Water. Desalination + reuse
Issue link: https://fhpublishing.uberflip.com/i/545366
| 32 | Desalination & Water Reuse | August-September 2015 RESEARCH BIOFOULING HAS BEEN described as the Achilles' heel of membrane-based desalination. Given this notoriety, continuing study of the complex combination of physical and chemical processes behind biofouling promises a profi table outcome in its prospect of a basis for means to combat what is a costly blight on membrane-based wastewater desalination. A group of researchers at Ben Gurion University of the Negev (BGU) in Israel led by Moshe Herzberg with assistance from researchers in Germany looked recently at a number of interactions in play when a reverse osmosis (RO) membrane comes into contact with permeate from tertiary pretreatment. The BGU group claims to have uncovered "an additional RO membrane fouling mechanism" arising from organic compounds present in ultrafi ltration (UF) permeate during wastewater desalination [1]. Herzberg's team identifi ed differences between the effects on RO membranes caused by substances in UF permeate and those caused by large molecules – extracellular polymeric substances (EPS) – secreted by microorganisms. Biofouling is, according to Herzberg's team, particularly prevalent [2] in water reclamation applications. He says the decrease in a membrane's performance on biofouling, and the challenges in cleaning the membrane derive more from the EPS than from the microorganisms themselves [3]. EPS, he says, ultimately act as the mortar holding biofi lms together [4,5]. Herzberg's fi ndings established that UFP constituents impaired irreversibly the fl ux in RO membranes and generated the build up of biofouling fi lm by providing a foothold for foulant bacteria. EPS binding was less tenacious and less damaging in the short term. While a full explanation for the observed differences is not established, the BGU researchers have suggested that a means to combat biofouling could lie in pinpointing the chemical and physical interactions behind the differences. In their paper the researchers say: "These effects are crucially needed to be defi ned during RO fouling scenarios in order to adopt the relevant fouling control strategies." ILL-UNDERSTOOD The researchers highlight the formation of a so-called conditioning fi lm - the fi rst thin layer of organic or inorganic substances deposited on the membrane surface in the initial stages of biofouling – as a crucial yet ill- understood stage in the fouling process (see box, Foul play). The conditioning fi lm has been noted in earlier research as introducing changes in the physicochemical characteristics of the membrane surface including its hydrophobicity, charge distribution, and roughness. All of these can have signifi cant effects on bacterial attachment [6] as well as the further adsorption of dissolved substances. Herzberg's study looked at two important interactions in play during membrane biofouling: • conditioning fi lm (from the UFP and EPS) interactions with the polyamide of the RO membrane surface; and • conditioning fi lm interactions with foulant bacteria. CONDITIONING FILM AND MEMBRANE Observations by the BGU team focused on the degree to which the capacity of the polyamide material used in RO membranes was affected by layers of UFP-borne substances and EPS. They looked at how foulants from Researchers in Israel have uncovered indicators of a new mechanism behind biofouling in reverse osmosis membranes. Progress in comprehension of the complicated interactions between chemical and biological species and their damaging effects on membrane permeability could spawn solutions to their costly repercussions. _________ Trevor Loveday editor, D&WR ___ Biofouling: a new start Another fi ne mess: might biofouling's beginings signpost a means to an end?