THE ECONOMICS OF SEAWATER REVERSE OSMOSIS DESALINATION PROJECTS. Abstract

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1 THE ECONOMICS OF SEAWATER REVERSE OSMOSIS DESALINATION PROJECTS Mark Wilf Ph.D., Tetra Tech, Von Karman Ave.# 400, Irvine, CA 92614, phone , Steve Tedesco, Tetra Tech, Irvine, CA Jill Hudkins, Tetra Tech, Orlando, Fl Abstract Desalination applications based on reverse osmosis (RO) technology today comprise over 50% of the capacity of all desalination systems worldwide and represent 75-85% of new desalination projects being implemented. The major reason for the shift in desalination projects to RO technology is the high energy efficiency of the RO process. There are three major application categories of large capacity, RO-based desalination projects: brackish RO; advanced municipal wastewater reclamation; and seawater RO. In the two first categories (brackish RO and wastewater reclamation), the systems configuration and equipment components are well defined. Therefore, project costs and operating expenses are fairly predictable. In seawater RO desalination systems, the RO process configuration is also very similar; however, some variability exists regarding the configuration of seawater water delivery and feed water pretreatment. The rest of the system s components and system operation methods are very uniform. However, an evaluation of published cost data of medium- to large-scale water RO desalination projects illustrates significant variability in costs of desalination systems. The variability in the desalination systems process is especially noticeable among RO seawater desalination systems built at different geographic locations. In some cases, the difference in product water cost produced by plants of a similar capacity but located in different countries can exceed 100%. The existence of such a great difference is very noticeable, considering that the desalination process, plant configuration and construction materials of seawater RO (SWRO) equipment are quite similar for the majority of plants. The critical components of RO desalination plants membrane elements and pressure vessels are standardized with respect to configuration, dimensions and capacity, and have similar market prices worldwide. All new SWRO desalination systems utilize similar types of high-pressure pumps and energy recovery devices, which brings energy usage to a similar range of 3-4 KWh/m 3. This suggests that the difference in economics of SWRO desalination projects is the result of localized factors such as local labor rates, cost of construction materials and sitespecific environmental regulations. To overcome issues of high local project cost components, it has become more common to construct the major components of a desalination system at locations having low labor rates and bring the assembled subsystem (via maritime shipment) to the desalination plant site for the final integration. An additional approach for reducing product water cost is to optimize the RO membrane unit configuration through the selection of membrane elements. The objective is to reduce feed pressure while still producing the required permeate quality. These efforts are leading to a shift in membrane element selection to the utilization of higher permeability membranes and applying a partial two-pass system configuration as required. This approach is encountering limitations inherent to high permeability membranes: unacceptably high permeate

2 Technology distribution, % salinity and/or concentrate pressure converging to the level of osmotic pressure of the concentrate, i.e., possible operation at negligible net driving pressure (NDP) at the concentrate end of the RO membrane unit. Therefore, an additional reduction in the power requirement of the RO process through a reduction in feed pressure is not very likely. Still, very high permeability SWRO membrane elements can be utilized to reduce the cost of the RO system while producing permeate of acceptable quality. In this system design approach, the very high permeability membrane elements will be utilized for operation at elevated permeate flux and enable a reduction in size of RO membrane units. This paper will analyze current economic conditions of seawater desalination, and highlight the limitations and possibilities of additional improvements of the economics of the SWRO desalination process. Energy requirement of seawater desalination processes Desalination processes Among commercial desalination technologies, reverse osmosis is the most energy efficient method of producing potable quality water. The difference in energy requirements between evaporation processes and RO-based desalination is so significant that the majority of new systems being constructed utilize RO technology and over 50% of desalination plants worldwide use the RO process. The preference toward reverse osmosis process is illustrated in Figure 1, showing distribution of new seawater desalination projects since year Figure 1. Distribution of contracted desalination projects according to desalination process 100 Contracted desalination capacity Membrane Thermal

3 The process energy cost is usually the highest components of the operating cost in seawater desalination processes. Figure 2 presents the different energy consumptions used for different commercial desalination processes. Among the pressure driven, membrane based, desalination processes, brackish water needs the lowest energy of all processes, yet the amount of brackish water available for desalination is limited and cannot be implemented in some locations due to the limitation of concentrate disposal. The reclamation of municipal wastewater for indirect potable use is steadily growing. Wastewater for advanced reclamation is available close to all large urban centers. However, indirect potable use requires specific configuration of underground aquifers, which are not available in all locations. Direct potable use of reclaimed wastewater faces significant problems of public acceptance. There is no limitation in using seawater for desalination, however, for acceptable economics and practical convenience of seawater supply and concentrate disposal, SWRO plants must be located at a short distance from the seashore. Three major RO-based desalination processes include: Seawater RO (SWRO) Brackish water RO (BWRO) Wastewater reclamation utilizing RO (WWRO) The energy requirement of SWRO processes, without the energy required for pumping product water, is below 3 KWh/m 3. Such low energy requirement is being achieved due to utilization of isobaric energy recovery devices that have high hydraulic efficiency, at about 96%. The total energy requirement of the SWRO desalination is usually in the range of 3 4 kwhr/m3 ( kwhr/kgallon). The energy requirements of BWRO and WWRO processes are similar for the RO section of the process, about 0.5 KWh/m 3. Figure 2: Energy requirements of various commercial desalination processes. 10 Specific energy, kwhr/m Electricity Steam 0 MSF MED VC SWRO BWRO WWRO

4 The additional energy requirement of the WWRO process, as shown in Figure 3, is for the membrane filtration pretreatment process and the advanced oxidation process (AOP). Figure 3: Energy requirements of desalination processes based on RO technology. 3.0 Specific energy, kwhr/m Aux. RO 0.0 SWRO BWRO WWRO The energy requirement of the SWRO process may increase by up to about 1 KWh/m 3 if longdistance pumping of product water is needed. Additional energy for the BWRO process is required if the recovery of the RO system is relatively low (due to the high scaling potential of RO concentrate), and the concentrated brine must be pumped a long distance for disposal. This is usually not the case for WWRO since WWRO plants are located close to wastewater treatment plants where RO concentrate is returned for disposal with the wastewater treatment plant outfall. Effect of permeability of RO membranes Energy usage in the RO desalination process is a function of feed water salinity, recovery rate, type of the pretreatment process, efficiency of high pressure pumping and energy recovery equipment, permeability of the RO membranes and the pressure required for product water distribution. In SWRO, where the energy usage required by high-pressure feed pumps is the highest, the lower limit of feed pressure is dictated by the osmotic pressure of the concentrate stream. The applied feed pressure must be high enough to provide some driving pressure at the concentrate exit from the SWRO membrane unit. Figure 4 illustrates the changes in osmotic and operation pressure along eight membranes elements. The net driving force is the difference between operating pressure and the feed concentrate osmotic pressure, continuously increasing along the RO membrane unit.

5 For the given site conditions in respect of feed water salinity and annual fluctuation of seawater temperature, the required feed pressure will depend on the permeability of RO membranes. All major membrane manufacturers offer RO membrane for seawater application in three ranges of permeability, as summarized in Table 1. The elements listed are of the same membrane area of 37.2 m2 (400 ft2) Figure 4. Distribution of feed pressure components in seawater RO desalination process The membrane elements listed have progressively higher nominal water permeate capacity but the same nominal rejection rate of 99.8%. Table 1. Offering of commercial RO seawater membrane elements (1) Manufacturer Membrane type Nominal flow, gpd (m3/d) Nominal Rejection Manuafacturer1 High salt rejection 6,500 (24.6) Manuafacturer1 High water permeability 9,000 (34.0) Manuafacturer1 Very high water permeability 13,200 (50.0) Manuafacturer2 High rejection 6,000 (22.7) Manuafacturer2 High salt rejection 8,200 (31.0) Manuafacturer2 High water permeability 11,000 (41.6) Manuafacturer3 Very high water permeability 6,500 (24.6) Manuafacturer3 High salt rejection 9,000 (34.0) Manuafacturer3 High water permeability 13,500 (51.0) Manufacturer 4 High salt rejection 6,500 (24.6) Manufacturer 4 Very high water permeability 13,700 (51.8) 99.80

6 Salt transport, cm/sec Intuitively, the most suitable membrane for a seawater desalination plant would be a membrane type of highest nominal permeate flow. However, as the nominal performance are determine at the same test conditions of feed salinity of 32,000 mg/l and feed pressure of 55.2 bar (800 psi), it became evident that increasing permeate production is being achieved by making the membrane more permeable to the water and also having higher passage of dissolved salts. The relation of between water and salt transport of commercial membrane is illustrated on Figure 5. Figure 5. Water and salt transport of commercial RO membranes. 2.0E E E-07 Seawateter elements Brackish elements 5.0E-08 y = 1.24E-08x E E Water permeability, l/m 2 /hr/bar As shown in Figure 5, the salt transport water permeability has almost linear relation in the whole range of commercial offering of RO membranes, seawater and brackish type. The interrelated transport property of RO membranes (higher water permeability is associated with higher salt transport) has direct effect on permeate salinity of product water and passage of some critical constituents, such as boron and bromide. An example of required feed pressure, permeate salinity and permeate boron concentration, desalting Atlantic/Pacific seawater utilizing the above mentioned three commercial types of SWRO membrane elements, is summarized in Table 2. Table 2. Projected feed pressure and permeate salinity in SWRO plant desalting Atlantic/Pacific seawater at recovery rate of 50%. Seawater temperature 25 C Seawater RO membrane type Feed pressure, bar (psi) Permeate salinity, mg/l Permeate boron concentration, mg/l High salt rejection 59.8 (867) High water permeability 55.1 (800) Very high water permeability 51.6 (748)

7 The membrane manufacturers usually add about 15% to the projected permeate salinity as a safety margin. Some small additional margin is usually added by system designers. During the post treatment, addition of calcium and alkalinity could increase the product water salinity by close to 120 mg/l. Therefore, for SWRO desalination projects with product water salinity limit of 400 mg/l, SWRO plants that plan to utilize the high permeability membranes would have to be configured as a partial two pass systems. Except for locations of consistently very low seawater temperature, very high water permeability membrane elements type can not be used, as the resulting salinity of product water could exceed the water quality limits. Currently, the membrane type being utilized in SWRO plants, processing Atlantic, Pacific or Mediterranean seawater, are of high water permeability type, in partial two pass desalination process configuration. Configurations of SWRO desalination process The large SWRO plants have quite uniform process configuration and equipment with the exception of the pretreatment process. At location of consistent good to average seawater quality, the pretreatment system usually consists of a single step filtration, schematically shown on Figure 6. Figure 6. Process flow diagram of a SWRO plant utilizing single stage filtration pretreatment (2) OCl A C P Media filters or membrane filtration units Intake SBS 4 Filtrate clearwell 5 6 CF 23 Solids management & dewatering 24 RO Pass 1 15 OCl AM CA Potable water storage Calcite reactor CO2 16 Permeate tank ERD OCl Hypochlorite SBS Sodium bisulfite A Acid CO2 Carbon dioxide C P Coagulant Polymer AM CA Ammonia Caustic Neutralization tank CIP tank CF Membrane cleaning unit

8 At locations of where seawater seasonally has elevated concentration of suspended particle, high seawater turbidity, clarification step is placed upstream of the filtration units. If periods of poor quality of seawater is related mainly to algae bloom, general assumption is that a dissolved air flotation step for preliminary clarification, is a more effective solution. This configuration is shown schematically on Figure 7. Some seawater plants with DAF as a preliminary clarification step have been built and operated in Singapore and Middle East locations. However, experience regarding effectiveness of DAF for algae removal in SWRO process is so far not conclusive. The filtration step can be either a multimedia filtration (gravity or pressure filters) or membrane filtration. Compared to multimedia filtration, membrane filtration represents a efficient barrier that will reject all particulate matter, presented in the seawater feed. Also, usage of pretreatment chemicals (coagulant and polymer) will be significantly lower than required for the effective operation of the multimedia filtration step. The footprint of membrane filtration system will be significantly smaller than the multimedia filtration system of the same filtration capacity. However, the membrane filtration process is more expensive and has higher energy usage than the conventional pretreatment process. Also, operation of membrane pretreatment in SWRO plant, frequently results in biofouling of RO membranes at higher extend than it is experienced with the conventional pretreatment. The consensus is that has a better rate of reduction of nutrients is seawater feed than the membrane filtration process. Operation of membrane filtration, conducted at higher differential pressure, results in damage of algae and some bacterial cells, creating organic matter that is poorly rejected by the MF/UF filtration membranes. This organic matter reaches the RO membrane elements, increasing concentration of nutrients and stimulating bacterial activity inside RO membrane elements. Figure 7. Process flow diagram of a SWRO plant utilizing dissolved air flotation and filtration pretreatment (2) Intake 1 OCl 2 A C P 3 DAF Media filters or membrane filtration units 4 Filtrate clearwell 5 6 SBS CF 28 Solids management & dewatering 29 RO Pass 1 27 OCl A C P 26 Hypochlorite Acid Coagulant Polymer Potable water storage 25 OCl AM SBS Sodium bisulfite CO2 Carbon dioxide AM CA Ammonia Caustic CA 24 Calcite reactor CO2 21 RO Pass 2 Permeate 19 tank Neutralization tank ERD CIP tank CF Membrane cleaning unit

9 The comparison of economic and operational characteristic of these two alternatives of feed water pretreatment utilized in SWRO desalination plants is summarized in Tables 3 5. Table 3. Comparison of major components of pretreatment processes based on multimedia and membrane filtration technologies Selected equipment Conventional pretreatment Membrane filtration pretreatment Microstrainers No Yes Flocculators Yes Yes/No (1) Filtration media Anthracite + sand Membranes Filtration basins 100% 50 60% Solids handling Yes Yes/No (1, 2) Cartridge filters Yes Yes/No (3) Footprint 100% 50 60% (1) Some MF systems operate without use of coagulants (2) According to (1) no solids handling is required (3) System operating without filtrate storage tank may operate without cartridge filters Table 4. Major cost components of alternative pretreatment process for SWRO plant of permeate capacity 100,000 m3/d (26.4 mgd) Cost component Media filtration Membrane filtration System cost, $M System footprint, m2 ~4,000 ~2,000 Annual power usage, kwhr ~1,350,000 ~4,800,000 Power usage, kwhr/m3 RO permeate Annual chemicals cost, $ ~350,000 Coagulant + polymer Annual chemicals cost, $ ~100,000 NaOCl, SBS, NaOH, Citric acid Annual sand replenishment, $ 15,000 Annual membrane replacement, $ 550,000 Other annual maintenance, $ 170, ,000

10 Table 5. Operating cost components of conventional and membrane filtration technologies Selected components of operating cost Conventional pretreatment Membrane filtration pretreatment Additional energy use kwhr/m3 Chemicals usage % Membrane replacement c/m3 Equipment maintenance cost + 100% Required level of operational Average High & maintenance skills Main advantages Proven pretreatment technology Absolute barrier to colloidal particles Major disadvantages Possibility of coagulant overdose Potential for biofouling of RO membranes Possibility of changing filtration technologies Possible Not practical Product water cost in SWRO desalination projects The water cost produced in SWRO plants shows a clear trend of price reduction. The driving force for this price reduction was a reduction in energy requirements through the introduction of efficient energy recovery devices, the introduction of higher permeability, and high rejection RO membranes that enable the operation of a SWRO plant at higher recovery rates and reduced feed pressure. An additional improvement in the process and equipment was gradually introduced by EPCs as competition increased with project implementation moving to Design-Build-Operate (DBO) projects delivery. The representative information on product water cost in recent SWRO projects is provided in Figure 8. This trend of price reduction has been reversed recently. In Europe, the Middle East and the Pacific Rim, the water process has remained low. In Australia and California, the water prices from SWRO plants are much higher than from SWRO plants built recently at other locations. The SWRO plant in Australia and the plant that recently started operation in California are configured in a similar manner and utilize very similar major equipment and membrane elements as the SWRO plants built recently in Singapore, Israel and North Africa. The process parameters are similar, resulting in similar plant operating expenses. The major difference is related to much higher project costs at these two locations. The high project cost can be attributed to the very lengthy project permit process, need to comply with strict environmental regulations, and higher

11 labor cost. There is no indication that this differential of significantly higher cost of development of desalination projects in US will be reduced in the near future. Summary RO-based desalination is a very reliable technology for augmenting potable water supply. The cost of major equipment and process consumables is similar worldwide; however, product water cost for the same application depends on the geographic/political location of the desalination plant. The variability of product water cost between various locations is the result of regulations, the permit process and local labor costs. The desalination process will continue to improve and some reduction in water cost can be expected. No major breakthroughs can be expected in the near future Figure 8. Water prices from recent large SWRO projects (3). Initial water price $/m Large seawater RO systems - water price Contract year

12 Figure 9: Water prices trend in large SWRO projects (3). Initial water price $/m Large seawater RO systems - water price Contract year References 1. Compilation of RO membrane elements performance data published by major manufacturers of RO membrane elements 2. Wilf, M. The Guidebook to Membrane Desalination Technology. Reverse Osmosis, Nanofiltration and Hybrid Systems. Process, Design and Applications, with chapters by C. Bartels, L. Awerbuch, M. Mickley, G. Pearce and N. Voutchkov, Balaban Desalination Publications (2006). 3. Compilation of cost information from published tenders of water desalination and wastewater reclamation projects.