This study presents the results of an exploratory experiment aimed at investigating the feasibility of microalgae-based tertiary wastewater treatment for the removal of phosphorus (P) and nitrogen (N) for decentralized systems. The effluent from a single-household on-site packaged activated sludge (AS) plant, not designed for targeted N and P removal, was used to feed an algal photo-sequencing batch reactor (Algae SBR; 4.5 L). The reactor was operated under laboratory conditions using natural sunlight (approximately 14 h light/10 h dark photoperiod) and a hydraulic retention time (HRT) of 20–21 h. The packaged AS plant exhibited pronounced effluent variability associated with changes in household occupancy (2–10 p.e.), with total phosphorus (TP) ranging from 5.1 to 13.1 mg/L and total nitrogen (TN) from 4.6 to 58.2 mg/L. These variations strongly influenced nutrient removal efficiency in the Algae SBR. Effluent nutrient concentrations were directly proportional to their influent concentrations, with R² values exceeding 0.99 for TN and 0.80 for TP. Phosphorus removal was significantly affected not only by influent phosphorus concentrations but also by influent nitrogen levels. Regarding compliance with legislative limits, nitrogen standards (TN ≤ 15 mg/L) were met only when influent TN remained below 20 mg/L. Phosphorus standards (TP ≤ 2 mg/L) were nearly achieved only when influent TN was below 8 mg/L and influent TP below 7 mg/L. These initial results indicate that algae-based tertiary treatment for on-site packaged plants is technically feasible. However, its performance appears to be highly sensitive to the effluent quality of upstream treatment units and therefore requires further in-depth investigation.