Controlling corrosion in desalination plants

Over the past ten years, Desalination and Reverse Osmosis (RO) systems have come into their own as an excellent alternative to treating drinking water, particularly in areas which have either a shortage of natural ground/surface water or are near seawater supplies. However, there are problems associated with producing such high quality drinking water.

The drive to produce water free from the chemical components found naturally in drinking supplies has been so successful that it has left us with a water quality almost totally devoid of anything but H2O - little to no alkalinity, little to no hardness, little to no solids, little to no anything.

This year, two desalination plants in the Virgin Islands have been the testing ground for Aqua Smart Inc, of Atlanta, Georgia, which has successfully applied its SeaQuest certified corrosion control drinking water additive to the distribution pipelines to protect them from aggressive desalinated and RO water. As there are few to no minerals, the typical SeaQuest treatment rate is 0.5ppm.

Although there are other polyphosphates and blended ortho/polyphosphates available, all must have calcium or calcium/magnesium present in the water supply to form calcium or calcium/magnesium phosphate coating. This is their only mechanism for providing corrosion control. Indeed, some products have been shown to de-optimise
corrosion control.

However, SeaQuest blended ortho/polyphosphate does not require the presence of calcium and/or calcium/magnesium, even without added pH control. SeaQuest forms a uniform continuous metal phosphate coating with the base metal of distribution piping that does not involve the presence of calcium or calcium/magnesium.

Aqua Smart created its proprietary SQ547 to make SeaQuest stable to reversion under varying pH, varying time, varying temperature, and also to enable it to form a uniform non-building metal phosphate. This is the corrosion protective mechanism on the inner surfaces of distribution pipes. Where corrosion pre-exists, SQ547 technology slowly removes the corrosion and replaces this with its own coating, solving low distribution disinfection-biofilm, and pipe corrosion problems. All this has been accomplished in real time, without shut downs or by-passes, and without affecting tap water quality.

Virgin Islands success
The two Caribbean islands service their customers almost exclusively with desalinated water. The maximum calcium and/or calcium magnesium content of these waters is 1ppm and the alkalinities are less than 5ppm. Both systems were failing mandated lead levels, compounded by corrosion and discoloured water.

The original thought was to run all the finished water through calcite in order to build up some level of alkalinity in the water along with any protection provided by the increased levels of calcium. However, SeaQuest was found to be easier to operate and less expensive to maintain. The Tables illustrate the results after only six months of SeaQuest use.

As desalination and RO systems become the norm for drinking water supplies, there will be an increasing need to complement their use by addressing the optimum corrosion control of the distribution piping.

To date in the US, only SeaQuest has proven to maintain optimum corrosion control in treated water of such high quality. Because SeaQuest does not rely on any minerals (soft or hard) for optimum corrosion control, there is no need for additional treatments, such as added pH control.


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