Fouling and forward osmosis

Leader

Challenge

Diminishing water resources and increasing water usages caused by population growth and industrial development have made insufficient fresh water supplies a worldwide issue. Membrane based water treatment processes are increasingly seen as a good option for water purification, yet high-energy consumption and problems related to membrane fouling remain as major issues in the application of membrane processes. Consequently, the development of high efficiency, a low-energy and low-fouling membrane technology for water purification is desirable.

The forward osmosis (FO) process uses natural osmotic pressure as the driving force, and does not require high hydraulic pressure. Although used for a number of applications already, it is not known how successfully it could be applied to treating difficult high scaling potential brackish water. When the direct use of reverse osmosis (RO) will result in rapid scaling that requires frequent cleaning, FO could be used as a pre-treatment process to remove scaling ions and matters that cause fouling.

However, it is not known how feasible a FO–RO hybrid system would be due to fouling behaviour and reversibility. While much information is available on fouling and cleaning of the membranes in pressure-driven membrane processes, fouling and cleaning of FO membrane in brackish water application is less reported. Most FO fouling studies have been limited to synthetic organic fouling and very few studies have been dedicated to investigating the biofouling of the FO membranes.

Investigation 

Compare the efficiency and energy consumption of the FO process against other pre-treatment processes, such as powdered activated carbon (PAC), ferric sulphate coagulation and ultrafiltration. Determine the mechanism of physical, chemical and biological factors affecting the formation and composition of the fouling layer that forms on the FO membrane during seawater, brackish and wastewater purification and determine the reversibility of the FO fouling by investigating different cleaning conditions. Optimise the operational parameters and conduct an analysis of the total energy balance of the FO-RO desalination system.

Outcomes

A bench-scale FO membrane system was designed, built and tested under optimised operating conditions using coal seam gas and brackish waters. The results indicated FO is able to efficiently remove scaling ions and biofoulants, protecting the downstream RO membrane. Analysis of the compositions of different foulants found protein-like substances and soluble microbial products were the predominant foulants. A combination of organic fouling and biofouling occurred on the FO membrane surface, where protein-like substance, microbial cells and their bioproducts were responsible for the fouling. Water flux declined by 65% compared to the initial rate however, was able to be restored after hydraulic cleaning. Compared to other pre-treatment processes, FO was one of the most efficient for removing scaling and biofouling agents and biofouling on the membrane was almost fully reversible without the use of chemical agents.

Future Direction

There is a disadvantage of using the FO-RO combination that needs to be overcome. This disadvantage is the presence of external and internal concentration polarisations, which act to significantly decrease the driving force of the process by reducing the effective osmotic pressure difference between the feed and the draw solution. In other terms, water flux itself is a self-limiting parameter of the FO efficiency. In order to get a reasonable water flux and a reasonable final draw solution, the surface of FO membrane area would need to be large in order to provide ample RO feed water. Development of new FO membrane material is still needed in order to improve water permeability, salt rejection, and anti-fouling characteristics.

Partners

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Total Value: $598,864 (cash and in-kind contributions)

Principal Investigator: Professor Linda Zou

Title: Forward osmosis as a low energy and high efficiency pretreatment process for reverse osmosis desalination

Length: 20 months

Personnel: 5 collaborators contributing 3.2 FTE

Further Information

FR4 UNISA Zou Pretreatment Summary Poster

Project Summary Poster – fouling and forward osmosis

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