Reverse Osmosis Hybrid System Research Articles

Size-dependent transport and fouling formation of organic matters in a pilot-scale PFFO–RO hybrid system for real wastewater treatment

The forward osmosis (FO)-reverse osmosis (RO) hybrid system has been gaining considerable interest because of its wide applicability and feasibility in both wastewater treatment and seawater desalination. However, as the FO process treats wastewater in the hybrid system, fouling of the FO membrane is a crucial factor in the FO-RO hybrid system, which determines the performance of the entire hybrid system. A few studies have reported that the configuration of the PFFO module has potential advantages, such as a higher cross flow velocity, higher fouling resistance, and lower pressure drop compared to conventional spiral wound-type modules. In this study, the first to investigate the performance of the PFFO-RO hybrid system in long-term operation, the performance and fouling characteristics of the PFFO-RO hybrid system in real wastewater treatment, and seawater desalination over two months were investigated. At the end of the process, organic compounds, mainly humic substances such as humic and fulvic acid, accounted for 89.32% and 91.46% of the foulant on the PFFO and RO membranes, respectively. Moreover, organic compounds with molecular weights over 500 Da were removed by PFFO, and those over 60 Da were removed by RO. This study can provide the information about maintenance of the PFFO-RO hybrid process in wastewater treatment.

Performance analysis of a hybrid system of multi effect distillation and permeate reprocessing reverse osmosis processes for seawater desalination

There is an increasing acknowledgement that the inevitable growth of population and industrialization necessitate an improvement of the associated seawater desalination technologies. This is due to a rapid growth of freshwater demand, especially in high capacity cities. Therefore, this study sought to present an adopted hybrid system of multi effect distillation thermal vapor compression (MED_TVC) and Reverse Osmosis (RO) process. Specifically, the permeate reprocessing design of RO process is introduced in several configuration of different upstream processes. The methodology of this paper is designed to implement an earlier model developed by the authors to simulate the proposed design. This will include the modelling of multistage permeate reprocessing RO process design. The performance of the proposed design is assessed and compared with several layouts of the MED_TVC + RO hybrid system based on the main performance metrics. The simulation data of the new design elaborated the characteristics of high-quality water at higher recovery, lower brine flow rate and salinity, in addition to a lower energy consumption compared to the suggested designs in the literature. Therefore, it is expected that the implementation of this design in seawater would lead to the production of high-quality drinking water in a more sustainable manner.

Forward osmosis membrane modular configurations for osmotic dilution of seawater by forward osmosis and reverse osmosis hybrid system

This study evaluates various options for full-scale modular configuration of forward osmosis (FO) process for osmotic dilution of seawater using wastewater for simultaneous desalination and water reuse through FO-reverse osmosis (RO) hybrid system. Empirical relationship obtained from one FO membrane element operation was used to simulate the operational performances of different FO module configurations. The main limiting criteria for module operation is to always maintain the feed pressure higher than the draw pressure throughout the housing module for safe operation without affecting membrane integrity. Experimental studies under the conditions tested in this study show that a single membrane housing cannot accommodate more than four elements as the draw pressure exceeds the feed pressure. This then indicates that a single stage housing with eight elements is not likely to be practical for safe FO operation. Hence, six different FO modular configurations were proposed and simulated. A two-stage FO configuration with multiple housings (in parallel) in the second stage using same or larger spacer thickness reduces draw pressure build-up as the draw flow rates are reduced to half in the second stage thereby allowing more than four elements in the second stage housing. The loss of feed pressure (pressure drop) and osmotic driving force in the second stage are compensated by operating under the pressure assisted osmosis (PAO) mode, which helps enhance permeate flux and maintains positive pressure differences between the feed and draw chamber. The PAO energy penalty is compensated by enhanced permeate throughput, reduced membrane area, and plant footprint. The contribution of FO/PAO to total energy consumption was not significant compared to post RO desalination (90%) indicating that the proposed two-stage FO modular configuration is one way of making the FO full-scale operation practical for FO-RO hybrid system.

Understanding the possible underlying mechanisms for low fouling tendency of the forward osmosis and pressure assisted osmosis processes

We investigated the possible underlying mechanism of the low fouling potential in the forward osmosis (FO) process during the osmotic dilution of seawater as part of the simultaneous desalination and wastewater reuse by FO and reverse osmosis hybrid system. Long-term experiments revealed an interesting water flux pattern highly dependent on the different operating parameters. The most interesting observation made was the spontaneous increase in the FO permeate flux at regular time interval during the FO operation using synthetic wastewater as feed and seawater. This sinusoidal FO flux pattern related well with the build-up of loose fouling layer and their natural peel-off from the membrane surface upon reaching certain layer thickness due to crossflow velocity shear. This flux pattern was more prominent at higher cross-flow velocity rates, lower feed water pH, for a smoother membrane surface and at lower operating pressure during pressure assisted osmosis (PAO) mode. Based on these results, membrane cleaning strategies were proposed by targeting a higher cross-flow velocity shear at a time when the permeate flux started to just increase. The approach of physical membrane cleaning was observed efficient and was able to almost fully restore the initial flux even under the PAO operation at 4bar.

An optimal design approach of gas hydrate and reverse osmosis hybrid system for seawater desalination

Gas hydrate (GH) desalination process is based on a liquid (salty water) to solid (GH) phase change coupled with a physical process to separate the GHs from the remaining salty water. However, GH process exhibits less than 90% of salt rejection, so reverse osmosis (RO) process is needed to finally meet the desalinated product water quality. In order to increase the total recovery of the GH and RO hybrid system, the concentrate of the RO process should return to the feed stream of the GH process. In this work, RO simulation was carried out to find an optimal RO recovery with which the energy consumption of RO was minimized. The optimal RO recovery values for GH processes with salt rejection of 78, 84, and 90% were 0.6, 0.8, and 0.8, respectively. The minimal total energy consumption appears at higher RO recovery rates than the optimal values to minimize the RO energy consumption because the portion of the GH energy consumption is inversely proportional to the RO recovery. The simulation also reveals that the maximum allowable energy consumption of GH process is 1.4 kWh/m3 (with GH salt rejection of 78%) to overcome seawater RO process with energy recovery device, and it can be increased up to 1.9 kWh/m3 when GH salt rejection increases up to 90%.

Optimization of hybrid system consisting of forward osmosis and reverse osmosis: a Monte Carlo simulation approach

Forward osmosis (FO) is a membrane process that has been studied as novel technology for treatment of a wide variety of aqueous solutions. FO uses a semi-permeable membrane to extract cleaning water from impaired water by an osmotic pressure gradient but it also needs to have a posttreatment so-called “draw solute recovery.” The focus of the current study was to investigate a FO–reverse osmosis (RO) hybrid process in which RO is being used to recover draw solutes in product water from FO. A Monte Carlo method was applied to optimize the system. The key parameters affecting the energy efficiency of the hybrid system were also identified. The results indicated that the FO–RO hybrid system has advantages over RO-only system under high fouling conditions. It was found to be essential to minimize the internal concentration polarization to ensure high-energy efficiency and smaller requirements of membrane surface area.

Toward a combined system of forward osmosis and reverse osmosis for seawater desalination

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 reverse osmosis (RO), FO does not require high pressure for separation, allowing low energy consumption to produce water. However, the internal concentration polarization in FO is an important factor affecting the performance of FO processes. This paper was intended to investigate the characteristics of FO and RO processes. A simple film theory model was applied to consider concentration polarization in FO and RO processes. This model allows the estimation of internal and external concentration polarization effects in FO process. A laboratory-scale FO device was used to find the model parameters for further calculations. The calculated flux was compared with experimental flux under a variety of operating conditions. It was found that the combination of FO and RO may result in a higher flux than FO-only process under some operating conditions. Further research will be required to investigate the effect of membrane materials on energy efficiency of FO and RO hybrid system.