Membrane Desalination Systems Research Articles

Biofouling control by hydrophilic surface modification of polypropylene feed spacers by plasma polymerisation

Biofouling compromises the efficiency of membrane desalination systems, especially energy and product water quality. Feed spacers employed in conventional spiral wound membranes create turbulence to minimise concentration polarisation. Research demonstrates that feed spacers can enhance biological growth within membrane elements. In combination with the strong link between fouling and surface hydrophobicity chemical modification of feed spacers could help counteract biofouling potential. In this study, an evaluation of feed spacers coated with diglyme using plasma polymerisation was assessed. Low energy density treatment was associated with an increase in biofouling, compared with the control, consistent with insufficient cross-linking of the diglyme monomer to the spacer surface. Increasing energy density treatment resulted in fouling performance approaching that of, or slightly better than the control. It is possible that batch recirculation type experiments may not best facilitate assessment of biofouling potential in single-pass cross-flow membrane systems. The study demonstrated that plasma treatment of conventional feed spacers has potential to reduce affinity for bacterial attachment, and may provide a viable and complementary approach to direct membrane surface modification for biofouling control. Further studies to quantify changes associated with diglyme plasma polymerisation and establish optimum conditions for biofouling minimisation are recommended.

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...

A feasibility study of municipal wastewater desalination using electrodialysis reversal to provide recycled water for horticultural irrigation

A membrane desalination system based on electrodialysis reversal (EDR) has been evaluated for its capacity to remove salt from treated municipal wastewater to provide a source of recycled water for horticultural applications. Economic and technical feasibility was determined using data collected from a pilot scale plant, from which the following parameters were calculated: salt removal, water recovery and overall process economics. The pilot plant consisted of a pre-treatment multimedia filtration unit (MMF) and an EDR system with a capacity of approximately 144kL/day. Treated effluent from a wastewater treatment plant (WWTP) was used as feed water for the desalination pilot plant. Water quality guidelines for horticulture specify an upper limit for total dissolved solids (TDS) of 375mg/L. The EDR process reduced the TDS from 1104mg/L to 328mg/L. Additionally, the process reduced the conductivity of recycled water by 72%, including the removal of 84% calcium, 76% chloride, 59% fluoride, 64% alkalinity and 60% phosphate, demonstrating that the EDR treated water is a viable alternative supply. The power consumption of the EDR plant was found to be 0.6kWh/kL and the media filtration 0.4kW/kL. The total operating cost was estimated to be 18cents/kL to deliver 82% water recovery.

Evaluation of new compact pretreatment system for high turbidity seawater: Fiber filter and ultrafiltration

Hyosung Goodsprings has embarked on development of new seawater reverse osmosis (SWRO) desalination technologies through construction and operation of SWRO research facility with 2000m3/day production capacity.The pilot plant includes a new compact pretreatment system to treat high turbidity seawater. The pretreatment system includes double stage stretching-releasing type fiber filter (FF) followed by ultrafiltration unit. This configuration shows successes in producing consistent quality of feed water to RO membrane of less than 3 SDI15 even during periods of high turbidity seawater reaching levels of 52NTU. The FF pretreatment has very small foot print and competitive operation cost in terms of chemical and energy consumption. However, the main concern of stretching-releasing type fiber filter is physical integrity of the fibers over the equipment lifetime. Field results for two years operation show that fiber bundles made of polypropylene break frequently and need to be replaced with new bundles annually. Improvement of durability of fiber material and filter equipments is necessary for application of FF in commercial desalination plants.The stable operation of RO unit provides support to the above conclusion that limited size pretreatment process evaluated in this study could be adequate for membrane desalination system treating high turbidity seawater.

Fluorescence spectrum-based biofouling prediction method for RO membrane systems

Monitoring reverse osmosis (RO) membrane conditions is an important task because it helps reduce the operation and maintenance cost in the RO membrane desalination systems by achieving long membrane lifetime and energy saving. As biological interactions between the membrane itself and microorganism cause the rapid degradation of membrane performance, it is crucial to identify and quantify potential biofoulants that are sensitive to each specific RO membrane. This study proposed a biofouling prediction method that indirectly quantifies the degree of biofouling by comparing the fluorescence excitation-emission matrix (EEM) of foulants sampled on the fully fouled RO membrane and those of brine samples from currently operating RO system. The experiment showed that the similarity distance measured from the comparison between the two fluorescence EEMs tends to increase when brine samples were secured from relatively clean RO membranes.

Solar desalination of Gas Refinery wastewater using membrane distillation process

Membrane desalination driven by solar energy is a new technology to purify high saline wastewater from oil fields. This system is based on pervaporation and its required energy is supplied by solar radiation. To evaluate the performance of this system in saline oily effluent treatment, one pilot scale membrane desalination system with a surface area of 40m2 is installed in Sarkhon zone near the port of Bandar Abbas in southern Iran. The system consists of a membrane evaporator located under a transparent plastic cover foil, which is UV resistant, placed on the ground. The feed for this unit is supplied from the deoiled effluent of the Sarkhon Gas Refinery. An average production rate of desalinated water for the period of spring 2005 is 1.3L/(d.m2). The quality of desalinated water meets the irrigation water standard of Iranian Department of Environment most of the time. The purification performance (contaminated versus purified water) of the system loaded with the Sarkhon Gas Refinery wastewater is total dissolved solids (TDS) from 1991 reduced to 91mg/L, conductivity from 3342 reduced to 150μS/cm, chloride from 1565 reduced to 6.6mg/L and Oil and Grease from 31 reduced to 1.12mg/L.

A new phosphorous-free antiscalant for membrane desalination

Phosphorous containing antiscalants, which are commonly used in RO desalination process, have become an environmental concern for RO concentrate disposal. These chemicals are suspected of contributing to algal blooms in the water bodies where RO concentrate is discharged. Therefore, phosphorous-free antiscalants are increasingly being required for both brackish and seawater membrane desalination systems. Nalco recently developed a phosphorous-free antiscalant (PC-1611T) to address this environmental concern. In the laboratory testing, this product performed similar to ATMP (commonly used phosphonate based antiscalant) for CaCO3 scale inhibition up to 3.0 LSI (Langelier Saturation Index). It also inhibited CaCO3 scale in presence of up to 1 ppm Fe3+ and residual levels of poly(DADMAC) based pre-treatment coagulant. It also showed 2–5× lower bio-growth contribution potential than polycarboxylates and is compatible with polyamide RO membranes. Full-scale evaluation of this product is in progress and the results for first 4 weeks show stable RO performance, indicating successful scale-control.

Ozone oxidation of desalinated seawater, with respect to optimized control of boron and bromate

One of the two main objectives of this study is to use two pass membrane desalination system and to oxidize permeate of the first pass with ozone, as disinfection by products (DBPs) formed by ozone are rather limited. Second objective of this work; is to increase the pH levels to 10 in the permeate of the first desalination pass in order to promote dissociation of boric acid (B(OH)3) to borate (B(OH)4−). Negatively charged borate can easily be removed by membrane systems because of the charge repulsion. Oxidation efficiency of ozone under pH 10 conditions and the buffer effect of borate were also focused on in this study. Double pass nanofiltration/ reverse osmosis (NF/RO) membrane filtration systems were used for the desalination of model seawater. Permeate of the first membrane filtration was adjusted to pH 10 and oxidized with ozone for 30 mins. Oxidized permeate was then fed to the second membrane filtration in order to investigate boron removal and bromate formation and removal. Boron removal was imp...

Submerged hollow fiber ultrafiltration as seawater pretreatment in the logic of integrated membrane desalination systems

Integrated membrane systems have been recently proposed as suitable alternatives to overcome existing limitations in traditional seawater reverse osmosis desalination plants. By this approach, submerged hollow fiber ultrafiltration have been chosen as pretreatment because of the lower energetic consumption and the great significance for reducing particles deposition on the membrane. This work focuses on the study of the influence of a set of operating conditions and the fouling dynamics when operating submerged ultrafiltration under subcritical flux conditions for relatively long-time.According with results, submerged hollow fiber ultrafiltration can be considered as a feasible pretreatment in reducing natural organic matter from seawater, provided that the opportune operating conditions were well identified and optimized.

Design of dual‐purpose membrane desalination systems

AbstractReverse osmosis (RO) and nanofiltration (NF) membrane plants are used extensively for brackish water desalination and industrial water purification. The operating range of RO plants is 60–80% product water recovery depending on the quality of raw water and feed water pretreatment. Typically, these plants operate at 75% recovery. The disposal of RO reject streams (20–40% of plant feed water) containing high concentration of salts is becoming an environmental problem. To reduce brine disposal costs and to increase product water recovery, several high‐recovery energy‐efficient membrane processes were analyzed. Generally speaking, a balance is required between brine volume reduction, capital and operating costs, energy consumption, and process simplicity. The analyses showed that up to 90% product water recovery can be achieved for low to medium brackish feed waters using RO/NF membrane systems with minimal feed water chemical pretreatment. The benefits of hybrid membrane systems integrated with fuel cell alternate energy systems are discussed. © 2009 American Institute of Chemical Engineers Environ Prog, 2010

An exergetic analysis of a membrane desalination system

In this paper, the exergetic analysis of a seawater membrane-based desalination plant has been carried out. The desalination plant has been described in detail, then the exergy of the various saline water streams has been determined and a comprehensive analysis towards the exergy distribution of the major process components has been conducted. The examination of the exergy losses throughout the plant revealed that exergy destruction was mainly due to pressure drops in the membrane modules, valves and brine lines. Moreover, 12.9% of the exergy input to the system was supplied by the heater. Therefore, the most reasonable way to reduce power input to the plant, thus improving its performance and cost, has been shown (i) to be replacing the valves on the reverse osmosis brine stream by an energy recovery system, and (ii) to have thermal energy available in the plant. With the identified technical changes, energy consumption decreased from 18.3 to 2.05 kWh/m 3, resulting in an annual saving of 0.17$/m 3.

Renewable Energy Powered Membrane Technology. 2. The Effect of Energy Fluctuations on Performance of a Photovoltaic Hybrid Membrane System

This paper reports on the performance fluctuations during the operation of a batteryless hybrid ultrafiltration--nanofiltration/reverse osmosis (UF-NF/RO) membrane desalination system powered by photovoltaics treating brackish groundwater in outback Australia. The renewable energy powered membrane (RE-membrane) system is designed to supply clean drinking water to a remote community of about 50 inhabitants. The performance of the RE-membrane system over four different solar days is summarized using four different NF membranes (BW30, NF90, ESPA4, TFC-S), and examined in more detail for the BW30 membrane. On an Australian spring day, the system produced 1.1 m3 of permeate with an average conductivity of 0.28 mS x cm(-1), recovering 28% of the brackish (8.29 mS x cm(-1) conductivity) feedwater with an average specific energy consumption of 2.3 kWh x m(-3). The RE-membrane system tolerated large fluctuations in solar irradiance (500--1200 W x m(-2)), resulting in only small increases in the permeate conductivity. When equipped with the NF90 (cloudy day) and ESPA4 (rainy day) membranes, the system was still able to produce 1.36 m(-3) and 0.85 m(-3) of good quality permeate, respectively. The TFC-S membrane was not able to produce adequate water quality from the bore water tested. It is concluded that batteryless operation is a simple and robust way to operate such systems under conditions ranging from clear skies to medium cloud cover.

Sustainable fuel cell integrated membrane desalination systems

According to the United Nations, between two and seven billion people will face water shortages by the year 2050. Further, it is estimated that the amount of water available per person will shrink by a third during the next two decades. Inadequate supply of good-quality water coupled with higher water demand due to rapid population growth and industrialisation in developing countries are among the major reasons for the worsening water situation. Current shortages of potable water around the world and looming water scarcity especially in the developing countries is the driving force behind the implementation of membrane technologies for seawater and brackish water desalination. Typical energy consumption in seawater reverse osmosis (RO) plants operating at 40–45% product water recovery and with energy recovery from the high pressure reject stream currently is about 3–4 kWh/m 3. The near-term goal of the industry is to reduce energy consumption to less than 2 kWh/m 3 by using a combination of energy efficient RO pumps, more efficient energy recovery devices, high performance low energy RO membranes, hybrid membrane systems, advanced pretreatment technologies and alternate energy integrated membrane systems. The beneficial aspects of using alternate energy systems such as on-site distributed fuel cell systems integrated with membrane desalination units in remote locations are discussed.

Pretreatment for desalination of seawater from an open intake by dual-media filtration: Pilot testing and comparison of two different media

The results of pilot testing of dual media filtration on seawater from an open intake in the Thermaikos Gulf, northern Greece, are presented in this study. Two different filter materials, FILTRALITE MC, a processed expanded clay material with particle range 1.5–2.5 mm, and anthracite coal with a similar particle range, were tested in parallel operation under various operating conditions (coagulant type and dose, filtration rate). Attention was focused on filtrate quality parameters relevant to feeding RO membrane desalination systems, that is, turbidity, silt density index (SDI), total organic carbon (TOC). In order to evaluate the effect of temperature variations on pretreatment performance, similar experiments were performed during a winter and a summer campaign of the same year. Despite the poor quality of feedwater, through an open intake, very low turbidities and SDI values lower than 5 could be obtained by means of coagulation and dual media filtration. Both filtration materials demonstrated almost similar performance in removing particulates from the feed water and produced permeates of acceptable quality for feeding RO systems (SDI 15 values lower than 5) during the winter and summer campaigns. It was observed that both filters were much more effective during the summer period due to the higher temperatures prevailing, which allowed the formation of larger aggregates and their subsequent effective capture within the filter media. Laboratory and on site pilot tests showed that optimal performance was achieved when polyaluminum chloride (PACl) was added to the feed water as a coagulant medium at a dose 1.8 mg Al/L. In the winter period similar SDI 15values were obtained by both filters when operating at a flow velocity of 10m/h, whereas at 5 m/h the sand/filtralite column produced water with lower SDI 15values and better quality compared to the one from the sand/anthracite column. On the other hand, during the summer period, and for the same coagulant dose, similar SDI 15values (usually close to 3) were obtained by both filters at all the tested flow velocities (5, 10, 15 m/h). Furthermore, due to the effect of temperature on aggregation, in the winter period the duration of filtration cycles was found to be determined by particle breakthrough. In contrast, in the summer no breakthrough was observed and the filtration cycles were determined by the available free space for hydrostatic head buildup above the filter media. Finally, regarding the hydrostatic head development, it was observed that the sand/filtralite column had a slower head buildup than the sand/anthracite column for the same duration of filter operation.

Energetic and exergetic analysis of an integrated membrane desalination system

An integrated reverse osmosis (RO) + membrane distillation (MD) system where MD operates on the RO brine is considered for desalination processes. In particular, an energy and exergy analysis of the RO unit alone and coupled to the MD unit is presented in order to determine the energy requirements related to the integrated system with respect to the RO one. This energetic analysis coupled to the analysis of the overall performance of the two different systems led to an evaluation of the potentialities of the integrated membrane processes in desalination operations. The analysis has been tentatively extended to a complete membrane integrated system where nanofiltration (NF) has been also considered in the RO feed pretreatment.

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.