Investigation and control of biofouling in seawater reverse osmosis desalination membranes by bacteria and their extracellular polysaccharides

Biofouling in seawater reverse osmosis desalination membranes is a phenomenon that needs urgent solutions to effectively mitigate the problem, mainly due to huge economic losses it incurs. To achieve this, a thorough understanding of the microbial community ecology and source of fouling organisms on RO membranes is important. Extracellular polysaccharides produced by bacteria form an important part of the biofilm matrix that govern physical properties and structural integrity of the biofilm. Information about the chemical composition of exopolysaccharides is necessary to employ good control methods. The objectives of this research were defined to better understand biofouling, especially with respect to polysaccharide fouling, and investigate control methods. They were achieved as follows i) Bacterial communities on industrially fouled RO membranes were characterized by next generation sequencing (NGS) on the Illumina Miseq platform; comparisons of microbial ecology were made between treatment groups of membrane samples. ii) Bacteria were isolated from membranes, prefilters and upstream locations of a full-scale desalination plant, and identified by 16S rRNA gene sequencing, matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) and Biolog Gen III systems. Suitable models were then selected from the culture collection based on their dominance in the genetic biofilm community. iii) Exopolysaccharides of model bacteria were purified by acetone precipitation methods and characterized by ion chromatography, Attenuated total reflectance-Fourier transformed infrared spectroscopy (ATR-FTIR) and enzyme linked lectin assay. iv) 2,2-dibromo-3-nitrilopropionamide (DBNPA), a biocide, and two free radical generating compounds, sodium nitroprusside (SNP) and xanthine oxidase, were tested as control agents to disperse biofilms by degrading polysaccharides on industrially fouled membranes. v) For potential use in biological control, bacterial isolates were screened for production of oxidizing enzymes using a xanthine oxidase nitroblue tetrazolium assay. The major findings of this research that contribute to scholarly knowledge are: i) The bacterial community on RO membranes was identified as being dominated by certain bacterial groups, which are known to be associated with unique biofilm forming abilities; mainly Caulobacterales, known to attach irreversibly with holdfast; Sphingobacterales, Rhizobiales and Sphingobacteriia that are known to produce glycosphingolipids; Burkholderiales, known for nitrate-reduction; and Pseudomonadales, proposed to be both primary and secondary colonizers, based on the literature. ii) The cultured bacterial population were dominated by Gammaproteobacteria. MALDI-TOF and 16S rRNA gene sequencing were the most efficient identification methods. The model bacteria were good representatives of biofouling organisms in large scale, within limitations of culture bias. iii) Polysaccharide structures of bacterial isolates revealed the presence of some rare sugars, which are known to form critical components of strong biofilms. iv) Free-radical-generating compounds, SNP and xanthine oxidase, were more effective than the biocide DBNPA in alleviation of fouling by degrading polysaccharides. v) Some bacterial strains like Microbacterium and Exiguobacterium produced xanthine oxidase to significant levels when exposed to hypoxanthine.