Chlorine can increase AOC in desalination - study

Addition of chlorine or antifouling chemicals in desalination pretreatment can increase levels of assimilable organic carbon (AOC) and accelerate problems, according to the results of a study presented at the 26th Annual WateReuse Symposium in Phoenix, USA, on 12 September 2011.

The study regarding pretreatment for desalination of seawater was reported on by Dr Mark LeChevallier, director of innovation and environmental stewardship, American Water. The results suggest that when terrestrial organic matter enters highly saline water, it undergoes complexation by manganese and/or calcium to form soluble complexes that control its subsequent surface chemistry.

Funded by the Water Research Foundation and subsidiary companies of American Water, the study specifically concerned the removal of AOC and total organic carbon during desalination pretreatment. Pretreatment for seawater desalination typically focuses on removal of particles, but many of the problems with membrane fouling are due to natural organic matter in water.

The study found that:

  • Particles in seawater contain both positive and negative charges, which is unusual and does not occur in freshwater. These charges are due to the presence of certain minerals (calcium and magnesium) naturally present in seawater, which interferes with the removal of particles in water and explains why removal of natural organic matter in seawater is so difficult.

  • Some common practices in seawater treatment (eg, the addition of chlorine, or antifouling chemicals) can increase the levels of AOC and actually accelerate problems due to bacterial growth on the membranes.

  • American Water developed a novel method to measure the AOC that bacteria feed upon in order to grow on the desalination membrane filters. The test uses a naturally bioluminescent marine organism for the assessment and monitors its growth by tracking the increase in light produced by the bacterium.

    "The results of these studies, suggest that when terrestrial organic matter enters highly saline water, it undergoes complexation by manganese and/or calcium to form soluble complexes that control its subsequent surface chemistry," stated LeChevallier. "When absorbed onto silts, these complexes become difficult to remove by charge neutralization and may require separation by enmeshment requiring high coagulant doses or coagulation at extremes of pH that impact the surface charge and allow for removal."

    Along with LeChevallier, American Water's Dr Orren Schneider, senior environmental engineer, and Lauren Weinrich, senior research analyst, were co-authors of the paper highlighting the study. The presentation will be available on the WateReuse Association's web site.

    Tags

    | Chlorine | Fouling


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