Desalination for Fresh Water Production

Transcription

1 Shelby Lynn Kindsvater University of Wyoming Technical Report American Society of Civil Engineers Rocky Mountain ASCE Student Conference March 31 April 2, 2016 Report Word Count: 1680

2 Table of Contents Introduction... 3 Fresh Water Production Needs... 3 Technologies... 3 Reverse Osmosis... 4 Multi-stage Flash... 6 Impacts... 7 Environmental Impacts... 7 Human Impacts... 8 Sustainability Analysis... 8 Conclusion... 8 References... 9 Appendix A: Effluent Properties P a g e

3 Introduction Water is a resource that all of life depends on. The amount of fresh water that is available is minute compared to the total amount of water. The hydrologic cycle indicates that total amount of water on earth never changes, but its location, and thus availability, changes as the cycle progresses. Fresh water is a limited resource and the effects of a necessary and severely limited resource are being seen in different areas. Some of these water shortages have been occurring over a long time period, although a more recent time frame is indicating shortages in newer areas where different means of producing fresh water are being undertaken. Desalination is a concept that has been around for ages. Evaporation is the simplest form of desalination, although on a large scale, it is not entirely practical. Technology has been used to ensure fresh, clean water for human use and the standards of fresh water have created the high quality drinking water that we see today. Desalination is able to be used, and is used in different parts of the world, although the expenses are part of the reason it is not used worldwide, or more specifically, in places in severe need of water. Fresh Water Production Needs In many different technological systems, there has been an increase in demand for lower costs with higher efficiencies. 1 The same is true for desalination. As the need for fresh water increases, the need for an unlimited source of water increases; however, the current sources of fresh water are being depleted as the hydrologic cycle precipitates more water in to the oceans and infiltration does not occur at a fast enough rate to replenish the sources of groundwater. Desalination has historically been a very expensive process, hence the reason it has not been done in the same capacity that freshwater treatment has been. As technology has improved, so has the cost for desalination, making it more cost effective to do so. Desalination is a process that needs to be sustainable economically, socially and environmentally. These factors will be discussed later, and are the driving forces behind the advances in the desalination technology. Technologies In the most basic process, even considering the different technologies, desalination forms two streams. One stream being fresh water to be distributed; the second stream being a waste stream consisting of concentrated brine that is pumped back into the ocean. Industrial production of desalinated water is dominated by two different technologies, reverse osmosis and multi-stage flash. There are other forms of desalination such as multiple-effect distillation, mechanical vapor compression and electro-dialysis, although these are much less common and not typically used in large-scale settings. These less common technologies are more costly in their construction 1 Lior, Advances in Water Desalination. 3 P a g e

4 materials and electricity usage. Appendix A shows a comparison of the effluent properties of reverse osmosis and multi-stage flash. Reverse Osmosis Reverse osmosis (RO) is the most common type of desalination and it dominates the industry in terms of production. RO has grown in the industry over the past decades because it is considered to be a cheaper option. Figure 1 2 below shows a basic set up of an RO desalination plant. It indicates all the major sources of energy consumption. The conceptual drawing of the plant indicates the proximity of the plant to the ocean, which is necessary for all desalination plants. Figure 1: Conceptual drawing of a seawater reverse osmosis desalination plant RO has made a lot of progress over the decades, making it the leader in desalination technology. These improvements have largely been focused on the membranes that are used and include better resistance to compression. Longer life, higher possible recovery, improved flux, and 2 Elimelech, "The Future of Seawater Desalination: Energy, Technology, and the Environment." Science 333, no (August 5, 2011): P a g e

5 improved salt passage. 3 These improvements have been able to significantly decrease the overall costs and energy usages of the desalination plants. The decrease in power consumption over the years, along with the theoretical minimum energy can be found in Figure 2 4 below. As indicative by (A) of Figure 2, since the 1970s there has been a drastic decrease in the amount of power that is need for RO desalination plants. The sub chart shows how much of a decrease of power Figure 2: (A) The change in power consumption for the RO stage. (B) Theoretical minimum energy for desalination. consumption occurred from 2000 to Depending on the amount of freshwater recovered during the desalination process, a different amount of minimum energy is necessary. It can be understood, through the data presented, that the more fresh water that is to be recovered requires a higher minimum energy use. RO, as mentioned earlier, uses membranes to desalinate the seawater. The seawater enters as a pressurized solution with the particles and dissolved salts being removed by membrane that is semi permeable. The semi permeable membrane lets only water through. After the water passes through the member, it is collected and sent for treatment to ensure it meets the necessary standards for fresh water. The brine is also capture and discharged back into the ocean off shore. If it is not discharged off shore, it can be diluted by a treated waste stream before discharge back into the ocean. 3 Khawaji, "Advances in Seawater Desalination Technologies." Desalination 221 (2008), Elimelech, "The Future of Seawater Desalination: Energy, Technology, and the Environment" P a g e

6 Multi-stage Flash Multi-stage flash (MSF) desalination is the second most commonly used type of desalination in the industry. Figure 3 5 below shows that basic process of MSF desalination. MSF operates using heat to turn the water into steam, where it is then pressurized and collected as fresh water. This evaporation process is done in flashes in order for the brine to cool and continue to flow through the system. Figure 3: Multi-stage flash process MSF is not as common as RO as it is a more expensive technology. When looking at Appendix A, there are significant differences between the effluents properties of the two types of desalination. These differences arise from the differences in processes, materials and chemicals used during the process. Appendix A also shows ambiguities in the properties because not all plants are constructed the same, thus having different effluent properties that either compare or contrast with that of the other desalination process. Figure 4 6 below indicates a breakdown of the desalination processes. It can be seen that combined, RO accounts for over 50 percent of the technology, while MSF, at least through Solar, accounts for 10 percent. When looking at the production of freshwater from the plant, studies indicate that the MSF desalination can produce more freshwater daily, although based on the energy use and the number of plants, it still appears to be more cost effective to use RO for desalination of fresh water. 5 Cipollina, Seawater Desalination: Conventional and Renewable Energy Processes (Heidelberg: Springer, 2009), Cipollina, Seawater Desalination: Conventional and Renewable Energy Processes, P a g e

7 Figure 4: Technology combinations of desalination plants Impacts The potential and known impacts of a technology are important to consider when implementing, as some impacts can be devastating while others lack any significance. Seawater desalination has environmental that are both direct and indirect; there are also human impacts associated with desalination. Environmental Impacts Desalination plants require more energy than traditional water treatment and, although technology has improved, the energy required to run a desalination plant still has impacts on the environment that should not be ignored. As with other technologies that use electricity, carbon dioxide is dispersed into the atmosphere when the plant is functioning. 7 From desalination, the brine is discharged back into the ocean and that causes problems for the marine life. The concentration of the brine is between 1.3 and 1.7 times the seawater concentration 8 and it is assumed that dilution of brine from mixing with the seawater after being pumped offshore will occur at a fast enough rate to prevent any significant problems. This higher concentration that is pumped into the ocean offshore can have a variety of effects depending on the vulnerability of the marine life, which varies significantly depending on the type of life. To minimize the adverse affects of high concentration brine, an appropriate technology has to be selected for pumping. The marine life could potentially be affected by the cleaning of the 7 Sadhwani, "Case Studies on Environmental Impact of Seawater Desalination." Desalination 185, no. 1-8 (2005), 3. 8 Ibid. 7 P a g e

8 systems. These chemicals have been discharged at extremely low doses, causing virtually no effect on the surrounding marine life. 9 Along with the marine life, there are also impacts on the land. Desalination plants are located in coastal areas where the ecosystems are found to be very diverse, both on land and in water. Constructing a desalination plant could negatively impact the ecosystem on land by removing or dislocating a significant portion of a species. Human Impacts Desalination plants are built away from the tourist areas to prevent obstruction of views. Tourist areas require buildings to meet some aesthetic requirements, and desalination plants can not become cost effective in the short term if they are to succumb to these requirements. Another human impact of desalination plants is the noise. The noise is produced by highpressure pumps along with the turbines for energy restoration. 10 The noise, similar to aesthetics, is another reason that the plants have to be located away from the tourist areas. Sustainability Analysis The technologies used for desalination have significantly improved over time to make them more efficient and sustainable. These advances were necessary in order for the technology to become accepted. Desalination is able to be a sustainable process providing the measures are taken and the correct technologies are used in the correct systems. Both reverse osmosis and multi-stage flash desalination are able to meet the needs of society by providing water to those in need. The technologies have improved to significantly lessen the adverse effects on the environment and the appropriate research appears to have been done to ensure that these impacts are in fact insignificant. Desalination will likely continue to be a more expensive process than traditional fresh water treatment; however, the improvements have made it more economical, and thus more accessible to those in need. Conclusion Desalination for fresh water production is a continuously increasing industry as the need for water becomes more prevalent. Many areas are facing water shortages and sources of ground water are depleting. Technology cannot replace the need for fresh water, but it can help to provide it to those in need. Reverse osmosis and multi-stage flash desalination are likely to continue to improve through the processes and technologies that they consist of making it easier for people, mainly along the coastal areas, to obtain fresh water. 9 Sadhwani, "Case Studies on Environmental Impact of Seawater Desalination." Einav, "The Footprint of the Desalination Processes on the Environment." Desalination 152 (2002), P a g e

9 References Cipollina, Andrea, Giorgio Micale, and Lucio Rizzuti. Seawater Desalination: Conventional and Renewable Energy Processes. Heidelberg: Springer, Einav, Rachel, Kobi Harussi, and Dan Perry. "The Footprint of the Desalination Processes on the Environment." Desalination 152 (2002): Elimelech, Menachem, and William A. Phillip. "The Future of Seawater Desalination: Energy, Technology, and the Environment." Science 333, no (August 5, 2011): Khawaji, Akili D., Ibrahim K. Kutubkhanah, and Jong-Mihn Wie. "Advances in Seawater Desalination Technologies." Desalination 221 (2008): Lattemann, Sabine, and Thomas Höpner. "Environmental Impact and Impact Assessment of Seawater Desalination." Desalination 220, no (2008): Lior, Noam. Advances in Water Desalination. Sadhwani, J. Jaime, Jose M. Veza, and Carmelo Santana. "Case Studies on Environmental Impact of Seawater Desalination." Desalination 185, no. 1-8 (2005): P a g e

10 Appendix A: Effluent Properties 11 Typical effluent properties of reverse osmosis (RO) and thermal MSF (multi-stage flash) seawater desalination plants 11 Lattemann, "Environmental Impact and Impact Assessment of Seawater Desalination." Desalination 220, no (2008), P a g e

11 Typical effluent properties of reverse osmosis (RO) and thermal MSF (multi-stage flash) seawater desalination plants (continued) 11 P a g e