Role of surface roughness in the algal short-term cell adhesion and long-term biofilm cultivation under dynamic flow condition

Abstract Surface roughness can enhance the deposition of algal cells to carrier surface and improve the production of algal biofilm. However, the corresponding underlying mechanisms remain unclear. In this study, pine sawdust, sugar bagasse, rice husk, elastic filler and quartz sand were selected as carriers and their surface roughness were measured using a confocal laser-scanning microscopy. Cell transportation experiments and long-term biofilm cultivation on these carriers were conducted in a self-designed thirty-channel algal biofilm reactor under dynamic flow condition. Results indicated that pine sawdust (0.420 to 0.595 mm, root-mean-square roughness Sq = 45.16 μm) was more suitable, efficient and cost-saving carrier for algal biofilm cultivation under the developed “regrowth mode” (using the residual suspended algal cells from the previous culture cycle as inoculum), with the productivity of algal biofilm achieved 14.75–15.75 g·m−2·day−1 after 8 days cultivation. Increase of carrier surface roughness (Sq: from 13.03 to 45.15 μm) enhanced the short-term (1 day) cell adhesion to surface (from 6.89 to 23.92 g·m−2·day−1). The productivity of the long-term (8 days) cultivated biofilm positively correlated with the rate of short-term cell adhesion and carrier surface roughness. Computational fluid dynamics (CFD) analysis revealed that these positive correlations were due to the created local hydrodynamic conditions by surface roughness. More asperities and hydrodynamic stagnant zones on the rougher surface promoted the interception and retention of algal cells and enhanced the cell adhesion strength, and further contributed to the intensive inoculation as well as the formation of robust and strengthened biofilm, thus finally leading to the improved biomass production indirectly. Findings of the present study are of particular importance for the manufacture of man-made rough carriers, the inoculation of biofilm systems and the enhancement of biomass productivity during the large-scale algal biofilm cultivation process.