Control of biofouling on reverse osmosis membranes using DBNPA

Ras, Gabriel Retief (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

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.

AFRIKAANSE OPSOMMING: Tru-osmose word wêreldwyd vir waterontsouting gebruik en is weens effektiewe energie gebruik en veilige bedryfsproses baie populêr. Bevuiling (waarvan biologiese bevuiling die problematiesste is) van die membrane is ‘n onvermydelike proses wat slegs bestuur kan word. Biologiese bevuiling onstaan weens mikrobiese groei wat binne die membraansisteem plaasvind en sodoende verskeie ongewenste probleme tot gevolg het. Doeltreffende voorbehandeling kan biologiese bevuiling verminder, maar nie noodwendig verhoed nie. Dit is a.g.v. mikro-organismis se vermoë om voort te plant en sekondêre kolonies regdeur die membraanstelsel te vorm, selfs as 99.99% van die organismes tydens voorbehandeling verwyder. Die meeste moderne tru-osmosis aanlegte is met dun film saamgestelde poliamied membrane toegerus. Biologiese beheer op die membrane is beperk aangesien oksiderende biododers, soos chloor, die membraan degradeer. Dit veroorsaak dan dat die soutdeurlating verhoog en membraanleeftyd afneem. Om hierdie rede word die gebruik van ‘n nie-oksiderende biododer, naamlik 2,2-dibroom-3-propionamide (DBNPA), op dun film saamgestelde poliamied membrane in hierdie studie ondersoek. Die primêre doel van die studie was om die invloed van drie DBNPA doseringsveranderlikes op die beheer van biobevuiling of tru-osmose membrane te demonstreer. Die veranderlikes was dosering (100 dpm tot 200 dpm), doserings intervalle (twee keer per dag tot elke tweede dag) en doseringstyd (30 min tot 2 ure). Die werk steun ook op die karakterisering van die biologiese bevuiling deur van biomassa parameters (proteïen- en polisakkariedkonsentrasies, totale sel telling en kolonievormende eenhede) gebruik te maak wat aan die afname in stroming gekoppel kan word. Toetse is in laboratoriumskaal, tru-osmose membraanblokke uitgevoer wat in staat was om dun film saamgestelde poliamied membrane saam met die toepaslike voerspasieerders te huisves, soos tipies in industriële stelsels aangetref word. Aangesien skoon munisipale water vir voerwater gebruik is, is voedingstowwe (natriumasetaat, natriumnitraat en natrium diwaterstofortofosfaat, in die verhouding van 100:20:10, gebruik om ‘n finale koolstofkonsentrasie van 100 μg/ℓ te gee) by die voerwater gevoeg om voldoende mikrobiese groei te stimuleer en sodoende ‘n sinvolle studie van die effek van DBNPA dosering uit te voer. Gedurende die verwydering van die biofilm vanaf die membraan, is gevind dat geen verwyderings- en homogeniseringstegniek (b.v. skraping van die biofilm vanaf die membraan, ultrasoniesebad en ultrasoniesestang behandeling) beduidend meer kolonievormende eenhede opgelewer het nie. R2A agar het wel beduidend meer kolonievormende eenhede opgelewer in vergelyking met voedingsagar. Die agar wat gebruik word is dus meer beduidend as die biofilm verwyderings- en homogeniseringstegniek wat gebruik word. DBNPA dosering het die hoeveelheid biobevuiling verminder, ongeag die doseringstrategie wat gebruik was. Binne die omvang van die studie, is biobevuiling die beste beheer deur ‘n DBNPA konsentrasie van 100 dpm vir twee ure elke dag. Deur dieselfde doseringsstrategie elke tweede dag toe te pas, was minder doeltreffend om die stromingsafname teen te werk. Dit was egter meer effektief as die ander doseringstrategieë wat getoets is. Die resultate ondersteun dus die idee van ‘n voldoende hoë doseringskonsentrasie vir ‘n optimale tyd, eerder as hoë doseringskonsentrasies op ‘n kort skok basis. ‘n Beduidende toename in biomassa parameters (totale seltelling, kolonievormende eenhede, proteienen polisakkariedkonsentrasies) is waargeneem wanneer voedingstowwe by die voerwater gevoeg was. Proteïenkonsentrasies (p=4.29 x 10-5, R2=0.71) en polisakkariedkonsentrasies (p=0.0053, R2=0.58) het ‘n sterk en beduidende verwantskap met die afname in stroming gehad. Dit maak die twee parameters geskik om vir die kwantifisering van biobevuiling gebruik te word. Kolonievormende eenhede het ook ‘n beduidende, maar minder sterk, (p=0.0011, R2=0.54) verwantskap met stromingsafname gehad. Totale seltelling het egter geen beduidende (p=0.14) verwantskap getoon nie. Proteïen- en polisakkariedkonsentrasies kan dus gebruik word vir kwantifisering van biobevuiling terwyl kolonievormende eenhede minder geskik is en totale sel telling glad nie geskik is nie. ‘n Destruktiewe studie is egter nodig om die parameters te bepaal. ‘n Praktiese manier is dus nog nodig om vroeë biobevuiling te identifiseer. Daar word aanbeveel dat ‘n groter verskeidenheid kruisvloei snelhede en drukke ondersoek moet word tesame met die effek van DBNPA dosering vir toekomstige studies. Dit sal ook meer gewens wees as die studies op spiraalgewikkelde membrane gedoen word om werklike prosesse beter te simuleer.

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