CLEANING BIOFOULED MEMBRANES EARLY IMPROVES PLANT OPERATION. COMPARISON OF AUTOPSY AND CLEANING RESULTS ON A SMALL BWRO AND LARGE SWRO

摘要

It is a fact that all new reverse osmosis membranes start to foul as soon as they start operation.The only variable is the rate at which they foul. Most RO plants operate at a differential pressure(ΔP) of 1.5 - 3 bar. Cleaning usually occurs when the ΔP is 10-15% above the designspecification. This paper explores the benefits of early identification of foulants through autopsyallowing predictive maintenance cleaning of the plant to be conducted at the first signs offouling. This prevents a build up of difficult to remove deposits which reduce membraneperformance and life expectancy.Autopsies were conducted on a 1,200 GPM (6,650m~3/day) brackish water reverse osmosis plant(BWRO) and a 22,000 GPM (120,000m~3/day) sea water reverse osmosis plant (SWRO).Foulants identified included biofilm, clay and iron. Different cleaning solutions were then testedon samples of the actual membranes to determine the best cleaning protocol.The authors present the results of autopsies, laboratory cleaning tests and full plant cleaningresults with a number of different cleaning agents. A strongly alkaline cleaner with a high ionicstrength appeared to be particularly effective at removing biofilm and clay. The authors theorisethat during the soaking process the high ionic strength of the cleaning solution on the feed side ofthe membrane encourages natural osmosis of permeate water through the membrane to the feedside thus causing some physical disruption to the biofilm and clay fouling layer. This allows thedetergent, surfactant and chelating molecules in the cleaner to break up the fouling layerallowing a more effective clean. Once biofilm is removed then an acid clean proved much moreeffective at removing iron deposits.Comparisons are drawn between the results of cleaning early on the SWRO plant and late on theBWRO plant. It is much easier to remove deposits before they have formed a substantial cakelayer resulting in more effective and less frequent cleaning which prolongs membrane life.