Biological and corrosion aspects of a multi-stage flash seawater desalination plant after deaerator modification

Deaeration units of a commercial multi-stage flash (MSF) cogeneration facility underwent venting modifications to increase system distillate productivity without increasing energy consumption or elevating top brine temperatures. Three MSF units were investigated after oxygen levels of deaerators spiked above design limits (20 ppb). The investigation found a significant biotic presence in the deaerator unit’s MSF heat rejection sections that included algae, bacteria, bivalves, and bivalve larvae. The bivalve taxa consists of Barbatia parva, Diodora funiculata, Pinctada margaritifera, Amiantis umbonella, and Acrostergma assimile. Galvanic corrosion exacerbated by biofilm was also observed in heat rejection sections. While attesting to the tenacity and adaptability of marine life, the presence of biological communities is symptomatic of the abnormally high oxygen levels in deaerator units. Elevated oxygen levels contribute to systems performance declines resulting from biological fouling of deaerator packing materials, partially responsible for inefficiencies in reducing the gas solubilities of MSF process feed waters and elevated concertation ratios [1,2]. Several thermal desalination units presented similar process issues, but one was selected for detailed inspection and evaluation.