Media filtration is being used as a pre-treatment to seawater reverse osmosis (SWRO) to reduce particulate and colloidal fouling. Ferric sulphate is used as in-line coagulant in media filtration, mainly to remove solids and colloids but also to remove the dissolved organics present in seawater. It is not efficient in controlling organic and bio-fouling. As a result, a number of desalination plants face the problem of biofouling on both cartridge filter and RO membranes downstream of the direct (media) filtration system. This in turn increases the operational and energy costs of RO plants and reduces the lifetime of the cartridge filters (usually used as a security barrier after direct filtration) and RO membranes.
Develop protocols for measuring organic and microbial parameters in seawater and pre-treated water at desalination plants. Understand the organic fouling and biofouling of the cartridge filter and RO membranes by developing representative testing methods to perform detailed autopsies for identifying and quantifying the organic and biofoulants. Optimise the direct filtration (backwash velocity, duration and frequency, and flocculation condition) in terms of specific biological and organic fouling reduction.
Methods were developed using LC-OCD and 3D-FEEM and it was found that there is a seasonal variation in organic fractions which may be linked to the seasonal variation of algal and biological activity and photo-degradation in the sea. Currently used pre-treatment steps were effective in reducing the particulates with a small decrease in high molecular weight organics. However, biofouling was not lowered by the pre-treatment systems and further study on methods for effective biological fouling reduction is needed.
Inorganic foulants present on fouled RO membranes either originated from the feed water, chemicals used in the pre-treatment processes or were found to be plant material. Humic substances and low molecular weight neutrals, which are the dominant fractions in seawater, were found to be the major organic foulants on fouled RO membranes.
A practical approach to evaluate the biofouling potential in the processes of seawater reverse osmosis was developed. In this test, a modified fouling index through an ultrafiltration membrane of seawater and pre-treated seawater were measured and compared with assimilable organic carbon (AOC) and low molecular weight organics. It showed that low molecular weight neutrals were responsible for the biological growth and levels correlated well with AOC concentration. Standard blocking index (Ks) calculated from MFI-UF10 was also related with AOC concentration. This indicated that the MFI-UF10 test can serve as a preliminary indicator of AOC and low molecular weight neutrals (biofouling precursors). A big advantage is that MFI-UF10 is that it can be used directly in desalination plants to monitor biofouling potential in pre-treatment processes.
The collaboration with the partner will continue with plans to automate the biofouling index test method.
Total Value: $520,000 (cash and in-kind contributions)
Principal Investigator: Professor Saravanamuth Vigneswaran
Title: The optimisation and improvement of direct filtration pre-treatment to reduce both organic and bio-fouling of reverse osmosis membranes
Length: 42 months
Personnel: 11 collaborators contributing 1.45 FTE
Related Projects: Membrane adsorption bioreactor hybrid system as a pretreatment to reverse osmosis desalination; Modelling, monitoring and control of reverse osmosis biofouling; Fibre-optic sensor for water quality monitoring (Phase 1 and 2)
- 2016. Li, S., et al. Marine bacterial transparent exopolymer particles (TEP) and TEP precursors: Characterization and RO fouling potential. Desalination 379:68-74.
- 2015. Jeong, S., and S. Vigneswaran. Practical use of standard pore blocking index as an indicator of biofouling potential in seawater desalination. Desalination 365:8-14.
- 2014. International Water Association World Congress. Lisbon, Portugal.
- 2014. 4th International Conference on Membrane Technology. Ho Chi Minh City, Vietnam.
- 2013. Naidu, G., et al. Microbial activity in biofilter used as a pretreatment for seawater desalination. Desalination 309 (0):254-260.
- 2013. Jeong, S., et al. A rapid bioluminescence-based test of assimilable organic carbon for seawater. Desalination 317:160-165.
- 2013. Jeong, S., and S. Vigneswaran. Assessment of biological activity in contact flocculation filtration used as a pretreatment in seawater desalination. Chemical Engineering Journal 228 (0):976-983.
- 2013. Jeong, S., et al. Bacterial community structure in a biofilter used as a pretreatment for seawater desalination. Ecological Engineering 60 (0):370-381.
- 2013. Shrestha, A., et al. Seawater biofiltration pre-treatment system: comparison of filter media performance. Desalination and Water Treatment:1-8.
- 2013. 2nd Water Research Conference. Singapore.
- 2012. Jeong, S., et al. The performance of contact flocculation–filtration as pretreatment of seawater reverse osmosis. Desalination and Water Treatment 43 (1-3):246-252.
- 2012. Singapore International Water Week. Singapore.
- 2011. 4th International Desalination Workshop. Jeju Island, Korea.