Slaughterhouse wastewater (SWW), which is rich in organic matter, ammonium, and phosphate, contributes to water pollution if not properly treated. Here we investigated non modified banana fibre (NMOD BF), and sodium hydroxide-modified banana fibres (MOD BF), as low-cost adsorbents for ammonium and phosphate in synthetic slaughterhouse wastewater (SSWW). Batch and continuous adsorption experiments assessed their performance, while scanning electron microscopy (SEM), scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDX), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) characterized the fibres. The SEM-EDX confirmed surface modification and FTIR showed structural changes due to hemicellulose and lignin removal, enhancing cellulose exposure. In both batch and continuous systems, ammonium removal reached more than 90% in MOD BF and NMOD BF. An efficient removal of ammonium was achieved in 2 h and fibre concentration of 1 g/l. Phosphate removal reached up to 50% in MOD BF and NMOD BF. The NMOD BF excelled slightly in phosphate removal. These findings suggest that banana fibres can be a viable solution for removal of ammonium and phosphate in slaughterhouse wastewater treatment (SWWT), with additional optimization warranted for real-world applications.