Study On The Rule Of AOX In The Traditional CEH Bleaching Wastewater Of The Wheat Straw Pulp

Pulp bleaching wastewater with chlorine, especially CEH three stage bleaching wastewater, contains substantive organic chlorides other than relatively high amount of COD and BOD. Maybe this is why CEH three stages bleaching has been replacing gradually, China is a country with plenty of straw pulp, in a short term, it is hard to substitute the traditional bleaching method—CEH with oxygen,ozone and peroxide bleaching methods. So it is urgent for us to study the rule of AOX in the traditional CEH bleaching effluents, with that we can explore a bleaching way that suits the situation of our country. According to the status of our nation's pulp and paper making industry, the paper did below research:Studied on the rule of AOX in the traditonal CEH bleaching effluents. The study revealed that:most of AOX in CEH three stage bleaching wastewater is presented in C and E stages. For non wood pulp, AOX generated at chlorination is about 2.5-4kg/t,in which about 2kg/t AOX dissolved in C stage bleaching wastewater,and 1-2kg/t in E stage wastewater,while there is a little AOX in H stage wastewater,commonly below 1kg/t. AOX produced in non-wood pulp bleaching is lower than wood pulp. For non-wood pulp,AOX total amount in CE stage wastewater has below relationship with chlorine usage in C stage: AOX(kg/t pulp)=0.089*Cl₂ In H stage bleaching, chlorine usage also has a relationship with AOX amount in wastewater: AOX(kg/t pulp)=0.014*Cl₂There is a relationship between AOX and time,temperature in bleaching, the study showed that prolong the bleaching time and rise the bleaching temperature can increase the AOX content in the effluent. In addition, the initial pH number in the H bleaching stage can influence the AOX greatly, the reduction of the pH number in the H bleaching stage can make both the AOX and the brightness increase. After we know the rule of the AOX in the traditional CEH three stages bleaching, the infection of ClO₂ bleaching on the reduction of AOX in the bleaching effluent was studied. The research revealed that ClO₂ bleaching can decrease the amount of AOX in the bleaching wastewater; Using chlorine dioxide to partly substitute the chlorine of the stage C in the CEH sequence was studied in this paper. The result showed that when the rate of the substitute was up to 70%,the amount of AOX can meet the demand of our nation.There may be little free chlorine in the chlorine dioxide solution. The study showed that in the process of chlorine dioxide bleaching, peroxide hydrogen can react with the free chlorine, so that the AOX in the bleaching wastewater can reduce. This method is feasiable,for the additonal cost is small. The amount of AOX can reduce about twenty persents compare to the traditional bleaching.The intensified CEH bleaching is also an effective way to reduce the amount of AOX. The pretreatment before traditional CEH bleaching can decrease the Kappa number of the pulp, so that the amount of the bleaching agent can reduce,and the AOX can decrease as well.(1) oxygen delignification before CEH bleaching can reduce the amount of AOX. The optimum condition of oxygen delignification was: bleaching concentration 10%, dosage of NaOH 4%,retention time 60min, reaction temperature 90℃,oxygen pressure 0.5 Mpa.Following these requisition,the delignification can reach 60%,so the charge of the subsequently bleaching agent can decrease 50%,and the AOX content of the effluent can also decrease 50% as a result without no distinctly reduction to the brighrness and intension.(2) xylanase pretreatment can reduce the dosage of the bleaching agent, CEH bleaching after xylanase pretreatment ,when the dosage of xylanase was 40IU/g, the dosage of chlorine can be reduced 40% compared to the traditional CEH bleaching ,without any reduction to the brightness. The amount of AOX can decrease 30%.In order to know the composition of the AOX correctly, the paper analyzed the chemical composition of AOX in the non-wood pulp bleaching wastewater by GC-MS. The study revealed that AOX in C stage wastewater contains poly-chlorinated acetone, poly-chlorinated propene,and poly-chlorinated cyclopropene as well; In E stage bleaching wastewater,mainly poly-chlorinated phenol and guaicol have been found ; But for AOX in H stage wastewater, there are only a few organic chlorides been found including trichlomethane and tetrachlorocarbon.