Browsing by Author "Ras, Gabriel Retief"
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- ItemControl of biofouling on reverse osmosis membranes using DBNPA(Stellenbosch : Stellenbosch University, 2016-03) Ras, Gabriel Retief; Burger, A. J.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: Reverse Osmosis (RO) is used throughout the world for water desalination and it has gained wide popularity due to its efficient energy consumption and the safe operating process. Fouling (of which biological fouling is the most problematic) of the membranes is, however, an inevitable process that cannot be avoided, only managed. Biological fouling is the growth of microorganisms in the membrane system, causing undesirable effects. The correct pre-treatment can reduce (but not necessarily prevent) biofouling. This is because microorganisms have the ability to reproduce and form secondary populations throughout the membrane system, even if 99.99% of the microorganisms are removed in the pre-treatment process. Most modern RO plants are equipped with thin film composite polyamide (TFC PA) membranes. However, biological control on such membranes is restricted, since oxidising biocides like chlorine degrade the membrane material, thereby increasing salt passage and reducing membrane life. Therefore, this study investigated the use of a common non-oxidising biocide, i.e. 2,2-dibromo-3- propionamide (DBNPA) to manage biological growth on TFC PA membranes. The primary aim was to demonstrate the influence of three DBNPA dosing variables on the control of biofouling on the RO membranes. These variables were dosage (10 ppm to 200 ppm), dosing frequency (twice daily to once every 2nd day) and dosing duration (30 min to 2 hours). The work also strongly relied on the characterisation of biological fouling through determination of biomass parameters (protein concentration, polysaccharide concentration, total cell count and colony-forming units) and linking it to flux decline. Tests were conducted in lab-scale RO membrane blocks, housing flat-sheet TFC PA membranes with appropriate flow spacers typically found in commercial spiral-wound membrane cartridges. Since clean municipal water was used as feed water, nutrients (sodium acetate, sodium nitrate and sodium dihydrogen orthophosphate, in the ratio of 100:20:10 to give a final carbon concentration of 100 μg/ℓ) were supplemented to stimulate sufficient microbial growth, thereby enabling a sensible study on the effect of DBNPA dosing. During the removal of the biofilm from the membrane, no combination of the removal and homogenisation techniques (e.g. scraping the biofilm from the membrane, ultrasonic bath and ultrasonic probe treatment) yielded significantly higher colony forming unit (CFU) counts. R2A agar, however, produced significantly higher CFU counts compared to nutrient agar. Therefore, the agar used during plate counts appears to have been of greater significance on cell enumeration than the combination of biofilm removal and homogenisation techniques, which had little effect on cell counts, irrespective of agar used. DBNPA dosing reduced the amount of biofouling, regardless of the dosing strategy used. However, within the scope of this study, biofouling was best controlled with a DBNPA dosage of 100 ppm for two hours once per day. Applying the same dosing strategy every second day, was not as effective in limiting flux decline, but still produced better results than the remaining dosing strategies. This supports the notion of a sufficiently high dosage for an optimal time, rather than high concentration shock-dosages for a short period. A significant increase in biomass parameters (cell count, colony forming units, and protein- and polysaccharide concentration) was observed when nutrients were added to the feed water. Protein concentration (p=4.29 x 10-5, R2=0.71) and polysaccharide concentrations (p=0.0053, R2=0.58) on the membrane had a strong and significant relationship with the flux decline, making it suitable parameters for biofouling quantification. CFU showed a significant, but not strong, (p=0.0011, R2=0.54) relationship to the flux decline, whereas total cell count did not provide a significant (p=0.14) relationship. Protein- and polysaccharide concentrations could therefore be used for the quantification of the biofouling. A destructive study should, however, be performed to determine these parameters. A practical tool is therefore still necessary for the early diagnosis of biofouling. For future studies, it is recommended that larger ranges of cross-flow velocities and pressures be investigated, together with the effect of DBNPA dosing. Ideally, the work should be performed on a membrane that is packed in a spiral-wound format to simulate real-life situations.