RO Process Research Articles

Assessment of fouling of an RO process dedicated to indirect potable reuse

Abstract Autopsies of three elements of a three-stages reverse osmosis process (lead, central and tail elements of respectively Stage 1, 2 and 3) treating microfiltered wastewater effluent were performed to quantify the propensity of each stage to foul. As expected, results showed fouling on Stages 1 and 2 to be predominantly biofouling whilst Stage 3, the most heavily fouled, was subject to most pronounced scaling, predominantly by calcium phosphate. Chloramine dosing in the RO feed water appeared largely ineffective for membrane biofouling mitigation due to its low rejection by the RO membrane leading to a very low residual in the RO retentate stream. Five antiscalants (three commercialised and two under development) were tested at pilot scale and their efficiency for scaling mitigation compared. Results showed wide variations in effectiveness, with the novel reagents showing the most promising results. However, none of the antiscalants tested succeeded in completely suppressing scaling without the addition of sulphuric acid. A cost analysis showed the operating cost to be most sensitive to pH adjustment, with reduced acid dosing requirement providing a signifi cant cost benefit.

The effects of feed water temperature and dissolved gases on permeate flow rate and permeate conductivity in a pilot scale reverse osmosis desalination unit

Feed water temperature is an important parameter in determining the optimum conditions for an efficient seawater reverse osmosis (SWRO) process. Increased feed water temperatures are known to increase the permeate flux rate in commercial SWRO systems. There are several factors which link feed water temperature to the operational efficiency of the fundamental membrane desalination process. In this study we have obtained precise data on these effects using two different types of RO membranes in a small scale pilot unit with feeds of seawater, brackish water and pure water. The mechanisms involved have been examined in this work. Pre-heating the feed water to enhance RO efficiency may lead to greater cavitation within the RO membrane. Vapour cavities formed by cavitation have the potential to hinder permeate flow by blocking sections of the polymer matrix in the skin layer of the membrane. In earlier work, it was identified that the presence of dissolved atmospheric gases in seawater leads to a potential for cavitation within the porous membranes used in high pressure RO processes. It was also established that the almost complete removal of these dissolved gases prevented this cavitation. The effects of de-gassing on the permeate rate in a small scale pilot SWRO system was reported recently. This work has been extended here to include more hydrophobic membranes, which are more likely to produce cavitation. In addition, there is new evidence to support the view that de-gassing the feed water can remove/reduce vapour cavities in the membrane for improved flow, which is maintained even when the feed water is re-gassed.

Application of membranes to treat wastewater for its recycling and reuse: new considerations to reduce fouling and increase recovery up to 99 percent

The RO process is very attractive for providing high product quality through the removal of a number of contaminants like oil, detergents, organics, nitrates, ammonia, phosphates etc. However the main disadvantages of the modern RO techniques are connected with membrane fouling, concentrate flow and pretreatment that increase operational costs, complicate design and create problems of brine utilization. Often pretreatment costs even exceed reverse osmosis facilities costs. Now a number of successful attempts were undertaken to modify spiral wound membrane channels to limit fouling and scaling potential. As presented in a number of publications, elimination of the spacer mesh from the feed channels eliminates dead regions that provide scaling and fouling conditions whilst also reducing the risk of particle “trapping” and associated dramatic pressure increase. Introduction of a new “open channel” configuration offers a new perspectives to escape fouling and development of novel techniques to treat water with high fouling potential (high suspended matter, organics and supersaturated solutions). This novel concept of spiral wound module with an “open channel” design is developed, field-tested and introduced into practice. Fouling control is achieved due to the elimination of spacer and the implementation of an optimum hydraulic mode providing sufficient cross flow velocities, flushings and cleanings. The main principles of high recoveries, low maintenance and zero discharge are ensured by concentration of brine through re-circulation by 50−100 times by volume. Several examples of water treatment flow diagrams are presented to demonstrate of zero concentrate flow discharge. Coagulated suspended matter after membrane flushes is collected, sedimented and finally dewatered. The concentrated solution containing salts and impurities together with the wet sludge constitutes no more than 1 percent of initial water in part due to high supersaturation values due to the strong stability of calcium carbonate solutions.

Ultrafiltration and reverse osmosis for clarification and concentration of fruit juices at pilot plant scale

The process of ultrafiltration of raw juices (peach, pear, apple and mandarin) through tubular polysulphone membranes (8 kDa) and finally concentrated by reverse osmosis membranes of tubular composite polyamide film (99 g/100 g NaCl rejection) with filtration area of 0.9 m2 was studied. Furthermore, in order to obtain a higher permeate flow, the fluids were previously treated with commercial pectinolytic enzyme.The following parameters were analyzed: permeate flow, soluble solids, viscosity, density, pH, acidity, formol index, starch, pulp content, color and presence of pectin. The use of a pectinolytic enzyme provides increased permeate flux of approximately 40%, retaining the juice properties. The highest total soluble solids content was clarified peach juice from 12.2 °Brix concentrated up to 30.5 and 21.52 °Brix by using RO with transmembranes pressures of 4 and 2 MPa respectively.The results show a cross-flow velocity (1.8 m/s) which corresponds with Reynolds number that is on the lower border of a laminar flow and above of transition rate (2090 < Re < 2900). Also the (VCF) for mandarin (1.49), pear (1.53), apple (1.56) and peach (1.58) was obtained for RO process. The membranes used were characterized morphologically using scanning electron microscopy.

Periodic control of a reverse osmosis desalination process

This paper addresses the forced periodic operation of a tubular reverse osmosis process for improved performance. The investigation is carried out through simulation of a previously validated model for the RO process. The feed pressure and feed flow rate are transformed into periodic behavior in the form of sinusoidal functions. A nonlinear model predictive control algorithm is utilized to regulate the amplitude and period of the sinusoidal functions that formulate the input signal. The control system managed to generate the cyclic inputs necessary to enhance the closed-loop performance in terms of higher permeate production and lower salt concentration. The proposed control algorithm can attain its objective with and without defining a set point for the controlled outputs. In the latter case, the process is driven to the best achievable performance. Similar successful results in terms of improved production and quality of water were also obtained in the presence of modeling errors and external disturbances.

Scaling propensity of seawater in RO boron removal processes

Boron contamination of permeate in seawater RO desalination is commonly prevented by increase of the concentrate pH level in a rather elaborate multi-pass processing system. The reason for this cumbersome processing is the need to control the scale precipitation tendency induced by the pH increase. The lack of sufficient information on the scaling propensity of seawater at various supersaturation levels hampers optimal design of boron controlled processes. The main objective of the present work was to provide scaling propensity data so as to assist design of boron reduction processes and explore the possibility of boron control by a single pass RO process. Membrane tests were conducted to determine induction periods and scaling threshold limits with and without an anti-scalant. The tests were carried out on Pacific Ocean seawater concentrates of 0% and 50% permeate recovery with both the presence or absence of Mg ions using either a continuous tubular flow or a spiral wound flow system. Results suggest that the concept of a single pass RO has a potential for simplifying the present rather cumbersome flow sheets of boron controlled processes.

Development of a simulation program for the forward osmosis and reverse osmosis process

Forward osmosis (FO) is an osmotic process that uses a semi-permeable membrane to effect separation of water from dissolved solutes by an osmotic pressure gradient. Unlike RO, FO does not require high pressure for separation, allowing low energy consumption to produce water. Therefore FO, a potential alternative to conventional membrane process, has been considered a novel technology for seawater desalination. There is no forward osmosis (FO) process simulation program yet, though. Therefore, the main objective of this paper is to develop such computer program based on the solution-diffusion model modified with the film theory for simulating and optimizing the FO, RO, and FO-RO hybrid process. The effect of concentration polarization on FO and RO process efficiency was also considered in the model. A MATLAB-based graphical user interface (GUI) program was used to develop the simulation program. Using the program, the effects of various factors, including the draw solution concentration, feed concentration...

Identifying, Evaluating, and Implementing Antiscalants in RO Reuse Applications

AbstractThe Orange County Water District (OCWD) operates a 70-mgd RO system as part of the Groundwater Replenishment System (GWRS), an advanced wastewater purification facility. Each of 15 5-mgd RO units consists of a three-stage array, operating at 85 percent recovery. As part of the operating strategy, antiscalant and pH suppression are used to limit third-stage precipitation of sparingly soluble mineral salts. Identifying successful antiscalants is extremely challenging. A variety of antiscalants with varying chemistries and capabilities present opportunities for optimizing RO processes, pricing, dosage, and pH adjustment. Recognizing the potential savings in RO pretreatment chemical cost, OCWD began antiscalant pilot-scale trials in 2009. However, failure to identify an effective product led to the trial's cancellation. Concurrently, the GWRS RO facility began experiencing third-stage permeability loss in all 15 RO units because the antiscalant was ineffective. This article discusses fast-track testin...

Energy analysis and efficiency assessment of reverse osmosis desalination process

A new concept of ideal RO process is introduced in this study for a more appropriate assessment of energy efficiency of water desalination, in which all the extra energy above the thermodynamic minimum is spent to maintain the required permeate flux. A pressure-recovery diagram was developed as a graphical method for better analyses and presentations of energy consumption in cross flow RO. It was demonstrated that the total energy input to a cross flow RO was much higher than the thermodynamic minimal energy for water desalination. Aside from the energy that remained in the retentate stream, a substantial amount of additional energy was needed in cross flow RO to maintain an economically meaningful nonzero permeate flux and overcome the elevated osmotic pressure due to salt accumulation along the membrane channel (configuration associated energy). The configuration associated energy became dominant at high recoveries and set the ultimate limit on the energy requirement of cross flow RO, which could not be reduced by further improvement in membrane permeability. Finally, the energy efficiency of cross flow RO was compared to an ideal RO process in which the configuration energy was completely eliminated.

Oily wastewater treatment using a hybrid UF/RO system

Oily wastewaters and oil–water emulsions are two of the major pollutants of the environment. Treatment of the oily wastewater using hybrid ultrafiltration/reverse osmosis (UF/RO) system was experimentally studied and the results were presented. Polyacrylonitrile and polyamide membranes were used as the UF and RO membranes, respectively. In this research, Taguchi method was used initially to plan a minimum number of experiments. An L9 orthogonal array was employed to evaluate effects of temperature (25, 37.5 and 5°C), TMP (1.5, 3 and 4.5 bar), CFV (0.25, 0.75 and 1.25 m/s) and pH (4, 7 and 10) on permeation flux, rejection and fouling resistance. According to the results, the optimum operating conditions of the UF process were found as following: TMP (3 bar), CFV (1 m/s), operating temperature (40 °C) and pH (9). The results indicated that the treated wastewater has sufficient quality to be introduced to the RO process as a pretreatment feed. Afterwards, the treated outlet water of the hybrid UF/RO sys tem...

Pilot-Scale MBR and RO Process for COD, Nitrogen and Phosphorus Removals of Mixed Domestic – Industrial Wastewater at 30°C in Singapore

Pilot-Scale MBR and RO Process for COD, Nitrogen and Phosphorus Removals of Mixed Domestic – Industrial Wastewater at 30°C in Singapore

Operating boundaries of full-scale advanced water reuse treatment plants: many lessons learned from pilot plant experience

Three Advanced Water Treatment Plants (AWTP) have recently been built in South East Queensland as part of the Western Corridor Recycled Water Project (WCRWP) producing Purified Recycled Water from secondary treated waste water for the purpose of indirect potable reuse. At Luggage Point, a demonstration plant was primarily operated by the design team for design verification. The investigation program was then extended so that the operating team could investigate possible process optimisation, and operation flexibility. Extending the demonstration plant investigation program enabled monitoring of the long term performance of the microfiltration and reverse osmosis membranes, which did not appear to foul even after more than a year of operation. The investigation primarily identified several ways to optimise the process. It highlighted areas of risk for treated water quality, such as total nitrogen. Ample and rapid swings of salinity from 850 to 3,000 mg/l-TDS were predicted to affect the RO process day-to-day operation and monitoring. Most of the setpoints used for monitoring under HACCP were determined during the pilot plant trials.

The energy demand for desalination with solar powered reverse osmosis units

There are few energy recovery devices (ERD) available on the market for small reverse osmosis units (RO). With reference to small units, installations have been considered which could be driven easily by a photovoltaic (PV) generator with an output of 200 litres up to 10 m3 desalinated water per day [1-14] . In this paper, a systematic analysis for typical reverse osmosis concepts is presented which allows better understanding of the energy demand of small RO processes with energy recovery and which can be used as a decision tool. In contrast to publications already in existence which compare energy recovery devices [ 9,10,12,15-27], the smallest devices are considered here. These are often positive displacement devices [3,10,16,28,29]. The new method of analysis takes as a basis the same physical parameters for all RO concepts, starting with the calculation of the osmotic pressure. Assumptions by an expert for possible operating parameters (recovery ratio, the feed and the concentrate pressure), known as...

Mass-transfer modeling of reverse-osmosis performance on 0.5–2% salty water

This work investigates the desalination efficiency from 0.5 to 2% salty water using the RO process and establishes mass-transfer models for water and salt transports. The water flux, salt passage rate, salt rejection and water recovery were studied under various operating conditions to validate the model adequacy. Salt permeability in the three-layer composite was lower than that predicted on polyamide–polysulfone top-intermediate layer using permeation cells, possibly due to the membrane compaction at a high pressure and/or additional mass-transfer resistance from the non-woven support. The permeate flux was 1.83–3.67 × 10 −5 m 3 m − 2 s − 1 for 0.5–2% NaCl feed solution, significantly higher than literature data. The salt concentration was reduced to 350–700 ppm from 1 to 2% NaCl solutions, rendering salt rejections of 91.0 to 98.4% under the tested operating conditions. The membrane intrinsic retention was 96.1 to 99.8% considering the increased solute concentration adjacent to the membrane surface resulting from concentration polarization phenomena. The salt flux did not show a particular trend with respect to the permeate flux resulting from various applied pressures, indicating a negligible flux coupling effect between the water and salt molecules. The salt transported mainly through solution-diffusion mechanism and water flow followed a pressure-driven process.

Life-cycle cost analysis of adsorption cycles for desalination

This paper presents the thermo-economic analysis of the adsorption desalination (AD) cycle that is driven by low-temperature waste heat from exhaust of industrial processes or renewable sources. The AD cycle uses an adsorbent such as the silica gel to desalt the sea or brackish water. Based on an experimental prototype AD plant, the life-cycle cost analysis of AD plants of assorted water production capacities has been simulated and these predictions are translated into unit cost of water production. Our results show that the specific energy consumption of the AD cycle is 1.38 kWh/m3 which is the lowest ever reported. For a plant capacity of 1000 m3/d, the AD cycle offers a unit cost of $0.457/m3 as compared to more than $0.9 for the average RO plants. Besides being cost-effective, the AD cycle is also environment-friendly as it emits less CO2 emission per m3 generated, typically 85% less, by comparison to an RO process.

Organic fouling of RO membranes: Investigating the correlation of RO and UF fouling resistances for predictive purposes

This study examines the characteristics of deposits from dead-end UF and cross-flow RO filtration of dilute mixtures of sodium alginate (SA) and humic acids (HA), which are representative of common foulants in RO and NF feed waters. The data are interpreted in terms of specific cake resistance, α. The presence of alginates in the mixed fouling layers, even in relatively small percentage, imparts characteristic properties to these deposits; i.e. a gel-like structure which is associated with reduced compressibility and permeability. With SA/HA mixtures, high rejection of organic species and high deposition factors are obtained in UF and RO tests, respectively. The specific resistance of mixed SA and HA deposits tends to increase with increasing SA concentration in solution. The resistances α exhibit fairly strong (power law) dependence on the pressure difference across the cake, ΔΡ c, for both RO and UF tests. The fairly satisfactory correlation of resistance α versus ΔΡ c data, obtained from both types of tests (for the mixtures of organic species tested at small salinity) is a significant result of this work. Based on this correlation, an approach is suggested for estimating fouling index I and initial membrane fouling rate of a specific RO process, relying on limited UF tests.

Concentrating primary reverse osmosis concentrate by direct contact membrane distillation

In the present work, a thermally driven membrane process-direct contact membrane distillation was investigated for concentrating the primary reverse osmosis concentrate to minimize the water loss. The primary reverse osmosis concentrate used in the work was obtained from the RO process of the direct drinking water preparation system designed for the 29th Olympic Games with 50% recovery. The feed water was Beijing tap water. Membrane fouling during the DCMD process was discussed, and acidification and accelerated precipitation softening was used as the pretreatment method to control the calcium scaling. Experimental results showed that acidification can alleviate and even eliminate CaCO3 scaling, but CaSO4 crystallization still occurred and led to a sharp decline of module efficiency. Accelerated precipitation softening enabled a high removal efficiency of Ca2 + , so both the CaCO3 and CaSO4 scaling was efficiently controlled. After proper pretreatment to control calcium scaling, the primary reverse osmosis concentrate could be concentrated 40 times and then the whole recovery was enhanced to 98.8%.

MBR 유출수 재활용을 위한 RO 막분리 공정에 대한 연구

S전자에서 발생되는 유기성 산업폐수의 생물학적 처리인 MBR 공정의 유출수를 LCD 제조공정의 용수로 재사용하기 위하여 <TEX>$32m^3/d$</TEX> 규모의 pilot-scale RO 막분리 공정을 구축하고 RO 막분리 공정의 운전압력과 투과유량에 따른 막간차압 및 CIP 주기 그리고 TOC와 전기전도도의 용질분리에 대한 영향을 분석하였다. MBR 공정의 유출수는 일반적인 처리수 재사용 수질기준을 만족하나 LCD 제조공정의 S전자 자체 재사용 용수 수질기준인 TOC<1mg/L와 전기전도도<<TEX>$100{\mu}S/cm$</TEX>를 만족하지 못하므로 후속 처리가 불가피하다. RO 막분리 공정의 회수율을 85%로 일정하게 유지한 상태에서 투과유량을 12.5 LMH에서 22.0 LMH로 증가시키면서 운전한 결과 모든 투과유량에서 RO 투과수는 자체 재활용 용수 수질기준을 만족하였다. 그러나 RO 막오염에 의한 막간차압이 상승되어 CIP 주기는 투과유량이 증가되면 짧아지는 효과가 나타났다. RO 막분리 공정의 최적 운전인자는 회수율 85%에서 투과유량 16.5~18.5 LMH이었으며 운전압력은 <TEX>$6.7{\sim}12.4kgf/cm^2$</TEX>, CIP 주기는 투과생산량/운전비에 적절한 20일~25일로 나타났다. Reuse feasibility of MBR effluent of S Electronic Company's organic wastewater as a LCD process water was investigated by a <TEX>$32m^3/d$</TEX> pilot-scale RO membrane process. The effects of operating pressure and permeate flux at constant 85% recovery of RO membrane process using MBR effluent were analyzed for transmembrane pressure and period for CIP by membrane fouling as well as rejection of TOC and conductivity. MBR effluent requires additional treatment to meet the LCD process water quality criteria of TOC<1 mg/L and conductivity<<TEX>$100{\mu}S/cm$</TEX> which is stringent as compared with those of conventional reuse water quality criteria. The RO process operated at 85% recovery with stepwise increasing of permeate fluxes from 12.5 LMH to 22.0 LMH was able to meet LCD process water quality criteria. However, the transmembrane pressure increased and the period of CIP decreased as increasing permeability fluxes due to fouling of RO membrane. The optimum operational conditions of RO membrane process were permeate fluxes of 16.5~18.5 LMH with operating pressure of <TEX>$6.7{\sim}12.4kgf/cm^2$</TEX> and CIP period of 20~25 days at constant 85% recovery.

Ultra-Nano Filtration: A Promising Practice to Supply Potable Water from Surface Resources

Membrane technology has grown rapidly in Iran in the last decade, however most of the installed capacities are to produce industrial water and rarely for potable water. Most of the installed plants use RO process to produce industrial and even potable water. In this study, Nanofiltration (NF) technology was employed for Karoon River desalination as an economic and energy saving treatment process. Unstable quality and high turbidity of Karoon River results in fouling on NF/RO membranes, leading to chemical washing once a month and replacing membranes within 6 month or per year. Thus, unlike of many water treatment plants which benefit traditional pretreatment processes; ultra-filtration (UF) has been used as an effective and helpful pretreatment process. To make the proposed system reliability more clear, UF-NF results after three month of operation were compared with an equivalent RO system. In fact, UF-NF system benefits a lower pressure drop due to lower fouling on membranes and also more suitable permeate quality for drinking than RO. Hence, the promising results of the present practice may offer a reasonable solution to solve potable water supply problems in some deprived and arid regions.

Energy Aspects in Osmotic Processes

Water, energy and environmental issues are on the top list of the world problems. Energy is needed for augmenting our water resources. Renewable energies are hardly the answer since innovative techniques based on biofuels and biodiesel consume an incredible amount of water. The modern desalination techniques in use consume different energy levels from different sources. Thermodynamics sets the absolute minimum limit of the work energy required to separate water from a salt solution. Unavoidable irreversibilities augment the actual energy consumption. Modern desalination techniques have succeeded in narrowing considerably the gap between actual and minimum energy levels. The implication of this small gap is that only marginal energy reductions are possible. Energy consumption of different desalination processes are reviewed. Forward osmosis is shown to be a high energy consumption process. It offers, however, advanced cost effective backwash techniques. The limitations of power generation by osmotic processes is discussed. Sidney Loeb together with Sourirajan were the pioneers that opened the door and introduced the RO process that allows us to desalinate seawater and brackish water at affordable energy consumption. Loeb continued during his last years to develop the osmotic energy machine, and even tried to develop an air condition system based on water evaporating from tubular membranes.