Drying Technology
General introduction to sludge mechanical dewatering
Mechanical dewatering of sludge mainly includes belt filter press dewatering, centrifugal dewatering and plate frame filter press dewatering. The main types and characteristics of mechanical dehydration process are as follows:
1. The belt pressure filter has low noise and power consumption, but the floor area and flushing water volume are large, and the workshop environment is poor. The water content of sludge in belt dewatering is generally less than 97.5%, and the water content of sludge out is generally less than 82%;
2. Centrifugal dewatering covers a small area, does not need to be filled with water, and has a good workshop environment, but it has high power consumption and high noise. The water content of sludge in centrifugal dewatering is generally 95% ~ 99.5%, and the water content of sludge out is generally 75% ~ 80%;
3. The moisture content of the plate and frame pressure filtration dewatering cement cake is low, but the land occupation and flushing water volume are large, and the workshop environment is poor. The moisture content of the mud in the plate and frame pressure filtration dewatering is generally required to be less than 97%, and the moisture content of the mud out is generally up to 65% ~ 75%;
4. Screw press dehydrator and rolling dehydrator have small floor area, small flushing water volume, low noise and good workshop environment, but the single machine capacity is small and the liquid-solid content of supernatant is high. The water content of sludge in screw press dewatering is generally 95% ~ 99.5%, and the water content of sludge out is generally 75% ~ 80%.
02. Pretreatment before mechanical dewatering of sludge
The purpose of pretreatment before mechanical dewatering of sludge is to improve the dewatering performance of sludge and improve the production capacity and dewatering effect of mechanical dewatering equipment. Pretreatment methods include chemical adjustment method, elutriation method, heat treatment and freezing method, but it should be distinguished. If the question is about common methods, it is chemical adjustment method + elutriation method.
Article 7.4.1 of code for design of outdoor drainage stipulates that the design of sludge mechanical dewatering shall comply with the following provisions:
1. The type of sludge dewatering machinery shall be selected after technical and economic comparison according to the dewatering nature and dewatering requirements of sludge;
2. The moisture content of sludge before entering the dehydrator shall not be greater than 98%;
3. The digested sludge can be washed before dehydration according to the nature of sewage and economic benefits;
4. The layout of the mechanical dewatering room shall be in accordance with the relevant provisions in the pump room in Chapter 5 of this specification, and the mud cake transportation facilities and channels shall be considered;
5. The dewatered sludge shall be stored in sludge storage yard or sludge silo, and the capacity of sludge storage yard or sludge silo shall be determined according to sludge outlet and transportation conditions.
6. The sludge mechanical dewatering room shall be equipped with ventilation facilities, and the air change times per hour shall not be less than 6 times.
03. Sludge filtration and dewatering
The basic principle of sludge filtration dehydration is that the pressure difference on both sides of the filter medium is used as the driving force. There are four methods to cause the driving force of the pressure difference:
1. Static pressure depending on the thickness of sludge itself (such as dehydration in drying plant);
2. Create negative pressure on one side of the filter medium (such as vacuum filtration dehydration);
3. Pressurized sludge presses water through the medium (such as pressure filtration dehydration);
4. Cause centrifugal force (such as centrifugal dehydration).
Sludge filtration and dewatering includes plate and frame filter press and belt filter press. The provisions on plate and frame filter press in code for design of outdoor drainage are as follows:
Article 7.4.5 stipulates that the design of plate and frame filter press and box filter press shall meet the following requirements:
1. The filtration pressure is 400 ~ 600kpa;
2. The filtration cycle shall not be greater than 4H;
3. Each filter press can be equipped with one sludge pressure pump, and the plunger pump should be selected;
4. The compressed air volume shall not be less than 2m3 / min per cubic meter of filter chamber (designed according to standard working conditions).
The provisions on belt filter press in code for design of outdoor drainage are as follows:
Article 7.1.4 stipulates that the number of sludge treatment structures shall not be less than 2. According to the design of simultaneous operation, one sludge dewatering machine can be considered for standby;
Article 7.4.3 stipulates that belt filter press, plate and frame filter press, box filter press or microporous extrusion dehydrator shall be adopted for the filter press, and the mud cake yield and water content of the mud cake shall be determined according to the test data or similar operation experience. The moisture content of mud cake is 75% ~ 80%.
Article 7.4.4 stipulates that the design of belt filter press shall meet the following requirements:
1. The air compressor shall be configured according to the requirements of belt filter press, and at least one set shall be standby;
2. Flushing pump shall be equipped with pressure of 0.4 ~ 0.6MPa and flow of 5.5 ~ 11m3 / [M (bandwidth) ▪ h] At least one standby shall be used for calculation.
3. The sludge dewatering load shall be determined according to the test data or similar operation experience, and the sewage sludge can be taken according to the provisions of table 7.4.4.
04. Sludge centrifugal dewatering
The basic principle of sludge centrifugal dewatering is to separate the solid and liquid in the sludge by using the rotation of the centrifuge. Article 7.4.7 of code for design of outdoor drainage stipulates that when horizontal screw centrifugal dewatering is adopted for sewage sludge, its separation factor should be less than 3000g (G is gravity acceleration). Article 7.4.8 stipulates that a sludge cutter shall be set before the centrifugal dehydrator, and the particle size of the sludge after cutting shall not be greater than 8mm.
For all kinds of sludge, it should be emphasized that the dewatering performance of activated sludge is the worst, but the dewatering performance is improved after the mixing of humic sludge, activated sludge and primary sludge.
05. Sludge drying and incineration
Sludge drying refers to the process of removing most of the water from concentrated sludge through infiltration or evaporation, including natural drying, heating drying and sludge incineration.
1. Natural drying includes natural filter drying and artificial filter drying field technology. The artificial filter drying field is divided into open drying field and covered drying field. Its main function is to reduce the water content of sludge through skimming, infiltration and evaporation. For the drying plant, the main influencing factors of dehydration are climate conditions, sludge properties and sludge conditioning. It should be emphasized that naturally dried sludge can not achieve the purpose of stabilization.
2. Heating drying includes full drying and semi drying, in which the moisture content of full drying is ≤ 15%, and the moisture content of semi drying is between 15% ~ 55%. The technical principle is that after sludge dewatering, under the external heating conditions, through the process of heat and mass transfer, the water in sludge is separated with phase change to become a dried product.
3. Sludge incineration is the process of completely mineralizing sludge into a small amount of ash by using incinerator. The factors affecting sludge incineration mainly include sludge moisture content, temperature, incineration event and the mixing degree between sludge fuel and air.
06. Final disposal of sludge
Sludge disposal refers to the final absorption process of treated sludge, generally including land use, landfill and utilization of building materials. Incineration is also a way of sludge disposal. Sludge recycling refers to the process of sustainable development of sludge treatment, which refers to the appropriate treatment of sludge as raw materials for making fertilizer, fuel and building materials.
