Control of organic fouling at two seawater reverse osmosis plants

Organic fouling of seawater Reverse Osmosis (RC) membranes is a phenomenon not well understood; it can result in a loss of membrane productivity and salt rejection properties. Two seawater RO plants using DuPont B-10 hollow fiber permeators had experienced organic fouling and were studied.The two plants used different sources of feedwater; one RO plant at Culebra, Puerto Rico, used open seawater; while the other RO plant at Grand Cayman Island, British West Indies, used a sea well. Both feed water sources possessed high concentrations of soluble organics (40–80mg/1) which were mainly humic acids. In an attempt to remove these organics with in-line cationic polyelectrolyte coagulation, the plants experienced organic fouling which caused excessive loss of productivity and salt rejection; both plants initially failed their acceptance tests.It was discovered that the fouling was actually caused by interactions between the humic acids and in-line cationic, polyelectrolyte coagulants which were not removed by in-dedth and cartridge filtration. Rather than remove the humic acid material, acid addition was initiated and in-line cationic coagulants use discontinued to keep the humic acids soluble. It should be noted that with the open seawater intake ferrous sulfate was still used to remove colloidal material and reduce the SDI. Both plants subsequently have passed their 720-hour acceptance test.Culebra and Grand Cayman plants have now exceeded design specification for both productivity and salt rejection. The aramid hollow fiber permeators on acidified feed have shown 100% rejection of these organics at both 25% and 50% conversion and organic fouling has not been evident.     

The malta seawater RO facility

The Polymetrics seawater reverse osmosis (RO) facility at Ghar Lapsi, Malta, is one of the largest seawater RO plants in the world, with an installed capacity of 20,000 m³ /d (5.3 U.S. mgd). A second plant, provided under the same contract, has a capacity of 4,500 m³ /d (1.2 U.S. mgd), utilizing highly brakish feedwater. The plants supply potable water for the island's 350,000 residents and its growing tourist trade.The seawater plant is a single pass design, utilizing DuPont permeators and is the first major installations incorporating energy recovery turbo-pumps.This paper summarizes the plant design, manufacture, assembly, and installation, and also reports up-to-date operation and maintenance data. The operating costs are presented.     

Reverse osmosis in the desalination of brackish water and sea water

To make RO an economical process, plant and operating costs have been reduced by developing compact modules, led by polyamide hollow fibre modules which recover energy from the reject when desalinating highly saline brackish water and seawater. Such plants only start to pay their way at product rates of around 400m³ /d +. Energy costs for desalination systems with lower capacities can also be reduced considerably by operating the RO plants at conversion rates of more than 30%. To what extent the required higher conversion rates can be obtained for a particular application depends on concentration of scales-forming content matter (calcium sulphate, strontium sulphate, barium sulphate and calcium carbonate), salt content of the raw water, concentration of fouling substances such as iron, manganese, anorganic and organic matter, risk of biological growth due to algae, shells and bacteria. The scope and type of conditioning and pre-treatment are based on these factors.     

A Comparative study of organic fouling in hollow fiber and spiral wound membranes

The occurrence of flux decline in brackish water reverse osmosis (RO) plants due to dissolved organics is a topic of ongoing research. This type of organic fouling has also been found in seawater RO plants. A study was undertaken to compare organic fouling in hollow fiber and spiral wound membranes using a seawater feed that possessed a high concentration of huraic acid. This study was undertaken at an RO plant on Grand Cayman Island, British West Indies. The feed water came from a sea well and possessed a concentration of humic acid that varied between 35 and 60 mg/l.The hollow fiber membrane was operated at a recovery of 25% while the recovery with the spiral wound membrane varied between 5 and 25%. The performance data which included permeate flow, salt rejection, pressure drops across the membrane and analysis of the membranes for organic fouling were undertaken. This study compared the performance data and organic fouling between the hollow fiber and spiral wound meembranes.     

Design aspects of large sea water reverse osmosis desalination plants

As the size of SWRO plants grow, more attention must be paid to the overall design aspects of the total plant. Some of these are simplifying and improving the pretreatment; reducing the pretreatment chemicals; systems optimization to match sea water conditions and; component size selections.The optimum conversion ratio is generally the maximum possible as limited by brine osmotic pressure and brine flow considerations. This is from 30% to 40% depending on feed temperature and pressure. An economic analysis of cooling of the feedwater is made. This was found to be beneficial for sea water temperatures over 27°C.To utilize larger single streams or “blocks” of permeators, larger high pressure pumps should be employed. These may be driven directly by medium speed diesel engines. A block size of 5 MIGD using a single H.P. pump of 2517 m³/h (11,080 U.S. GPM) at 701m is envisioned.     

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.     

The performance of the 12,000 m3/day seawater ro plant at Jeddah,Saudi Arabia

This paper outlines the performance of the 12,000 m³/day seawater RO plant at Jeddah, Saudi Arabia. The plant started operation in February 1979 and has been operating since then. The feedwater to the plant has a TDS of 42,000 ppm and the maximum seawater temperature is 32°C. The plant is made up of a first and a second stage. The first stage includes nine units, eight operating and one stand-by, and the second stage containes three units. The nine first stage and the three second stage RO units are each composed of 56 Fiberglass pressure tubes (Model 66 FRP-6) 150 mm diameter by 6.5 m long, each containing six spiral wound elements manufactured by UOP Fluid Systems Division. The plant uses Fluid System's polyamide thin film composite membrane element, Model 1501 TFC. The pretreatment is composed of dual media filtration, acid injection, sodium hexametaphosphate injection and cartridge filtration.The performance from the start of operation till the present time is shown both for the overall plant and for each unit. The product rate of each unit and its water quality throughout its operating period is indicated showing the times when the unit was shut-down and the reason for it. Deterioration in unit performance is analyzed and explained. Actual plant consumption of fuel and chemicals is given and estimates of the plant operating costs are made.     

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.     

Osmotic power production from seawater brine by hollow fiber membrane modules: Net power output and optimum operating conditions

This study analyzes the net energy output and optimum operating conditions for osmotic power generation from seawater brine based on the currently available hollow fiber membranes in the module scale. Factors that are influential on membrane performances, such as external concentration polarization, internal concentration polarization, salt reverse diffusion, and dilution have been taken into account. Net power density is defined and applied to characterize the efficiency of the PRO system, in terms of power production minus pumping energy, pretreatment cost and energy consumption by pressure drop in the membranes. When using 1 M NaCl as the draw solution and 10 mM NaCl as the feed, it is found that up to 7 W m^?2 net power density can be harvested by the PRO system depending on the water sources. Coupling with the existing RO plant is highly beneficial in terms of readily available high pressure source, high salinity and less or negligible pretreatment costs for the draw solution. Sources with higher salt concentrations are preferred. The optimum hydraulic pressure, module length, flow rate to membrane area ratio and feed to draw flow rate ratio have also been analyzed to maximize the net power output. In addition, implications on hollow fiber development are discussed. Fibers with high water permeability, lower structural parameter, good mechanical stability, better fouling resistance, and outer‐selective configurations are recommended for further studies. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1216–1225, 2016     

Operating experience on RO sea water desalination plant at chigasaki laboratory

A demonstration plant with a capacity of 800 m³/d has been operating satisfactorily in Chigasaki Laboratory since September 11, 1979. The plant has two sets of modules, namely hollow fiber and spiral wound types. These two types of modules have been operated alternately every other month, being supplied sea water from the same pretreatment facility and the same high pressure pump. The accumulated operation time of each module has exceeded 5,000 hours. The operation result shows high recovery ratio, low energy consumption and good quality of product water.     

New chlorine‐resistant polyamide reverse osmosis membrane with hollow fiber configuration

New asymmetric hollow fiber reverse osmosis (RO) membrane was developed from a new chlorine‐resistant copolyamide [4T‐PIP(30)] with a piperazine moiety by a conventional phase‐separation method. The new 4T‐PIP(30) hollow fiber membrane has the same low‐pressure RO performance as cellulose triacetate hollow fiber membrane (FR = 205 L/m² day, Rj = 99.6%) and superior chlorine resistance as well as pH resistance to conventional aramid RO membranes. Structural analysis and viscoelastic study revealed that the new hollow fiber consisted of a top skin, dense layer, and microporous layer, and that it began to decrease its elasticity at 80°C in water, which is possibly related to its good and stable RO performance around room temperature. Several kinds of RO modules were made from the new hollow fiber membranes, for which RO performances were stable for 2 years in chlorinated feed water desalination (the free residual chlorine ranged from 0.l to 1.1 mg/L). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 517–527, 2001     

Optimization of solar-powered reverse osmosis desalination pilot plant using response surface methodology

A solar thermal and photovoltaic-powered reverse osmosis (RO) desalination plant has been constructed and optimized for brackish water desalination. The central composite experimental design of orthogonal type and response surface methodology (RSM) have been used to develop predictive models for simulation and optimization of different responses such as the salt rejection coefficient, the specific permeate flux and the RO specific performance index that takes into consideration the salt rejection coefficient, the permeate flux, the energy consumption and the conversion factor. The considered input variables were the feed temperature, the feed flow-rate and the feed pressure. Analysis of variance (ANOVA) has been employed to test the significance of the RSM polynomial models. The optimum operating conditions have been determined using the step adjusting gradient method. An optimum RO specific performance index has been achieved experimentally under the obtained optimal conditions. The RO optimized plant guarantees a potable water production of 0.2 m³/day with energy consumption lower than 1.3 kWh/m³.     

中空纤维膜N_2-H_2分离器分离性能计算方法

本文提出了计算中空纤维膜N_2-H_2分离器及器组分离性能的数学模型;在计算机上考察了各操作参数和膜性能参数对渗透气中H_2浓度及H_2的回收率的影响;井以三根φ100 ×3000mm分离器现场试验结果进行了验证.     

The analysis and design of a both open ended hollow fiber type RO module

The use of seawater desalination plants using RO technology has spread and the scale of the plants has increased. In such a situation, a larger‐sized RO module has been strongly required. The conventional hollow fiber type RO element is a single open‐ended (SOE) structure. That is, one side of the hollow fibers in the modules is opened and the other side is closed. In this SOE structure, the increase in the flow pressure loss of the permeated water which flows in the bore side of the hollow fibers prevents development of a large‐sized (longer) RO element. In this work, a both open‐ended (BOE) element was devised which can reduce the flow pressure loss of the permeated water. It has been confirmed by analysis and experiment that the permeate flow rate of BOE is greater by about 30% than that of SOE. Furthermore, the large‐sized RO module with high volume efficiency was designed using the performance analysis method that was confirmed to be applicable to BOE. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Facilitated Transport Membrane Hybrid Systems for Olefin Purification

Abstract

A new membrane system has been developed by BP for refinery and chemical plant olefin purification and recovery. This facilitated transport system, coupled with distillation, offers lower capital and operating costs than conventional distillation alone. Initial results on lab scale hollow fiber devices indicate membrane flux ranging from 8.75×10^?6 to 8×10^?5 m³/m²/sec (2.5 to 23 scfd/ft²) and selectivities from 150 to 300. Pilot plant experiments on propylene/propane and ethylene purge gas recovery over three to six months duration show membrane stability and product purity of 98.5% or greater using refinery grade propylene feed. Hybrid system optimization data for membranes and distillation indicate that using a side draw from the distillation tower provides advantages in terms of membrane area, purity of feed to the membrane, and low per-pass recovery coupled with high overall propylene recovery. Membrane performance data under various conditions will also be presented. In addition to performance data, economic evaluation and energy savings will be discussed.     

Status of reverse osmosis desalination technology

Reverse osmosis (RO), a relatively new technology, is gradually becoming an established and economical method for demineralization of saline waters. Over 50 commercial plants ranging in size from 50,000 gpd to 2 million gpd (2 mgd) are producing fresh quality water for municipal and industrial uses from brackish water sources. The U.S. Congress has authorized construction of a 100 mgd plant in Yuma, Arizona to demineralize otherwise unusable high salinity irrigation return flows as part of the Colorado River Salinity Control. Engineering design and operation details together with cost information on some commercial plants and the planned 100 mgd plant will be presented.A review of the plant operation data indicates that is imperative for the plant owners and equipment suppliers to place due emphasis on providing adequate feed water pretreatment facilities and trained plant operation personnel to ensure trouble-free operation and to achieve furthur economy in desalting costs.Significant advances have been made in the development of RO process for sea water desalination. Soaring energy costs are providing incentive for plant owners to prefer RO plants (up to 100,000 gpd) over vapor compression distillation hardware. Results of the Federal Government Desalting R & D Programs clearly indicate that RO desalting costs will be at least 20–30% lower than distillation.     

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.     

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.     

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.