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.

    過濾是將含固體顆粒的溶液通過某種固體支持或過濾介質，由此使固體物質從液體中分離出來，這是固液分離一種最簡單的步驟。

    但微小的微生物使得發酵液和其他生物性溶液的過濾變得復雜得多。

    一般來說，發酵液和其他生物性溶液都是非常難以過濾的。因為料液通常都是高的非牛頓黏度流體或者是高度可壓縮濾餅。也就是說，我們必須對這些生物分離程序進行改良。

    最簡單，最便宜的預處理方法是對料液進行加熱。加熱不但能夠提高料液的操作性，而且可以對其進行殺菌。但這種方法具有局限性，它只能用于具有耐熱性的產物。第二種預處理方法是通過加入電解質來促使原始溶液的凝聚和絮凝，試劑有簡單的電解質，酸，堿，以及合成聚合電解質。

    簡單電解質屏蔽了膠體粒子間的靜電排斥，靜電排斥的降低使范德華引力引起主導作用，這樣就會聚合成較大的的膠粒。粒子的密度越大，越容易分離。