Microalgal attachment and attached systems for biomass production and wastewater treatment

Microalgae are promising feedstocks for food, feed, fuel, fertilizers, fine chemicals and other value-added products, and are considered of great potential in wastewater treatment. Convenient acquisition of the cultivated microalgal biomass or easy separation of microalgae from the treated effluents, however, are the prerequisites of these applications. Biomass harvesting and dewatering in traditional suspended cultivation systems are costly and energy intensive, since volumes of liquid medium needs to be separated from the obtained diluted microalgal biomass. Attached systems can effectively overcome such limitations by introducing substratum into the liquid medium for microalgae attachment, and the attached biomass with magnitudes higher solid content can be collected via simple and straightforward harvesting methods such as mechanical scraping. As an emerging field with less than ten years of research history, systematic information on microalgae attachment and attached systems for biomass production and wastewater treatment has not been thoroughly reviewed. This review, therefore, analyzed the two-step mechanisms of microalgae attachment, discussed the influencing factors of attached microalgal growth including properties of substratum, properties of microalgal cells, turbulence of liquid medium, frequency of biomass harvesting, etc., and their possible impacts on microalgal biofilm formation and biomass production in attached systems. Classification of attached microalgal systems based on different criteria such as substratum orientation, relative position of microalgal cells and the culturing medium, system scales and culturing medium, was summarized. Performances of attached microalgal systems for biomass production and wastewater treatment were evaluated, with commonly used parameters identified and compared as well. Recommendations for future theoretical research and practical applications were also addressed in the manuscript. This review could offer much useful information on microalgal attachment and attached systems for biomass production and wastewater treatment, and may serve as guidance for future studies in related fields.