Simple solution, imprison less people. NYC prisons are notorious for overcrowding for decades now [1] [2].
It's shocking for me as a German that this seems to have been a consistent fact of life that's just accepted by everyone and nothing is being done - no new prisons are built to offer more space for prisoners, and nothing is being done to reduce the amount of people heading towards prison.
NYC average daily jail population has been decreasing almost continuously for the last 30 years. Conditions at Riker's Island are much more to do with the problems between the civilian oversight function and the COs unions than they are to do with an actual excess of inmates with respect to cells etc.