This document discusses tower fermenters, which are elongated fermentation vessels with a height to width aspect ratio of 6:1 or more that allow for the unidirectional flow of gases. There are several types of tower fermenters including bubble columns, vertical tower beer fermenters, and multistage fermenter systems. Tower fermenters have been used for the production of products such as citric acid, tetracycline, beer, and to cultivate organisms like yeast and E. coli. They provide a simple design for aerobic fermentation of cells and enzymes.