Biofilm-based microalgal cultivation has recently received great attention owing to its low harvesting cost, but the main problem in practicing it is the low rate of attachment on solid carriers. The aim of this research is to introduce novel physical and wet chemical surface engineering methods to provide more favorable polymeric surfaces for microalgal adhesion. PET threads were used as a substrate in the treatments. The surface of the threads was treated with chromic acid, sodium hydroxide and sandpaper. The chemical composition, surface morphology, topography and contact angle of the threads were characterized. The threads were placed in a biofilm-based cylindrical photobioreactor as a bed for attachment. Two freshwater single-cell microalgae, Scenedesmus dimorphus and Chlorella vulgaris, were cultivated in the photobioreactor to assess the attachment rate of the threads. The analysis of SEM and AFM images confirmed the creation of new grooves. The AFM image analysis showed 323%, 184% and 11.5% increase in the surface roughness, while there were 73%, 51%, and 30% rates of reduction in the contact angles for the treatments with acid, sandpaper and base, respectively. Creation of new grooves, increase of the surface roughness and decrease of the contact angle led to an increase in the microalgae attachment rate. The best results were achieved with acid treatment. It led to a remarkable increase in the attachment rate of S. dimorphus. However, the attachment of C. vulgaris cells was not efficient. This research is the first to apply a surface engineering method to increase the microalgal attachment rate in biofilm-based systems. The insight that is provided can be of benefit for further studies in this field.