Aquifer composition and the tendency toward scale-deposit formation during reverse osmosis desalination — examples from saline ground water in New Mexico, USA

Desalination is expected to make a substantial contribution to water supply in the United States by 2020. Currently, reverse osmosis is one of the most cost effective and widely used desalination technologies. The tendency to form scale deposits during reverse osmosis is an important factor in determining the suitability of input waters for use in desalination. The tendency toward scale formation of samples of saline ground water from selected geologic units in New Mexico was assessed using simulated evaporation. All saline water samples showed a strong tendency to form CaCO₃ scale deposits. Saline ground water samples from the Yeso Formation and the San Andres Limestone showed relatively stronger tendencies to form CaSO₄·2H₂O scale deposits and relatively weaker tendencies to form SiO_2(a) scale deposits than saline ground water samples from the Rio Grande alluvium. Tendencies toward scale formation in saline ground water samples from the Dockum Group were highly variable. The tendencies toward scale formation of saline waters from the Yeso Formation, San Andres Limestone, and Rio Grande alluvium appear to correlate with the mineralogical composition of the geologic units, suggesting that scale-forming tendencies are governed by aquifer composition and water-rock interaction.     

Forecasting the economic costs of desalination technology

Government policy, in the form of grants and contracts for desalination technology, has had a major impact on steadily declining costs of desalination. The process, reverse osmosis (RO), exhibits economies of scale, which increases its feasibility as a water treatment technology for large populations. Ultrafiltration, an RO pre-treatment, also shows economies of scale. The real economic costs of desalination technology can be forecast using an ARIMA model. If these costs fall below those of conventional water treatment processes, RO and ultrafiltration become competitive with conventional water treatment technology. Our ARIMA forecasts are validated by using independent plant level cost data.     

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.     

Operational water cost and productivity improvements for small-size RO desalination plants

This detailed study was carried out for the determination of the water cost in small size RO desalination plants in remote areas. Data from desalination plants in three Greek islands were used for a period of three years. The actual expenses for these plants were examined thoroughly and as a result the real cost per cubic meter was estimated. An evaluation of the efficiency of two different energy recovery systems was conducted by using factual operating data. The introduction of new technologies, automation, data acquisition and remote operation can be used to reduce the labor and maintenance cost for small size RO desalination plants. The increase in labor productivity has proved to be the direct result of the introduction of a low cost SCADA system to the RO plant.     

Development of the electrochemical scale removal technique for desalination applications

The possibility of alkaline scale precipitation and removal by electrolytic devices has long been recognized. The scale removal principle of the electrochemical technique is based on the creation of a high pH environment around the cathode by water and oxygen reduction reactions which release hydroxyl ions. The alkaline environment induces precipitation of the calcium hardness in the form of CaCO₃ and of the magnesium hardness, in the form of Mg(OH)₂. Despite the commercial availability of such equipment, the use of electrochemical scale control methods is quite limited. Currently, the main field of application of electrolytic devices is for reducing the hardness of water recirculating in cooling towers. The lack of authoritative technical information on electrochemical scale removal reflects the paucity of research and development efforts in a technology which holds considerable promise for expanding the rather limited scope of viable scale control techniques. The objectives of this research project are to evaluate the potential of the electrochemical technique for RO desalination processes in general and for increasing water recovery levels in particular. The paper summarizes results of the first phase of the research. Models describing cell resistance in the absence and in the presence of a deposit on the cathode are presented. The effects of several parameters on the deposition rate and on the electric energy consumption are investigated. Results show that the higher the water hardness, the higher the scale precipitation rate and the lower the specific energy consumption. An increase in the flow velocity augments the scale deposition rate. Analysis of the velocity effect data indicates that the scale precipitation reaction is mass transfer controlled. The main optimization parameter is the current density. As may be anticipated, a low specific electrical energy is consumed when the electrolyzed solution is exposed to a large electrode surface and a high specific energy is consumed when the solution is exposed to a small electrode surface. The energy consumption can be rather low. For instance, in the electrolysis of a typical concentrate stream of a brackish desalination plant at a current density of 25 A/m², the energy consumption is of the order of 4 kWh per kg of precipitated CaCO₃ and the scale precipitation rate is of the order of 25 g CaCO₃/h m². Finally, a flow scheme is presented indicating the possibility of beneficial increase of the water recovery level in brackish water RO desalination, by partial recycle of the concentrate after electrochemical precipitation of the scale forming ions held in solution by the anti-scalant.     

Feasibility of reverse osmosis desalination of brackish agricultural drainage water in the San Joaquin Valley

The technical feasibility of reverse osmosis (RO) desalination of agricultural drainage (AD) water in California's San Joaquin Valley (SJV) was evaluated based on systematic analysis of water quality monitoring data and field water desalting tests in a laboratory plate-and-frame RO (PFRO) system. Thermodynamic solubility analysis and diagnostic PFRO desalting tests served to determine the feasible range of water recovery limits and to assess the mineral scaling potential. Analysis of the recovery limits imposed by scaling due to sparingly soluble salts (e.g. calcite, gypsum, silica) suggested feasible recoveries in the range of 46%-69%. Diagnostic PFRO desalting tests with five representative field water sources from the SJV (having gypsum and calcite saturation indices in the range of 0.12-1.03 and 2.9-9.5, respectively) confirmed the above recovery range. Mineral scale coverage was consistent with the observed flux decline. Deployment of RO technology for treatment of brackish SJV AD water would require site-specific process optimization given the geographic and temporal water quality variabilities. Therefore, RO operation with variable feed water quality (with respect to salinity and scaling propensity) and at sufficiently high recovery would require effective plant control, enabled by real-time mineral scale detection and adaptable process operation to mitigate mineral scaling.     

High recovery system in seawater reverse osmosis plants

Seawater desalination by the reverse osmosis (RO) method is an energy-saving system compared with the evaporating method, and can perform seawater desalination efficiently. Seawater RO desalination technology has been established and become a reliable system. Seawater desalination plants using RO technology have spread and the scale of the plants has increased significantly. More economical and efficient RO method seawater desalination systems have come to be required. A high recovery system, which offers reduction of plant construction cost and running cost was devised. Towards realization of this high recovery system, simulation and the field tests were done to confirm the practicality. Furthermore, a high recovery system was adopted for the biggest desalination plant in Japan, and it is performing favorably. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Implications of desalination for water resources in China — an economic perspective

China is a country with severe water shortages. Water is becoming scarcer due to population growth, industrialization and urbanization. Recent studies show that by the next 50 years water resources per capita will go down to around 1700 m³, which is the threshold of severe water scarcity. Especially in North China, water shortage has become a critical constraint factor for socioeconomic development in the long run. To solve or eliminate water shortage problems, seawater desalination draws more and more attention as an alternative water supply source. The objective of the study is to assess the potential of desalination as a viable alternate water source for China through analysis of the costs of desalination, the water demand and supply situation as well as water pricing practices in China. Based on the investment costs and estimated operation and maintenance costs, an economic appraisal for the costs of desalination for two main processes, MSF and RO, has been conducted. The study shows that there is a decline of unit cost of desalination over time and the average unit cost of the RO process was lower than that of the MSF process. A unit cost of 0.6 $/m³ for desalting brackish water and 1.0 $/m³ for seawater are suggested to be appropriate for the potential application of desalination in China. Future trends and challenges associated with water shortages and water prices are discussed, leading to conclusions and recommendations regarding the role of desalination as a feasible source of water for the future.     

RO,MSF and VTE/VC Desalination: A comparative evaluation

For sea water conversion in single-purpose plants, reverse osmosis (RO) is shown to be more economical than multistage flash (MSF) desalination over a wide range of unit size and fuel cost. Attention is focussed on vertical tube evaporation/vapor compression (VTE/VC) a self-contained, low-energy consuming thermal process which is potentially competitive with RO.     

Reliability analysis of reverse osmosis plant

The increasing complexity of desalting plants demands consideration of operational and maintenance factors in the design phase. Applications of the fault tree techniques to RO sea water desalination systems are illustrated; critical failure problem areas are identified and methods of improving the performance of the RO desalination plants are recommended. Failure data has been extracted from operation and maintenance reports of the operating RO desalination plants in Jeddah, Saudi Arabia. An assessment is also made of the impact of failures and outages on the availability of the plant.     

Nanofiltration and reverse osmosis of model process waters fromthe dairy industry to produce water for reuse

In dairy plants the process waters generated during the starting, equilibrating, interrupting and rinsing steps contribute to the production of effluents. They correspond to milk products (milk, whey, cream) diluted with water without chemicals. The treatment of these dairy process waters by nanofiltration (NF) or reverse osmosis (RO) operations was proposed to concentrate dairy matter and to produce purified water for reuse in the dairy plant. The study reports one-stage and two-stage (NF + RO and RO + RO) spiral-wound membrane treatments with five model process waters representative of the main composition variations observed in dairies. Performances (permeate flux, milk components rejection, purified water characteristics) of the different operations were compared. Discussion was focused on the comparison between quality of produced waters and vapour condensates (from product drying and evaporation processes) reused in dairy plants. Accordingly, both total organic carbon (TOC) and conductivity of water treated by a single RO or NF + RO operations were convenient for reuse as heating, cooling, cleaning and boiler feed water. With the two-stage RO + RO process, a more purified water complying with the TOC drinking water limit was achieved.     

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。     

Reliability of the jeddah seawater RO desalination plant based upon operations experience

Demand on commercial RO seawater desalination plants can be assured with increase in reliability and with high levels of performance. Such improvements can best be achieved through “lessons learned” from experience gained in available RO systems such as the large Jeddah RO plant.In this study, critical components which affect plant performance have been identified and methods of improving the design have been recommended. Failure data has been extracted from operation and maintenance reports.An assessment is made of the impact of failures and outages on the availability of the RO desalination plant. The single largest influence on the effective capacity of a water supply system based on RO desalination of seawater is found to be plant availability and maintainability. The design configuration of the plant is also found to have an impact on the acceptability of water supply system performance.     

反渗透系统中Zn2+对CaCO3结垢的阻滞

Scaling of reverse osmosis （RO） membrane surface is one of the main problems in desalination processes. To mitigate scales, organic anti-scalants are often used. If the dosages of anti-scalants are reduced, by using other much cheaper scale inhibitors, RO running cost will decrease greatly. The present paper investigated the inhibition of CaCO3 precipitation by zinc ions in RO system. The results show that the zinc ion concentration of 2mg·L^-1 was able to exert a marked suppression effect on both bulk precipitation of CaCO3 and on membrane scaling on waters of moderate hardness.     

Demonstration plant for seawater desalination by reverse osmosis (RO-2 hollow fibre module line)

A reverse osmosis (RO) seawater desalination demonstration plant (3000 m³/d with three lines) is being operated in Kuwait to investigate the suitability and optimum operating conditions for polyamide hollow fibre compact modules, and spiral-wound modules.The plant described herein has hollow fibre modules with one high pressure stage of 56–63 bar operating pressure and salt passage of about 1.5%, and a second desalination stage with an operating pressure of 28 bar. Drinking water is made up as mixed permeate from the two stages.During the first year, the RO units operated perfectly, and results obtained exceeded the design values.     

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.     

A thermodynamic benchmark for assessing an emergency drinking water device based on forward osmosis

Following the creation of the first reverse osmosis (RO) membrane in the 1960s, the technique has been widely used for the purposes of both small scale and municipal seawater desalination. Forward osmosis (FO) is now also emerging as a possible contender, with the potential for much lower energy consumption. In this study, we have developed a thermodynamic benchmark for use in assessing the suitability of a potable water system for purifying small amounts of brackish water in emergency situations. The light, portable and re-usable purification system is driven by FO. A pouch is filled with draw solution and immersed in brackish water; the pouch incorporates a traditional RO membrane. The ‘draw solution’ contains digestible salts and/or sugars to provide an osmotic pressure difference across the membrane, thus drawing in purified water across the membrane. Three such draw solutions were produced and tested, allowing the osmotic potential of the solution to be determined over a succession of dilutions. The results could be fitted with a power law function. In order to take account of the solution non-ideality and the non-linearity of flux rates, a thermodynamic relationship was used in conjunction with a membrane transport model to develop a benchmark which describes the ideal behaviour of a FO water system. This benchmark, in conjunction with the power law function, showed that such a system could be used in an emergency to provide safe, potable water in a reasonable time interval and without the need for a power source. The study has also suggested the possibility of a continuous water purification system based upon this principle, and has drawn attention to the benefits of novel draw solutes in such a system.     

Hybrid Reverse Osmosis‐Capacitive Deionization versus Two‐Stage Reverse Osmosis: A Comparative Analysis

Reverse osmosis (RO) is a high‐pressure single‐phase desalination process used to obtain freshwater from seawater/brackish water. The RO system shows high energy consumption for a given unit volume of pure water produced. The reported hybrid system of RO and capacitive deionization (CDI) aims at improving the RO water recovery and minimizing energy consumption. The RO‐CDI system is simulated and compared with two‐stage RO, to determine the effectiveness of the new hybrid system. The energy recovery from RO brine was also studied. The specific energy consumption by two‐stage RO for two different arrangements of the energy recovery device is higher than for RO‐CDI. The hybrid RO‐CDI system is energy efficient for the production of freshwater from brackish water.     

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.     

海岛高含铁苦咸水的淡化

本文和反渗透技术处理海岛高含铁量苦咸水的淡化工艺。针地铁的去除做了一系列的予处理研究,使之在最佳经济运行成本的条件下,使原水经予处理后能满足足渗透进水的要求。