Sewage Treatment Technology of Paper Mill

There are three main types of sewage discharged from alkaline papermaking:

1. The wastewater produced by cooking wood pulp (or straw pulp) is also called black water.
2. The sewage discharged from the pulp machine and the refining machine is called pulping wastewater.
3. The paper machine sewage that can be used directly is called white water.

• Suspended solids include sedimentable suspended solids and non-precipitable suspended solids, mainly fibers and fiber fines (ie, broken fiber fragments and miscellaneous cells);
• Biodegradable organics include hemicellulose, methanol, acetic acid, formic acid, sugar, etc. in low molecules;
• Non-biodegradable organic matter mainly comes from lignin and macromolecular carbohydrates in fiber raw materials
• Rosin acid and unsaturated fatty acid in black liquor of toxic substances.
• The pH value of alkaline pulping wastewater is 9-10, and the pH value of acid pulping wastewater is 1.2-2.0;
• The residual lignin contained in the chroma pulping wastewater is highly colored.

Pretreatment methods for papermaking wastewater:

The pretreatment process mainly includes: grid and screen, fiber recovery system, adjustment of water flow rate and water quality, etc. According to the different quality of wastewater in the paper industry, different pretreatment methods can be used to remove some pollutants and improve the quality of wastewater, thereby optimizing the treatment effect of the entire wastewater treatment system.

1. Grid and Screen

Paper industry sewage often contains fine floating substances such as bark, wood chips, plastics and pulp fibers. Therefore, before entering the water pump and the main body treatment system, it is cut off, and the grid is set to cut off the large floating matter, and the screen is set to cut off the fine floating matter. Grids are generally used to treat papermaking wastewater containing a large amount of water. Screens are usually used when the amount of water is relatively small and the effluent contains a large amount of fine suspended solids.

2. Fiber recycling system

Papermaking wastewater contains a large amount of pulp fibers. The fiber recovery system in the factory is mainly used for fiber recovery of papermaking white water, and white water circulation is carried out to reduce white water discharge.  The pulp fibers were recovered by means of filtration, floatation and precipitation. Off-site fiber recovery often adopts the method of screen filtration to recover paper fibers.

3. Adjustment

Due to the diversity of wastewater discharge in the paper industry, the quality and quantity of discharged wastewater can vary within a day. Therefore, it is necessary to adjust the sewage, balance the water quality, and make it evenly enter the subsequent treatment unit to improve the treatment effect. Sewage regulation mainly includes water volume regulation and water quality regulation.

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