Desalination - Team B
Reverse Osmosis Desalination Plant in Durban, South Africa
Authors: Lauren Burke, Cindy Chen, Osman Jamil, Natalia Majewska
Instructors: Fengqi You, David Wegerer
March 11, 2016
An imminent global shortage of water will greatly affect the lives of millions of people. The situation in South Africa is predicted to exacerbate in the upcoming decades. This growing need for fresh drinking water motivated the design of a reverse osmosis desalination plant in the suburbs of Durban on the eastern coast of South Africa. A reverse osmosis process was selected based on the water salinity in the area and recent technological advancements in membrane efficiency.
The process is designed to operate for 350 days a year, 24 hours a day to yield 132,000 m3 of water per day. Prior to reverse osmosis, the seawater is pretreated to prevent membrane fouling using a cascade of filters and high-pressure pumps. Reverse osmosis is carried out at 37 bar with two banks containing 900 and 4000 membranes, respectively. After reverse osmosis, the pressurized concentrate that contains the unwanted salts and ions is sent to an energy recovery device to pressurize the reverse osmosis feed. The deionized water from the reverse osmosis system is remineralized and disinfected before being sent to the consumer.
Based on a thorough economic analysis, it has been determined that the process is not feasible at this stage. The annual estimated revenue is $37.5 MM. In addition, the 20 year NPV of the plant is -$23.5 MM with an IRR of 2.9%. A sensitivity analysis indicated that an increase in the price of water could potentially increase profits. Therefore, the design may become profitable implemented with the aid of government subsidies or if a drastic increase in water scarcity increases the selling price of water.
Close to 1.2 billion people, approximately one fifth of the world’s population, reside in areas of physical water scarcity . Although there is enough water in the world for a population of seven billion people, it is unevenly distributed, and many regions have been experiencing water shortages for more than a decade. Given the current rate of population growth and water consumption, many studies predict that this problem will be exacerbated in the years to come. Water desalination is a process used to purify water so that it meets drinking standards for consumption. The two main technologies used for desalination are thermal desalination and reverse osmosis. While thermal desalination is a robust method that scales well, it is typically preferable for inlets of very high salt concentrations and very low energy costs, such as the gulf coast region. Reverse osmosis plants are more prevalent elsewhere, making up 80% of all desalination plants in the world . Given the salinity of the inlet flow for this process and advances in technology, reverse osmosis has been selected as the optimal choice for this project.
The Durban area in South Africa has been selected as the location of this project since it is currently experiencing approximately 25% water withdrawal as a percentage of total available water. This value is projected to increase in the upcoming years to 40% water withdrawal due to physical water scarcity [3,4] as can been seen in Figure 1.
In addition to the increasing water scarcity in the next decade, this location was selected due to relatively low location costs and salinity levels as seen in Appendix A. South Africa has a location cost factor of 1.1, which is only 10 % more than the Gulf Coast basis. The country also has relatively low cost of labor and has a labor productivity factor of 1.3 . To provide energy for this process, hydroelectric power from a plant located approximately 200 km away from the proposed location will be used, allowing for a more sustainable energy source at a stable price point. Five desalination plants already exist in the country; all located on the western coast, in suburban areas as shown in Appendix B. A plant located on the eastern side, specifically in Amanzimtoti, near the large metropolis of Durban, would supplement these existing facilities by increasing the amount of available drinking water and provide an economic boost to an area that has one of the largest percentages of agricultural households in the country.
Preliminary Market Analysis and Process Selection
There are currently five large scale water desalination plants in South Africa, run by Veolia Water Technologies, as shown in Appendix B . Similar to 80% of desalination plants in the world, South Africa largely utilizes reverse osmosis technology. The 20% of facilities that utilize thermal desalination account for 50% of all water desalinated, but these plants are largely concentrated in the Gulf States due to the high salt concentration of the sea water present and the abundance of cheap fuel. Reverse osmosis technology continues to improve, increase in efficiency, and recently gained market popularity as a desalination technology . Furthermore, the salt concentration in South Africa is average, so multi-stage flash distillation is not required to reach the required drinking water purity. Therefore, reverse osmosis has been selected for this project because at lower inlet salt concentrations, it is more economically viable.
Process Feed and Composition
The salinity of the ocean water around the coast of South Africa is approximately 35 ppt, at 35 g of salt per every 1000 g of water . The seawater inlet of the proposed desalination plant will have the chemical composition shown in Appendix C . In addition to inorganic species in seawater, the inlet will also include heterotrophic bacteria at a concentration of 2.01*109 cells/L  and viruses at a concentration of 50*109 viruses/L . Typical limits of total dissolved solids (TDS) are at 1 g/L for drinking water . The products of this desalination process will be lower than this limit, at approximately 0.75 g/L. Individual mineral components must be lower than the upper limits shown in Appendix C to prevent negative health effects . Based on the population of the greater Durban area, the total water intake of the city is estimated to be 2.9 million m3 of water per day . To satisfy the drinking needs of 3.5% of this population, which is the typical output of a desalination plant, the goal of this process is to produce at least 100,000 m3 of freshwater per day.
Pretreating seawater is important to prevent particle, colloidal, organic, mineral, oxidant, and biological fouling. Particle fouling is caused by sand, clay, and suspended solids in seawater. This can be avoided by using filtration. Colloidal and organic fouling can be avoided by using a combination of coagulation and filtration, although the preferred method is ultrafiltration. To remove any final particulates, nanofiltration (5-micron) is required before the feed water enters the RO membranes . A summary of these fouling types and methods to prevent them can be seen in Appendix D.
Biological fouling is caused by bacteria, viruses, microorganisms, and protozoa, which contribute to biofilm formation on the membrane surface. Chlorination during pretreatment kills these microorganisms to prevent biofouling on the membranes. Existing plants typically use sodium hypochlorite injections of 3 mg/L to achieve this. Other options for chlorination include the use of chlorine gas or chlorine dioxide, but sodium hypochlorite is preferred at this stage because it has the lowest operating cost. After killing bacteria, it is necessary to dechlorinate the water because chlorine oxidizes the membranes in the RO process. One possible method for dechlorination is the injection of sodium bisulfate, which results in harmful byproducts, including hydrochloric acid. To prevent this, dechlorination can be achieved using an activated carbon filter, which has been selected for this process. These filters have a very high probability of adsorbing chlorine, making them the optimal choice for dechlorination .
Water treatment processes use several types of membranes, including microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. Microfiltration membranes have the largest pores, followed by ultrafiltration, nanofiltration and finally reverse osmosis . RO membranes filter out monovalent and multivalent ions, viruses, bacteria, and suspended solids . This makes RO membranes the ideal choice for the process, especially if the water is to be consumed.
RO membranes are either made of cellulose acetate or polysulfone coated with aromatic polyamides . Cellulose-based membranes have NaCl rejection values of up to 99.5% at 2000 psig and are often made from acetylated cellulose. Thin Film Composite (TFC) membranes are based on aromatic polyamides, and their characteristics surpass those of acetylated cellulose membranes. They have an average rejection of 99.5% at only 225 psig of feed pressure, making the process more economical and energy efficient. The membranes are made from a highly permeable support of polysulfone and are coated with a cross-linked aromatic polyamide thin film, which allows for the high salt rejection percentage .
There are four types of modules for membrane arrangements: plate-and-frame, tubular, spiral wound, and hollow fiber. An illustration of these types can be seen in Appendix E. The plate-and-frame module is the most simple, containing two end plates, a flat membrane, and spacers. The tubular module is an annulus with the membrane lining the walls and solution flowing through the tube. The spiral wound option has a flat membrane in the form of a sheet that is wrapped around a perforated collection tube. Finally, hollow fiber modules are bundles of fibers in a pressure vessel .
TFC membranes in the spiral wound arrangement have been chosen for this process. TFC membranes reject small organics that could pass through cellulose-based membranes, withstand a larger pH range and can be used at higher temperatures. The spiral wound arrangement was chosen because it has a large surface area and is easier to clean. Hollow fiber membranes provide a higher surface area, but they also incur high maintenance and replacement costs.
There are two main methods for energy recovery: direct recovery of mechanical energy, and conversion to electricity. Direct recovery of the mechanical energy from the high pressure concentrate is the most common method of energy recovery . The devices used to accomplish this can have up to a 98% efficiency. With this type of energy recovery, the high pressure from the concentrate can only be used to pressurize another stream of water. Alternatively, conversion of energy of the high pressure concentrate into electrical energy is much more versatile, but much less efficient. While energy capture from water can be as high as 95% as in hydroelectric power plants, the devices using that energy will likely have a much lower efficiency . For example, a typical pump has an efficiency from 40% to 60%, resulting in much higher overall energy loss. Therefore, for the proposed design, since versatility is not as big of a concern as energy conservation, a direct recovery system will be used for the mechanical energy from the high pressure concentrate to pressurize the filtered water for reverse osmosis.
Reverse osmosis membranes cannot selectively remove certain species. Since typical drinking water contains some salts, it is necessary to reintroduce them before water distribution and use. There are four major methods used for water remineralization after reverse osmosis. These methods utilize a mixture of three smaller processes: mixing with saltwater, mixing with salts or gases, and percolation with salt-based filters. Typically, percolation yields the best water quality, but also requires the largest investment and has the highest operating costs . Method overviews are given in Appendix F. All of the methods commonly used bring the water to an acceptable range for drinking water in South Africa. For the proposed design, the process of mixing the reverse osmosis product with brine will be used. While this slightly produces lower water quality than its alternatives, the process still meets all federal guidelines for drinking water and chlorine concentration will be kept below 100 mg/L to prevent corrosion.
Although the product stream should not contain any bacterial or viral materials due to the effectiveness of the filtration and RO steps, disinfection is typically done as a precaution. This ensures that water quality is maintained despite any upstream issues with membrane integrity. Chlorine gas, sodium hypochlorite, ozone, and UV light are options for disinfection . UV treatment and ozone are environmentally friendly and have no chemical byproducts, but the costs tend to be much higher. Chlorine gas and sodium hypochlorite have low capital investment and operating costs, but the byproducts can be difficult to manage and are not environmentally safe. Chlorine dioxide treatment is highly effective, requires very low capital investment, and produces byproducts that are easier to manage. While the operating costs are slightly higher than that of chlorine gas and sodium hypochlorite treatment, chlorine dioxide treatment has the best combination of environmental friendliness, overall investment, and effectivity .
An essential factor of the proposed reverse osmosis plant is waste treatment. Outlet streams of desalination plants include hyper-saline solutions at densities higher than circulating ocean water. This difference in density will cause the brine to settle to the ocean floor, potentially resulting in negative effects on the fauna and flora . Conventional brine disposal methods include surface water and sewer discharge, deep well injection, and evaporation ponds .
Surface water discharge directly disposes hyper-saline solutions to surface water. This method has low capital, operating, and manufacturing costs. Although South Africa has less stringent policies on waste eliminations into surface water, ethical considerations should still be taken into account to eliminate negative environmental effects. Sewer discharge directly disposes brine outlet solutions into the sanitary sewer system. This method is cost effective when there are sewer systems nearby the plant. However, this is not the case for our location . Deep well injection disposes brine into porous subsurface rock formations. While this method may be more environmentally friendly, it requires extremely high capital costs. Site evaluation would require time and money, and an extensive scale up analysis would be required to ensure that this method would be feasible. Evaporation ponds utilize solar energy to reduce the water content in brine solutions. This method incurs low costs and is reliable, as few pieces of equipment are required. However, depending on the amount of brine removed from the plant, large amounts of land may be needed to contain the effluents.
The most common technique for brine disposal used by water desalination plants is to discharge back into the ocean . This is legal in South Africa as evidenced by the largest desalination plant in the country at Mossel Bay . According to regulations, a permit must be acquired to discharge large amounts of waste into the ocean , but the precedent from Mossel Bay demonstrates that South Africa will grant such a permit to a water desalination plant. In addition, research of nearby protected marine environments was conducted, and it was concluded that the proposed plant location near Amanzimtoti is at least 25 km from the nearest Marine Protected Area . Therefore, direct brine disposal is the chosen alternative for the process.
Process Flow Diagram
Mass and Energy Balances
Equipment Sizing and Trade-offs
Sensitivity Analysis and Optimization