The tower effluent flows by gravity through a glass wool filter. Medium then enters the lower reservoir, where it remains until a level-switch activates an induction-driven pump. Medium is then forced through a 15 micro cellulose acetate filter cartridge (Commercial Filters Corp. Fulflo Model F 15-10) in series with a 0.45 micro pleated membrane cartridge (Gelman Instrument Co. Cat. No. 12505) and into the upper reservoir. Both filters are supplied on polypropylene cores. The processed medium is then delivered by gravity to inlets on the bottom of each culture tank. Flow rate is governed by pinch valves located on the delivery tubes.