Microalgae attract considerable interest as a source of lipids, carbohydrates, proteins, and high-value compounds, which may be utilized in many sectors, including biofuels, bioplastics, animal feed, and nutraceuticals. There is a great need to improve the cost and efficiency of microalgal processing to advance the feasibility of its large-scale implementation. In this study, we proposed the use of carrot pomace waste to support the heterotrophic cultivation of Auxenochlorella protothecoides using a carrier material to promote attached biomass growth. Various materials were compared for microalgal attached growth, and cotton string was chosen for yielding the highest biomass attachment. String-attached biomass could be harvested easily by straining, and the procedure for direct lipid extraction of attached biomass was optimized, yielding a maximum lipid content of 23.2% DCW. Carrot pomace waste was utilized with a one-time addition of 5 g/L glucose and without mineral supplementation, resulting in a total biomass of 13.2 g/L, of which 98.5% was attached to the string. String-attached biomass had high (49.2%) solid content, making it potentially ready to use. The combination of attached cultivation, heterotrophic conditions, and waste utilization presents a novel microalgal processing scenario with the potential to improve efficiency, reduce costs, and advance overall feasibility.