There are two denoising options available for rendering with Arnold:
Arnold Denoiser (Noice)
The Arnold Denoiser (Noice) can also be run from a dedicated UI, exposed in the Denoiser tab of the Render Settings window, or as an imager. To use the Arnold denoiser, you will need to render images out first via the Arnold EXR driver with variance AOVs enabled. It is also available as a stand-alone program (noice.exe).