Fixed bed reactors The liquid feed is either pumped through or allowed to trickle over the surface of the solids where the immobilized cells convert the substrates into products. Once steady state has been reached there will be a continuous cell loss from the solid surfaces. These types of fermenters are widely used in waste treatment In other types of fixed-bed fermenters, the cells are immobilized in solidified gels such as agar or carrageenin In these fermenters, the cells are physically trapped inside the pores of the gels and thus giving better cell retention and a large effective surface area for cell entrapment. In order to increase the surface area for cell immobilization, some researchers have investigated the use of hollow fibres and pleated membranes as immobilization surfaces. Industrial applications of fixed bed reactors include waste water treatment, production of enzymes and amino acids, and steroid transformations One advantage fixed bed reactors is that non-growing cells can be used. In such systems, the cells enzymatically act on substrates in the feed. The cells can be either inactivated or not fed nutrients required for growth. 3、流化床反应器（Fluidized bed reactors）