Boron removal from saline water: A comprehensive review

Boron is an essential micronutrient for plants and animals as well as a useful component for numerous industries. It is necessary to produce low boron containing water from RO desalination plants for both human consumption and for agriculture. For plants, a small amount of boron is necessary for their growth and development, but boron becomes toxic if the amount is slightly greater than required. Desalinated seawater from RO plants often contains high boron content and, when used for irrigation, has been proven to be damaging to crops including blackberry, lemon, and grapefruit. Apart from the toxic effects of boron on plants, boron should be removed from RO desalination plants to comply with the current guideline value, 0.5 mg/L, for potable water issued by the World Health Organisation (WHO). Currently there is no simple method to remove boron from saline water. The use of multi-pass reverse osmosis membrane (RO) with pH modification and the use of ion exchange using boron selective resins (BSRs) have both been considered as effective methods for the removal of boron. A hybrid process, Adsorption Membrane Filtration (AMF), has received attention as an emerging technology for boron removal with a high efficiency and low operating costs. The purpose of this review is to give an overview on boron in general and to discuss its toxicity. The problems of boron in the MENA (Middle East and North Africa) region are discussed as well as technologies, current and future, for the removal of boron from seawater. The focus is placed on current RO and ion exchange methodologies using BSRs as well as the future for the AMF method. The fundamentals of each process, the effects of experimental parameters, and findings are discussed.     

Field testing and economic evaluation of RO seawater technology

An experimental RO seawater system has been in operation for two years at Eilat on the Red-Sea shore. Simplified seawater pretreatment, comprising only sand filtration and acid dosing was applied. Hollow fiber and spiral wound membranes were tested. Evaluation of pretreatment effectiveness and comparative membrane performances are reported. Investment and water cost analysis of a 4,000 m³/day RO seawater desalting system under various operating conditions is given. The analysis is based on the results of experimental site operation, current technology development and updated industrial quotations. The resulting water production cost from a one-stage system, adequate to produce desalted water acceptable for local drinking water standards, is approximately $ 1.1/m³.     

A comparative study for boron removal from seawater by two types of polyamide thin film composite SWRO membranes

In this paper, field performance of a small-scale seawater reverse osmosis unit installed in Urla Bay-Izmir, Turkey was analyzed and presented. The design of SWRO system in Urla consists of two types of FilmTec polyamide thin film composite spiral wound seawater reverse osmosis membranes (high rejection FILMTEC XUS SW30XHR-2540 RO membrane and FILMTEC SW30-2540 RO membrane) which could be operated in parallel. To make a comparative study between two types of membranes regarding their desalination performances and boron rejections, each membrane was operated individually for each set of experiments. This comparison was made via investigation of the effects of feed seawater temperature (10–16 °C), operating pressure (55, 60 and 62 bar), and pH adjustment on the feed side (pH 7.0–7.5).     

Crevice corrosion behavior of high-alloy stainless steelsin a SWRO pilot plant

This paper presents the results of a study which has been carried out to investigate the crevice corrosion behavior of high-alloy stainless steel in a SWRO pilot plant. The study evaluated the corrosion performance of some austenitic and duplex steels in a crevice-forming environment created in a RO plant, especially in high-pressure feed and brine lines. The study of the effect of chemical dosing on crevice corrosion in the RO plant and electrochemical testing of crevice corrosion in the laboratory were the main objectives of this test program. High-alloy stainless steels, namely AL 6XN® and 254 SMO (superaustenitic), 2205, 2507 and DP3W (Duplex) were used in the test program. The tests were carried out in natural seawater and RO concentrate (conductivity: 75,000 to 80,000 μS/cm) at ambient temperature by operating the test plant at normal SWRO operating feed pressure of 54 bar. FeCl₃ was added as coagulant to maintain a silt density index of ∼3, and H₂SO₄ was added to feed in order to maintain the feed pH of ∼ 6.5. Chlorination and subsequent dechlorination agents were not added to the feed. For crevice corrosion tests in RO unit, the exposure periods were 6 and 12 months, respectively. The results of the tests showed that alloy DP3W has the best pitting resistance in crevice forming environment of seawater. In RO concentrate, alloys 2507 and 254 SMO showed lowest “maximum pit depth”. The results of potentiodynamic cyclic polarization (PCP) indicate that all the alloys have high pitting potential and small hysteresis loop. The results of critical crevice solution pH (CCSpH) indicated excellent resistance of alloys 254 SMO and DP3W against crevice corrosion attack and 2205 had least resistance in most aggressive sodium chloride solution.     

Optimal operation of RO system with daily variation of freshwater demand and seawater temperature

The optimal operation policy of flexible RO systems is studied in this work. The design and operation of RO process is optimized and controlled considering variations in water demands and changing seawater temperature throughout the day. A storage tank is added to the system layout to provide additional operational flexibility and to ensure the availability of freshwater to customer at all times. A steady state model for the RO process is developed and linked with a dynamic model for the storage tank. The membrane modules are divided into a number of groups to add flexibility in operation to RO network. The total operating cost of the RO process is minimized in order to find the optimal layout and operating variables at discreet time intervals for three design scenarios.     

Reverse osmosis desalination plants — marine environmentalist regulator point of view

The environmental characteristics of the brine and its impacts on the marine environment were always the “backyard” and the less concern while planning and operating RO desalination plants. The lack of drinking water in Israel made desalination a national goal. The first huge RO plant was initially operated in Israel on 2005 — 100 Mm³/y and by the end of 2010 305 Mm³/y potable water expected to be produced. Construction and installation of desalination plants requires applying suitable environmental solutions for protecting and preserving the marine and coastal environment from ruin or deterioration.

The Israeli environmental legislation is described including the marine environmental policy and regulations as well as the acquired operational experience during the last two years. Results from the first year of operation and monitoring are shown along with the new appearance of the red phenomenon discharge of VID desalination plant in Ashkelon.

The environmental policy based on the precautionary principle is performed and includes environmental requirements and guidelines for pretreatment, discharge composition, planning marine outfall, background and compliance marine monitoring program and discharge quality standards.     

The field assessment of a high temperature scale control additive and its effect on plant corrosion

Distillation of seawater, currently the only aćcepted, method, for the large scale production of fresh water from seawater, produces two serious problems that cannot be easily “engineered” out of the process. These are scale formation on heat transfer surfaces and corrosion of the plant constructional materials and are a direct result of the nature and chemical composition of seawater.Scale formation can he controlled by the use of additives or acid. However, where fuel costs are high, it is important for efficiency to operate distillation plants at as high a temperature as possible. Until recently high temperature operation has only been possible by the use of acid treatment, as additives based on polyphosphates do not work at temperatures greater than 90°C because of hydrolysis of the polyphosphate to inactive phosphate. Use of acid, unless very carefully controlled, can lead to serious corrosion of the plant. Ideally acid addition should be controlled to leave a residual alkalinity in the seawater feed of 10–20 ppm, which in practice is very difficult, because of the problems in accurately controlling the pH of a large flowing volume of seawater. If acid is underdosed, to avoid corrosion problems then scale formation will result.The result of a joint programme between CIBA-GEIGY (UK) LTD and the UKAEA has been the introduction of a high temperature scale control additive. Results of plant trials with this chemical will be described and calculation of fouling factors from plant operational data show its effectiveness in controlling alkaline scale formation.Simple laboratory tests have shown the likely benefits, in terms of lower corrosion rates, to be gained from the use of additives which result in high recirculating brine pH. These observations have been substantiated by plant experience.     

Use of membrane technology for potable water production

Excess fluoride in drinking water is harmful for human health, so it is necessary to be removed. In this study thepotential of a reverse osmosis (RO)membrane for defluoridation of underground water samples at different solute concentrations was studied. Because of the variation in feed water composition, its chemical properties, pH and operating conditions, effect of these parameters on the membrane separation process was studied. The results showed that pH, feed water composition, flow rate and pressure affect the membrane efficiency, and thus proper control of these factors is essential for successful operation, Optimum membrane properties and proper rejection were determined percentage for the RO system to minimize the overall cost of water treatment. The samples were collected from three villages in the Gurgaon district and treated at optimized conditions. The results indicate that RO membranes can be successfully used for the treatment of underground water to the desired purity level, as they remove up to 95% of fluoride present in water and also take care of other ions present in water.     

Complete elimination of acid injection in reverse osmosis plants

Antiscalants with broad activity spectra are available today. When properly chosen, a single antiscalant can efficiently and simultaneously control calcium carbonate, calcium sulfate, strontium sulfate, barium sulfate and calcium fluoride scales as well as inorganic foulants resulting from iron, aluminum and reactive silica present in any given water or wastewater. The effectiveness of many antiscalants towards controlling calcium carbonate scaling in reverse osmosis (RO) plants has allowed us, in the past five years, to successfully help eliminate the continuous injection of acid in all RO systems operating with polyamide membranes. A minority of systems with cellulose acetate membranes however still require acidification due to hydrolytic sensitivity of cellulose acetate towards feedwater pH higher than 6. The chemistry behind the main reason for acidification — the prevention of scaling by calcium carbonate — is reviewed. The mechanisms of calcium carbonate scaling and its avoidance by acidification or with antiscalants are discussed. Major seawater and brackish water RO plants around the world are designed and operated with the continuous injection of concentrated sulfuric or hydrochloric acid for scale control, sometimes simultaneously with an antiscalant. We present a case study in southeastern USA of a 5 million gallons permeate per day plant as a typical example among many for the conversion of major municipal RO plants. By suitable selection of an antiscalant, acid elimination was successfully attained with a simultaneous reduction in the antiscalant dosage, in this case to 2-3 mg/1 in the feedwater. While process optimization continues in the plant, current chemical cost savings due to the deletion of 93% sulfuric acid and the associated caustic soda neutralizer alone amounts to 67% of the annual cost of all chemicals used. This does not include savings through benefits accrued in equipment, operation, maintenance and safety in the plant.     

Boron removal from seawater using high rejection SWRO membranes — impact of pH, feed concentration, pressure, and cross-flow velocity

The main objective of this work was to investigate boron removal from seawater using two commercial high rejection SWRO membranes. The impact of solution pH, feed concentration, pressure, and cross-flow velocity on boron rejection and permeate flux was determined. The membranes used were the Toray™ UTC-80-AB and Filmtec™ SW30HR. A lab-scale cross-flow flat-sheet configuration test unit was used for all RO experiments. Seawater sample was collected from the Mediterranean Sea, Alanya-Kızılot shores, south Turkey. For all experiments, mass balances were between 91% and 107%, suggesting relatively low loss of boron on membrane surfaces during 14 h of operation. Operation modes did not have any impact on boron rejection, indicating that boron rejection were independent of feedwater boron concentrations up to 6.6 mg/L. For both membranes, much higher boron rejection were obtained at pH of 10.5 (>98%) than those at original seawater pH of 8.2 (about 85–90%). Permeate boron concentrations less than 0.1 mg/L were easily achieved at pH 10.5 by both membranes. The dissociated boron species are dominant at this pH, thus both electrostatic repulsion and size exclusion mechanisms are responsible for the higher boron rejection. The rejection of salts in seawater did not correlate with boron rejection at constant conditions. For each membrane type, permeate fluxes at constant pressure were generally lower at pH of 10.5, which may be partially explained by membrane fouling and enhanced scale formation by Mg and Ca compounds from concentration polarization effect at higher pH values. While somewhat higher boron rejection was found for one membrane type as the pressure was increased from 600 to 800 psi, increasing pressure did not affect boron rejection for the other membrane. Feed flowrate thus the cross-flow velocity (0.5–1.0 m/s) did not exert any significant impact on boron rejection at constant conditions.     

Performance study of RO systems in Saudi Arabia

Desalination of water is one of the most important engineering tasks facing Saudi Arabia today. Lacking unlimited supplies of potable Water, the Kingdom must depend upon the desalination process of brackish well water or sea water to satisfy the ever increasing demands. These demands are expected to reach the 430 MGD figure in 1980 and 700 MGD in 1985.

Abstract

until quite recently, the reverse osmosis played a rather modest role in the country's desalination program and was almost exclusively limited to brackish water applications and plants not exceeding a capacity of 1 MGD. Within the last year, significant changes took place in favor of these processes. The estimated total output of RO units to be commissioned this year will be about 70 MGD. This figure includes the 3.2 MGD plant for desalting the sea water in Jeddah and the 55 MGD plant in Riyahd from brackish well water.

This paper deals primarily with the performance of RO systems, their cost of operation/maintenance and the factors affecting the cost. These factors may be divided into three groups as related to feed water, system design and system operation. The quality of feed water and good pretreatment plays an extremely important role in performance of a RO system. This study pays special attention to these parameters in expectation that more cost effectiveness will evolve in the future either through a better understanding of the processes or an improved process design match to materials employed.     

Two seawater reverse osmosis plants operating at high pressure and 50% conversion

At the Second World Congress on Desalination and Water Re-Use (Bermuda) an evaluation was made on a Reverse Osmosis (RO) plant operating at high pressures (1100-1500 psig) using Middle East conditions; it was concluded that such an RO plant could obtain 50 percent conversion of the feedwater and incur lower capital and operating costs.Two RO seawater plants using DuPont B-10T hollow fiber permeators were studied; these plants, one a 400m³/day facility and the other a 115m³/day facility, were designed for 50% conversion at a maximum 1200 psig operating, pressure. The plants used direct one to one brine staging and have operated at 1140 psig since their start-up.These plants have continually exceeded design specifications for both productivity and salt rejection. Both the first and second stage permeators combined have maintained 50% conversion with a TDS of the product water less than 350 ppm. These hollow fiber aramid permeators have maintained excellent performance at high pressure despite the fact that the feedwater possessed a very high concentration of organics (humic acid).The capital and operating costs of this plant have been significantly less than that of a comparable RO plant using lower pressure (900 psig). The performance data and cost analysis of this high pressure RO plant support the hypothesis that high pressure RO can provide excellent performance at a lower cost.     

Effects of feed water temperature on inorganic fouling of brackish water RO membranes

A bench scale RO process simulator was operated in a batch concentration mode to determine the effects of product water recovery and feed water temperature on flux, rejection, and inorganic fouling by gypsum scale formation for simulated brackish water. As feed water temperature increased, salt rejection and concentration polarization decreased (reducing scale formation potential at a given recovery). However, gypsum crystal nucleation and growth rates increased with temperature. Specifically, at 15 and 25°C gypsum scale formation resulted in slow, steady flux decline at recoveries as low as 10–20%. At these temperatures, many small crystals formed over the entire membrane surface. In contrast, at 35°C flux decline was due to the increasing feed solution osmotic pressure — up to a recovery of about 70%. At this recovery, we observed a sudden, rapid loss of flux and a concomitant spike in feed water turbidity. Relatively few (in number), large crystals formed on the membrane towards the brine outlet of the RO simulator, but the entire membrane surface was covered with “needle-like” crystal fragments. The crystal fragments broke off from growing gypsum rosettes and re-deposited uniformly across the membrane forming a “cake layer” that caused the massive flux decline. These results suggest that high temperature operation of brackish water RO processes could enable higher recovery and lower energy consumption, but operating near the limiting recovery (at elevated temperature) creates an increased risk of a catastrophic fouling event.     

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.     

Energy production at the Dead Sea by pressure-retarded osmosis: challenge or chimera?

In recent years two types of very large-scale plants have been proposed for handling seawater brought to the Dead Sea, both processes taking advantage of the 400 m drop to Dead Sea level and both sized to replenish the 3,000,000 m³/d evaporation rate of the Dead Sea. Pressure-retarded osmosis (PRO), the process discussed herein, would use the replenishment stream to produce an appreciable amount of benign and renewable electric power. If the seawater plant prior to PRO would be reverse osmosis (RO), handling 5,000,000 m³/d to produce 2,000,000 m³/d of fresh water, PRO could produce 48,000 kW from the RO-concentrated seawater feed at a capital cost for power of about $4,000 per kilowatt and a PRO plant cost of $190,000,000. The electrical energy would be produced at a cost of about $0.07/kWh. The PRO plant would use a DuPont B-9 type or similar hollow fiber modified to have 110 and 320 micron internal and outer diameters (instead of 40 and 90). Osmotic permeation of half of the 3,000,000 m³/d RO reject brine into Dead Sea brine would produce 35 atmospheres of hydrostatic pressure relieved by passage of an equivalent volumetric rate of diluted Dead Sea brine through a hydroturbine/generator set. The second type of plant prior to PRO would use 3,000,000 m³/d of seawater to produce hydropower, estimated at about 130,000 kW. The permeation rate in PRO could then be 2,000,000 m³/d enabling power production in PRO of 70,000 kW at a capital cost for power of $3,300 per kilowatt and a PRO plant cost of $230,000,000. The cost of produced energy in PRO would be $0.058/kWh. It is believed that the Great Salt Lake should also be examined as a site for PRO.     

Trends in the major cost components of MSF desalting and their implications for future thermal desalting plant

The capital and operating costs of MSF seawater desalters have increased drastically over the past decade. Analysis of trends in their major construction and operational costs permits conclusions to be drawn concerning the direction of future developments.If present trends continue, arid, oil-producing countries will require plants of low initial cost, high reliability and ease of operation, rather than new and advanced desalting concepts. On the other hand, oil-importing countries needing desalting must develop less fuel-intensive processes.Although seawater RO can meet these needs, a market for dual-purpose plant of high Performance Ratio will persist, particularly where flexibility of operation is required. Hybrid desalters, combining power-using and thermal desalting techniques, will be developed in these countries. Mechanical vapour compression combined with multiple-effect evaporation and MSF feed preheating is one such process.     

PACl: A simulation of the change in Al concentration and Al solubility in RO

Polyaluminium chloride (coagulant) is a very complex compound. The change in the ratio HCO₃/CO₂ during coagulation cause a change in pH in the subsequent RO unit (because CO₂ is not rejected by the RO membrane), and it was found that this change is constant for any given recovery, e.g. at 75% recovery, the pH change between the RO feed water and concentrate water is 0.6. The simulation of aluminium solubility and concentration along RO systems were performed assuming the presence of three aluminium species, Al₁₃⁷⁺, Al(OH)₄⁻, Al(OH)²⁺. The simulation of aluminium solubility was performed at two temperatures, 20°C to represent the summer period and 5°C to represent the winter period. Results showed that for winter (5°C) an RO feed water pH of at least 7.2 and for warm weather (20°C), a feed water pH of at least 6.7 are required respectively, to avoid scaling of aluminium on the RO membranes.

In conclusion, theoretical simulation of aluminium solubility and results of deposition factor showed that aluminium more than likely plays a role in fouling of the RO membranes and could be a reason for the frequent cleaning.     

Foam Control Additives In MSF Desalination

Excessive foaming in multistage flash desalination plants can give rise to salt-contaminated distillate and reduced operating efficiency. Plant operating variations in pressure, temperature, flow, and seawater feed composition and alkalinity can perturb vapor/liquid equilibria, resulting in increased levels of foam. Since such foaming is not always controlled by mechanical means, additives are often needed to control foaming level.In addition to controlling foam, a useful additive must not interfere with heat transfer or with the performance of the scale control additive; it must not contain steam-volatile components that can contaminate the distillate; it must be stable at MSF operating temperatures and residence times; and it should be compatible with the antiscalant feed solution.Antifoam performance data for over 60 candidates in 7 chemical classes were determined according to these criteria. The most effective structural class was found to be the polyglycol monoalkyl ethers. Foam control effectiveness within this class was shown to be a function of ethylene oxide content and alkyl chain length. The most effective additive found in this study is offered commercially as FLOCON Antifoam AF-4. FLOCON AF-4 has been used successfully in high-temperature MSF plants.     

Reverse osmosis to achieve water control and recycle

A 648,000 GPD reverse osmosis (RO) facility at ERDA's Rocky Flats Plant near Golden, Colorado will convert tertiary sewage plant effluent for recycle as cooling tower makeup to reduce external water demand and achieve “zero discharge” off-site of tertiary sewage effluent.

Design parameters for the facility, determined by three years of pilot plant testing, include 98% feedwater recovery, 100 ppm T.D.S. product water, and minimum brine production for evaporation to dryness.

Pretreatment consists of RO feed attenuation in a large pond, chlorination, sand filtration, softening, diatomaceous earth filtration, feed-water heating and pH adjustment. The RO plant will have three 150 GPM trains, each with a combination of HFF modules producing about 90% of the permeate, followed by SW modules producing the final 10%. Permeate from the SW modules can be combined with permeate from the HFF modules or returned to the RO feed stream.

Unique design considerations include heating the 40–70°F fee to 77°F by means of heat recovery from the permeate and supplemental steam heating, recycling of pretreatment backwash streams wherever possible to reduce the volume of brine, and precautions to avoid silica scaling of the modules.     

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.