Hybrid membrane system for desalination and wastewater treatment: Integrating forward osmosis and low pressure reverse osmosis

Since more than 97% of the water in the world is seawater, desalination technologies have the potential to solve the fresh water crisis. The most used desalination technology nowadays is seawater reverse osmosis (SWRO), where a membrane is used as a physical barrier to separate the salts from the water, using high hydraulic pressure as the driving force. However, the use of high hydraulic pressure imposes a high cost on operation of these systems, in addition to the known persistent fouling problems associated with reverse osmosis (RO) membrane filtration systems. Forward osmosis (FO) is an alternative membrane process that uses an osmotic pressure difference as the driving force. FO uses a concentrated draw solution to generate high osmotic pressure, which extracts water across a semi-permeable membrane from a feed solution. Afterwards, fresh water can be obtained when the diluted draw solution is regenerated in a second treatment step, e.g., low pressure reverse osmosis (LPRO). Research has identified the potential for hybrid forward osmosis/low-pressure reverse osmosis (FO/LPRO) systems for several applications, including seawater desalination, and to reduce the cost and fouling propensity of producing fresh water from impaired-quality water sources, compared to conventional high pressure RO systems. One of the main advantages of FO is the limited amount of external energy required to extract water from the feed solution, using only a very low amount of energy to recirculate the draw solution on one side of the membrane, while the feed solution is passively in contact with the other side of the membrane. The objective of this research is the evaluation of a hybrid desalination system using forward osmosis, where the feed water is a primary or a secondary wastewater effluent, and the draw solution is seawater, with the purpose of recovering fresh water from impaired quality sources with the use of minimum hydraulic pressure. This hybrid system has two clear advantages: (i) the diluted seawater resulting from the FO dilution process is further treated in a LPRO unit to produce fresh water, using less energy than conventional high pressure SWRO systems; (ii) the concentrated wastewater effluent produced by FO enables low-cost processing. The results show that forward osmosis recovers water from wastewater, rejects nutrients and micropollutants, outperforms traditional SWRO systems in terms of fouling resistance and control, having a high flux recovery when applying physical cleaning methods. Water recovery A study revealed the ability of a FO process to integrate seawater desalination and municipal wastewater treatment for drinking water production (Chapter 2). The FO process showed a high rejection for chemical oxygen demand, phosphate and trace metals, and moderate rejection for ammonia and total nitrogen. Organic carbon analysis revealed that the membrane tested was unable to reject low molecular weight acids and low molecular weight neutral compounds, such as sodium acetate and urea. Biopolymer-like substances played a dominant role in the formation of fouling on the membrane surface. The study showed that FO is a reliable barrier to effectively reject most wastewater contaminants and salts from either the wastewater as feed solution or seawater as draw solution while allowing clean water to pass through, providing a possible significant energy-saving strategy to combine (integrate) municipal wastewater treatment and seawater desalination to further promote sustainable urban water management and water reuse in coastal cities. Furthermore, in another study (Chapter 3), applying practical conditions of water reuse applications, FO membranes were able to reject most of the organic micropollutants spiked in the feed water; rejections were moderate for hydrophilic neutral compounds (44 – 95%), moderate for hydrophobic neutral contaminants (48 – 92%), and high for the hydrophilic ionic micropollutants (96 – 99%). FO coupled with LPRO was effective in rejecting low molecular weight hydrophilic neutral micropollutants, with rejections exceeding 89%. For the rest of the compounds, rejections were greater than 99%. A hybrid FO/LPRO system serves as a double barrier against micropollutants, including pharmaceutically active compounds, hormones and other pollutants. Organic fouling and cleaning Characterization of the organic foulants in both wastewater and seawater was performed (Chapter 4). Organic carbon analysis (liquid chromatography coupled with organic carbon detection (LC-OCD) and three-dimensional fluorescence excitation emission matrices (3-D FEEM)) suggest that biopolymers and protein-like substances, present in the feed water, form a fouling layer on feed side of the FO membrane, reducing the water flux and thus, affecting the efficiency of the seawater dilution process. Transparent exopolymer particles (TEP) were identified in the support layer of the FO membrane in contact with the seawater, which contains a significant amount of these particles, reducing the flux of the FO membrane. Physical and chemical methods were used and compared in an effort to set an effective protocol for FO membrane cleaning (Chapter 5). Natural organic matter fouling showed high hydraulic reversibility, up to 90% when in-situ air scouring for 15 minutes was used as a cleaning technique. Chemical cleaning with a mixture of Alconox, an industrial detergent containing phosphates, and sodium ethylenediaminetetraacetic acid (EDTA) showed to improve the reversibility further (93.6%). Osmotic backwashing using a 4% NaCl solution and deionized (DI) water proved to be ineffective to recover the flux due to the salt diffusion phenomena occurring at the active layer (the membrane separation layer). The same detergent solution used to clean the active layer was used to clean the support layer; 95% of flux was recovered, showing that the chemically irreversible fouling of the FO membrane is in the order of 5.5%, which might be associated with the adsorption of biopolymers on the active layer and some TEP residuals on the support layer. Physical cleaning (air scouring) proved to be the most effective way to control organic fouling. Biofouling The study on the influence of feed spacer thickness (28, 31 and 46 mil, 1 mil = 0.0254 mm) on performance and biofouling development on the feed side of FO membranes (Chapter 6) led to the following conclusions: (i) the biomass amount alone does not determine the flux decline: the same amount of biomass was found for all spacer thicknesses after the same run time at the same feed flow, while the flux reduction decreased with thicker spacer; (ii) the flux decline caused by biomass accumulation can be reduced by using a thicker spacer; (iii) spatial distribution of the biofilm differed with feed spacer thickness. Findings are in agreement with reported data for high pressure reverse osmosis cross-flow systems: thicker spacers reduce the impact of biofouling on performance. This result clearly contradicts observations obtained with particulate and colloidal fouling in forward osmosis. Outlook Forward osmosis (FO) is an emerging membrane technology with a range of possible water treatment applications (desalination and wastewater recovery). An overview of applications, advantages, challenges, costs and knowledge gaps is given (Chapter 7). With current commercial technology, hybrid FO systems for both desalination and water recovery applications have proven to have higher capital cost compared to conventional technologies. Nevertheless, due to the demonstrated lower operational costs of hybrid FO systems, the unit cost for each m3 of fresh water produced with the FO system are lower than conventional desalination/water recovery technologies (i.e. ultrafiltration/RO systems). There are key benefits of using FO hybrid systems compared to RO: (i) chemical storage and feed systems may be reduced for capital, operational and maintenance cost savings, (ii) reduced process piping costs, (iii) more flexible treatment units, and (iv) higher overall sustainability of the desalination process, while producing high quality water. The major challenges of FO to be a commercially viable technology are: (i) developing a higher flux membrane, capable of maintaining an elevated salt rejection and a reduced internal concentration polarization (ICP) effect, (ii) the availability of appropriate draw solutions, which can be recirculated via an efficient recovery process, (iii) better understanding of fouling and biofouling occurrence, (iv) assuring the high quality of the water produced, (v) hybridization with other technologies that can increase the benefits of FO use (i.e. water recovery, energy production, etc.). Numerical modeling can be a useful tool to understand biofouling in FO membrane processes and to suggest potential approaches for fouling prevention/reduction. Along with this, future experimental studies should focus on the use of modified spacers and novel cleaning strategies. It is strongly suggested to upscale the process into a pilot scale facility in which a comprehensive evaluation of water quality and energy parameters can be done, facilitating a life cycle assessment and a cost cycle assessment of a hybrid process (i.e. FO-LPRO), which will give important information on the direction that should be taken to develop robust low cost water treatment hybrid systems to produce high quality water