Control of Rising Salinity in the Salton Sea by Desalination Technologies

Kevin QiKQi@ucla.edu

UCLA Civil and Environmental Engineering

CEE 155: Unit Operations and Processes for Water and Wastewater TreatmentDr. Linda Y. Tseng

Submission Date: 12/6/13

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Abstract

The rising salinity of the Salton Sea has become a huge issue. The Salton Sea is the largest lake in Southern California and is home to many birds and very little aquatic life. It is important in both environmental and economic standpoints to its area in Southern California. The increased salinity is due to the inflow of various riverine, high rate of evaporation and low rainfall. The discharge of saline waste from agricultural sources also cause an increase in salinity. A wide variety of methods have been proposed to manage the salinity. Multi-stage flash distillation is one such method that is effective but is also energy intensive and therefore expensive. Reverse osmosis is another method that is less energy intensive as MSFD but is more vulnerable to disturbances in water. Lastly, solar technologies could possibly be effective and efficient in controlling the rising salinity in the Salton Sea.

Keywords: Salinity, Desalination, Treatment Efficiency, Treatment Energy Usage, Contaminants

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Introduction

Aquatic life and marine animals are essential to ecosystems such as the Salton Sea. Unfortunately, the salinity levels in the Salton Sea have crossed a threshold whereby it cannot support most types of life. The Salton Sea is the largest lake in California and is approximately 964 km2. Like Death Valley, it is below sea level. There are 400 species of birds that spend migration seasons at the Sea. However, due to the high salinity levels, there is barely any aquatic life. It’s currently salinity is around 45 g/L which fluctuates depending on the season. For comparison, the Pacific Ocean’s Salinity is only 35 g/L. Regardless, the Salton Sea salt concentration continues to increase around one percent a year. It is a saline sea with no outlets and scientists and engineers are working on various methods to remove the contaminant (salt).

The act of removing salt is called “desalination”. It has been used throughout history to treat water and it is still the primary method in the modern day. There are various methods that can be considered to decrease the salinity in the Salton Sea. Three methods will be explained and analyzed in this paper: reverse osmosis, multi-stage flash desalination (MSFD), and solar thermal desalination. Reverse osmosis uses pressure to force the saline water through a specially designed membrane that separates the salt and fresh water so that the salt is on one side of the member and the fresh water passes through. MSFD removes salt from water by segmenting portions of water into steam through multiple stages of heat exchanges. Solar thermal desalination is the least used but potentially effective method that uses sunlight for desalination purposes. Saline water is put in cones where it is evaporated by sunlight. These methods all have various pros and cons whether it be efficiency, effectiveness, or cost.

Contaminant Description

Salt is the primary contaminant that needs to be dealt with in the Salton Sea. Salts in the Sea are washed into the oceans from rocks on land. When water is in constant contact with rocks, a process called “weathering” occurs. Salts such as Calcium carbonate (limestone), table salt (sodium chloride), and magnesium sulfate (Epsom salt) are produced by this process. These solutes are brought to the Sea by rivers and precipitation. These salts are removed in various ways, usually by marine organisms that use them as nutrients or other reasons. For example, clams use salts to build their shells. Weathering is an extremely slow process when compared to the human life cycle but is very noticeable in the geological time scale.

In the beginning of the 20th century, the area now known as the Salton Sea was a dry lake bed named the “Salton Sink” (Figure 1). The California Development Company utilized irrigation from the Colorado River to make the area fertile for farmers while the Liverpool Salt Company mined the salt. However, only a few years into operation (1905), heavy precipitation caused the Colorado River to flood the Salton Sink thus creating the Salton Sea (1). The Salton Sea initially had high salt levels which continued to increase at a steady rate. This was primarily due to the riverine input combined with a high evaporation rate of 1.8 m/year. The precipitation in the region is only .07 m/year. The lake levels are maintained by the 1.7 km3/year of annual runoff from agriculture. The residual salts from the pre-flood area of the Salton Sink are still suspended in the lake-bottom sediments too which contribute to the salinity. Studies in the early 20th century indicated the existence of a brine reservoir located in the sediments at the bottom of

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the Salton Sea. Because the lake does not have any outflows, salt can only be removed by marine organisms in the water and by human efforts. The problem is, the salt levels have gotten to a point so high that no aquatic life lives in the Sea anymore.

Detection Methods

There are numerous methods of detecting levels of salinity including evaporation, a hydrometer to measure density and electrical conductivity. Evaporation is the oldest and most straightforward technique for measuring salinity. Salinity is defined as the weight of salt over the weight of water. If the water is all evaporated, then the leftover salt can be weighed. This is usually done by measuring out one kilogram of water, evaporating it, and weighing the precipitates to determine the salinity.

A hydrometer measures density of the water to determine salinity. Density is simply defined as mass over volume of the water. The density of saline water is dependent on temperature and salinity so as long as density and temperature is measurable, then salinity can be determined. To collect measurements of mass and volume of a water sample, it is most efficient to use Archimedes’ Principle and displace the water with a glass ball. The higher the glass ball floats, the greater the density of the water. Hydrometers are devices specifically designed to measure fluid density when more accurate measurements are needed than those provided by the floating glass ball (Figure 2). It has fixed mass that is concentrated at the bottom of the tube so that it will always float upright and when it is placed in the water sample. As the density of the water rises, the tube will displace less water. This is due to the same concept as the glass ball; the hydrometer sinks less in higher density fluid due to Archimedes’ Principle. The narrow stem at the top of the tube allows even the smallest volume changes to be measurable (2).

Salinity of water can also be measured using electrical conductivity. Pure water has low electrical conductivity. When salt is added, positive and negative ions donate their ions which causes the water to have higher electrical conductivity levels. It is then possible to use these measurements to correlate between salinity and electrical conductivity. Using a conductor and light bulb, it is possible to use this method to determine salinity. When connected to a light bulb, pure water does not have enough electricity to complete a circuit and light a small light bulb. Water with dissolved salts contains positive and negative ions that constantly transfer which will complete a circuit and light the bulb. The brighter the bulb is, the greater the conductivity, and ultimately the higher the salinity.

The detection measurements of the Salton Sea were made in the early 21st century (3). The North Basin was first sampled twice in 2000 and 2001 at a depth of 10 m. In 2002, three different sites were sampled in the South Basin at depths of 8, 12, and 14 meters. The measurement of different depths was essential to this study to the varying salinities varying with lake depth (Figure 3). The depths of 0 to 14 m of the Salton Sea were sampled with most of the data coming from 8-14 m. The samples were collected in stainless-steel boxes that were specifically designed for sediment samples. These box-core samples were lowered into the sediment-water interface and until completely submerged before they were let go into the sediment for measurements. These box-core had dimensions of 7.5 cm in diameter and a height of 50 cm. After the box-cores collected data and were brought back on board, they were inspected for complete closure and even penetration of the sediment. This sediment was extracted through tape-covered holes distributed evenly down the length of the core. Each year,

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core-liners with different spacing’s were used whereby sediments were extracted using plastic syringes. The sampling started at the sediment-water interface and finished at the down-core section. Afterwards, the samples were put in airtight containers surrounded by ice so the temperature could be maintained. They were then transferred to the lab and stored at the same temperature until all samples were ready to be analyzed. The pore waters were extracted by centrifugation from the box-cores and the resulting supernatant was filtered through a 0.2 µm filter.

Treatment Methods

Reverse osmosis is the most straightforward and oldest treatment method for desalination. It is a process that uses pressure to force a solution through a membrane whereby salt is retained on one side and the pure water is passed to the other side. Osmosis is defined as the natural movement of solvent moving from an area of low concentration (high water potential) through a membrane to an area of high concentration (low water potential) with no external pressure. It is done through a semi-permeable membrane so that the solvent may pass but the solute is retained. Reverse osmosis is the direct opposite in that it forces the material with external pressure from a high concentration through a membrane to a region of low solute concentration. The membrane in this case is an extremely dense barrier in the polymer matrix where the separation occurs. In this specific case, the membrane is designed so that only pure water passes through the dense barrier layer while the salt ions are blocked. This process require high pressure levels on the high concentration side of the membrane dependent on the type of water being tested on. Fresh water requires less pressure than sea water due to the concentration of salt levels. Usually, 2-17 bars (30-250 psi) of pressure are required for fresh/brackish water while 40-70 bars (600-1000 Psi) are required for seawater. Seawater requires a much higher pressure exertion because it has a 25 bar (350 psi) natural osmotic pressure and higher salt concentration levels which must be overcome.

Reverse osmosis is quick and effective but requires high amounts of energy and electricity. Because of this, this treatment method is mostly used places in the Middle East such as Saudi Arabia where there are abundant oil reserves that is cheap and easily transportable to these treatment plants. Desalination plants using reverse osmosis are typically located near power plants to reduce energy losses in transmission and allows waste heat to be used in the desalination process which ultimately reduces the amount of energy needed for the desalination of water and provides cooling for the power plant (4). The most essential components of a reverse osmosis plant is that of system automation and reliability. The operational priorities are maintaining water quality and personnel safety while also meeting economic and environmental demands. Reverse osmosis processes should also constantly be operated at a high recovery rate so most of the feed water volume is treated to low salinity product water due to decreased economic and environmental costs that comes with brine disposal (5). While high recovery promotes better efficiency and reduces costs, the plants are more vulnerable to disturbances in feed water quality, temperature, and total dissolved solids concentrations. These disturbances can cause negative effects in product flow-rate, brine flow-rate and internal system pressure.

The majority of desalinated water in the Salton Sea is produced in multistage flash plants. Multi-stage flash distillation (MSFD) is a treatment process that desalinates water by flashing water into steam in multiple stages that are regenerative heat exchangers. The process is more

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complicated than other methods but it is highly efficient and economic. First, saline water is heated in a “brine heater” by condensing steam on a set of tubes carrying saline water. The heated water is then passed to a container called the “stage” where the outside pressure is lower than the pressure in the brine heater. This sudden introduction of water into the lower pressure stage causes the water to boil so rapidly that it flashes into steam. Only a small portion of the water is converted into steam during this flash of steam. The remaining water is then sent through a series of additional stages each with lower ambient pressures than the previous one. As the steam is generated in each stage, it is condensed into tubes of heat exchangers. The MSFD processes combined with power generation systems reduces the energy requirement for the entire treatment by more than half when compared to a single purpose desalination plant using the same fuel.

A simulation was study was carried out to determine the relationships between number of stages, performance ratio, top brine temperature, energy losses and the terminal temperature approach (6). The terminal temperature approach was defined as the difference in temperature between the condensing vapor and the recirculating stream leaving the condenser. This parameter is an essential design parameter depending on the overall heat transfer coefficient and the area of the heat transfer. The relationship between these parameters are analyzed in a graph for TTD of 2ºC (Figure 4a) and 4ºC (Figure 4b) respectively. As seen through these figures, increasing the number of stages will decrease the temperature drop per stage whereby the irreversibility of the system is reduced due to the reduction in condenser and flash energy losses which ultimately results in an improved thermal performance. Increasing the performance ratio will reduce steam consumption and will consequently minimize operating costs while an increase in surface area results in increased costs. Therefore, the optimum performance ratio is determined by the cost tradeoffs between the cost of the process and the capital cost of the process. Conversely, increasing top brine temperature while keeping the number of stages constant will also increase the performance ratio and decrease the specific condensing area with no decrease in available energy. The simulation analysis shows that the dependence of available energy loss is more dependent on the number of stages than the due to the variation of top brine temperature.

The MSFD process has many advantages such as a simple facility design, high reliability compared to reverse osmosis, and a high water capacity for each testing unit. However, its energy consumption is the highest out of all the methods mentioned in this paper. To combat this issue, an enhanced multistage flash (E-MSF) desalination system can be used (7). The main improvement of the E-MSF over MSFD is that a portion of the flash vapor in the flash room is extracted into the next stage to heat the flashing brine and part or all of the inlet crude seawater is replaced by the cooling seawater of the power unit. To determine the exact improvements of the E-SMF, analytical models of technical and performance were set up and compared to MSF treatments. The models shows that the gain ratio potentially increased by up to 74.1%, the mean annual capital cost of fresh water production would decrease by 10.7%, the brine concentration during each stage would reduce by an average of 21.8% and the extracting quotient of flash vapor would increase to .773. These improvements made in an E-SMF would greatly decrease the energy consumption and further utilize the advantages of cogeneration when compared to a MSFD.

Using solar technologies as a treatment method for salt is one of the lesser used methods but shows great potential as more research and technological improvements are made. The most positive aspect of solar technologies is that the energy source, the sun, is unlimited. However,

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the efficiency of solar energy is highly limited by the system by the method in which it is harnessed. It can be taken directly using solar collectors or panels. Collecting direct sunlight is generally considered inefficient because it requires large land areas. In most cases, sunlight is collected indirectly with other desalination methods like MSFD or reverse osmosis. However the design with direct sunlight collection is simple and has relatively low costs when used in small-scale production. When small-scale production is effective, direct solar radiation is collected through solar stills (8, 9). Solar stills use natural methods such as evaporation and condensation to assist in the solar collection. A transparent cover covers a pan of saline water which traps solar energy. This causes the water to evaporate and condense on the inner face of the sloping transparent cover. The resulting water is clean and drinkable as the salts, inorganic and organic components are left in the bottom of the bath. The sunlight, if high enough, should also kill the pathogenic bacteria in the evaporating water too. The bottom of the tank will eventually have sludge buildup and need to be cleaned consistently (10). Nevertheless, this process is straightforward and effective in small areas.

Conclusion

The high salinity levels of the Salton Sea is a significant but manageable issue. There are various desalination methods available with different advantages and disadvantages to treat the salt contaminates. Solar energy combined with desalination offers a cheap method that covers needs of power and water for small areas of areas that are analyzed. It can either be a direct or indirect process. The direct method uses solar energy to produce distillate directly into the solar collector while the indirect method uses two different processes, usually a solar energy collector and a separate desalination process such as reverse osmosis. By using the indirect method, solar energy collection can greatly reduce costs of desalination in the Salton Sea. In addition to improvements in solar technology, technological development in waste heat utilization in the desalination process over the past decade have improved the reliability and performance of the process and reduced the unit cost of desalination. Because of this, the MSFD process is the most ideal desalination process for a large scale treatment of the Salton Sea. Since there are no outlets in the sea, the only way water is removed is by evaporation. Spraying salty water into the air to speed up evaporation would assist in the desalination process. The Salton Sea has been building up salt for the past century and it is an issue that needs to be addresses as soon as possible. Salt is one of the oldest contaminants known to mankind with various treatment methods, some that are old and some that are still being developed. In the case of the Salton Sea, it is essential that the MSFD process is used in combination with solar technologies as long as the brine from the basins are continuously cleaned. This would be a significant step in dealing with the rising salinity in the Salton Sea.

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References:

(1) Laflin P. The Salton Sea: California's overlooked treasure. The Periscope. 1995, 43, 66.

(2) Mohammad A. K. Seawater desalination-SWCC experience and vision. Desalination. 2001, 135, 121-139.

(3) Wardlaw G. W.; Valentine D. L. Evidence for salt diffusion from sediments contributing to increasing salinity in the Salton Sea, California. Hydrobiologia. 2005, 533, 77-85.

(4) Shih H.; Shih T. Utilization of waste heat in the desalination process. Desalination.2007, 204, 464-470.

(5) McFall C.W.; Bartman A.; Panagiotis D.C.; Cohen Y. Control of a reverse osmosis desalination process at high recovery; American Control Conference, Seattle, WA, 2008.

(6) Hamed O.A.; Mohammad A. K.; Imam M.; Mustafa G. M.; Bamardouf K. Simulation of multistage flash desalination process. Desalination. 2001, 134, 195-203.

(7) Junjie Y.; Shufeng S.; Jinhua W.; Jiping L. Improvement of a multi-stage flash seawater desalination system for cogeneration power plants. Desalination. 2007, 217, 191-202.

(8) Hazim M.Q.; Banat F. Solar thermal desalination technologies. Desalination. 2008, 220, 633-644.

(9) Velmurugan V.; Deenadayalan C.K.; Vinod H.; Srithar K. Desalination of effluent using fin type solar still. Energy. 2008, 33, 1719-1727.

(10) Machine tested for reducing Salton Seasalinity; Los Angeles Times, March 2, 2000, p A26.

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Figure 1: The Salton Sink 1905-1908 (1)

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Figure 2: Hydrometer: As density of water increases, the hydrometer displaces less water. The upper tube is extremely narrow so any change in density and volume will be easily measurable. (2)

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Figure 3. Sediment depth distributions of pore water salinity from the two samples collected at the north panel (closed values) and the three from the south panel (open values) basin in the Salton Sea. (3)

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Figure 4a. Dependence of performance ratio, available energy (exergy) losses, and condensing area on top brine temperature and number of stages for TTD=2ºC (3)

Figure 4b. Dependence of performance ratio, available energy (exergy) losses, and condensing area on top brine temperature and number of stages for TTD=4ºC (3)

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