RO Plant Research Articles

Chemical elements of brine discharge from operational Tajoura reverse osmosis desalination plant

Despite the fact that desalination is an important technology for providing desalted water, it has some disadvantages that have not been taken seriously into consideration. Such technology can have undesirable effect on the environment. Environmental related impact could occur from the early days of plant construction and, subsequently, when the plant is in operation. Tajoura reverse osmosis desalination plant is the largest desalination plant of its kind in Libya. It has been under operation for nearly three decades without serious consideration of its impact on the environment. This paper is the first attempt in that direction. The main objective of this study is to measure the chemical components of brine concentrate discharged from Tajoura plant, and to highlight their impact on the environment. This study was conducted in the Water Desalination Department in Tajoura Nuclear Research Center. Samples were collected from feed source (seawater), and from reject stream (brine discharge point). Samples were analyzed at water department’s facility using suitable instrument. The obtained results show differences in most of the measured chemical components between seawater and brine discharge which was highly expected. It was found that brine discharge contained traces of metals such as chromium and none of some other metals such as iron. Therefore, contamination with metals is below a critical level. This can be attributed to the fact that metal equipment is not used in Tajoura RO plant. The major concern of brine discharged from Tajoura plant is represented by its high salinity as its increase from feedwater exceeds by far 1 psu.

A Cost-Effective Wind Power-Driven RO Plant for Treatment of Brackish Water

Desalination has often been perceived as a high cost means of producing water. Reason being that some of the water treatment methods use electricity to run a pump as in Reverse Osmosis system and some uses thermal energy to produce steam as in Multi Effect Distillation. High cost and less availability of conventional fuels has attracted the focus of researchers to explore other unconventional means and methods to purify water. This paper explores the possibility of using wind power for water treatment. In costal countries, tremendous wind power is available at the coastline which may be used to drive a RO plant. In India particularly, the average wind speed calculated along the coastlines of Gujarat state gives a fair idea about potential of energy that can be generated. This energy can be harnessed to run a rotating pump, or a piston-cylinder arrangement coupled with RO module to provide the necessary pressure required to push saline water through the membrane. Along the western coast of India there is an abundance of wind energy to capitulate high amount of energy in terms of electrical Power. If the rotor diameter is 4 feet (1.3 m) and located at a height of 8 feet (2.5 m), each wind turbine may produce power of around 2 KW. This power may be used to run the reciprocating pump which provides pressure to the RO Apparatus. The paper describes recent advancements in the wind industries technology to produce electrical energy at low cost but it concentrates more on coupling wind energy directly with RO module to save energy conversion cost which is a seldom or different approach than converting electrical energy to mechanical energy for running of RO system.

Optimal operations for large-scale seawater reverse osmosis networks

Optimal operation of large-scale seawater reverse osmosis (SWRO) systems, which include multiple RO plants and storage tanks, are studied to reduce the energy cost; and at the same an effective computing method was introduced for the purpose of real time optimization. With well developed models of the RO process and storage tanks based on the first principle, the optimal operation problem is subsequently formulated in the form of differential-algebraic optimization problems (DAOPs). Simultaneous collocation method was used to transform the problem into a large-scale nonlinear programming problem, and then IPOPT solver, combined with an efficient initial-value and finite-element meshing technique, was used for fast solution. Then the optimal operation problem with different variable parameters that significantly affect the performance and the energy saving of the SWRO system were investigated through case studies. Computational results show that the optimal operation problem under different conditions can be solved with high efficiency and stability, and significant energy-saving potential can be obtained through the optimal operation proposed in this paper.

Improved method for measuring transparent exopolymer particles (TEP) and their precursors in fresh and saline water

Transparent exopolymer particles (TEP) and their precursors produced by phyto-/bacterio-planktons in fresh and marine aquatic environments are increasingly considered as a major contributor to organic/particulate and biological fouling in micro-/ultra-filtration and reverse osmosis membrane (RO) systems. However, currently established methods which are based on Alcian blue (AB) staining and spectrophotometric techniques do not measure TEP-precursors and have the tendency to overestimate concentration in brackish/saline water samples due to interference of salinity on AB staining. Here we propose a new semi-quantitative method which allows measurement of both TEP and their colloidal precursors without the interference of salinity. TEP and their precursors are first retained on 10 kDa membrane, rinsed with ultra-pure water, and re-suspended in ultra-pure water by sonication and stained with AB, followed by exclusion of TEP-AB precipitates by filtration and absorbance measurement of residual AB. The concentration is then determined based on the reduction of AB absorbance due to reaction with acidic polysaccharides, blank correction and calibration with Xanthan gum standard. The extraction procedure allows concentration of TEP and their pre-cursors which makes it possible to analyse samples with a wide range of concentrations (down to <0.1 mg Xeq/L). This was demonstrated through application of the method for monitoring these compounds in algal cultures and a full-scale RO plant. The monitoring also revealed that concentrations of the colloidal precursors were substantially higher than the concentration of TEP themselves. In the RO plant, complete TEP removal was observed over the pre-treatment processes (coagulation-sedimentation-filtration and ultrafiltration) but the TEP precursors were not completely removed, emphasising the importance of measuring this colloidal component to better understand the role of TEP and acidic polysaccharides in RO membrane fouling.

Heating seawater: the solution or the pipe dream of reverse osmosis electrical consumption

The desalination market based on reverse osmosis is a growing market thanks to much lower energy consumption compared to thermal desalination. This leading position implies that a lot of efforts should be done to improve both its acceptability and cost to thwart its detractors. For those reasons, the further improvement of the energy consumption is a key factor and one of the main objectives of the desalination community. It is well known that temperature has an impact on membranes’ feed pressure. For equivalent feed water quality and operational conditions, high temperature water will require less pressure to produce the same amount of permeate. Based on this reality, some projects are considering the opportunity to recover cooling waters from cooling towers or MSF/MED cooling system as seawater feed to RO plants to reduce electrical consumption, in particular on hybrid, IWPP, or co-location projects. But temperature has also an adverse effect on the permeate water quality that could result in increasing the 2nd Pass flow requirement or the use of tighter membranes. The present paper analyses the impact on electrical consumption and design of the complete RO plant using a heated seawater source. Through case studies based on typical middle east seawaters, 41 g/l and 45 g/l and two different permeate water quality targets, 200 mg/l and 500 mg/l TDS, it attempts to conclude if overall benefits can be identified in such conditions.

Technical feasibility of a seabed gallery system for SWRO facilities at Shoaiba, Saudi Arabia, and regions with similar geology

Subsurface intakes can be used as part of the pretreatment system for seawater reverse osmosis facilities. Wells of various designs and galleries are being used for intakes at many sites globally to reduce pretreatment costs, chemical usage, biofouling potential, and environmental impacts (entrainment and impingement). The goal of using a subsurface intake is to reduce or replace conventional pretreatment processes. A field and laboratory investigation was undertaken to assess the feasibility of developing a seabed gallery intake offshore near the Shoaiba seawater RO (SWRO) plant site (150,000 m3/d treatment capacity) that could be used to replace the current intake used for only the RO capacity. A survey of the beach and offshore area was made and 51 sediment samples were collected from the seabed for laboratory analysis of grain size distribution, porosity, and hydraulic conductivity. Field observations showed that the marine bottom has a low slope from the shoreline seaward a distance of about 100 m to a depth of about 1.0 m before it steepens to a depth of over 2 m at 150 m from shore. The site has a relativity thin cover of unlithified carbonate sands, 2–5 cm thick, sitting on a soft coralline limestone of Pleistocene age. The sediments investigated were found to be clean carbonate or slightly muddy carbonate sands with mean grain diameters ranging mostly between 0.25 and 0.5 mm. Most of the area investigated contained a mud percentage between 0 and 5%. The measured porosity values range between 0.33 and 0.45 and measured hydraulic conductivity values from about 1.6 to 79.5 m/d with 64.6 m/d being the maximum offshore value. A preliminary design was developed to meet the full operational capacity of the Shoaiba SWRO treatment plant which would require 375,000 m3/d of raw water to produce 150,000 m3/d of permeate (40% conversion assumed for Red Sea water with a TDS of 41,000 mg/L). The design of the RO plant uses 10 trains to produce the 150,000 m3/d of product, but at full operational efficiency, only nine trains would produce the required capacity. The engineered gallery design included five layers with a total thickness of 3 m. The proposed infiltration rate is 7 m/d with the possibility of increasing it to 10 m/d. The gallery design consists of 10 cells (one for each train) producing 42,000 m3/d each. The cells have dimensions of 30 × 200 m and an aggregate area of 60,000 m2. It is believed that use of the seabed gallery at this site is feasible and would reduce environmental impacts by eliminating impingement and entrainment of marine organisms, the use of chlorine in the process, the use of coagulants in the pretreatment filtration process, the disposal of marine debris and coagulant sludge, and may eliminate the need for use of any additional pretreatment before the cartridge filters.

A Dynamic Optimization Strategy for the Operation of Large Scale Seawater Reverses Osmosis System

In this work, an efficient strategy was proposed for efficient solution of the dynamic model of SWRO system. Since the dynamic model is formulated by a set of differential-algebraic equations, simultaneous strategies based on collocations on finite element were used to transform the DAOP into large scale nonlinear programming problem named Opt2. Then, simulation of RO process and storage tanks was carried element by element and step by step with fixed control variables. All the obtained values of these variables then were used as the initial value for the optimal solution of SWRO system. Finally, in order to accelerate the computing efficiency and at the same time to keep enough accuracy for the solution of Opt2, a simple but efficient finite element refinement rule was used to reduce the scale of Opt2. The proposed strategy was applied to a large scale SWRO system with 8 RO plants and 4 storage tanks as case study. Computing result shows that the proposed strategy is quite effective for optimal operation of the large scale SWRO system; the optimal problem can be successfully solved within decades of iterations and several minutes when load and other operating parameters fluctuate.

Energy-Optimal Control of RO Desalination

A novel model-based control system for the operation of a spiral-wound RO membrane desalination system was developed with a focus on maintaining energy-optimal operation. The control scheme utilized an operational model for spiral-wound RO desalting with a supervisory controller providing real-time updates of membrane permeability and the appropriate feed pressure set-points for maintaining the target permeate productivity at the lowest feasible specific energy consumption. System RO feed pressure and flow rates were controlled by a lower-level RO controller through adjustment of the RO high pressure feed pump, variable frequency drive, and RO concentrate valve. Seawater desalination tests with an RO plant, capable of permeate productivity up to 18 000 gallons/day, demonstrated effective self-adaptive energy-optimal operation, subject to feed salinity fluctuations, constraints imposed by the system’s physical limitations (i.e., minimum and maximum feasible operational pressures and flow rates), and the th...

Ultrafiltration versus sedimentation-based pretreatment in Fujairah-1 RO plant: Environmental impact study

Commonly, the main contributors to the environmental impacts of desalination are energy consumptions and chemical usage. In this work, two alternatives for pretreatment of seawater feed to Fujairah-1 seawater reverse osmosis (SWRO) plant, in United Arab Emirates, were compared: sedimentation-based and ultrafiltration (UF) membrane-based pretreatment. Life cycle analysis (LCA) methodology was applied in this case study research, based on real data from the Fujairah-1 plant for the sedimentation-based system and full design data for the alternative UF system, provided by X-Flow®. By use of the LCA, a comprehensive environmental modeling of the considered options was conducted. The pretreatment systems, including their components (e.g., equipment) and material and energy flows throughout their lifetime, were modeled. Environmental impacts of these systems, using alternative energy mixes, were also modeled.Our findings indicate that a membrane-based pretreatment has a lower environmental impact, throughout the system's life cycle, than the existing sedimentation-based pretreatment in the Fujairah-1 desalination plant. This result holds under all alternative energy mix assumptions; including renewable, natural gas, oil, and nuclear energy. Our findings also show that the most significant contributor to the overall environmental impacts of both pretreatment systems is energy consumption.

Data-driven models of steady state and transient operations of spiral-wound RO plant

The development of data-driven RO plant performance models was demonstrated using the support vector regression model building approach. Models of both steady state and unsteady state plant operation were developed based on a wide range of operational data obtained from a fully automated small spiral-wound RO pilot. Single output variable steady state plant models for flow rates and conductivities of the permeate and retentate streams were of high accuracy, with average absolute relative errors (AARE) of 0.70%–2.46%. Performance of a composite support vector regression (SVR) based model (for both streams) for flow rates and conductivities was of comparable accuracy to the single output variable models (AARE of 0.71%–2.54%). The temporal change in conductivity, as a result of transient system operation (induced by perturbation of either system pressure or flow rate), was described by SVR model, which utilizes a time forecasting approach, with performance level of less than 1% AARE for forecasting periods of 2s to 3.5min. The high level of performance obtained with the present modeling approach suggests that short-term performance forecasting models that are based on plant data, could be useful for advanced RO plant control algorithms, fault tolerant control and process optimization.

Rapid field assessment of RO desalination of brackish agricultural drainage water

Rapid field evaluation of RO feed filtration requirements, selection of effective antiscalant type and dose, and estimation of suitable scale-free RO recovery level were demonstrated using a novel approach based on direct observation of mineral scaling and flux decline measurements, utilizing an automated Membrane Monitor (MeMo). The MeMo, operated in a stand-alone single-pass desalting mode, enabled rapid assessment of the adequacy of feed filtration by enabling direct observation of particulate deposition on the membrane surface. The diagnostic field study with RO feed water of high mineral scaling propensity revealed (via direct MeMo observation) that suspended particulates (even for feed water of turbidity <1 NTU) could serve as seeds for promoting surface crystal nucleation. With feed filtration optimized, a suitable maximum RO water recovery, with complete mineral scale suppression facilitated by an effective antiscalant dose, can be systematically and directly identified (via MeMo) in the field for a given feed water quality. Scale-free operating conditions, determined via standalone MeMo rapid diagnostic tests, were shown to be applicable to spiral-would RO system as validated via both flux decline measurements and ex-situ RO plant membrane scale monitoring. It was shown that the present approach is suitable for rapid field assessment of RO operability and it is particularly advantageous when evaluating water sources of composition that may vary both temporally and across the regions of interest.

Voltammetric Determination of Boric Acid Using the Ternary Complexation System with Salicylaldehyde and H‐Acid

AbstractA square‐wave voltammetric method for the determination of boric acid in water has been described based on the new understanding of the electrochemical behavior of boric acid‐Azomethine H complexation. Salicylaldehyde and H‐acid were used as the starting materials of boric acid‐Azomethine H complex and their concentrations were optimized for boric acid determination in water. A glassy carbon electrode, instead of a conventional mercury electrode, was used in the measurement. The detection limit of the proposed method was 0.10 mg B dm−3. The proposed method was successfully used for boric acid determination in the water from a seawater desalination RO plant.

Incipient calcium carbonate scaling of desalination membranes in narrow channels with spacers—experimental insights

Significant uncertainty exists regarding incipient membrane scaling (a criterion for determining water recovery and antiscalant use) with negative impact on optimization of RO plant design and operation. Experimental data are reported on the initial phase of RO-membrane scaling by weakly supersaturated brackish water (calcite supersaturation ratio S∼1.9 to 3.1) in a narrow spacer-filled channel, simulating local (constant flux) conditions in spiral-wound membrane (SWM) modules. Scanning electron microscopy image analyses of numerous membrane samples, combined with complete physico-chemical characterization of feedwater and retentate, provide new insights into early-phase membrane scaling. The unavoidable presence of small particles in feedwater (despite the use of tight cartridge filter) apparently promotes heterogeneous nucleation. Additionally, there is a clear evidence that, under these conditions, induction period for membrane scaling is practically non-existent. The initial deposit flux [mgCaCO3/(m2min)] on the membrane, obtained through measured crystal size distributions, exhibits strong dependence on supersaturation ratio; moreover, measured deposit fluxes are substantial and (if not mitigated) would be eventually detrimental to SWM performance. These data were obtained, while none of the frequently used parameters (bulk fluid turbidity, pH, Ca concentration, permeate flux) indicated particle/scale deposition on membranes. Implications of the new results, on practical and theoretical aspects of scaling, are discussed.

Operational experience of the brackish water reverse osmosis of the water treatment plant in Sant Joan Despí

To ensure correct operation of any facility that uses membrane treatment technology (ultrafiltration, reverse osmosis, etc.) requires a set of maintenance and cleaning operations, which allow correct operation and optimum duration of membranes, and also ensure the quality of water produced. In 2009, the existing drinking water treatment plant in Sant Joan Despí started operating a treatment combining ultrafiltration and reverse osmosis technologies to ensure compliance with the health limits set by the Spanish Royal Decree RD140/2003. Specific antiscalant was developed especially for controlling calcium carbonate, calcium phosphate, and barium sulfate scales, which are the most problematic insoluble species in the brine of the reverse osmosis at the water treatment plant in Sant Joan Despí. Specific software was developed to calculate the optimum dose of antiscalant based on the feedwater chemistry, the temperature, and the RO plant’s conversion, thus minimizing operating costs. Furthermore, Adiconline RO was installed to measure online and in real time the concentration of antiscalant in the feed. This system analyzes the active ingredients of the antiscalant and allows controlling the dosage level of antiscalant at the required level. The operational data of the RO and the results of autopsy of different elements confirmed that actions taken had been appropriate to provide efficient operation.

Self-adaptive feed flow reversal operation of reverse osmosis desalination

The technical feasibility of operating a spiral-wound RO plant in a cyclic mode of feed-flow reversal (FFR) was evaluated for brackish water desalting under conditions of high mineral scaling propensity. Scale-free and continuous permeate productivity was demonstrated, with calcium sulfate as the model scalant, in an automated spiral-wound RO pilot system in which FFR was triggered by scale detection in an external membrane monitor (MeMo). Real-time detection of mineral scaling in an external RO MeMo cell, receiving its feed from the concentrate of the RO plant tail element, enabled cyclic FFR operation in a self-adaptive mode accomplished by feed-back RO plant control in which permeate productivity was maintained. Membrane permeability was restored after each FFR cycle even with the initiation of membrane cleaning (i.e., via FFR) after a measurable level of scale formation in the MeMo and spiral-wound RO pilot. FFR cycle periods varied in length given the stochastic nature of crystal nucleation on the membranes in both the RO plant and in the MeMo RO cell. Scale-free FFR operation was demonstrated, without anticalant addition, with the RO plant operating (up to 81% recovery) such that the gypsum saturation index was up to 3.45 at the membrane surface of the tail element.

Validation of model-based optimization of brackish water reverse osmosis (BWRO) plant operation

A previously developed model-based optimization methodology is validated in an industrial brackish water reverse osmosis (BWRO) desalination plant in Southern California. A close match between model predictions and plant data in numerous cases indicates that the mathematical model accurately describes the main characteristics involved in typical industrial BWRO processes. It is demonstrated in this desalination plant that a 10% reduction in energy consumption can be achieved while maintaining the same permeate flow rate by adjusting operating conditions as suggested by the model. Furthermore, an opportunity for significant savings in disposal cost is identified due to a reduction in brine volume. This work clearly shows the effectiveness of model-based optimization in RO plant operation.

Drinking water supply by reverse osmosis plants: three years of experience at El Prat de Llobregat Municipality

Deep coastal aquifer of Llobregat delta constitutes the main source of drinking water supply to El Prat de Llobregat municipality (64,000 inhabitants) near Barcelona (Spain). Since 70s this aquifer initiates a gradual salinization process by seawater intrusion, magnified by an exhaustive and not ordered water extraction. Also, aquifer pollution by volatile organic compounds (trichloroethylene and tetrachloroethylene) appeared in 90s. In order to remove groundwater pollution, three air stripping plants were built 15 years ago. When groundwater salinity achieved non-drinking water standards, two water treatment plants (Sagnier WTP and Mas Blau WTP) were built using reverse osmosis technology. Both WTPs by RO has a pre-treatment based in volatile organic compounds removal by air stripping, in-line coagulation (only for one WTP), multimedia filtration, cartridge filtration and scaling inhibitor dosage. Chlorination and dechlorination is available but it is not used. Each WTP has two RO lines with two stages and 75% recovery design. As energy recovery device, Turbo Charger is working as booster pump between first and second stage. Drinking water quality according to Spanish regulations is achieved using filtered water by-pass blend and postchlorination before distribution tank. Operation and maintenance aspects are monitored using supervisory control and data acquisition register and remote/local control by O&M team of Aigües del Prat (public company). Water quality (raw water, filtered water, RO feed, permeate, blend, distribution tank and point of use) is monitored daily by Aigües del Prat laboratory (accredited under ISO 17025) according to drinking water Spanish regulations and municipality health criteria. Since January 2009, Aigües del Prat produces without interruption the 90% of drinking water of El Prat municipality using RO plants. This paper presents the experience acquired on operation and maintenance, reliability, drinking water quality distributed control, groundwater quality evolution, network changes and public perception.

RO membrane mineral scaling in the presence of a biofilm

Abstract The influence of a pre-existing biofilm on RO membrane mineral scaling was evaluated using gypsum as a model scalant. The biofilm was established using microfiltered secondary treated wastewater effluent on-site in a wastewater treatment facility. Mineral scaling was then monitored via direct visual observation of crystal growth on the membrane surface in a transparent plate-and-frame RO cell. SEM imaging of the membrane surface and sectioned biofilm revealed gypsum crystals within the biofilm matrix, including the protrusion of crystal rods from within the biofilm. Both mineral scale surface coverage and crystal number density were greater in the presence of the biofilm. Moreover, individual mineral crystal growth rate within the biofilm was significantly higher relative to growth in the absence of a biofilm. Analyses of crystal growth and nucleation rates, within the biofilm and in its absence, suggest concentration polarization enhancement within the biofilm. The present study suggests that mineral scaling may not be restricted to tail elements in RO plants, but could also occur in lead elements where biofilms can enhance concentration polarization. Therefore, understanding of the coupling of biofouling and mineral scaling may be of particular significance to the operation and design of RO plants in water reuse applications.

Techno-economic analysis of combined concentrating solar power and desalination plant configurations in Israel and Jordan

Combined concentrating solar power (CSP) and desalination plants represent a realistic future option for the production of electricity and fresh water for countries of the world’s sunbelt. In this paper, parabolic trough power plants for electricity production have been analysed in combination with multi-effect distillation (MED) and ultrafiltration/reverse osmosis (RO) desalination plants for two sites in Israel (Ashdod) and Jordan (Aqaba). Both RO and MED desalination plants were designed for a fresh water production capacity of 24,000 m3/d. The power block of the CSP plant was selected to meet the steam consumption of the MED plant at the design point, which led to a gross electrical power generation capacity of the power block of 42 MWel. Due to the low availability and generally high cost of coastal land, the CSP + RO plant consists of two separate units. It was assumed that the CSP plant is located at an inland location where there is land available. The RO plant is located at the sea, while the MED plant is located at the CSP site. The pumping of the seawater and the water transmission system add about 0.2 $/m3 to the levelized water costs of the CSP + MED plant compared with a plant located at the sea. Three different sizes of high temperature heat storages (0 h, 6 h and 12 h of additional full load operation of the steam turbine) were applied to find the most economic setup. At current prices for heat storage units, systems with huge heat storage capacities become economic only at high feed-in tariffs for electricity and thus high revenues. The price of the electricity generated by the CSP plant was varied to show the influence of the feed-in tariff on the water generating costs. The levelized costs of water (LCOW) strongly depend on the electricity price. Water costs in Ashdod are higher than those in Aqaba due to the lower solar irradiance. For Aqaba, LCOW of about 1 $/m3 can be realized if a feed-in tariff of about 0.24 $/kWh for electricity from renewable energy sources is established.

The relation between the RO fouling membrane and the feed water quality and the pretreatment in Djerba Island plant

The reverse osmosis system is exposed to many problems that affect its efficiency. The major problem is fouling membrane which is frequently related to the interaction between the quality of feed water and the pretreatment process. This paper treats two problems of RO plant in Djerba Island, the sulfuric water and the fouling membrane. For the first problem, the anaerobic process showed better performance than the aerobic process. Furthermore, the aerobic process of non-sulfuric water was affected after 6 years of operation with a fouling phenomenon at R.O membrane. This problem was correlated with the increase of the operating pressure, pressure drop and permeates conductivity. To identify causes of these problems, different investigations are carried out. Membrane autopsy, chemical analyses of feed water and follow-up of the operation parameters are performed. The membrane autopsy was achieved by SEM/EDS. The results show that the film is composed mostly with organic matter, silica, metallic matter and aluminum phosphate. This result is correlated with the phosphonate based antiscalant composition. Therefore, antiscalant selection and dosage appear to be critical for membrane fouling development.