Algal toxin removal in seawater desalination processes

Most marine algal species are beneficial, not harmful, as algae are the foundation of the food chain and provide the bulk of Earth's oxygen through photosynthesis. Mankind also commercially harvests algae for a myriad of uses in the food, pharmaceutical and medical industries to name but a few. However, the sudden prolific growth in algal cell numbers, referred to as harmful algal blooms (HAB), can constitute an operational and/or health risk to desalination plants, threatening water supply security and safety, respectively. The excessive biomass and organics associated with HAB can lead to the closure of desalination plants, particularly sea-water reverse osmosis (SWRO) plants due to overloading of the pretreatment facilities or potential irreversible RO membrane fouling. While these impacts are well documented, the removal of potent marine algal toxins, which represent a potential public health risk if not removed by desalination plant processes, is not well researched. The incidence of HAB has escalated throughout the world with algal specialists reporting that "compared to 30 years ago, we have more algal toxins, more toxic algal species and more areas affected". Therefore, this paper examines the major marine algal toxins that may be present at the intake of a desalination plant, their fate in thermal and SWRO desalination plant processes and the potential residual risk to public health in desalinated drinking water. Toxin removal in the various process steps is predicted based on the physico-chemical properties of these marine toxins. Results from bench and pilot studies investigating the efficacy of barriers in the desalination technology processes to remove cell-bound toxins and extracellular toxins from ruptured algal cells are also reviewed.