Filtration separates solid from a liquid by forcing the liquid through a solid support or filter medium. This is a straightforward procedure for well defined crystals. 

    However the small size of microorganisms make filtration of fermentation beers or other biological solutions considerably more complicated.
Generally, fermentation beers and other biological solutions are notoriously hard to filter. They are often hard to filter because of high non-Newtonian viscosity or highly compressible cakes. That is to say, we must modified these procedures for bio-separations.

    The simplest pretreatment and the least expensive method is to heat the feed. Such heating can not only improve the feed’s handling characteristics, but also may pasteurize it. The chief constraint of this approach is the thermal stability of the product.( lead to irreversible reaction).The second pretreatment method is the addition of electrolytes to promote coagulation and flocculation in the initial solution. Useful agents rang from simple electrolytes through acids and bases to synthetic polyelectrolytes. 

    Simple electrolytes act by screening the electrostatic repulsion which commonly exists between colloidal particles. When this electrostatic repulsion is reduced, attractive London-vander waals forces predominate. The colloids can then coagulate as larger, dense particles which are more easily filtered.

    过滤是将含固体颗粒的溶液通过某种固体支持或过滤介质，由此使固体物质从液体中分离出来，这是固液分离一种最简单的步骤。

    但微小的微生物使得发酵液和其他生物性溶液的过滤变得复杂得多。

    一般来说，发酵液和其他生物性溶液都是非常难以过滤的。因为料液通常都是高的非牛顿黏度流体或者是高度可压缩滤饼。也就是说，我们必须对这些生物分离程序进行改良。

    最简单，最便宜的预处理方法是对料液进行加热。加热不但能够提高料液的操作性，而且可以对其进行杀菌。但这种方法具有局限性，它只能用于具有耐热性的产物。第二种预处理方法是通过加入电解质来促使原始溶液的凝聚和絮凝，试剂有简单的电解质，酸，碱，以及合成聚合电解质。

    简单电解质屏蔽了胶体粒子间的静电排斥，静电排斥的降低使范德华引力引起主导作用，这样就会聚合成较大的的胶粒。粒子的密度越大，越容易分离。