The objective of this study was to enhance the phosphorus (P) removal performance of raw steel slag (RS) using surface modification and CO2 stripping methods. To reduce the alkalinity of the wastewater, CO2 stripping was performed through pH adjustment. To reduce the effect of the crystal barrier, carbon nanotubes (CNTs) was used for increase in the specific surface area of RS. As the alkalinity was reduced through CO2 stripping, the P removal efficiency (PRE) improved by approximately 40%. It was shown that slag surface modification using CNTs PRE by approximately 25%. RS to which both methods, CO2 stripping and surface modification, were applied had a P removal capacity approximately 2.1 times higher than that of RS. When sludge dewatering filtrate was treated using RS, PRE was found to be approximately 34% higher than that of RS. In addition, it could be applied in a continuous process because surface modification has a sustainable effect in batch series tests. The P removal performance of the RS filter is mainly influenced by alkalinity and chemical clogging. Few studies have simultaneously considered these two factors to improve the performance of P removal. In this study, improved methods were suggested through CO2 stripping and surface modification using CNTs, and their applicability to sludge dewatering filtrate was confirmed.