Water. desalination + reuse

DWR MayJune 2015

Water. Desalination + reuse

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| 26 | Desalination & Water Reuse | May-June 2015 RESEARCH How could an understanding of the chemical language of microbes unlock the enigma of biofouling? Studies looking at the communications between bacteria that lead to the build up of biofoulant reveal opportunities to scramble the messages and possibly remove this thorn in the side of membrane desalination. _________ Professor Sunny Jiang and Leda Katebian University of California, Irvine ___ The word on the sheet: bioflm has a script AdvAnces in desAlinAtion to improve its performance and effciency are growing in value and importance as California, Texas and other regions of the developed and developing world struggle to tackle advancing water scarcity. And among the greatest challenges for operators and developers of the chief seawater desalination technology – reverse osmosis (RO) – is the congestion of the membranes by bacteria and other microbes known as biofouling. The biological, chemical and physical factors in play during the build up of biofoulants on an RO membrane are complex. So intense scientifc scrutiny is vital if we are to unveil a way to curb biofouling. And one of the more rewarding areas of scrutiny could be in the way bacteria talk to each other. At the biochemical level, the formation of bioflm – the frst thin layer of foulant that forms on an RO membrane – as well as the subsequent build up on the membrane to cause biofouling - are regulated by a bacterial communication system, called quorum sensing (QS). TaLKing SenSing QS is a communication system based on chemical messages transmitted constantly by each bacterium in a population. Once the population density has reached a level – the quorum – to produce a threshold density of chemical messages, the messages are translated into functions carried out by each bacterium – such as bioflm formation. The QS communication is seen in many different types of bacteria in and beyond biofouling. It provides genetic signals that trigger functions such as light emission (luminescence) or toxin release in various bacteria. As with bioflm formation these functions are dormant until in the bacteria reach a population density that fres up the QS process. The best understood QS mechanism deploys one or both of two types of messenger – known as autoinducer 1 (AI-1) and autoinducer 2 (AI-2) – see box, Message in a molecule. Marine bacteria isolated from biofouled RO membrane surfaces have been shown to produce several different types of AI-1 molecules (fgure 1). Indeed the frst AI-1 was identifed in a marine bacterium. This suggests that inhibition of QS autoinducers could strike at the heart of the offending bacteria's means to proliferate to form biofouling in seawater RO. Winning inhibiTionS We have investigated the effectiveness of known QS inhibitors (QSIs) – vanillin, which blocks AI-1 systems and cinnamaldehyde which inhibits AI-2 – and found both to inhibit bioflm production by a number of bacteria. However vanillin treatment of eight single species extracted from sources in California and Perth, Australia showed a range of effects from virtual extinction of bioflm formation to, in one species, apparent stimulation (fgure 2). On treating natural marine bacteria populations from three sites in California with cinnamaldehyde, and using the same optical spectroscopy method for bioflm density measuring used with the AI-1, the QSI was shown as highly effective in each case (fgure 3). The disparity in the AI-1 results suggested that there are signifcantly different QS pathways in play. But the evidence was strong that treatment of bulk feed water in an RO system could subdue bioflm production by natural marine bacteria populations. This was supported on a laboratory scale. Adding vanillin to the feed in a laboratory cross-fow reverse osmosis set up and subsequent analysis showed that bioflm production was signifcantly reined in. Using confocal scanning laser microscopy with depth of feld enhancement (Z stacking) showed a halving of the thickness of bioflm produced and a similar reduction in the thickness of extracellular polymeric substances – the "mortar" that forms around biofouling bacteria. bound To WorK beTTer Incorporation of QSIs into an RO membrane composite could concentrate the agent at the front line of the battle with biofoulants and enhance signifcantly its capacity to repel bioflm. We have succeeded in depositing vanillin and cinnamaldehyde on an RO membrane. Our future work

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