The main structure of the sludge dewatering and natural drying method is the sludge drying field, which is a flat land surrounded and separated by earth embankments. Rely on infiltration and evaporation to reduce the water content of the sludge discharged to the field. The infiltration process is completed in about 2 to 3 days, and the water content can be reduced to about 85%. After that, it mainly depends on evaporation, and it can drop to about 75% after a few weeks. The dewatering effect of the sludge drying field is affected by the local rainfall, evaporation, temperature and humidity. Generally suitable for use in dry, little rain, sandy soil areas.
Usually the sludge is pretreated first to improve the dewatering performance and then dewatered. The most common pretreatment method is to add inorganic salts or polymer coagulants. In addition, there are elutriation method and heat treatment method. Mechanical dehydration methods include filtration and centrifugation. Filtration is to filter the wet sludge with a filter layer, so that the water permeates the filter layer, and the sludge is dehydrated and trapped on the filter layer. The centrifugation method achieves the separation of mud and water by different centrifugal tendencies caused by the difference in specific gravity of solid and liquid in the sludge. The equipment used in the filtration method includes a vacuum filter, a plate and frame filter press and a belt filter. The vacuum filter continuously feeds and discharges mud, and runs smoothly, but there are many auxiliary facilities. Plate and frame filter press is a common equipment in chemical industry. It has large filtration driving force, low moisture content of mud cake, intermittent mud feeding and mud discharge, and low productivity. Manually operated plate and frame filter presses are very labor-intensive, and most of them are now switched to mechanical automatic operation. Belt filters are new types of filters with various designs and different dewatering principles (gravity filtration, pressure filtration, capillary water absorption, granulation), but they all have revolving belts that carry mud while dewatering, or only mud movement.
They are similar in complexity and energy consumption. Centrifugal method commonly used horizontal high-speed sedimentation centrifugal dehydrator, which consists of inner and outer drums, one end of the drum is cylindrical, and the other end is conical. The speed is generally around 3000 rpm or higher, and there is a certain speed difference between the inner and outer drums. Centrifugal dehydrators are continuously produced and automatically controlled, with good sanitary conditions and small footprint, but require higher sludge pretreatment. The mechanical dewatering method is mainly used for primary sedimentation tank sludge and digested sludge. The moisture content of sludge dewatering is related to sludge properties and dewatering methods.
The water granulation dehydrator is a new equipment developed. Its main body is a horizontal cylinder made of steel plate, which is divided into a granulation part, a dehydration part and a compaction part, and rotates slowly around the horizontal axis. The sludge after adding polyamine coagulant first enters the granulation section. Under the action of the sludge's own gravity, it is flocculated and compressed, and rolled into mud balls in layers. Then the mud balls and water enter the dewatering section, and the water is drained from the oblique slit of the annular drain. Finally, it enters the compacting section, and the mud pellets are further compressed and dehydrated under their own weight to form large and dense mud pellets, which are pushed out of the cylinder. The granulator has a simple structure, is not easy to wear, has low power consumption, and is easy to maintain. The moisture content of mud pellets is generally about 70%.
The filtrate produced in the sludge dewatering process of the sewage treatment plant contains a high concentration of pollutants, except that the filtrate of the drying bed contains less pollutants. Therefore these filtrates must be treated, generally together with the influent wastewater.