A fluid processing system for processing fluid and returning processed fluid and/or a replacement fluid to a fluid recipient, the fluid processing system being configured to cooperate with a fluid flow circuit including a fluid recipient line for drawing fluid into the fluid flow circuit and returning processed fluid and/or a replacement fluid from a fluid processing chamber to a fluid recipient, the fluid processing system comprising: a fluid recipient pump operable in a first direction to pump processed fluid and/or a replacement fluid through the fluid recipient line toward the fluid recipient and in a second direction to pump processed fluid and/or a replacement fluid through the fluid recipient line away from the fluid recipient; an air detection assembly configured to monitor the contents of the fluid recipient line and produce an output indicative of the presence of air in the fluid recipient line flowing toward the fluid recipient; and a controller programmed to receive the output from the air detection assembly and, upon receiving said output, automatically instruct the fluid recipient pump to operate in the second direction to pump processed fluid and/or replacement fluid containing air through the fluid recipient line away from the fluid recipient at least until the air in the fluid recipient line is removed from the fluid recipient line, and wherein the fluid processing system further comprises a processed fluid weigh scale configured to measure the weight of at least a portion of the fluid flow circuit and the processed fluid and/or replacement fluid therein, wherein the controller is further programmed to instruct the fluid recipient pump to pause operation in either the first direction or the second direction after the air in the fluid recipient line has been removed from the fluid recipient line, compare the weight registered by the processed fluid weigh scale at a time before the air detection assembly produced said output to the weight registered by the processed fluid weigh scale at the time when operation of the fluid recipient pump is paused, and if the weights compared by the controller are substantially the same or if the weight registered by the processed fluid weigh scale at the time when operation of the fluid recipient pump is paused is greater than the weight registered by the processed fluid weigh scale at the time before the air detection assembly produced said output, instruct the fluid recipient pump to operate in the first direction to pump processed fluid and/or replacement through the fluid recipient line toward the fluid recipient. a fluid recipient pump operable in a first direction to pump processed fluid and/or a replacement fluid through the fluid recipient line toward the fluid recipient and in a second direction to pump processed fluid and/or a replacement fluid through the fluid recipient line away from the fluid recipient; an air detection assembly configured to monitor the contents of the fluid recipient line and produce an output indicative of the presence of air in the fluid recipient line flowing toward the fluid recipient; and a controller programmed to receive the output from the air detection assembly and, upon receiving said output, automatically instruct the fluid recipient pump to operate in the second direction to pump processed fluid and/or replacement fluid containing air through the fluid recipient line away from the fluid recipient at least until the air in the fluid recipient line is removed from the fluid recipient line, and wherein the fluid processing system further comprises a processed fluid weigh scale configured to measure the weight of at least a portion of the fluid flow circuit and the processed fluid and/or replacement fluid therein, wherein the controller is further programmed to instruct the fluid recipient pump to pause operation in either the first direction or the second direction after the air in the fluid recipient line has been removed from the fluid recipient line, compare the weight registered by the processed fluid weigh scale at a time before the air detection assembly produced said output to the weight registered by the processed fluid weigh scale at the time when operation of the fluid recipient pump is paused, and if the weights compared by the controller are substantially the same or if the weight registered by the processed fluid weigh scale at the time when operation of the fluid recipient pump is paused is greater than the weight registered by the processed fluid weigh scale at the time before the air detection assembly produced said output, instruct the fluid recipient pump to operate in the first direction to pump processed fluid and/or replacement through the fluid recipient line toward the fluid recipient.