Use the Arnold Denoiser imager when rendering a single frame.
An example denoising scene can be downloaded here.
For the denoiser to work you first need to setup AOVs by using a combination of ArnoldOutputChannelDefine and RenderOutputDefine node pairs, where you set up the AOV in the former, creating a channel, and then the latter uses that channel to set up an actual render output.
Once they are defined add an ArnoldDenoise node where you can specify which outputs are to be denoised. It creates the basic required AOVs automatically (note: not render outputs, just "AOVs"). However, you need to create additional outputs if you want them to be denoised or if you want them to be used as a denoiser: albedo, etc.
Example denoising setup