I would suggest looking at ...

I would suggest looking at the sludge storage on site as in my experience, Nocardia is often caused by problem of sludge backlog i.e. sludge storage tanks in high level, leading to ssludge solids introduced to the treatment process via liquor return.  This situation needs to be checked and rectified first.  Controlling SRT and DO levels as described by others here is worth trying but if you have floating sludge or foam on your final tanks, this may need to be physically removed, possibly by suction to prevent the Nocardia entrained in the foam/floating sludge being re-introduced to the aeration process via the scum removal system.