The present study was conducted to investigate the elimination of Reactive Black 5 (RB5) through the photocatalytic process alone and a combination of photocatalysis and cavitation using nanotitanium dioxide as a photocatalyst. To use a new form of cavitation, hydrodynamic cavitation (HC) was generated by an orifice plate with a 7 mm hole diameter at the inlet pressure of 4 bars. First, the photocatalytic process was optimized by changing the parameters of pH, nanotitanium dioxide concentration, irradiation power and dye concentration; then, by adding cavitation and producing combined photocatalysis-cavitation and with the same optimal conditions obtained in the previous stage, the amount of nanophotocatalyst was adjusted, and the amount of nanotitanium dioxide required to yield an efficiency equal to that of the single process was calculated. Finally, by measuring the amount of current consumed in these processes, the cost of nanotitanium dioxide and the electrical energy consumption, the total cost of both the single and the combined processes was estimated in the same efficiency. The results showed that using cavitation significantly reduces the amount of nanomaterials used from 100 to 8.4 mg/L and the total cost of the process to one-seventh, which suggests that cavitation is a very promising process. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.