Sludge Treatment and Disposal Technology (9)

37. Selection of Sludge Dewatering Equipment

After sewage sedimentation, a large amount of sludge will be produced. Even after concentration and digestion, the water content is still as high as 96%, the volume is very large and difficult to dispose of. It must be dewatered to increase the solid content of the mud cake to reduce the floor space occupied by the sludge.

Generally, large and medium-sized sewage treatment plants use mechanical dewatering. There are many types of dewatering machines, which can be divided into three categories according to the dewatering principle: vacuum filtration dewatering machine, filter press dewatering machine and centrifugal dewatering machine. This article introduces filter press dewatering machine (including belt filter press,  plate and frame filter press) and centrifugal dewatering machine commonly used in domestic sewage treatment plants.

37.1 Belt Filter Press Dewatering Machine

The belt filter press is composed of two tensioned filter belts, which carry the sludge layer and pass through a series of regularly arranged roller cylinders in an S shape. The tension of the filter belts will form a squeezing and shearing force on the sludge layer, squeeze out the capillary water in the sludge layer, and obtain a mud cake with a high solid content.

Generally, the belt filter press is composed of filter belt, roller cylinder, filter belt tensioning system, filter belt deviation adjustment system, filter belt washing system and filter belt driving system. When choosing the model, the following aspects should be considered:

(1) Filter belt. It is required to have high tensile strength, resistance to bending, acid and alkali, and resistance to temperature changes. At the same time, the specific properties of the sludge should be considered, and the appropriate weaving texture should be selected to make the filter belt have good air permeability and interception performance of sludge particles.

(2) Roller cylinder deviation adjustment system. Generally completed by pneumatic device.

(3) Filter belt tensioning system. Generally controlled by pneumatic system. The tension of filter belt is generally controlled at 0.3-0.7MPa, and the commonly used value is 0.5MPa.

(4) Belt speed control. Different types of sludge have different requirements for belt speed, that is, there is an optimal belt speed control range for any specific sludge. Within this range, the dehydration system can both guarantee a certain processing capacity and obtain high-quality mud cake.

The belt filter press is less affected by sludge load fluctuations. It also has the characteristics of low sludge moisture content, stable operation, low startup consumption, relatively simple management and control, and low requirements on the quality of operating personnel.

37.2 Centrifugal Dewatering Machine

The centrifugal dehydrator is mainly composed of a drum and a screw conveyor with a hollow shaft. After the sludge is fed into the drum by the hollow shaft, it is immediately thrown into the drum chamber by the centrifuge force. Due to the large proportion of sludge particles, the centrifuge force is also large, then the sludge is thrown stick on the inner wall of the drum to form a solid layer. The water density is small, and the centrifuge force is also small, the liquid layer is formed inside the solid layer. Due to the slow promotion of the screw conveyor, the solid layer is transported to the cone end of the drum , and continuously discharged through the solid outlet. The liquid layer is continuously overflowed from the liquid outlet.

When the direction of sludge inlet is the same as that of sludge solid transportation, it is called forward flow centrifugal dehydrator. Conversely, it is called reverse flow centrifugal dehydrator. The centrifugal dehydrator usually controls the moisture content of dehydrated sludge below 80%. It is mainly compose of drum, screw pusher, machine shell, main bearing, drive system and base. The overall service life is more than 15 years, the operating life of the pusher blade is more than 15000h, and the minimum service life of the main bearing is 100000h.

The centrifugal dehydrator has excellent sealing performance. The sludge, water and odor will not overflow from the machine to pollute the operating environment. The working process of feeding, separating and discharging filtrate and sludge cake is continuous and can run 24 hours every day with high efficiency. The centrifugal dehydrator has an automatic cleaning device, which can automatically clean the drum every time when it stops. However, the centrifugal dehydrator is expensive, requires a lot of investment and high maintenance costs.

37.3 Plate and Frame Filter Press Dewatering Machine

The plate and frame filter press achieves the purpose of dewatering by extruding the plate and frame to discharge the water in the sludge through the filter cloth. It is mainly composed of concave filter plate, frame, automatic-pneumatic closing system, plate suspension system, filter plate shaking system, air compression device, filter cloth high-pressure washing device, and photoelectric protection device on one side of the machine body. When choosing the model, the following aspects should be considered:

(1)Requirements for the solid content of the mud cake. Generally, compared with other types of dewatering machines, the plate and frame filter press has the highest solid content of the mud cake, up to 35%. If considering the factor of reducing the land occupation of sludge, the plate and frame filter press should be the preferred option.

(3) Material of the frame.

(3) Material of the filter plate and filter cloth. It is required to be corrosion-resistant and the filter cloth must have a certain tensile strength.

(4) Movement of the filter plate. It is required to be fully automatic or semi-automatic through a hydraulic-pneumatic device to reduce the labor intensity of the operator.

(5) Filter cloth oscillation device. Make the filter cake easy to fall off.

Compared with other types of dewatering machines, the biggest disadvantage of the plate and frame filter press is that it occupies a large area.

37.4 Conclusion

According to the comparative analysis of the performance of different types of dewatering machines, it can be seen that different sewage treatment plants should make comprehensive considerations from aspects such as sludge characteristics, operating conditions, personnel quality, requirements for mud cake, funds, and costs, in order to make relatively reasonable choices.