Water. Desalination + reuse
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RESEARCH | 34 | Desalination & Water Reuse | August-September 2015 other. That would suggest that the UFP conditioning layer might actually block EPS binding which was not Herzberg's finding from QCM-D observations. He proposes that EPS adsorption may have taken place on patches of the sensor that were free from conditioning film. He says also it may remain possible that despite the steric and electrostatic hindrances, the conditioning film layer was still able to adsorb EPS. Perhaps more importantly he reminds us that bacterial binding was greatly enhanced by the conditioning film and that EPS – a bacterial product - is less important during the initial stages of biofilm formation and highly significant in binding together mature biofilms. "There is still a great lack of understanding regarding the influence of the conditioning film and the EPS on bacterial cells attachment to surfaces as well as their role in the resistance of the biofouling layer to cleaning agents and shear," Herzberg reports. He points out other research that has established important contributions to the chemistry of biofouling including the role of calcium ions in boosting organic matter fouling by "complexation and subsequent formation of intermolecular bridges within the organic fouling layer". He says efficient chemical cleaning was achieved only when calcium ions were eliminated. The team at BGU has uncovered what appears to be a fouling mechanism that involves different species in differing roles. Herzberg proposes his team's finding of reduced swelling mediated by organic foulants as an entirely new RO fouling mechanism. Clearly the chemistry of the flux-arresting effects of the conditioning film is complex. Herzberg argues that further study is called for to possibly unlock new and better ways to counter their costly impact on wastewater reuse. "A good understanding of the physicochemical interactions between the conditioning film and the membrane surface, the conditioning film and bacteria, bacteria and EPS and within the EPS is important for the development of biofouling control and cleaning strategies," he says. l REfEREnCES [1] W. Ying, N. Siebdrath, W. Uhl, V. Gitis, M. Herzberg, New insights on early stages of RO membranes fouling during tertiary wastewater desalination. J. Membr. Sci. 466 (2014) 26−35. [2] H.-C. Flemming, G. Schaule, T. Griebe, J. Schmitt, A. Tamachkiarowa, Biofouling – the Achilles heel of membrane processes, Desalination. 113 (1997) 215-225. [3] M. Herzberg, S. Kang, M. Elimelech; Role of Extracellular Polymeric Substances (EPS) in Biofouling of Reverse Osmosis Membranes, Environ Sci Technol. 43 (2009) 4393-4398. [4] H. Ridgway, K. Ishida, G. Rodriguez, J. Safarik, T. Knoell, R. Bold, Biofouling of membranes: Membrane preparation, characterization, and analysis of bacterial adhesion, Method Enzymol. 310 (1999) 463-494. [5] J.S. Vrouwenvelder, D. van der Kooij, Diagnosis, prediction and prevention of biofouling of NF and RO membranes, Desalination. 139 (2001) 65-71. [6] H.J. Busscher, A.H. Weerkamp, Specific and Nonspecific Interactions in Bacterial Adhesion to Solid Substrata, Fems Microbiol Rev. 46 (1987) 165-173. [7] W. Ying, K. Rajender, M. Herzberg, R. Kasher, Diminished swelling of cross-linked aromatic oligoamide surfaces revealing a new fouling mechanism of reverse-osmosis membranes. Environ Sci Technol. 49 (2015) 6815-6822. [8] M.P. Tong, P.T. Zhu, X.J. Jiang, H. Kim, Influence of natural organic matter on the deposition kinetics of extracellular polymeric substances (EPS) on silica, Colloid Surface B. 87 (2011) 151-158. foul play Biofouling in reverse osmosis (RO) has three identified stages. First, a so-called conditioning film of nutrients and other substances attaches to the membrane surface; second, bacteria gain a hold; and third the bacteria release large molecules that also adsorb onto the membrane to consolidate the biofilm. The conditioning film is the first thin layer of largely organic substances deposited on the membrane surface. The film changes the surfaces' physicochemical nature, including its roughness, its water-repelling (hydrophobic) characteristics, and the surface distribution of electrical charge. In wastewater desalination, RO conditioning films are formed from dissolved organic matter in the permeate from tertiary treatment – typically membrane bioreactor ultrafiltration permeate (UFP). This effluent organic matter – along with some ionic species includes substances that have not been broken down during MBR treatment. These include natural organic matter already present in drinking water, natural or synthetic organic matter from surface runoffs, synthetic organic compounds from domestic use, and soluble products produced by the microorganisms in the biological wastewater treatment process. Together the changes to the membrane surface once the conditioning film has adhered make it easier for biofouling bacteria to attach. As the bacteria proliferate they release large molecules – extracellular polymeric substances (EPS) – which adsorb onto the membrane surface to form a mortar for the building blocks of the biofilm that ultimately fouls the RO. The measure of the matter To measure the effects of conditioning film build up on RO membrane polyamide, Herzberg and his team deployed quartz crystal microbalance with dissipation (QCM-D) – which measures changes in mass per unit area of a test substance. In QCM-D the test substance (polyamide) is fixed to a vibrating quartz sensor and changes in the dissipation of acoustic energy caused by changes in the mass of the test are expressed as a frequency change with increased mass producing a reduction in frequency. Biofouling effects on flux were studied using an RO flat-sheet laboratory unit, receiving the UFP from a hybrid growth membrane bioreactor (HG-MBR) as feed solution and operated continuously. Herzberg's team used samples from the biofouling layer on the HG-MBR as a representative source of EPS for subsequent use in testing the effects of EPS conditioning on RO membranes and the binding of EPS to RO membranes. The thin begininng: conditioning film forms from effluent organics in the permeate