Therefore most mills have a pretreatment stage to take out the inorganics prior to the P-stage. This is an acidic process with chemistry similar to that of elemental chlorine. It is typically used after an O-stage and the effluent can be sent directly to the recovery boiler. In fact some mills produce TCF pulps via a four-stage OZEP process. A biotechnological approach to bleaching involves the use of the enzyme xylanase.
Fungi naturally produce xylanase to breakdown wood xylans into carbohydrates.
The carbohydrates then become the energy source for the organism to grow. Scientists have found fungi that produce alot of xylanase, and they harvest the enzyme and sell it to the mill. It has also been theorized that xylanase breaks lignin-xylan covalent bonds, thereby releasing the lignin. Alkaline extraction . An E-stage is used to solubilize lignin degradation products.