Microbial removal in pre-treatment systems

Leader

Challenge

Reverse osmosis (RO) membrane technology is used in the desalination industry to produce high-purity water that is microbiologically safe and biologically stable. However, RO membranes are subject to biofouling which leads to (1) a decline in membrane performance, (2) a reduction in system efficiency and (3) a reduction in the lifetime of the membranes. Given the complexity of biofilm formation in marine (saline) environments, it is essential to better understand the main precursors of biofouling, particularly if prevention is to be achieved.

Transparent exopolymer particles (TEP) have been identified as one of the main precursors of biofouling in desalination membranes. TEP comprises of sticky organic microgels with varying size range from 0.4 to 200 μm, including exopolymeric substances (EPS), made of polysaccharides and of other constituents (i.e., proteins, uronic acids, sulphates). Various studies have shown that microalgae produce EPS and in the ocean these biopolymers play a crucial role in the formation of marine gels, marine snow and biofilms.

It is essential to understand what is influencing the formation of biofilms and to evaluate the characteristics of water from natural ecosystems that is used to feed the desalination systems. It is also necessary to assess the characteristics of water used to clean the system as this can also be a source of microbial input into the system.

Investigation

Physical and biological characteristics of water in the Penneshaw desalination plant were assessed over a 12 month period, taking samples fortnightly from five points in the pre-treatment system. Specifically, TEP, phytoplankton, bacteria and viruses were examined after each pre-treatment stage; medium pressure-UV disinfection, multimedia filtration and cartridge filtration. The biofouling potential of microbial communities present in the feed tank and the effect of biofilm growth inhibitors were also examined.

Outcomes

Medium pressure-UV disinfection had no effect on the phytoplankton, bacterial or viral cell counts. In contrast, multimedia filtration was shown to be the most efficient step in removing TEP and microorganisms from seawater, while this removal was less significant for viruses. Cartridge filters had limited efficiency. Phytoplankton was observed to be more efficiently removed compared to bacteria. Although phytoplankton removal rates varied over time and were dependent upon cell size and shape, most of the microorganisms were removed from seawater throughout the period of study.

The production of TEP was reflective of the growth phase of the biofilm, it increased over time as the size of the biofilm increased. In a planktonic state within the natural environment the production of TEP is relatively controlled, in particular by the availability of nutrients however, within the desalination system microbial composition and turbulence determine the generation of TEP.

Therefore, both direct and indirect approaches are needed to reduce the biofouling capacity of microorganisms present within the RO feed tank and make the system more economical.

Future Direction

To examine the production of EPS from the bacteria isolated from the feedwater tank and the response of the biofilm under chemical and physical stresses.

Partners

SA Water Nanyang
 


Total Value: $1,703,046 (cash and in-kind contributions)

Principal Investigator: Dr Sophie Leterme

Title: Assessing the biofouling role of microbes in the desalination system: from the intake pipe to the reverse osmosis membranes

Length: 55 months

Personnel: 16 collaborators contributing 12.15 FTE

Further Information

FR3 FLINDERS Leterme Biofouling Summary Poster

Project Summary Poster – microbe removal

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