1 waste liquid of cooking section
That is, black liquor produced by alkali pulping and red liquor produced by acid pulping. Black liquor is produced by alkaline pulping in most paper mills in our country. Black liquor is the main research object here. The pollutants contained in black liquor account for more than 90% of the total emission of paper industry, and it has the characteristics of high concentration and difficult degradation, so its treatment has always been a big problem. There are three main components in black liquor: lignin, pentose and total alkali. Lignin is a kind of nontoxic natural polymer material, which has a wide range of uses as chemical raw materials. Pentosan can be used as livestock feed.
2 middle water
The effluent from the middle stage of pulping refers to the effluent from the screening, washing and bleaching process of the pulping pulp extracted from the black liquor. The color of the effluent is dark yellow, accounting for 8% ~ 9% of the total pollution discharge of the paper industry. The COD load per ton of pulp is about 310kg. The ratio of BOD to cod is between 0.20 and 0.35. The biodegradability is poor, and the organic matter is difficult to be biodegraded. The organic matter in the middle water is mainly lignin, cellulose, organic acid and so on, with soluble COD as the main component. Among them, the most serious environmental pollution is the chlorine containing wastewater produced in bleaching process, such as chlorinated bleaching wastewater, hypochlorite bleaching wastewater, etc. Hypochlorite bleaching wastewater mainly contains chloroform and more than 40 kinds of other organic chlorides, among which chlorophenols are the most, such as dichlorophenols and trichlorophenols. In addition, dioxin, a highly toxic carcinogen, is contained in bleaching effluent, which poses a serious threat to the ecological environment and human health.
3 white water
White water is the waste water of papermaking section, which comes from the papermaking process of papermaking workshop. White water mainly contains fine fibers, fillers, coatings and dissolved wood components, as well as added rubber, wet strength agent, preservatives and so on. It is mainly insoluble cod with low biodegradability. The preservatives added in white water have certain toxicity. The amount of white water is large, but its organic pollution load is far lower than that of black liquor and middle stage wastewater. At present, almost all of the paper mills adopt partial or total closed system to reduce water consumption, save power consumption, improve white water reuse rate and reduce excess white water discharge.