A simplified simulation model of RO systems for seawater desalination

Membrane technology has emerged as a dominant solution to seawater desalination due to its superior advantages such as stable output water quality, lower energy consumption, ease of operation and smaller footprint. However, the design of spiral wound reverse osmosis (RO) membranes used in desalination does not allow for backwash or air scouring, thus rendering the RO membrane highly susceptible to fouling. Pretreatment for the RO system is therefore essential to ensure a long service life of the RO membranes. For waters containing suspended solids of up to 75 mg/L (such as that in the SingSpring Desalination Plant at Tuas, Singapore), conventional pretreatment methods (such as dissolved air floatation and filtration (DAFF), chemical dosing and cartridge filtration) require regular operator intervention to produce a permeate of reasonably quality. Ultrafiltration (UF) as a pretreatment for seawater desalination can offer better treated water, lower operating costs, a smaller footprint, and flexibility in dealing with poor or varying feed water quality. By improving the pretreatment permeate water quality, reducing operating costs and the footprint, capital expenses can be lowered. Greater stability is also achieved during times of poor or variable feed water conditions (such as periods of algalbloom). A pilot study was conducted at SingSpring to track the performance of Hyflux's Kristal® 2000 hollow fiber UF membranes as pretreatment for the seawater reverse osmosis (SWRO) system. The results of the pilot study will enable the design of future large-scale UF-SWRO membrane projects for seawater desalination.

The State of Qatar and the Gulf Cooperation Council (GCC) region are located in a hyper-arid area with no rivers, over-abstracted groundwater supply and limited rainfall. Consequently, with the discovery of oil and gas and the associated economic prosperity, the State of Qatar and the GCC region have relied on desalination of seawater from the Arabian Gulf. As of 2013, the GCC region held a 70% share of total global desalination capacity.Multi-Stage Flash (MSF) desalination technology has been the source of water supply in the State of Qatar and the GCC region for the past few decades due to the low cost of energy in these countries and the problems historically faced by Reverse Osmosis (RO) membrane processes in dealing wih the high salinity of the Arabian Gulf. MSF is a thermal process that distills water through stages based on high temperature and changing pressures. The systems suffer from high energy requirements and low recovery rates resulting in significant discharge of brine with elevated temperature to the ambient receiving water. RO on the other hand relies on applying a positive pressure to pass permeate through a fine polymer filter material against the osmotic pressure gradient. RO is widely adopted worldwide due to its lower energy consumption and increased product recovery. With recent developments the technology can cover the high salinity of the Arabian Gulf (40,000 mg/L to 55,000 mg/L total dissolved solids). Nevertheless, RO systems require extensive pretreatment to ensure the integrity of the membrane and to prevent blocking of the fine pores. This makes the process susceptible to surface water quality fluctuations such as during algal blooms and therefore its application in Qatar is still challenging due to the shallow and enclosed nature of the Arabian Gulf.Subsurface intake processes for RO have the potential to reduce the effects of fluctuations in source water quality and reduce the energy intensity of the process, since they provide natural filtration of the source water and simplify the extensive pretreatment requirements necessary to protect the RO membranes. However, significant tradeoffs occur by using subsurface intakes. For instance, intake pumping may be increased to overcome the additional headloss through the intake media while the construction phase also involves increased civil works. This research investigates the environmental impacts associated with the operation phase of RO systems using both open intake and beach well intake systems theoretically located in the State of Qatar, since operational phase impacts typically comprise most of the environmental loads in cradle-to-grave assessments.The study utilizes Life Cycle Assessment (LCA) methodology to assess a wide range of effects from the systems. The ReCiPe lifecycle impact indicator approach is utilized with mid-point impact indicators including climate change, marine eutrophication, terrestial acidification, photochemical oxidant formation, particulate matter formation, marine ecotoxicity, water depletion, mineral resource depletion and fossil fuel depletion. The RO system, its pretreatment and intake will be sized and modelled for a desalinated water output of 100,000 m3/d using a combination of fundamental process equations and commercially available software. The results will show a clear direction from an environmental perspective on which type of intake system Qatar should consider if implementing seawater RO as a preferred desalination technique.

Desalination is a method for producing water for human consumption, irrigation or industrial utilisation. In this study, a reverse osmosis (RO) system for brackish water desalination was theoretically investigated to produce both potable drinking and agricultural water with a lower overall and specific energy consumption. As a case study, the Main Outfall Drain in Iraq is used as the brackish water source. A numerical model based on solution-diffusion theory was developed in Matlab Simulink and used to analyse the design and performance of an RO system. The effect of feed water temperature, pressure, salinity and recovery ratio on the efficiency of the whole RO system was investigated for a wide range of design considerations. The design of an RO system for this application was optimised and economic assessment carried out. Results show that with boosting recovery ratio from 30% to 60%, the specific energy of desalinated water production below 400ppm was reduced from 2.8kWh/m3 to a more economically favourable value of 0.8kWh/m3, when utilizing a pressure exchanger as a recovery device. Salt rejection was reduced from 97% to 88% to obtain large quantities of water for irrigation with an acceptable salinity (<1600ppm), for agricultural use. The reduction in salt rejection is influenced by the feed water temperature and pressure; also the average pore diameter of the RO membrane and in turn determines the reduction in system energy consumption. It was found that the total cost to produce 24,000m3/d of water from a feed salinity of 15,000ppm and a water quality of <400ppm would be 0.11£/m3 with a corresponding investment cost of £14.4million for the drinking water, and for irrigation) obtained product <1600ppm) are £0.9/m3 and £11.3million.

Reverse osmosis (RO) technology has been widely applied to water treatment such as seawater desalination, and large RO plants are many in operation around the world. Moreover, much larger plants will be required to secure sufcient water resource in the near future because global water shortage and quality problems are still getting more serious. Mega-ton Water System project was carried out for sustainable management of water environment and for low-carbon path to develop advanced key technologies of water treatment. Lowpressure RO membrane for seawater desalination has been studied in the project as a part of the core technologies to realize mega plant that is capable of producing 1,000,000 m3 of freshwater per day. Fundamental and scientifc research for RO membranes based on fne structure analyses by means of transmission electron microscopy with a special technique was conducted, and practical tools for designing new innovative RO membrane were acquired by the structure analyses to quantify the physicochemical and chemical properties of RO membranes. As the result of studying on structural design of RO membrane, low pressure SWRO membrane was obtained to reduce energy consumption compared to conventional ones in the past of SWRO. The vision of the “Mega-ton Water System” is sustainable desalination and reclamation. The missions are: 1) energy reduction (20-30%), 2) water production cost reduction (50%), and 3) low environmental impact (fewer chemical operations). Water cycle in “Mega-ton Water System” is separated into two parts including i) Seawater RO (SWRO) system, and ii) Seawater RO system with PRO system. The main challenge of development goal is the construction of mega-ton-scale system for seawater desalination for half the current cost. Accordingly, we developed the world’s frst low-pressure, multi-stage, high yield RO system, using a low-pressure seawater desalination membrane, and as a result of incorporating into it the elemental technologies gained from research in subthemes, such as highly-efcient pressure energy recovery, low-cost and highly durable plastic piping, pretreatment without the use of chemicals.

Radionuclides (uranium, thorium, radium, radon gas etc.) are found naturally in air, water, soil and rock. Everyday, we ingest and inhale these radionuclides through the air we breathe and through food and water we take. Out of the internal exposure via ingestion of radionuclides, water contributes the major portion. The natural radioactivity of water is due to the activity transfer from bed rock and soils. In our surveys carried out in the past few years, we have observed high concentrations of uranium and total dissolved solids (TDS) in drinking waters of some southern parts of Punjab State exceeding the safe limits recommended by national and international agencies. The main drinking water source is the underground water procured from different depths. Due to the highly saline taste, disorders in their digestive systems and other ailments, people are installing reverse osmosis (RO) systems in their houses. Some RO systems have been installed on commercial basis. The state government is also in the process of installing community RO systems at the village level. As high values of uranium are also undesired and may pose health hazards due to radioactivity and toxicity of uranium, we have conducted a survey in the field to study the performance of various RO systems for removal of uranium and TDS. Water samples from about forty RO systems from Faridkot, Mansa, Bathinda and Amritsar districts of Punjab State were collected and analyzed. Our results show that some RO systems are able to remove more than 99% of uranium in the underground waters used for drinking purposes. TDS values are also reduced considerably to the desired levels. So RO systems can be used to avoid the risk of unduly health problems posed by high concentrations of uranium and TDS in drinking water.

Chapter 4 - Transport Models, Membrane Materials, and Basic Flow Patterns

Evaluation of economic feasibility of reverse osmosis and membrane distillation hybrid system for desalination

A new approach for freshwater production and energy recovery from an oil field

The reverse osmosis (RO) system is widely applied to produce reclaimed water for high-standard industrial use. Chlorine disinfection is the main biofouling control method in the RO systems for wastewater reclamation. However, researchers reported the adverse effects of chlorine disinfection which aggravated biofouling in laboratory-scale RO systems. In this study, four parallel 4-inch spiral wound RO membranes were used to study the effect of chlorine on biofouling in a pilot-scale RO system. The free chlorine dosages in four experimental groups were 0, 1, 2 and 5 mg/L, respectively. After continuous chlorination and dechlorination, the feed water entered the RO system. It was found that chlorine pretreatment caused a 1.9–36.7% increase in relative feed water pressure of the RO system, suggesting that chlorine aggravated the membrane fouling in the pilot-scale RO system. The microbial community structures of living bacteria in the feed water of the RO system were determined by the PMA (propidium monoazide)-PCR method and showed that the relative abundance of chlorine-resistant bacteria (CRB) was significantly increased after disinfection. Nine major genera which maintained higher relative abundance in experimental groups with high chlorine dosage were considered as possible key species causing membrane fouling, including Pedobacter, Clostridium and Bradyrhizobium.

Fabrication of high boron removal reverse osmosis membrane with broad industrial application prospect by introducing sulfonate groups through a polyvinyl alcohol coating

Ground water in both the northeastern and southwestern coast areas of Taiwan may contain high concentrations of arsenic. Since no central water supply system is available in some of those areas, point-of-use (POU) water purification devices are considered as an option for providing safe drinking water. In this study, removal of arsenic, using two types of POU purification devices, reverse osmosis (RO) systems and distillers, was investigated. Three commercially available RO systems and two distillers were selected to test their removal efficiency of arsenic from synthetic and real ground water. Experimental results of the three RO systems using synthetic ground water showed that only one system had good removal efficiency for arsenic. In subsequent experiments using real ground water with 0.7 mg l−1 arsenic, only one RO system was able to meet the drinking water standard after producing about 1,000 l of treated water. For the distilling systems, 99% of the arsenic was removed from both synthetic and real ground water. The arsenic concentrations in the finished water of both distillers were all below the standard for drinking water. Although systems with higher arsenic removal efficiency seemed to have better removal of total dissolved solids (TDS), no correlation could be found after analysis.

The abundance of transparent exopolymer particles (TEPs) in seawater has been reported for many years. Recently, however, TEPs have been implicated as one of the leading causes of biofouling in reverse osmosis (RO) systems. A pilot plant study of a seawater ultrafiltration (UF)–RO system was conducted to monitor seasonal variation of TEPs, removal through pretreatment processes, and deposition in the RO system. Operational performance of the UF and RO systems was also monitored to evaluate the effects of TEPs on plant operations. A spectrophotometric assay measured TEPs in the particulate and colloidal size range at different stages of the plant. TEP monitoring of raw water indicated a significant increase of p-TEPs and c-TEPs from late March until early May, after which TEPs subsided to lower levels. The period was also marked by increased chlorophyll a and total organic carbon, indicating algal blooms in the seawater source. TEPs in the raw water were partially removed by microstraining and UF p...

Effectiveness of household reverse-osmosis systems in a Western U.S. region with high arsenic in groundwater

Removal of metals and assimilable organic carbon by activated carbon and reverse osmosis point-of-use water filtration systems

Cost evaluation and optimisation of hybrid multi effect distillation and reverse osmosis system for seawater desalination