Soluble Algal Products (SAPs) in large scale cultivation of microalgae for biomass/bioenergy production: A review

Nowadays, microalgae-based bioenergy has been commonly considered as a potential substitute of the non-renewable traditional energy resources with increasing attention. Nevertheless, microalgal cells will secrete/release some Soluble Algal Products (SAPs) into the culture medium during microalgal biomass cultivation. Studies on the influences, especially the negative ones, of SAPs in large-scale cultivation of microalgae for biomass/bioenergy production remain limited. The negative effects, properties, production and treatments of SAPs were therefore investigated and summarized in this paper. It was found that SAPs may inhibit microalgal growth, raise the cost of microalgal recovery and lower the performance of the downstream discharged water treatment process after microalgal cultivation, among others. The production of SAPs could be as high as 70 mg/L in terms of dissolved organic carbon (DOC), causing substantial environmental issues. The chemical properties of SAPs significantly varied among different microalgal species, growth phases and nutrient concentrations. The distribution of molecular weight of SAPs varied from less than 1 kDa to more than 100 kDa, meanwhile the hydrophilic/hydrophobic property of SAPs showed a consistency. All studies showed that more than 60% of SAPs are hydrophilic. After comparing three different SAPs treatment methods, it was concluded that biodegradation could be a feasible and economical process to remove SAPs based on the properties of SAPs. The mechanisms of SAPs formation and the mitigation of their undesirable effects are still not clear. Therefore, a systematic and deep study on those topics is urgently needed.