Cleaning biofouled membranes early improves plant operation. Comparison of autopsy and cleaning results on a small BWRO and large SWRO

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Abstract

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 design specification. This paper explores the benefits of early identification of foulants through autopsy allowing predictive maintenance cleaning of the plant to be conducted at the first signs of fouling. This prevents a build up of difficult to remove deposits which reduce membrane performance and life expectancy.

Autopsies were conducted on a 1,200 GPM (6,650m³/day) brackish water reverse osmosis plant (BWRO) and a 22,000 GPM (120,000m³/day) sea water reverse osmosis plant (SWRO). Foulants identified included biofilm, clay and iron. Different cleaning solutions were then tested on samples of the actual membranes to determine the best cleaning protocol. The authors present the results of autopsies, laboratory cleaning tests and full plant cleaning results with a number of different cleaning agents. A strongly alkaline cleaner with a high ionic strength appeared to be particularly effective at removing biofilm and clay. The authors theorise that during the soaking process the high ionic strength of the cleaning solution on the feed side of the membrane encourages natural osmosis of permeate water through the membrane to the feed side thus causing some physical disruption to the biofilm and clay fouling layer. This allows the detergent, surfactant and chelating molecules in the cleaner to break up the fouling layer allowing a more effective clean. Once biofilm is removed then an acid clean proved much more effective at removing iron deposits.

Comparisons are drawn between the results of cleaning early on the SWRO plant and late on the BWRO plant. It is much easier to remove deposits before they have formed a substantial cake layer resulting in more effective and less frequent cleaning which prolongs membrane life.

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