Studies on desalination by reverse osmosis. III. Mechanism of solute rejection

The mechanisms by which cellulose acetate membranes can be effective in removing salt from aqueous solution have been considered in relation to available experimental data. It is proposed that diffusive flow is involved, through a pore-free layer in the membrane, and that solute rejection occurs as a result of the high pressures used, and of the fact that the solute is present at relatively low concentration. The effect of pressure on the activity of a component present at low concentration is small, and consequently the flow of solvent is increased to a greater extent at high pressures than is the flow of solute. Rejection thus approaches 100% asymptotically, as the pressure on the system is increased.     

Bench-scale evaluation of seawater desalination by reverse osmosis

In bench-scale tests of seawater reverse osmosis desalination it is important to carefully consider osmotic pressure effects and determine the extent of concentration polarization so that sources of flux variation—whether from fouling, compaction, or osmotic pressure changes—can be properly assessed. Rigorous modeling of concentration polarization is difficult because of the complex geometries and flow regimes in RO modules; typically, concentration polarization must be measured. However, concentration polarization measurement usually requires knowledge of membrane permeability, which can vary from coupon to coupon. In this study a method is presented to determine both the membrane permeability and the concentration polarization regime in a single test. The key to the test is to allow the salt concentration to vary over time in a predictable way and extract parameters from a model fitted to the flux data. The usefulness of this technique is highlighted by evaluating results from several seawater experiments. It was found that specific flux decline in the experiments was caused by changes in osmotic pressure and membrane compaction. RO fouling by seawater organic-matter was not significant for the several seawater samples tested.     

Effects of acceleration on reverse osmosis desalination

Hollow fine-fiber reverse osmosis elements were run in a continuous-flow centrifuge at accelerations up to 1000 G. Under acceleration, the elements produced more desalted water and passed less salt, showing a reduction of concentration polarization. Reduced data showed that the mass transfer co-efficient of salt from the membrane surface to the bulk of the brine increased with acceleration to the 0.3 power. Acceleration also improved flow distribution by eliminating regions of high-density stagnant brine. Use of a centrifuge will allow utilization of higher transport membranes.     

Composite membranes for seawater desalination by reverse osmosis

A composite reverse-osmosis membrane has been developed for seawater desalination having a 400-Å semipermeable barrier. The membrane is prepared by directly forming a very thin film of a polymer, generally cellulose triacetate, upon the finely porous surface of a supporting membrane. The composite membrane, capable of desalinating seawater in a single pass, has demonstrated improved flux stability at high pressures over modified membranes currently used to desalinate brackish water.     

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.     

海水反渗透淡化技术的分析与探讨

反渗透海水淡化技术是一种高效、节能、先进的液体分离技术.论述了目前国内外海水反渗透淡化技术的应用现状,着重介绍了反渗透膜材料及特点、膜污染及清洗、典型的海水反渗透淡化流程,探讨了反渗透海水淡化技术目前存在的问题及未来发展趋势.     

Sea water desalination by reverse osmosis in chigasaki laboratory

For the purpose of constructing a reverse osmosis [RO) sea water desalination plant of 800 m³/day capacity, a series of tests on the following themes have been carried out in the Chigasaki Laboratory:

1. Performance and durability of 8B modules made in Japan

2. Simplification of pretreatment system

3. Establishment of energy recovery system.

Domestic modules showed good and stable performance during long term operation, and water recovery ratio of these modules have been raised to 40%.

In-line coagulation and filtration system has been established for the pretreatment of feed sea water, instead of coagulation, sedimentation and filtration system.

The energy recovery equipment is consisted of a high-pressure pump, a motor and a hydraulic turbine on a common base. Recovered energy from pressurized brine is used for the auxiliary motive power of the high-pressure pump. The experimental data show that about 20% of required power for the pump was recovered.     

Practical aspects of sea water desalination by reverse osmosis

The cost of reverse osmosis equipment for sea water desalination is now equal or lower than the cost of multistage flash evaporators up to 6000 MTD capacity, depending on conditions. The high temperatures and salinity of the sea in several locations where fresh water is in short supply are important in the design of reverse osmosis plants; high temperatures will increase the flux of the membranes but will also increase the rate of compaction and flux decline. As a consequence, the most rugged membranes are required for the desalination of warm sea water. Experience has shown that the addition of aluminum sulfate followed by dual media filtration will provide a suitable feed for reverse osmosis treatment.     

Considerations of energy consumption in desalination by reverse osmosis

The primary factors affecting the energy consumption of a reverse osmosis plant are considered. These are the osmotic pressure of the feedwater, the feedwater temperature, the water recovery, and the relationship between the water flux and salt flux characteristics of the membrane. In addition, the required permeate quality may have several indirect effects on the energy consumption. Permeate quality standards may impose minimum operating pressures, limit the recovery, and/or require treatment with a full or partial second stage. As a general rule, the energy required increases with increasing feed salinity and increasing permeate quality.For any given recovery, a single stage system will require less energy than a partial two stage system. However, for a specified permeate quality, a partial two stage system can operate at a higher overall recovery and a lower energy consumption than a single stage systemEnergy recovery systems can recover between 50 and 90 percent of the available energy in a reverse osmosis unit, thus significantly lowering the energy consumption. Studies have shown that with an energy recovery system, the minimum energy consumption occurs at a first stage recovery of 30 to 35 percent. Currently, very few energy recovery systems are in use due to their high capital cost, but as energy recovery systems become more available and reliable, they will greatly increase the energy efficiency of reverse osmosis plants.     

Pre-treatment strategies for seawater desalination by reverse osmosis system

A good quality pre-treatment process is instrumental to the successful operation of a seawater reverse osmosis (SWRO) plant. The compounds that are susceptible to foul the reverse osmosis (RO) membranes are inorganic suspended solids, sand, oil, clays, bacteria, and dissolved organic matters. In order to prevent the fouling, a pre-treatment of the raw water needs to be conducted. The pre-treatment technologies to prevent membrane fouling and to extend the lifetime of the RO membrane are commonly grouped into two categories, conventional and non-conventional. Both of these treatments are currently applied in SWRO plants in the world. The pre-treatment system applied is highly site specific and depending on the type of the feed water. This paper reviews the recent representative researches that are related to SWRO antifouling strategies and answers the most crucial questions about design and operating parameters of SWRO and its pre-treatment process. Also the economic evaluation of the SWRO system in regards to antifouling strategies' experience is discussed.     

Seawater desalination and reverse osmosis plant design

Plant site, water intake, pretreatment, choice of materials of construction, design alternatives and energy recovery are important variables to be considered in the design of seawater desalination plants employing “permasep” B-10 reverse osmosis modules. Techniques are outlined to permit custom design of seawater desalination plants which offer reliable long term performance as well as competitive economics.     

冷冻-重力脱盐与反渗透结合的海水淡化分析研究

随着我国沿海液化天然气(LNG)接收站的迅速发展,LNG冷能有望成为冷冻法海水淡化的低成本冷源。在此前提下,研究了人工海冰在不同融化率下的重力脱盐效果,结果表明,当融化率达到79.53%时,Cl-、TDS、总硬度这3项指标可达到饮用水标准。当融化率为39.81%时,将冷冻-重力脱盐产水作为反渗透系统进水的运行模式,相较单纯冷冻-重力脱盐模式,饮用水产水率由20.47%上升到45.1%;相较直接将原海水进行反渗透处理,水泵吨水能耗从3.39 kW·h下降到0.87 kW·h。     

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     

An alternative design concept in reverse osmosis desalination

A highly adaptable plate system for reverse osmosis and ultrafiltration with easily accessible flat membranes is introduced, employing a straight-channel construction of plastic components, designed to tolerate comparatively bold operation conditions at the calculated expense of membrane service life. Pilot installations are illustrated.     

Explicit flux expressions in tubular reverse osmosis desalination

Calculation of actual fluxes in reverse osmosis desalination through membrane tubes involves considerable iteration due to the existence of concentration polarization. Explicit flux expressions have been developed for such cases on the basis of the flux expressions of Lonsdale , Sourirajan, and Johnson to eliminate iteration. Low levels of solvent flux in existing membranes allow approximations leading to such explicit expressions. The computer flow diagram for reverse osmosis plant design with membrane tubes in series has been simplified by means of the explicit flux expressions.     

Fresh water by reverse osmosis based desalination: simulation and optimisation

The reverse osmosis (RO) desalination process to make fresh water from seawater has been studied here. First, a model for the process is developed. Sensitivity of different operating parameters (feed flow rate, feed pressure) and design parameters (internal diameter, total number of tubes) on the recovery ratio are studied via repetitive simulation. Finally, an optimisation framework for the process is developed so as to maximize the recovery ratio or a profit function using different energy recovery devices, subjectto general constraints. The optimal operating parameters (feed flow rate, feed pressure) and design parameters (internal diameter, total number of tubes) are determined by solving the optimisation problem using an efficient successive quadratic programming (SQP) based method. The optimal values for the decision variables depend on the constraints introduced, and are also sensitive to variations in water and energy prices, as well as feed concentration. The use ofthe emerging energy recovery devices is widely justified, reporting much higher reductions in operating costs than the traditional technology used for this purpose. Using a pressure exchanger device, it is possible to reduce energy consumption by up to 50%.     

Effect of recovery on desalination of the black sea water by reverse osmosis

The effect of fresh water recovery was studied during Black Sea water RO desalination with cellulose acetate tubular membranes. Two different schemes of brine pretreatment before RO were investigated.It was shown that ultrafiltration can be applied as an effective alternative method for Black Sea water pretreatment. Up to 30% recovery, the product flux as well as the salt and the specific ionic rejection are all fairly stable. Special experiments showed that in a stable turbulent regime Ca ions removal from Black Sea water before RO with tubular membranes hold out no technological advantages.