Mechanical Dewatering of Papermaking Sludge

There will be a large number of sludge produced in papermaking sewage process, if the sludge is not properly treated, it will cause secondary pollution. In order to treat and utilize the sludge economically and reasonably, it is necessary to reduce the water content, so as to reduce the sludge volume for further treatment.

1. Classification of Papermaking Sludge

The main components of paparmaking sludge are organic substances with fine particles, small specific gravity (about 1.006-1.02), colloidal structure, high water content and difficult to be dewatered. According to the properties of papermaking sludge, it can be roughly divided into deinking sludge, primary sludge, biochemical sludge and chemical sludge.

2. Characteristics of Papermaking Sludge

Compared with municipal sludge, the fiber content of papermaking sludge is larger, its dewatering performance is better, the dosage of flocculants is relatively less.

3. Papermaking Sludge Treatment Technology and Equipment

The sludge treatment process in papermaking sludge treatment is as follows: sludge thickening → sludge pretreatment → mechanical dewatering or natural drying → final disposal (land recovery, sanitary landfill, incineration, etc.). At present, sludge thickening is combined with  sludge dewatering in papermaking sludge treatment.

The dewatering methods of papermaking sludge can be divided into natural drying and mechanical dewatering. Natural drying was widely used in the past, but now it is gradually replaced by mechanical dewatering, due to its large occupation of land, bad odor and easy to pollute groundwater.

4. Sludge Mechanical Dewatering Equipment

At present, there are many kinds of sludge mechanical dewatering equipment in China. Such as plate and frame filter press dehydrator, centrifugal dehydrator, screw press dehydrator and belt filter press dehydrator. These mechanical machines have their own characteristics, and suitable for different kinds of suspensions.

4.1 Common Mechanical Dewatering Process and Equipment Combination

There are three common ways of mechanical dewatering in papermaking sludge treatment:

(1) The sludge with a concentration of of 4%-5% will be pre-concentrated by pre-concentration equipment, and then dewatered by screw press dehydrator, after dewatering, the sludge dryness will be 40%-50%.

(2) Belt filter press dehydrator is used to dewater the sludge to 25%-40% dryness. If it can not meet the further production requirement, pre-concentration equipment or screw press dehydrator can be added to improve the sludge dryness.

(3) Plate and frame filter press dehydrator can dewater the sludge to about 40% dryness.

4.2 Precautions for Use of Flocculants

The type and dosage of flocculants are related to sludge properties, dewatering methods and other factors. The flocculants must be reasonably selected in order to achieve the best dewatering effect.

(1) The common flocculants used for papermaking sludge are inorganic flocculants and organic flocculants. The common inorganic flocculants include polyaluminium chloride (PAC), aluminum sulfate, ferric chloride, polymeric ferric sulfate, etc. Polyacrylamide (PAM) is the common organic polymer flocculant.

(2) When using the powdered flocculants, it is not allowed to directly add them into the sewage, they should be dissolved in the clean water first, then the water solution will be added into the sewage in a certain proportion. If the water solution is not uniform or insufficient, the flocculation effect will be affected. In order to avoid the shear degradation of polymers caused by the rotating blades of centrifugal pump, screw pump is usually used.

(3) For papermaking sludge, the effect of combined use of PAC and PAM is better than the use of PAM alone, and the cost of sludge treatment will be reduced. The correct step is to mix PAC and sludge evenly before adding PAM. If PAC and PAM are mixed before adding into the sludge, the flocculation effect will be greatly reduced. The consumption of flocculants depends on the property of sludge and the quality of flocculants. Only the reasonable combination of PAC and PAM can realize the purpose of low dosage of flocculants, good treatment effect and high efficiency.