Selection of Sludge Dewatering Equipment in Municipal Sewage Treatment Plant

1. Introduction

The sludge generated in the process of municipal sewage treatment has the characteristics of large output, high moisture content, special smell, perishability and so on. How to optimize the sludge treatment process and reasonably select the sludge dewatering equipment, is an important issue to be considered at present.

2. Sludge Dewatering Equipment

At present, in the sewage treatment plant, the sludge is mainly composed of three types: primary sedimentation tank sludge with moisture content of 95%～99.2%, residual sludge in biochemical tank and chemical sludge in advanced treatment. However, the volume of sludge is usually dozens of times to the volume of dry matter contained in it, and it is not conducive to transport. So it is necessary to treat the sludge with effective technical measures.

The sewage treatment plants usually treat the sludge through two links: sludge concentration and sludge dehydration. Sludge concentration mainly refers to the utilization of gravity to effectively improve the solid content of sludge. But if only using gravity concentration tank to treat sludge, it will take a lot of time, and the organic matter in the sewage will be rotten and stink due to the action of microorganisms, which will seriously affect the sanitary condition of the sewage treatment plant. Therefore, in the actual treatment process, when the moisture content of sludge is reduced to 94%～96% by using the gravity concentration tank, it is necessary to use the sludge dewatering equipment to reduce the moisture content to less than 80%, so as to facilitate the later transportation and disposal work.

At present, the common sludge dewatering equipment are belt filter press dehydrator, centrifugal dehydrator and plate frame filter press dehydrator.

3. Introduction of Dehydration Equipment

3.1 Belt Filter Press Dehydrator

The belt filter press dehydrator is generally composed of filter belt, roller, belt tension system, belt deflection adjustment system, belt washing system and belt drive system. The sludge is clamped by two tensioned filter belts, and passes through a series of regularly arranged roller cylinders in S-shape bending. Depending on the tension of the filter belt to form the squeezing force and shear force, the capillary water in the sludge is squeezed out to obtain sludge cake with high solid content, so as to achieve dehydration. The moisture content is usually controlled below 80%.

The belt filter press dehydrator should use the filter belt with strong permeability, and the sludge should be chemically treated. The belt filter press dehydrator has a higher requirement for the flocculation effect of the incoming sludge. If the incoming sludge quality can not meet the requirement, the quality of the outgoing sludge and the dehydration effect will be seriously affected. The belt filter press dehydrator is relatively mature and reliable, with low initial investment and maintenance costs.

In the operation process, high pressure water is needed to continuously wash the filter surface, and the operation environment can not be completely closed. Therefore, water and gas spillage exists in the working room, which makes the production environment bad. The belt filter press dehydrator has the phenomenon of uneven feed, which will affect the dewatering effect partly, so it requires high technical level of operators.

3.2 Centrifugal Dehydrator

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.

3.3 Plate Frame Filter Press Dehydrator

The plate frame filter press dehydrator is a kind of pressure filtration equipment operated intermittently, which is mainly compose of five parts: frame part, filtration part, hydraulic part, blow off system, discharge device and electrical control part. It includes filter plate pressing, low-pressure sludge feeding, high-pressure sludge feeding, pressing, back blowing, filter plate loosening, discharging, washing and other processes.

Its operation process is as follows: after conditioning, the sludge is injected into the filter press through the diaphragm pump or screw pump to be separated. The maximum pressure of sludge feeding is 0.80MPa, and the time of sludge feeding is generally 1.5～2.5h. After the sludge feeding is stopped, it is pressurized by the multi-stage centrifugal pump, and the diaphragm in the filter press forcibly compresses and dehydrates the sludge, the time of filter pressing is generally 20～50min, and the pressure is 0.8～1.2MPa. After that, use 0.6MPa high-pressure air to blow off the sludge in the central sludge inlet pipe and the filtrate in the cavity. Then slowly loosen the filter press to drain the remaining filtrate. Finally, remove the sludge cake in the machine.

The plate frame filter press dehydrator is suitable for the solid-liquid separation of various suspensions, with wide application range and good separation effect. It can control the moisture content of dehydrated sludge below 60%.

However, there are many disadvantages in the actual operation of the plate frame filter press dehydrator: the feeding hole is very easy to be blocked, and it is difficult to take out the filter cake sandwiched between the filter plate and the filter frame. The dehydrator can not be operated continuously, and its working capacity is small. During operation, the consumption of the filter cloth is large, more manpower and material resources are needed to clean the filter cloth. Otherwise, the cleanliness of the workshop environment can not be guaranteed.

4. Conclusion

In summary, municipal sewage treatment plants should select sludge dewatering equipment in combination with its own actual situation and use requirements. The belt filter press dehydrator can meet the requirement of sludge dewatering with low project investment. It is a relatively economic and practical sludge dewatering equipment. If it is not sensitive to the investment and has high requirement for continuous operation of sludge treatment and the quality of dehydrated sludge, the centrifugal dehydrator is a better choice. If the requirement for moisture content of dewatered water in subsequent treatment is low, plate frame filter press dehydrator will be another choice.