See the denoise_teapots.katana example file in RfK's Examples directory to see how to setup denoising. 

Setup:

Step 1:

Create a PrmanDenoiseChannelDefine node which is a macro for creating all the specific DisplayChannels required for the denoise utility.

Step 2:

Create a PrmanDenoiseRenderOutputDefine node which is a macro for rendering a multichannel OpenEXR file for the beauty pass combined with the AOVs.

You can command-click to see inside these macros and change settings if necessary. On the RenderOutputDefine node, make sure asrgba is set off, and that the type is set to raw to avoid any post-processing.

Step 3:

Set pixelfiltermode to importance if you changed it from the default. This is done in PrmanGlobalStatements.

If your pipeline does not support multichannel EXR, you can use $RMANTREE/bin/exrmerge to combine the AOVs into a multichannel EXR. Then use this merged multichannel EXR for the denoise utility.
Note that each AOV's type should be set to "raw" because this preserves the channel name in the EXR which the denoise utility will look for. Combined EXR using Katana's RenderOutputDefine's merge will not work with the denoise utility.

Running the denoiser:

1. Running Denoise Via RenderOutputDefine Script (Single Frame)

   Create a RenderOutDefine. Wire this node to the previous RenderOutputDefine node. Select script for the type, select primary for the scriptInput, and set the scriptCommand to the following:

   $RMANTREE/bin/denoise_batch -o /path/to/directory/for/denoised/images $INPUT

   
   
   

2. Running Denoise Via commandline (Crossframe or Single Frame)

   Render out your images with katana --batch or katana queue. Then run the following command 

   
   

   $RMANTREE/bin/denoise_batch -o /path/to/directory/for/denoised/images imagename.*.exr

   
   
   

3. Running Denoise UI (Crossframe or Single Frame)
   
   The PRMan 25 shelf has a script called RenderMan Denoiser. You can open this to select and denoise images that you have already rendered out
   
   
   

Users must run a disk render in order to denoise an image. Denoising is not currently supported in interactive rendering.