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The simultaneous ultrasonic-assisted removal of Congo red (CR) and phenol red (Ph R) dyes from the aqueous solutions was accomplished by using titanium oxide (TiO₂) nanoparticles (NPs) loaded onto activated carbon (AC) prepared from bio-waste watermelon rind (WR), referred to as TiO₂–NPs–ACWR. The TiO₂–NPs–ACWR was characterized by FT-IR, FESEM, SEM, EDX, BET, and TGA techniques. To simplify the optimization process, the pH effect was first studied via one-variable-at-a-time method. The influences of the other parameters such as sonication time, initial dyes concentrations, and adsorbent dosage were then evaluated by central composite design (CCD) combined with desirability function. The optimum conditions consisted of pH 4.3, 0.04 g of TiO₂–NPs–ACWR, 8.22 min of sonication time, and initial concentrations of 45.9 mg L⁻¹ and 14.7 mg L⁻¹ for Ph R and CR, respectively, which …