Processes and systems for maintaining a live culture of activated sludge in the activated sludge unit of a petroleum refinery during an extended refinery shut-down. In the absence of a source of refinery wastewater, a living culture of activated sludge comprising aerobic prokaryotes and/or eukaryotes that nitrify ammonia is maintained by redirecting at least a portion of clarified wastewater leaving the clarifier portion of an activated sludge unit. The redirected clarified wastewater is mixed with an organic food source that may be ammonia and/or ethyl acetate, a source of phosphate ion, and the pH is adjusted to produce a growth media that is returned to the aeration chamber of the activated sludge unit. After a time exceeding one week, the input of refinery wastewater to the aeration chamber is resumed while discontinuing the redirecting of the clarified wastewater.