Paper industry wastewater is a kind of difficult-to-treat organic wastewater with large water volume, high color, large suspended solids content, high organic matter concentration and complex components. The main sources of wastewater are pulping waste liquid, intermediate water, and paper machine white water. The wastewater discharged during washing is black-brown in color, called black water, and the water has a high concentration of pollutants and contains a lot of fiber, inorganic salts and pigments. The middle section of the washing and bleaching process produces the largest amount of water, and the pollutants have higher concentrations of lignin, cellulose and resinate and other components that are difficult to biodegrade, and the color is dark. The wastewater discharged from the paper machine, known as white water, contains a large amount of fibers and fillers and rubbers added during the production process.

1. Introduction to papermaking wastewater 

The wastewater discharged from the paper industry contains a large amount of cellulose and lignin, which has a high COD value and is very difficult to degrade. Its wastewater also contains a variety of chemical substances at the same time, and the composition is very complex, and the treatment of papermaking wastewater is a special wastewater that is difficult to treat. In China, the amount of industrial wastewater discharged by the paper industry accounts for 1/6 of the total industrial wastewater, which shows that the pollution of papermaking wastewater in China is extremely serious. According to the papermaking process, the wastewater generated at different stages is different, and the pulp and papermaking wastewater can be roughly divided into three categories: cooking waste liquid, pulping wastewater and papermaking wastewater.
（1）Cooking waste
    The cooking waste liquid is the ultra-high concentration waste liquid generated in the pulping and cooking process, and the waste liquid in this section is very difficult to treat, including the black liquor of alkaline pulping and the red liquor of acid pulping. At this stage, most of China's paper mills use alkaline pulping, and the pollutants discharged from the black liquor in the whole pulping process belong to the wastewater with the highest concentration and the darkest color, which is brown-black. The cooking waste liquid almost concentrates 90% of the pollutants generated in the pulp and paper making process, which contains a large number of degradation products such as lignin and hemicellulose, pigments, pentose sugars, residual alkalis and other leachables, and its discharge is smaller than that of the other two waste liquids, and about 10t of black liquor can be discharged per 1t of pulp produced, but this kind of waste liquid is difficult to treat.
（2）Wastewater in the middle section of pulping
    The wastewater in the middle of pulping is the wastewater discharged from the cooking slurry in washing, screening, bleaching and pulping after the black liquor is extracted. This part of the wastewater volume is large, and each ton of pulp can produce about 50-200t of mid-section wastewater. The pollution volume of the middle wastewater accounts for about 8%-9%, and the COD load is about 310kg, which contains more lignin, cellulose and other products that are very difficult to degrade, as well as organic substances such as organic acids, most of which belong to soluble COD.

（3）Papermaking wastewater
    Papermaking wastewater, also known as white water, is produced in the process of making paper, mainly containing fine fibers and fillers, rubber and chemicals added when making paper, etc., the amount of water in this part of the wastewater is larger, the amount of white water produced per ton of paper is 100-150t, but the pollutant load of this kind of wastewater is low, its organic matter is insoluble in water, mainly insoluble COD, easy to treat, and the treated water can be reused while recovering fibers.

With the diversification and complexity of pulp raw materials and the increase in the use of additives in recent years, there are more and more organic substances in papermaking wastewater, and it is difficult to meet the requirements of national discharge standards by relying only on traditional treatment methods, which puts forward new requirements and challenges for the formula design, synthesis process and flocculation performance of flocculants.

2. Main pollutants

（1） Suspended solids: including sedimentable suspended solids and non-sedimentable suspended solids, mainly fibers and fiber fines (i.e., broken fiber fragments and miscellaneous cells)

（2）Easily biodegradable organic matter: including low molecular weight hemicellulose, methanol, acetic acid, formic acid, sugar, etc.

（3）Refractory biodegradable organic matter: mainly derived from lignin and macromolecular carbohydrates contained in fiber raw materials.

（4）Toxic substances: rosin acid and unsaturated fatty acids contained in black liquor.

（5）Acid-alkali poisons: the pH value of alkali pulping sewage is 9~10; The pH value of acid pulping wastewater is 1.2~2.0.

（6）Color: The residual lignin contained in the pulping sewage is highly colored.

3. Process description

The concentration of SS and COD in waste paper and papermaking wastewater is higher, and COD is composed of two parts: insoluble COD and dissolved COD, usually non-soluble COD accounts for most of the total COD composition, and when SS is removed in sewage, most of the insoluble COD is removed at the same time. Therefore, the main problem to be solved in waste paper and paper wastewater treatment is the removal of SS and COD. Based on years of engineering experience, our company adopts physicochemical combined biochemical treatment process for papermaking wastewater to achieve the standard discharge of wastewater. The main processes include: gravity sedimentation method, air flotation method, advanced oxidation technology, membrane separation technology and biochemical treatment process.