A product concentration device for modification or concentration of a cell-depleted filtrate, said device comprising: 1) a hollow fiber filter element comprising an entrance end and an exit end, said filter element further comprising a plurality of filtration retentate chambers, each filtration retentate chamber being an open-ended hollow fiber, wherein each hollow fiber comprises a semi-permeable outer wall, said filter element further comprising a filtrate chamber comprising a filtrate chamber inner wall and a filtrate chamber outer wall, such that said filtrate chamber encloses said hollow fibers but does not block their open ends, such that the semi-permeable outer walls of the fibers are also part of the filtrate chamber inner wall, said filtrate chamber outer wall optionally comprising pores or outlet(s), said filtrate chamber outer wall enclosed in a filtrate collection chamber, said filtrate collection chamber comprising an outlet for flowing filtrate from the filtrate collection chamber to an intermediate or final collection vessel; 2) an alternating flow pump connected to the exit end of the filter element so as to permit fluid from the pump to enter the filtration retentate chambers and fluid from the retentate chambers to enter the pump, wherein said pump comprises a pump housing, two chambers (77, 88), and a diaphragm separating the chambers (77, 88); and 3) a reservoir connected to said filter element entrance end, so that fluid can flow between said reservoir and the filtration retentate chambers; wherein the retentate chamber fibers are disposed in parallel to the center axis of the filter element, wherein those fibers each have an entrance at the entrance end of the filter element and an exit at the exit end of the filter element, wherein the filtration retentate chambers and the reservoir are located within a housing, wherein the reservoir is directly connected to the filtration retentate chambers, and wherein the reservoir is connected to a sensor that senses one or more parameters of the fluid within the reservoir. 1) a hollow fiber filter element comprising an entrance end and an exit end, said filter element further comprising a plurality of filtration retentate chambers, each filtration retentate chamber being an open-ended hollow fiber, wherein each hollow fiber comprises a semi-permeable outer wall, said filter element further comprising a filtrate chamber comprising a filtrate chamber inner wall and a filtrate chamber outer wall, such that said filtrate chamber encloses said hollow fibers but does not block their open ends, such that the semi-permeable outer walls of the fibers are also part of the filtrate chamber inner wall, said filtrate chamber outer wall optionally comprising pores or outlet(s), said filtrate chamber outer wall enclosed in a filtrate collection chamber, said filtrate collection chamber comprising an outlet for flowing filtrate from the filtrate collection chamber to an intermediate or final collection vessel; 2) an alternating flow pump connected to the exit end of the filter element so as to permit fluid from the pump to enter the filtration retentate chambers and fluid from the retentate chambers to enter the pump, wherein said pump comprises a pump housing, two chambers (77, 88), and a diaphragm separating the chambers (77, 88); and 3) a reservoir connected to said filter element entrance end, so that fluid can flow between said reservoir and the filtration retentate chambers;