Treatment Of Whitewater In Papermaking With Whole-cells Biocatalysts Of Aspergillus Oryzae And Aspergillus Niger

Whitewater closure is an important step in the papermaking process that can save fresh water, decrease losses in fiber and chemical additives, save resources, and reduce production cost. It can also reduce the discharge of pollutants, thus benefiting the environment. However, with the closure of the whitewater, various dissolved and colloidal substances(DCS) will accumulate in the mill process waters. Anionic DCS compounds in whitewater interfere with the papermaking process as well as the product quality. Paper mills have begun to use enzymes to solve many problems that arise in the paper making process. Due to the high cost or complexity of the production of enzymes, the large-scale commercial application in paper industry was limited. Recently, whole-cell catalysis method has attracted more attention. Wholecell biocatalysis utilizes enzymes naturally located in or exuded from the cells to catalyze reactions.In this dissertation, two types of fungus Aspergillus oryzae and Aspergillus niger was oriented cultured to produce the lipase and pectinase, respectively. After cultured, the fungus cells were used as whole cell catalyst for treating stickies and DCS in whitewater. In addition, immobilized cells were also investigated.Results showed that whole cells of Aspergillus oryzae 3.5 were used as biocatalysts for the degradation of glycerol trioleate(TG), which largely contributes to pitch deposits in papermaking. Different types of inducers in culture media showed different effects on both the biomass and lipase activity of whole-cell biocatalysts. The cells of A.oryzae 3.5 cultured with Tween 80 showed higher catalytic activity than the others. The effects of several key factors on A.oryzae 3.5-catalyzed treatment of a TG-containing model whitewater were investigated, and the optimal p H value, reaction temperature, substrate concentration, and shaking speed were determined to be 7.5, 45 °C, 50% and 200 rpm, respectively. Results showed that the mean particle size of the original whitewater from paper mill was sharply reduced from 534 nm to 356 nm after treatment by A.oryzae whole cell catalyst.Pectins or polygalacturonic acids(PGA) were originated from alkaline peroxide bleaching of mechanical pulps and considered as the dominant troublesome substance. Research on A.niger whole cell catalyst treatment of dissolved and colloidal substances(DCS) present in whitewater was investigated. Pectinases originated from Aspergillus niger, which was the major genera of microorganism producing pectinases in industry, can collaboratively degrade pectins. The optimum temperature, time and dosage of the fungal pectinase when treating simulated whitewater were 40 ℃, 40 min and 40g/L, respectively, under which the enzyme showed high enzymatic activity of 119.67 U ·g-1. The whitewater treated by immobilized A.niger whole cell catalyst were also studied. Results showed that the immobilized A.niger whole cell can effectively decompose PGA; under the optimal reaction conditions, the cationic demand of PGA can be reduced about 85%. And the recycled property of immobilized A.niger whole cell is good: after fourth reuse, it can still degrade the whitewater cationic demand to about 50%. In a treatment of whitewater from paper mill by immobilized A.niger whole cell, cationic demand can be reduced about 33%.Results showed that A.oryzae and A.niger whole-cell catalysts prepared in this research have good catalytic performance through the strict control of culture conditions and can reduce the harmful substances effectively. The biocatalysts have the potential to be used in paper industry in the further.