Methods of Solid Liquid Separation (SLS) in Pulp and Paper Industry


Solid-liquid separation is a process that is critical to various industries such as pharmaceuticals, food and beverage, pulp and paper, and mining. The purpose of the process may be to either recover the solids, liquids, or both. In some cases, it may be to recover neither. It may just be to separate the solid from the liquid thus, cleaning it before discharging from the system.

Different methods are used for separation depending on the characteristics of the solid, liquid, and other conditions. In a pulp and paper manufacturing unit, the solid-liquid separation principle is used at various stages. 3 Major methods used are Dissolved Air Floatation(DAF), Poseidon, and Effluent Treatment.

Dissolved Air Floatation (DAF)

In a paper mill, the water which is removed from the retention aid process still has some amount of fibers in it. To recover the fiber, it is passed through a floatation tank. Dissolved Air Floatation separates liquid and solids through dissolving air and releasing it in a floatation tank. The suspended solids get attached to the air bubbles during the aeration process. They begin to float on the top of the tank from where they are separated using a skimmer. The suspended solids in the systems may be small in size causing difficulties in bringing them onto the surface. Thus, chemicals are added to alter the charge of these particles to combine them so that they grow in size and get attached to the bubbles. The fibers recovered in the process are then pushed back into the system to manufacture paper.


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The application of Poseidon is in the pulping stage. Paper manufacturing uses immense amounts of water, and efforts are taken to reduce the intake by reusing it. To be able to reuse, water should be free of any contaminants. After the washing stages, the water contains ink and other suspended solids, which should be removed so that water is reusable. The Poseidon is used here. Chemicals are dosed to group the particles. Water is then saturated with air so that bubbles are formed. The particles get attached to them and float. These are then removed through a skimmer and the water is reused in the system.

Effluent treatment

Wastewater collected at the end of the manufacturing process is treated using the settlement process. The contaminants are allowed to sediment. During treating the effluent from the plant, coagulants are added so that the particles come together. They then become heavy and settle in the bottom of the tank. This helps to clear the water of the contaminants and then water can be removed from the top. Different methods like decanters, centrifuges, and screw press are also used for sludge separation

Effluent Treatment

HABER’s Approach

The use of coagulants and flocculants remains critical through all three methods. HABER helps its clients in determining the chemical dosage through sampling and generating insights in real-time. Dosage is also automated, making the entire process free of dependency on subject matter experts and manual intervention. It helps to achieve the required turbidity of water.

Different ways of separating the solid from the liquid are practiced within a pulp and paper manufacturing unit. It is critical to recover costly fibers as well as treating water so that it can be reused. Similarly, in other industries, different methods are used depending on the characteristics of liquid, solid, and other conditions.

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