This article reviews the occurrence of HABs in seawater, their effects on the operation of seawater
reverse osmosis (SWRO) plants, the indicators for quantifying/predicting these effects, and the
pretreatment strategies for mitigating operational issues during algal blooms. The potential issues in
SWRO plants during HABs are particulate/organic fouling of pretreatment systems and biological
fouling of RO membranes, mainly due to accumulation of algal organic matter (AOM). The presence
of HAB toxins in desalinated water is also a potential concern but only at very low concentrations.
Monitoring algal cell density, AOM concentrations and membrane fouling indices is a promising
approach to assess the quality of SWRO feedwater and performance of the pretreatment system.
When geological condition is favourable, subsurface intake can be a robust pretreatment for SWRO
during HABs. Existing SWRO plants with open intake and are fitted with granular media filtration can
improve performance in terms of capacity and product water quality, if preceded by dissolved air
flotation or sedimentation. However, the application of advanced pretreatment using ultrafiltration
membrane with in?line coagulation is often a better option as it is capable of maintaining stable
operation and better RO feed water quality during algal bloom periods with significantly lower