【正文】 定化后，細胞仍保持較高活力。 二、性質(zhì) 1，最適 pH 細胞經(jīng)固定化后，其最適 pH 因固定化方法不同而有一些調(diào)整。 3，穩(wěn)定性 一般而言，細胞經(jīng)固定化后，其穩(wěn)定性會有所提高。 thus minimising cell removal from the fermenter Cell recycle system Cell recycle is used in activated sludge systems. A portion of the cells are separated in a settling tank and returned to the activated sludge fermenter. Biomass recycling for product or biomass production is more difficult due to the need for maintaining sterility during cell separation. Centrifugation w hich is a faster process than settling would be used to separate the cells. Biomass recycle systems can be easily modelled. 固定床反應器（ Fixed bed reactors ） In fixed bed fermenters, the cells are immobilized by absorption on or entrapment in solid, nonmoving solid surfaces. In one type of fixed bed fermenter, the cells are immobilized on the surfaces of immobile solid particles such as plastic blocks, concrete blocks, wood shavings or fibrous material such as plastic or glass wool. 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 fixedbed 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 nongrowing 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. 流化床反應器（ Fluidized bed reactors） The fluidisation of the particles in the reactor leads to the surface of the particles being continuously turned over. This also increases the mass transfer rate. Fluidised beds are typically categorized as either being a 2 phase system which are not aerated and 3 phase system which is aerated by sparging Fluidized bed reactors Fluidized bed bioreactors are used widely in wastewater treatment. Fluidized bed bioreactors are also used for animal cell culture. Animal cells are trapped in gels or on the surface of special particles known as microcarriers. Fluidized bed reactors are one example of perfusion culture technology used for animal cell culture. Comparing fluidised bed and fixed reactors Fluidised bed reactors are considerably more efficient than fixed bed reactors for the following reasons: (1) A high concentration of cells can be immobilized in the reactor due to the larger surface area for cell immobilization is available (2) Mass transfer rates are higher due to the larger surface area and the higher levels of mixing in the reactor. (3) Fluidised bed reactors do not clog as easily as fixed bed reactors. Fluidised bed reactors are however more difficult to design than fixed bed reactors. Design considerations include: (1) Setting the flow rate to achieve fluidisation (2) Ensuring that bubble size remains small during the fermentation. (3) Prevention of the cells from falling or sloughing off the particles. (4) Minimising particle damage. 絮凝細胞系統(tǒng) (Flocculated cell systems) In flocculated cell reactors, the cells are trapped in the reactor due to an induced or natural flocculation process. In flocculation cells tend to group together causing t