It is estimated that one liter water sent into space by shuttle expend $22,000, so the research for recycle of water is important to spaceflight. A pilot-scale, closed-loop water recycling system has been run to imitate wastewater produced by space crew. A mathematical model of the individual reactor based on plug flow, as well as system level simulation has been developed for nitrification and denitrification. Two important parameters, Monod constant and reactor efficiency constant, are discussed. The system simulation combines nitrification, denitrification, recycle, and degassing steps, and it accurately reproduces the ammonia, nitrite, nitrate, and total nitrogen (TN) variation observed experimentally at a 20/1 recycle ratio. Both experiments and simulations indicate that biological closed loop wastewater treatment system has significant potential for long-duration manned space flight.