 36 h, et al., 1994) and alum (KAl(SO ) 12H O), for our follow- respectively. It is indicated in Fig. 2 that at the dose of 4 2 2 3 ing turbidity control experiments. 0.78 kg/m (ratio of PAC to bittern), the turbidity reduced The same three samples of 10 L subsided and filtered to its minimum. On the other hand, however, after subsid- seawater were respectively put in three exactly same buck- ing for 12 h the turbidity only reduced to 15.9 ntu, which ets each of which has a diameter of 20 cm and a height of does not meet the standard. In the case of 36-h subsidence, 35 cm. In one bucket, we added 0.6 ml of a 12% sodium the turbidity reduced further to 5.7 ntu, which is a satisfac- hypochlorite solution (7 ppm chlorine, the best dosage tory result for lower convective zone (LCZ) of the pond. based on our experiments). Ten grams of alum solid was But the problem is that subsiding time is too long to be added to another bucket, and no chemicals were added suitable for the filling-up of the pond. The coagulant aid, to the left one. After well mixed, three samples were put polyacrylamide was therefore also added in our study. In outdoor without covers. We measured variation of the tur- our experiment, 0.65 kg polyaluminium chloride and 5 g

30 25 20 No chemicals 15 Sodium Hypochlorite 10 Alum Turbidity (ntu) 5 0 0153045607590105 Time (h)

Fig. 1. Turbidity variation with time in three seawater samples. 中国科技论文在线 http://www.paper.edu.cn N. Li et al. / Solar Energy 84 (2010) 289–295 291

60 25 Brine after desalination 12 hour 50 Seawater 36 hour 20 40 Bittern

30 15

20 Turbidity(ntu) 10

10 Turbidity (ntu)

0 5 0 0.5 1 1.5 2 2.5 3 PAC/bittern (kg/m3) 0 Fig. 2. Relation between the dose and the turbidity. 0 5 10 15 20 25 30 35 40 45 Days (d) polyacrylamide were used for 1 m3 bittern with a density of 1241 kg/m3 and a turbidity of 35 ntu. After more than 12-h Fig. 3. Turbidity profiles of three solutions. subsidence, the turbidity reduced to below 7 ntu, suitable for the filling-up of LCZ of the pond. There are great dif- cess, and the brine after being further concentrated was put ferences in the properties of the pond water in different geo- in NCZ of the solar pond. Therefore, the treatment cost for graphic regions. The components are also different in water the pond water is dramatically reduced using brine to fill treatment agents even with the same names. For this rea- the solar pond. son, a specific experiment is needed for the selection of water treatment agents when the specific solar pond is 2.3. Diffusions of turbidity and salinity studied. The residual brine after desalination cannot be directly 2.2. Turbidity profiles of bittern, brine and seawater used in LCZ of solar pond due to its relatively low salinity. On the other hand, concentrating the brine is really time- Generally, there are three optional sources to fill up sea- consuming. As a quick thought, the bittern might be a bet- water-based solar ponds. They are bittern left after the pro- ter source to fill LCZ. But the question is whether the tur- duction of salt, concentrated residual brine left after bidity can diffuse upwards along with the salinity. We, desalination and directly concentrated seawater. The therefore, conducted the following experimental study on advantage is that we cannot only save a large amount of diffusion of turbidity and salinity in the bittern. salt and freshwater and reduce the cost of the solar pond, The raw bittern with a turbidity of 27 ntu and a density but also prevent high salinity brine from discharging to of 1268 kg/m3 was filled into a large beaker (35 cm high the sea and protect the coastal seawater from the possible and 18 cm in diameter) up to a depth of 15 cm. To prevent pollution. To know their thermal performance, we studied the breeding of algae during the experiment period, sodium turbidity variation with time for equal amount of seawater, hypochlorite solution was added to make chlorine concen- residual brine after desalination and bittern, respectively. tration in the bittern reach 1 ppm. The water with a turbid- Fig. 3 shows turbidity profiles of three solutions during ity of 0.2 ntu and a density of 1000 kg/m3 was added to the 45 days in the outdoor condition. surface of the bittern by a depth of 15 cm. The water-add- From Fig. 3, it can be seen that the turbidity of residual ing method is as follows. A floating sheet was first put on brine after desalination increases moderately during the bittern surface before adding the water, and then the 45 days, however, the turbidities of both bittern and seawa- water was poured slowly on the sheet, so that the sheet rose ter show a rapid increase, most probably due to quick mul- up together along with water surface and a very clear inter- tiplication of algae and bacteria in two solutions under the face was formed between the water and the bittern layers. appropriate growth environment conditions. Residual After adding the water, the beaker was kept still in the lab- brine after desalination not only has a high salinity after oratory at a room temperature of 17 °C. Meanwhile, as a going through a concentrating process, but it also has a purpose of comparison, another beaker filled with the low turbidity due to the strict pretreatment (Wen et al., water only was also studied. Fig. 4 shows the density and 1999) of water-supply to the desalination process, including turbidity diffusion profiles in the bittern after depositing sterilization, coacervation and flocculation (Eran et al., for 15 days and 45 days, respectively. From Fig. 4(a), it 2008). In addition, the pretreatment process is also able can be clearly seen that density quickly diffuses in the to depress the breeding of algae and bacteria. All of these upward direction in the beaker, which is driven by salinity ensure a low turbidity in brine for a long time. gradient. For the case of 15-day deposition, the turbidity in In our experiment, bittern was directly filled into the the upper layer is 0.9 ntu which is a little higher than its bottom layer of solar pond without any concentrating pro- initial value (0.2 ntu), as seen from Fig. 4(b). The same var- 中国科技论文在线 http://www.paper.edu.cn 292 N. Li et al. / Solar Energy 84 (2010) 289–295

1300 their turbidities and the reduction of their thermal perfor- deposited for 15 days mance. It would be nice to know the sedimentation rates initial of the dust in the layers of the pond with diﬀerent salinities. )

3 So the following experiments were conducted. The same 1200 deposited for 45 days four beakers (35 cm high and 20 cm in diameter) were ﬁlled up to a depth of 25 cm respectively with four diﬀerent NaCl solutions with the initial salinities at 1.0%, 5.7%, 10.0% and 1100

Density (kg/m 15.0%. On each NaCl solution surface, it was then filled to another 5 cm height with a high turbidity solution (98 ntu), the salinity of which is the same as the NaCl solution 1000 below. All four solutions then remained still without any 0 0.1 0.2 0.3 disturbance from the environment. Fig. 5 shows the pro- Hight (m) files of turbidity reduction, measured at the solution sur- (a) Density profiles face. From Fig. 5, we can see that the turbidity decreased 50 dramatically at early time in all four studied solutions. It deposited for 15 days dropped below 10 ntu within the first 48 h. Then the turbidity reduced slowly in all four solutions. We can see that initial 40 the turbidity in the lower salinity solution decreased rela- deposited for 45 days u) tively more rapidly than in the higher salinity solution.

(nt 30 The reason is that the sedimentation of dust is slowed down in a higher salinity solution by corresponding larger 20 buoyancy. Fig. 5 also shows that it takes the turbidity a

Turbidity not-short time (around 10 days) to drop below 5 ntu in 10 all the solutions studied in this paper. From our following experiment, we found that the sub- 0 siding speeds of dust were also different in the NaCl solu- 0 0.05 0.1 0.15 0.2 0.25 0.3 tion and in the bittern with the same salinity. We filled Hight (m) four 250 ml volumetric cylinders with four different fluids (b) Turbidity profiles up to 22 cm depth: fresh water, NaCl solution, bittern with turbidity reduction and bittern without turbidity reduction. Fig. 4. Density and turbidity diffusion profiles. The last three have the same density 1116 kg/m3. The initial turbidities were 0.9 ntu, 4.1 ntu, 4.5 ntu and 8.8 ntu respec- iation in turbidity after 15-day deposition was also tively before superfluous soil was added into these fluids observed in the comparison sample (water), that is, the tur- and fully mixed. Then the cylinders were kept still to let bidity in the water-only sample also increased from 0.2 ntu the sediments subside. The curves in Fig. 6 show that in to 0.9 ntu. Fig. 4(b) also shows that the turbidity level shot the fresh water and NaCl solution, the turbidity went down up considerably at the bottom of the beaker on day 45, as to 1.9 ntu and 5.2 ntu respectively in 48 h, close to their ini- the sediment was seen there. However, the turbidity shows tial values. On the other hand, in the raw and treated bit- little change in the upper layer of the beaker. It had a value tern solutions, after 282 h subsiding, the turbidities were of 0.4 ntu, smaller than the turbidity in the comparison 24 ntu and 34 ntu, respectively, which are much higher sample (water) with a value of 1 ntu. The slight increase than their original turbidities. This experiment indicates in the turbidity of the comparison sample might attribute to the growing of microorganism because the water sample was not added sodium hypochlorite solution. From Fig. 4b, we can conclude that turbidity does not diffuse 60 Salinity 1.0％ upward and the sediments go down, which is driven by 50 gravity. Salinity 5.7％ u) t 40 We obtained the consistent results with the above using Salinity 10.0％ the treated bittern taken from the bottom of the solar 30 Salinity 15.0％ pond. Therefore, it is not necessary to reduce the turbidity in LCZ, thus the cost would be reduced. 20 Turbidity (n

10 2.4. Subsidence of turbidity 0 0 50 100 150 200 250 300 As solar ponds were exposed to outdoor usually without Time (h) covers, it is inevitable for the ponds to get mixed with the dust from the rain and wind, which causes the increase of Fig. 5. Subsidence of turbidity in diﬀerent salinity solutions. 中国科技论文在线 http://www.paper.edu.cn N. Li et al. / Solar Energy 84 (2010) 289–295 293

200 cation. The treated rainwater was then injected to the sur- fresh water face of the pond. Thus, the dust mixed in rainwater and NaCl solution wind was stopped out of the pond. Limpidity of the pond 150 treater bittern water was ensured and disturbance between UCZ and bittern NCZ was avoided. It also saved freshwater through the 100 rainwater recycle.

Turbidity (ntu) 3. Thermal performance simulation of solar ponds with 50 diﬀerent turbidities

0 Turbidity of water is an important factor affecting ther- 0 100 200 300 mal performance of a solar pond. Wang and Seyed-Yagoobi Time (h) (1995) introduced turbidity in their radiation transmission Fig. 6. Dust subsiding speed in four kinds of liquid. model, so called W.S. model. This model was then well-recognized and referred by many researchers. Under the help of the W.S. model, we simulated the thermal performance of that dust subsides very fast when it is dropped into fresh solar ponds with different turbidities in this paper. water. In the pure salt water, the subsiding gets a little The model we used to describe the heat transfer inside bit slower, still in the relatively very fast speed. But in bit- the solar pond is based on a one-dimensional unsteady heat tern the subsiding is very slow. Usually, NCZ of a solar conduction equation with internal heat sources, pond is a mixture of seawater and fresh water with bittern @T @ @T dhðxÞ after subsiding in various ratios. So, once the dust mixed qc ¼ k H q ð1Þ into the solar pond, it is very hard to clarify naturally. In @t @x @x s dx ext this point, it is quite different from a salt-water solar pond. where T is temperature, t is time and x is the depth from the water surface in the pond. q; c; k are the density, specific

2.5. Utilization of rainfall heat and thermal conductivity, respectively; qext is heat extraction. H s is solar radiation energy on water surface. Influence of rainfall on solar ponds has two sides. One According to Beijing Meteorological Station (Zhang and side, rain is good for solar ponds because it does a good Wang, 1991), H s can be calculated by job for surface flushing (Agha et al., 2004). But on the H ¼ 177:6 þ 86:2 sinð2pD=365ÞðMJ=m2 dayÞð2Þ other side, the dust mixed into the rain also may cause s more turbid of ponds. The turbidity of rainwater is highly where D is the days starting from the vernal equinox related to regions. In Dalian, China, the turbidity of rain- (March 21st). water is around 0.5–2.5 ntu sometimes in summer, but The hðxÞ in Eq. (1) is the radiation transmissivity at the sometimes it can reach 15–20 ntu. It would definitely cause depth x, that is surplus solar radiation energy at the depth turbidity problems in ponds. x. It is determined by the W.S. model. For the case of uni- In summer, Dalian city has plenty of sun shine, but it form turbidity in the solar pond, the W.S. model can be also rains a lot. Usually, another rainfall comes before tur- expressed as bidity caused by the first rainfall subsides completely. The hðh; xÞ¼hð0:3; xÞRðh; xÞð3Þ frequent rainfalls would keep the salinity gradient solar pond in a very high turbidity situation and thus reduce where, the transmissivity of LCZ and the efficiency of the solar hð0:3; xÞ¼0:58 0:076 lnð100xÞ pond. Summer’s rainfall is usually much heavier, which 2 tends to more easily destroy UCZ and get it mixed with Rðh; xÞ¼1 0:1975xðh 0:3Þþ0:0144xðh 0:3Þ NCZ. As a result, it destroys the stability of the solar pond. where hðh; xÞ is a dimensionless reference radiation trans- How to prevent UCZ and NCZ from damaging from mission function at turbidity h; hð0:3; xÞ is dimensionless strong rainfall and how to avoid the salinity gradient solar reference radiation transmission function at h ¼ 0:3 ntu; pond from getting turbid has been a very important Rðh; xÞ is the ratio of hðh; xÞ to hð0:3; xÞ at a given turbidity. research topic. Expanding the W.S. model, the radiation transmission In this paper, we have taken some steps to prevent rain- function for nonuniform turbidity is fall from directly dropping into the solar pond and to ! ! ! Xi Xj Xi1 Xj Xi recycle the treated rainwater to the pond as one freshwater hðf ðhiÞ;xÞ¼ h hj; lk h hjþ1; lk x¼ lj source. The flow sheet is shown in Fig. 7. The pond was j¼1 k¼1 j¼1 k¼1 j¼1 covered with plastic sheet before the strong rainfall came. After the rainfall, the rainwater was pumped out of the where f ðhiÞ is the integrated turbidity throughout i layers sheet to a container and then added the chemicals for the brine; hj; hjþ1 are the turbidities of jth layer and the treatments, including sedimentation, filtration and clarifi- j þ 1th layer, respectively; lk is thickness of the kth layer. 中国科技论文在线 http://www.paper.edu.cn 294 N. Li et al. / Solar Energy 84 (2010) 289–295

Rain

Pump Back to Surface of Original Valve Solar Pond UCZ NCZ Plastic Film

LCZ Filter Paper Rain Water Clean And Fresh Chemical Water

Fig. 7. Preventing and using rainfall.

To solve Eq. (1) numerically, we need to know the atmo- sphere temperature (temperature at x ¼ 0), taken from Bei- jing Meteorological Station

T a ¼ 11:6 þ 15:4 sinð2pðD 27Þ=365ÞðCÞð4Þ

It is very difficult for the salinity gradient solar pond to reach and keep the low turbidity while the pond is filled up with bittern or seawater. So, LCZ of the solar pond is filled with bittern and NCZ of the solar pond is filled with the naturally concentrated residual brine after desalination. Based on the experimental result in Section 2.3 of this paper, the turbidity in LCZ (the storage zone) has little effect on the solar pond’s thermal efficiency. Therefore, the bittern to fill in LCZ is usually roughly treated by subsiding and chemicals. Instead, most focus is put on largely reducing the turbidity in NCZ. We calculated the heat storage performance of the solar Fig. 8. Temperature profiles with different turbidities. pond with different turbidity distributions. Fig. 8 shows calculation results for the cases with an inhomogeneous top down. According to the method of average volume of distribution of turbidity: 4.5 ntu in LCZ, a linear profile turbidity, the average volume of turbidity of UCZ and from 4.5 ntu to 1.0 ntu in NCZ and 1.0 ntu in UCZ, as well NCZ is 2.4 ntu for the first case and 3.1 ntu for the other as a homogeneous turbidity of 0.5 ntu, 1.0 ntu, 2.0 ntu, and case. The bottom reflectivity of pond is neglected and the 4.5 ntu, respectively. The curves show the temperature pro- collection efficiency is not affected by the turbidity of files on the 150th day from vernal equinox. It can be seen LCZ. The collection efficiency profile with different turbid- that the higher the turbidity was, the lower the temperature ities is shown in Fig. 9. in the heat storage zone was and the less efficient the solar Collection efficiency of all four homogeneous turbidity pond was. Fig. 8 also shows that in the case with the cases is connected by a curve, as shown in Fig. 9.Itis turbidity of 4.5 ntu in LCZ and 4.5–1.0 ntu in NCZ, the clearly seen that the collection efficiency of the solar pond temperature of LCZ was higher than that with turbidity is reduced with the increase of turbidity. For the case with of 2.0 ntu in all the water. Obviously, the former is more inhomogeneous turbidity of 1.0 ntu in UCZ and a linear easily achievable and lower in cost than the latter one. distribution between 1.0 ntu and 4.5 ntu in NCZ from the Some calculations on relation between collection top down, the corresponding collection efficiency, point efficiency and turbidity have also been done. The collection A, is above the collection efficiency profile of homogeneous efficiency after the 250-day continuous operation on the turbidity case. For the case with inhomogeneous turbidity solar pond was calculated for both homogeneous and inho- of 4.5 ntu in UCZ and a linear distribution between mogeneous turbidities cases. For the homogeneous case, 4.5 ntu and 1.0 ntu in NCZ from the top down, the corre- the turbidities we studied are 0.5 ntu, 1.0 ntu, 2.0 ntu, sponding collection efficiency value, point B, is below the and 4.5 ntu, respectively. The collection efficiency of inho- curve. This can be explained by the following. When tur- mogeneous turbidity was calculated by two cases. One case bidity is high in the upper layer of the pond, most solar is that turbidity is 1.0 ntu in UCZ, and a linear profile from radiation has difficulty reaching the lower layer and instead 1.0 ntu to 4.5 ntu is used in NCZ from the top down. it is stored in the upper layer where heat is easy to lose to Another case is that turbidity is 4.5 ntu in UCZ, and a lin- the environment. Thus, lower collection efficiency is pro- ear profile from 4.5 ntu to 1.0 ntu is used in NCZ from the duced in this case. 中国科技论文在线 http://www.paper.edu.cn N. Li et al. / Solar Energy 84 (2010) 289–295 295

22 rainwater dropping into a solar pond and reusing the treated rainwater as a water-supply to a solar pond. This method not only avoids the turbidity 19 A increase of a solar pond caused by rain, but also saves the freshwater source needed to maintain the salinity gradient of a solar pond. 16

B Acknowledgment Collection Efficiency (%) Collection Efficiency 13 012345The authors are thankful to the National Natural Sci- Turbidity (ntu) ence Foundation of China (NSFC) for ﬁnancial support Fig. 9. Correlation of collection eﬃciency and turbidities. of the present work (Grant No. 50676016).

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