Sludge Dewatering

What is sludge dewatering?

Sludge dewatering separates sludge into liquids and solids, minimizing the volume of waste and making it easier and more cost effective for final disposal. Sludge is a semi-solid slurry that is produced from various industrial and municipal water treatment processes. Dewatering sludge with sludge dewatering equipment can significantly reduce its weight and volume prior to disposal.

How does the sludge dewatering process work?

Before the dewatering process begins, the sludge must be passed through any of the mineral chemicals or organic chemicals, such as coagulants and flocculants. After conditioning the sludge, it is then thickened by dissolved air flotation, gravity belt, thickening drum/spiral drum or centrifuge.

Once the conditioning step is complete, the analysis of the most suitable sludge dewatering technology can begin. The choice of sludge treatment method depends on several factors, including characteristics, volume, time and disposal options available. Common dewatering options are plate and frame filter presses, membrane filter presses, chamber filter presses, belt filter presses, centrifuges and screw presses.

Benefits of sludge dewatering

Waste volume reduction

The focus of sludge dewatering is to reduce weight and volume in order to minimize waste disposal costs. Therefore water removal is the primary means of volume reduction until the sludge by-products can be economically disposed of.

Reduce transportation costs

A dry, high solids content cake means lower costs. The use of dewatering equipment is an effective tool for managing sludge accumulation and can reduce transportation costs and storage costs.

Wastewater recycling

Once the sludge is dewatered and treated water is free of hazards and contaminants, it can be recycled by industry and communities for reuse. For example, steam power plants use reclaimed water to provide power services to communities.


Many people think of filter presses as labor-intensive equipment. In fact sludge dewatering filter presses can be automated with automatic cloth washers, mechanically assisted cake release devices and sophisticated control systems that allow communication with the main plant control system.

Protecting the environment

Reduces the volume of waste to a more compact and manageable size for disposal. While the dry cake is disposed of, the reuse of the filtrate reduces the need for new water sources required for treatment in municipal and industrial wastewater plants, as well as the overall amount of water required for treatment.


Sludge dewatering is the separation of mud and water from sludge by mechanical-physical principles (centrifuges, screw presses, filter presses) or by natural sedimentation. This results in a sludge cake with a very low water content.

Sludge thickening processes are usually gravity thickening, mechanical thickening, and air flotation thickening, of which gravity thickening, which uses natural gravity to separate sludge liquids, is a widely used and simple thickening method. Sludge thickening is the process of reducing the water content of sludge by gravity or air flotation to thicken the sludge. It is an effective method to reduce the water content of sludge and reduce the volume of sludge. Sludge thickening mainly reduces the interstitial water of sludge, and the thickened sludge is nearly paste-like and still maintains fluidity. It is an economical and effective method to reduce sludge volume, but thickening is usually difficult to reduce sludge water content below 90%.

Sludge thickening is to increase the concentration of sludge from about 1% to 3-4%. Sludge dewatering is the removal of most of the water from sludge of about 3% to increase its dryness significantly. There is a big difference between the two in terms of effectiveness.

Commonly used sludge treatment methods are: thickening, digestion, dewatering, drying, incineration, solidification and integrated use. There is also the more final disposal method of sanitary landfill in landfills.

A sludge treatment method that removes water from primary, thickened or digested sludge in a fluid state and converts it into a semi-solid or solid sludge mass. After dewatering, the sludge moisture content can be reduced to 55 to 80 percent, depending on the nature of the sludge and sludge and the effectiveness of the dewatering equipment. Further dewatering of sludge is called sludge drying, where the moisture content of dried sludge is less than ten percent. The main methods of dewatering are natural drying, mechanical dewatering and granulation. The natural drying and mechanical dewatering methods are applicable to sewage sludge. The pelletizing method is applicable to sludge from coagulation and sedimentation.

First, the sludge is treated by sludge pumping from the secondary settling tank to another pool, where it is separated from the supernatant. The original sludge usually has a moisture content of 99.5%, so the sludge needs to be thickened and there are several ways to reduce the volume of the sludge. For example, mechanical treatment methods such as vacuum filtration and centrifugation are often used before the sludge is disposed of in a semi-solid form. Generally, these methods are used in preparation for sludge incineration treatment. If biological treatment is planned, most of them are thickened by gravity settling or floating methods. These two cases correspond to slurries that remain fluid.

There are three main factors affecting the effect of sludge dewatering – sludge nature, sludge dewatering chemicals, and on-site equipment.

1. Factors of sludge nature

The sludge type and sludge mixing ratio directly affect the sludge dewatering effect and sludge treatment cost, and the moisture content of sludge cake mainly depends on the nature of sludge. If the combined moisture content of sludge is high, it is impossible to continuously reduce the moisture content of sludge cake by using any chemicals and equipment, and the combined moisture is mainly determined by the comprehensive nature of sludge, therefore, the moisture content of sludge cake mainly depends on the nature of sludge. The nature of sludge mainly depends on factors such as influent water quality, use of chemicals, treatment process and management, etc. There are more factors affecting the nature of sludge, and it is not easy to detect changes in the nature of sludge, so it is difficult to stabilize the nature of sludge during the daily operation of the sewage plant, and the sludge dewatering changes a lot, which requires long-term observation, analysis and testing to make adjustments in order to handle the dewatering effect to the best and reduce the water content to the lowest.

2. The factor of dewatering agent

Sludge dewatering agent plays the most important role in the sludge dewatering process, and the quality of the sludge dewatering agent directly affects the condition of the sludge, which in turn affects the sludge dewatering effect. The sludge dewatering agent polyacrylamide (PAM) is generally used in sewage plants, because there are many types of PAM, and the type chosen is different according to the nature of the sludge, and the charge type, molecular weight and the size of the cation degree of polyacrylamide will directly affect the sludge conditioning, and then affect the sludge dewatering effect. The preparation and dosing of the agent are usually carried out by using a polymer dosing system.

Generally speaking, sludge particles in municipal wastewater treatment plants have a negative charge on the surface, so cationic polyacrylamide can be used as a dewatering agent, and the specific ionicity and molecular weight size are determined according to experiments. The sludge dewatering agent not only plays the role of electric neutralization to destabilize the sludge particles, but also plays the role of adsorption and bridging to accelerate the formation of sludge particles flocculation, so that the flocculation increases and improve the degree of separation of sludge particles and water.

3. Equipment factors

The operation management of the equipment, for example, the treatment effect of the centrifuge will be affected by the operating rate, inlet sludge load, agent dosage, concentration and dosing point. If the nature of the sludge changes and the operation of the equipment is not adjusted, the effect may not be so ideal. The low speed of the centrifuge is good for the mud cookie degree, but has a certain range of limits. The equipment structure of the centrifuge directly affects the treatment effect and the applicability to different sludge, such as the length-to-diameter ratio, the speed, the way of adjustment and sensitivity of the differential speed, the height of the discharge weir plate, etc. Therefore, the same sludge treated with different centrifuges will produce different treatment results.

If pumping is not continuous, it may cause difficulty in pumping.

The water content of the sludge dewatering machine is between 60% and 80%.

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