Optimal design of hybrid RO/MSF desalination plants Part II: Results and discussion

This paper presents the results of an optimization study, based on minimum water cost, to explore the feasibility of the hybridization of RO and MSF processes. The study explores the possible improvement of MSF process economics. Nine different scenarios for the production of the same capacity of desalted water are presented and compared from the standpoint of minimum water cost, specific capital cost and water recovery. The process and cost models, formulation of the optimization problem and solution outlines were previously presented in the first part of this study. In this work, results show that RO technology is recommended when building new desalination plants. RO technology becomes preferable at low feed concentrations and for brackish water desalination. Although they come in second position after the RO process, some hybrid plants economically exceed by far the MSF process. Computations gave a water cost of 1.1 $/m³ for the brine recycle MSF process against 0.75 $/m³ for the two-stage RO process. Water cost of the MSF process can be reduced by 17 to 24% through hybridization with RO technology.     

Economical study of a small-scale direct contact humidification-dehumidification desalination plant

Economical study of humidification-dehumidification desalination (HDD) pilot plant was made in order to estimate the economic benefits of the process in comparison with a small-scale reverse osmosis (RO) system. The energy recovery of the unit was investigated to be 75%. Some theoretical modifications were made to the HDD system in order to reduce the energy costs of the unit. Exact and clear economic analysis results were obtained using COMFAR ΙΙΙ software including fixed investment costs, production costs, internal rate of return on investment, operating costs, energy costs and some other economical parameters. Based on the energy prices in Iran, the total fresh water production cost was calculated to be $ 6.4/m³ that was nearly the same as produced by an imported RO plant. Finally some points were recommended whether to choose HDD or RO plant in different cases and capacities.     

Distillation vs. membrane filtration: overview of process evolutions in seawater desalination

The worldwide need for fresh water requires more and more plants for the treatment of non-conventional water sources. During the last decades, seawater has become an important source of fresh water in many arid regions. The traditional desalination processes [reverse osmosis (RO), multi stage flash (MSF), multi effect distillation (MED), electrodialysis (ED)] have evoluated to reliable and established processes; current research focuses on process improvements in view of a lower cost and a more environmentally friendly operation. This paper provides an overview of recent process improvements in seawater desalination using RO, MSF, MED and ED. Important topics that are discussed include the use of alternative energy sources (wind energy, solar energy, nuclear energy) for RO or distillation processes, and the impact of the different desalination process on the environment; the implementation of hybrid processes in seawater desalination; pretreatment of desalination plants by pressure driven membrane processes (microfiltration, ultrafiltration and nanofiltration) compared to chemical pretreatment; new materials to prevent corrosion in distillation processes; and the prevention of fouling in reverse osmosis units. These improvements contribute to the cost effectiveness of the desalination process, and ensure a sustainable production of drinking water on long terms in regions with limited reserves of fresh water.     

Optimal design of hybrid RO/MSF desalination plants Part III: Sensitivity analysis

This is the last paper in a series of three parts entitled “Optimal design of hybrid RO/MSF desalination plants”. This research is concerned with exploring the feasibility of hybridization of multi-stage flash (MSF) and reverse osmosis (RO) technologies in order to improve the performance characteristics and process economics ofthe conventional MSF process. The research project involved an optimization study where the water cost perunit product is minimized subject to a number of constraints. In the first part, the design and cost models were presented, the optimization problem formulated and solutions for a number of cases were outlined. In the second part, results were presented and discussed. In this paper we discuss the sensitivity of water cost from the alternative plant designs to variations in some cost elements and operating conditions. In general, it is concluded that, for the same desalting capacity, hybrid RO/MSF plants can produce desalted water at a lower cost than brine recycle MSF plants, while hybrid plants are characterized, by lower specific capital costs and higher water recovery fractions. Reduction in steam cost allows MSF to compete more with hybrid RO/MSF plants. This result explains the advantage of coupling MSF plants and steam power plants where the exhaust steam from the back pressure turbine represents a relatively cheaper source of heat for the MSF process. Results showed that the RO technology exceeds all other designs over the whole range of energy, chemicals and membrane costs studied here. However, water cost of the RO process was the most sensitive to variations in membrane and electricity costs compared to other hybrid configurations.     

Performance study of water desalination methods in Saudi Arabia

Saudi Arabia is an arid desert country without rivers or sweet water lakes, however, it does have vast amount of groundwater and seawater. In order to make these waters suitable for human consumption and industrial use, most of their salts must be removed by some means. The desalination methods most frequently used in the Kingdom are: Multistage flash (MSF) evaporation, reverse osmosis (RO), and electrodialysis (ED).During the last decade, we have witnessed a spectacular growth of desalination plants. This growth is expected to continue in the next decade. The present production capacity of all desalting plants in Saudi Arabia is estimated to be 750,000 m³/day; this figure will be most likely tripled in the next five years.This paper is a report on a performance study of the most significant desalination plants in the Kingdom. The plants, which include MSF and seawater and brackish water RO plants, were selected either because of their size or their importance to the desalination technology. The plants are briefly described and their performances are discussed.     

Selection of small desalination plants for specific locations — reverse osmosis,flash distillation,vapour compression

Selection criteria are given for small desalination plants (2000–3000 m³/d) for specific locations using RO, MSF or mechanical VC processes. Of prime economic importance to customers, consultants and manufacturers are peripheral location conditions and operating suitability. Choice by the manufacturers is especially complicated due to the fact that the decision for small plants is left to the bidder. However, based on knowledge of local conditions, manufacturers can anticipate costs.Examples given show that under the assumed conditions the MSF process cannot possibly be the most cost-effective. This applies also to RO when the required product is high-purity process water instead of drinking water. For drinking water any choice between MVC and RO is dependent on energy costs and the capitalization factor.     

Integrating hybrid systems with existing thermal desalinationplants

In Gulf countries, most power plants are co-generation power desalting plants (CPDP) that generate electric energy and also produce fresh water through the desalination of seawater. This paper provides detailed technical and economical analyses to evaluate a new generation of dual purpose technology that includes the integration of reverse osmosis (RO) processes with existing thermal desalination processes and power generation (triple hybrid system) at Layyah plant, Sharjah, UAE. Hybridization of sweater reverse osmosis (SWRO) and the multi-stage flash (MSF) technology was considered to improve the performance of latter and reduce the cost of the produced water. Moreover, “idle” power in winter (seasonal surplus of unused power) was mainly utilized by RO to further reduce the cost of the hybrid system for six months of the year. Spinning reserve was also used to further reduce the cost of the proposed hybrid system. Integration ofthe three processes of MSF, MED, and RO desalination technologies could be made at different levels through which the resulting of water cost will depend on the selected configuration and the cost of materials of construction, equipment, membrane, energy, etc. Thus, the capital and annual operating costs were calculated for all potential alternatives for various plant capacities. It was found that for all plant capacities, integrated hybrid systems resulted in most cost effective solution. For example, at a capacity of 50 MIGD, the present worth of the cost was calculated to be 588.7, 443.2, and 380 million US$ for MSF, MED, and hybrid RO systems, respectively.     

脱盐过程的比较和选择

本文简要介绍世界上三种主要海水淡化方法一多级闪蒸、低温多效蒸馏和反渗透的原理、特点和方法选择。将这三种方法进行比较并结合实践对选择海水淡化方法的依据进行探讨。预计21世纪的海水淡化市场会有更大发展。     

用海水淡化法解决海岛用水

我国多数海岛属于资源性缺水,解决用水困难的途径有两条,海底管道从大陆调水和海水淡化就地制水。文章详细分析了影响反渗透海水淡化成本的诸因素,对不同生产容量反渗透海水淡化工厂的投资及生产成本进行了预测。针对舟山某岛群的地理条件对海底管道引水和就地海水淡化制水进行了综合比较。最后提出解决该岛用水短缺的可行方案。     

考虑不同水质输入的水电联产优化设计

结合已有的反渗透/多级闪蒸混产系统和发电-多级闪蒸联产系统,设计了一个新的包括热力发电系统、反渗透海水淡化系统和多级闪蒸海水淡化系统的水电联产超结构,对以不同盐度苦咸水、海水为原水的水电联产系统进行了优化设计,通过求解系统以年费用最小为目标函数的非线性数学模型,得到不同盐度下联产系统优化的生产结构. 结果表明,在低盐度(≤25000 mg/L)下采用冷凝式发电和一级反渗透产水,高盐度下采用抽汽冷凝式发电和热膜混合产水,可降低联产系统的年费用,获得较低成本的淡化水. 在本工作所定的生产规模下,优化联产方案的年费用可降低23%~36%.     

Membrane processes for water supply and reuse - a question of energy consumption and cost

This paper is limited primarily to reverse osmosis which is the dominating membrane process in commercial plants. Desalination of brackish water and seawater with reverse osmosis, with special emphasis on costs and energy consumption, is the primary subject discussed in the paper. Some aspects of and development trends in industrial and domestic applications of membrane processes are also taken up, particularly with regard to by-product recovery and water reuse in connection with advanced wastewater treatment.The first RO plant to be brought into operation in Riyadh, Saudi Arabia, is located at Salbukh. The investment and total operation costs for this plant have been calculated in the paper. The water cost is at least twice as high as in a continental U.S. location. The main reason for this is the very high cost of civil and local works in Saudi Arabia. A similar calculation has been made for RO seawater desalination.Increased energy costs during the last decade have directed research and development work for all desalination methods towards reducing energy consumption. It is shown in the paper that energy recovery in connection with RO seawater desalination is particularly feasible. Different methods for energy recovery have been investigated and reported, the preferred methods depending on the size of the RO plant. A large underground RO plant for energy recovery, based on utilization of the static pressure instead of high pressure pumps, has also been studied.Another possible energy-saving, but also water quality improving method has been proposed, viz . a combined MSF-RO dual purpose plant. Excess power for reverse osmosis seems to be more and more available in Saudi Arabia due to the high power/water ratio in MSF dual purpose plants compared to the real demand for power and water.     

Characterization of uncertainties in the operation and economics of the proposed seawater desalination plant in the Gaza Strip

In the Gaza Strip, the available freshwater sources are severely polluted and overused. Desalination of seawater through reverse osmosis (RO) has become the most realistic option to meet a rapidly growing water demand. It is estimated that the Gaza Strip will need to develop a seawater desalination capacity of about 120,000 m³/d by the year 2008, and an additional 30,000 m³/d by the year 2016 in order to maintain a fresh water balance in the coastal aquifer and to fulfill the water demand for different uses in a sustainable manner. Cost and reliability of a large RO facility are still subject to much uncertainty. The cost of seawater desalination by RO systems varies with facility size and lifetime, financing conditions, intake type and pre-treatment requirements, power requirements, recovery rate, chemicals cost, spare parts cost, and membrane replacement cost. The permeate salinity is a function of feed water temperature, recovery rate, and permeate flux. The quantity of water produced depends mainly on plant size, recovery rate, and operating load factor. Many of these parameters are subject to a great deal of uncertainty. The objective of this work is to develop a probabilistic model for the simulation of seawater reverse osmosis processes using a Bayesian belief network (BBN) approach. This model represents a new application of probabilistic modeling tools to a large-scale complex system. The model is used to: (1) characterize the different uncertainties involved in the RO process; (2) optimize the RO process reliability and cost; and (3) study how uncertainty in unit capital cost, unit operation and maintenance (O&M) cost, and permeate quality is related to different input variables. The model utilizes information from journal articles, books, expert opinions, and technical reports related to the study area, and can be used to support operators and decision makers in the design of RO systems and formulation of operational policies. The structure of the model is not specific to the Gaza Strip and can be easily populated with data from any large-scale RO plant in any part of the world.     

Nitrate removal with reverse osmosis in a rural area in South Africa

The nitrate-nitrogen concentration (>6 mg/l) and the salinity (>1000 mg/l TDS) of many borehole waters in rural areas in South Africa are too high for human consumption. Therefore, an urgent need for water denitrification and water desalination exists in these areas. Reverse osmosis (RO), electrodialysis (ED), ion-exchange (IX) and certain biological technologies can be very effectively applied for water denitrification. Each of these technologies, however, has its own advantages and disadvantages. Reverse osmosis technology, however, has been selected for this study because the technology is well known in South Africa and because it can be very effectively applied for water desalination. The objectives of this study were: (a) to transfer RO technology through process demonstration performance for water denitrification and water desalination to people living in rural areas; (b) to build capacity regarding the operation and maintenance of an RO application in a rural area; (c) to produce a preliminary operational and maintenance manual for the operation of an RO unit in a rural environment; (d) to train local operators to operate and maintain an RO plant in a rural environment; (e) to evaluate stock watering as brine disposal option; and (f) to determine the preliminary economics ofthe process. The following conclusions were drawn. It was demonstratedthatthe RO process could be very effectively applied for water denitrification and water desalination in a rural area. Nitrate-nitrogen was reduced from 42.5 mg/l in the RO feed to only 0.9 mg/l in the RO product water. The TDS of the RO feed was reduced from 1292 mg/l to 24 mg/l in the RO permeate. Therefore, an excellent quality water could be produced for potable purposes. The RO brine at approximately 50% water recovery should be suitable for stock watering if the conditions for stock watering are met in terms of nitrate-nitrogen concentration, TDS and other constituent concentrations. The capital cost for an approximately 50 m³/d output RO plant is approximately US $29,900. Preliminary cost estimates have shown that the operational cost for water denitrification is approximately US $0.50/m³. This cost, however, should be significantly reduced by optimisation of the chemical dosing and by blending borehole water with RO product water.     

Design, simulation and economic analysis of a stand-alone reverse osmosis desalination unit powered by wind turbines and photovoltaics

The fresh water shortage is a significant problem in many areas of the world such as deserts, rural areas, Mediterranean countries and islands. However, renewable energy potential in these areas is usually high using solar and wind energy. A desalination unit powered by renewable energy sources is a promising solution for this problem. This paper presents the design of a stand-alone hybrid wind-PV system to power a seawater reverse osmosis desalination unit, with energy recovery using a simplified spreadsheet model. A daily and monthly simulation and economic analysis were also performed. The calculated fresh water production cost was 5.2 ?/m³, and the realized energy saving was up to 48% when a pressure-exchanger-type energy recovery unit is considered.     

Water desalination: An imperative measure for water security in Egypt

Water Desalination is an indispensable industry for the most of the Arab countries. In the last four decades, the number and capacities of desalination units have increased dramatically (45% Multi-Stage Flash (MSF) and 42% Reverse Osmosis (RO) of world capacity); especially in the Gulf States. Almost all available conventional water resources in Egypt - represented by the Nile water, renewable groundwater, and some scant annual precipitation- have been exhausted. Further development measures require review of current water allocations in order to raise efficiencies and protect against pollution, in addition to exploring new options of non-conventional water resources to narrow the gap between water supply and demand. These measures are the pillars of Egypt's integrated water policy and have been clearly postulated in its National Water Resources Plan 2017. The objective of this paper is to study and investigate water desalination as a solution for water scarcity in Egypt. Moreover, the present work demonstrates the significance of seawater desalination for national development in Egypt. At present, Egypt is encouraging, not only the public sector but also the private sector, to apply modern technologies for desalination, which historically started with Distillation then Electrodyalisis and followed by RO. The great achievements in desalination technology have now moved the costs for desalting in many applications from the realm of "expensive" to "competitive". Current technology is feasible for tourist villages in the north coast and the Red Sea, due to its far distances from conventional sources that makes the cost of water conveyance very high and subject to pollution problems. The results indicated that, in spite of research and developments, still the energy requirement and membrane know-how are limiting factors. Thus, Egypt's future vision is non-traditional in the field of desalination. It is based on a real breakthrough towards the use of renewable energy, namely, solar energy to be harnessed for operating high compression pumps needed for reverse osmosis modular systems. The reasons are obvious, since Egypt has great potential of brackish water wells, immense amounts of solar radiation in remote areas and future integrated development projects are located at a distance from the Nile water. This trend is what Egypt is focusing on as a prospective future for wide applications of desalination. Finally, this research concluded that, the water desalination as a conventional water resource should be considered as an imperative measure for water security in Egypt. The future use of such resource for different purposes will largely depend on the rate of improvement in the technologies used for desalination and the cost of needed power.     

热膜耦合海水淡化系统的优化设计

采用混合节点和分配节点的概念,建立了多级闪蒸(MSF)和反渗透(RO)海水淡化集成系统的超结构模型,以年度总费用最小为目标,引入产水比的概念,并将该参数作为集成系统的一个优化变量,采用改进的遗传算法进行求解,获得了集成系统的最优结构及相应的操作条件。实例结果表明:集成海水淡化系统的淡水成本比独立运行的RO和MSF低,产水比为0.45时集成系统的费用最小,流程结构为MSF-RO。     

The role of desalination in bridging the water gap in Jordan

The water resources of Jordan have always been scarce. The demand for water in the country exceeds the available resources and the gap between the water supply and demand is continuously increasing. Furthermore, the quality of water resources has dramatically deteriorated in recent years, which prevented the utilization of some resources. To secure the economic sustainability of the country, it is important at this stage to consider non-conventional water resources, such as sea and brackish water desalination. This study examines the water resources of Jordan, projected water demand, and the gap between the demand and supply based on two scenarios. The possibility of closing this gap by water desalination was investigated. Both sea and brackish water desalination were considered. The analysis revealed that seawater desalination process is economically infeasible for domestic water uses at this stage. This is attributed to that Jordan is not an oil producing country and the location of sea shoreline in Aqaba is on the Red Sea, away from the consumption centers. However, seawater desalination may be a viable option for tourist and industrial enterprises that will be established as a result of the new Law of Aqaba Free Zone. The most convenient and realistic option for Jordan in the short term, is the desalination of brackish water by reverse osmosis (RO). The brackish water with total dissolved solids between 2000-10000 mg/l is available from many aquifers and springs of the country. The study concluded that Jordan is facing a severe water crisis. Until recently, the water planners have not given the water desalination the required attention as a potential source of water supply. Taking into account the progress that is being achieved in the field of desalination technology, Jordan has to consider the desalination option more seriously and to start building a national capacity in the field of water desalination. It was emphasized that desalination is not a substitute to traditional water resources, it is rather a supplementary source that can contribute in bridging the water gap of the country.     

Energy advantages of reverse osmosis in seawater desalination

The very rapid increase in energy costs during the past three years is causing a change in the preferred process technology for seawater desalination. The phase changes, evaporation, and condensation, required in the distillation processes make them more energy intensive than the ambient temperature liquid separation that occurs in the reverse osmosis (RO) process. This paper describes the RO process and how to calculate its energy consultation.The RO process requires only 5–7 KWh/m³ of product water compared to 15–16 KWh/m³ required by the most efficient distillation process. The productivity of a large dual purpose electricity/RO water plant is compared to the productivity of a commercially purchased state-of-the-art dual purpose electric/distillation water plant that is currently under construction. The RO potable water productivity is about 2X the distillate flow at the same fuel rate     

Economic aspects of membrane processes

Care must be exercised, in predicting operating costs for membrane desalination systems for electricity, labor, chemicals, etc. The assumptions made in establishing unit costs can greatly affect the predicted operating cost and hence the ultimate investment decision. The use of data estimating the quantities of chemicals, power, etc., expected to be used per unit of production can assist in predicting costs, as then site-specific cost data can be used. The fact that 0.2 pounds of acid per kgal of permeate is required in a Florida RO plant is more useful for predicting operating costs of an RO plant on the Island of Anegata than is the fact that acid costs at the Florida plant are $0.03/kgal of water produced. Data based on units rather than costs are presented for seven RO facilities and one ED facility in this paper for use in this type of analysis.     

考虑水需求的水电联产海水淡化系统的优化设计

水电联产不仅能缓解淡水资源不足的问题,而且可有效降低能耗和淡化成本。建立了水电联产系统数学模型,将优化设计描述为一个混合整数非线性规划(MINLP)问题,并采用混合编码的遗传算法进行求解,结果表明,以水定电模式下水电联产系统最优操作模式为发电、多级闪蒸(MSF)和反渗透(RO)三者的集成,且MSF和RO的产水比存在最优值,发电采用背压式蒸汽轮机。随着淡水需求量的增加,联产系统的淡水成本逐渐降低,MSF与RO的产水比呈现出逐渐降低的趋势。