Experimental and Theoretical Investigation of Forward Osmosis and Membrane Distillation Hybrid System

Lead Research Organisation: University of Oxford
Department Name: Engineering Science

Abstract

With increasing global population and economic growth during recent years, the rise in demand for clean water has presented a major concern for many countries. Various studies have shown that water shortages have the potential to affect billions of people around the world, with a recent study by Burek et al. (2016) suggesting that up to 5.7 billion people could be living in areas that are facing water scarcity by 2050. As a result, increasing research attention is being invested into the sustainable water engineering field to find novel technologies that can contribute to meeting the global demand for clean water.
Water purification technologies can generally be classified into membrane separation and thermal separation techniques. Thermal separation techniques have been used in industry for many years and primarily operate based on the process of distillation. However, the development and application of membranes that can offer superior separation to that of thermal separation techniques has led to a shift in research towards membrane based technologies. Currently, reverse osmosis (RO) is the most widely used membrane separation process and has been used for applications such as seawater desalination. RO works through the application of hydraulic pressure to a feed solution, which results in a flow of water through the membrane. Whilst this process is effective due to the high selectivity of the membrane, a significant amount of energy is required to generate and maintain the hydraulic pressure that can be in excess of 60-70 bar. This, in conjunction with RO pre-treatment requirements and membrane fouling issues (e.g. deterioration of membrane performance with time), can lead to significant capital, operating and maintenance expenses that can result in RO being impractical for impoverished communities. Thus, further research is necessary to find a reliable and cost effective water purification technology in order to establish the long-term availability of clean water.
The work being carried out in this project, which falls within the EPSRC Engineering research area, focuses on a relatively new membrane separation process known as forward osmosis (FO). FO has the potential to offer similar separation capabilities to that of RO. However, unlike RO, FO utilises a natural osmotic pressure gradient that is generated through the use of a concentrated 'draw' solution in order to separate water from the feed solution as opposed to the application of hydraulic pressure. As a result, FO could present a promising low-energy solution that can help to alleviate the global shortages of clean water.
Significant research has been invested into finding more effective draw solutions (e.g. generation of higher osmotic pressures) and improving membrane performance. However, few research studies have been conducted to combine FO with a suitable draw solution regeneration technology, which is essential to separate the resulting draw solution and water mixture to isolate the water product and recycle the draw solution. Therefore in this project, a novel Aquaporin InsideTM FO membrane has been coupled with a promising thermally driven regeneration technology, membrane distillation (MD), which can be operated at relatively low to moderate temperatures. The main aim of this project is to investigate the feasibility of applying the FO-MD hybrid process for the purpose of water purification. Four key objectives have been identified to achieve this aim and each of these objectives have been designed to explore novel features of the FO-MD hybrid process through theoretical and/or experimental investigations. The objectives and novel work in this project include:
(i) Mathematical modelling of the FO-MD hybrid system and balancing of FO and MD processes within the hybrid system
(ii) Application of FO-MD hybrid system to wastewater treatment
(iii) Analysis of FO-MD hybrid system energy requirements and feasibility of using low-grade energy sources

Publications

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publication icon
Zohrabian L (2020) Hybrid forward osmosis-membrane distillation system: Demonstration of technical feasibility in Journal of Water Process Engineering

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
2102584 Studentship EP/N509711/1 01/10/2017 30/09/2021 Linnet Zohrabian
 
Description At present, the main findings of this research study include:

- A mathematical model has been developed and verified to predict the behaviour of the forward osmosis (FO) - membrane distillation (MD) hybrid system
- Short-term testing revealed that fouling of the FO membrane is more significant than that of the MD membrane
- Results from the experiments and developed model show that clean water production can be optimised through the balancing of FO and MD water transfer rates
- It was found that the MD feed solution temperature is the primary operating parameter that can be used to balance the FO and MD water transfer rates
- Long term experimental results (25 days) indicated that the FO-MD hybrid system can achieve promising performance for long-term operation
- The long-term accumulation of solute in the FO feed solution suggests that mitigation strategies could be required to control feed solution properties such as concentration and pH
- For long-term operation, parameters that require greater control were identified as the MD feed temperature and FO draw solution concentration
- Process control can be applied to improve FO-MD water transfer rate balancing for longer periods of operation and therefore maximise permeate production
Exploitation Route The current outcomes of this research study suggest that the forward osmosis (FO) - membrane distillation (MD) hybrid system offers promising potential for the purification of water. Due to the lack of pilot studies in FO-MD area, it would be beneficial to continue this research by constructing a pilot scale plant that can be used to verify the laboratory scale results.

Since the developed mathematical model can accurately predict the behaviour of the lab-scale FO-MD system, applying the model for larger scale FO-MD system performance prediction can offer significant benefits for system design and provide insight into operating a larger commercial scale FO-MD water purification plant. The model can also be used for predicting system behaviour under different conditions (including feed and draw solution concentrations, velocities and temperatures). This can aid with selecting optimal operating conditions to run the system, in addition to contributing to meeting the clean water drinking requirements and managing variations in the system during operation.
Sectors Agriculture, Food and Drink