We didnt replace the leach lines since it passed a 40 minute test.The tank relies on bacteria to break down the solids, i believe the process is called anaerobic.Flushing down bleach kills the bacterias so just be aware of that.

I dealt with this in a condo I bought from a smoker.1) Open windows2) Definitely rip up any carpet that may be there3) Put on a mask-  Clean and scrub walls/windows/floors/trim every surface in the house with bleach/water mixture.

change them, clean the spots/areas with bleach-water, your done! 

Don’t use bleach, as it only kills live mold, but not mold spores, leading to faster mold growth.

Can these be bleached clean or is it typical to replace them? 

I was planning to bleach it and put darker stain on it but wondering whether it's still worth it.

as stated its every where bleach and elbow great gets ride of it.

I've removed metal scratches and rust from tubs and sinks with both when no amount of cleaners, bleach or Lime Away would work.

They only issue with the carpet on the move in checklist were some bleach spots on the carpet next to the washer/dryer. 

One tenant had a cat and he was rather unclean and yes his litter box was smelly but once he moved out, no problem some spray of Clorox bleach did the trick in the areas where the litter box was.