1. Composition and working principle
The horizontal screw centrifugal sludge dewatering unit is mainly composed of horizontal screw centrifuge, automatic flocculant preparation and dosing device, sludge crushing and cutting machine, sludge feed pump, dosing pump, single screw sludge transfer pump, flowmeter and automatic control system.
1.2 process flow
Horizontal screw centrifuge is the abbreviation of horizontal screw discharge centrifuge. It is mainly composed of high-speed rotating drum, screw and differential with the same rotation speed as the drum.
When the sludge enters the drum cavity of the centrifuge, the high-speed rotating drum generates strong centrifugal force. Due to the high density and centrifugal force of sludge particles, the sludge is thrown on the inner wall of the drum to form a solid ring layer; The density of water is small and the centrifugal force is small, so the liquid ring layer can only be formed on the inner side of the solid ring layer. Due to the different rotating speeds of the screw and the drum, there is relative movement between them (i.e. differential rotating speed), which pushes the sludge deposited on the inner wall of the drum to the outlet of the small end of the drum, and the separated water is discharged from the other end of the drum. The function of the differential is to form a certain slip between the drum and the screw. Flocculant is added to the sludge to produce flocculation, so that the dispersed sludge particles gather to produce large flocs and accelerate the separation of sludge and water.
2. Adjustment of operating parameters and treatment effect of horizontal screw centrifuge
2.1 selection of drum speed
2.2 determination of dry solid load
Dry solid load refers to the weight of nonvolatile solid treated per hour, expressed in kgds (dry sludge) / h. Adjusting the dry solid load of the centrifuge has a great impact on the sludge dewatering effect. When the mud inlet flow (i.e. "hydraulic load") reaches a certain degree and the suspended solids content brought in exceeds the maximum dry solid load that the centrifuge can bear, it will increase the moisture content of the mud cake and increase the sludge carrying capacity of the supernatant. At this time, the mud inlet flow should be reduced to make the supernatant separated from the centrifuge clear. In actual operation, the hydraulic load must be adjusted to ensure that the load of dry solids entering the centrifuge does not exceed the maximum bearing capacity of the centrifuge. Otherwise, the excess dry solids will be discharged from the supernatant, and the suspended solids in the supernatant will increase sharply, but the output of cement cake will not increase. When the rotating speed of the brick drum of the centrifuge increases, the dry solid load will increase accordingly.
2.4 determination of liquid ring layer thickness (set the height of liquid level baffle)
When the amount of sludge is certain, if the thickness of the liquid ring layer is large, the sludge will stay in the centrifuge for a long time, and the longer the sludge is separated in the liquid ring layer, more sludge will be separated, which can reduce the possibility of some small particles being disturbed and losing with the separation liquid. However, if the thickness of the liquid ring layer is too large, water will overflow from the sludge outlet with the dewatered sludge; If the thickness of the liquid ring layer in the centrifuge is small, the residence time of the sludge in the centrifuge is short, and the working pressure is not easy to increase, but the moisture content of the dewatered sludge is also low.
Based on the above two aspects, the thickening of the liquid ring layer will generally improve the solid recovery rate of dehydration (supernatant and clear liquid), but the thickening of the liquid ring layer will shorten the shore area accordingly. As shown in the above figure, the sludge separated from the liquid ring layer will not have enough time to be dumped dry, so the solid content of the mud cake will decrease. When controlling the thickness of liquid ring layer, the time should be balanced between high solid recovery and solid content of mud cake. In addition to incineration after sludge dewatering, it is generally unnecessary to pursue too high solid content of sludge cake, and the higher the solid recovery rate, the better. Therefore, the thickness of liquid ring layer should be increased as much as possible.
Adjust the thickness of liquid ring layer of centrifuge by changing the position of liquid level baffle. The adjustment of the liquid level baffle of the centrifuge is very important, which directly affects the dehydration effect and the vibration degree of the centrifuge. When adjusting the height of the liquid level baffle, pay attention to ensure that all liquid level baffles are at the same height, otherwise it will cause great imbalance and violent vibration of the centrifuge, and ensure that the height tolerance of the liquid level baffle is ± 0.25 mm.
"Differential speed" refers to the difference between the rotating speed of the drum and the screw speed, that is, the relative speed between the two. Increasing or decreasing the "differential speed" will change the residence time of sludge in the drum, which has a very important impact on the treatment effect.
"Differential" shall be selected according to the following principles: when the amount of sludge is certain, if the differential is relatively low, the sludge will stay in the centrifuge for a long time, the dewatered sludge will be drier, the supernatant will be turbid and the treatment capacity will be relatively low; If the differential speed is high, the sludge will stay in the centrifuge for a short time, and the dewatered sludge will be wetter, but the treatment capacity is also high; At the same time, the sludge dried by the centrifuge is pushed out by the screw in time, and will not return to the supernatant due to the long residence time. The solid recovery rate is also greatly increased, and the supernatant is clear. However, if the differential speed is too large, the greater the relative movement between the drum and the spiral will increase the disturbance to the liquid ring layer, and the sludge separated from the solid ring layer will be flooded to the liquid ring layer again, and may be lost with the separation liquid (the supernatant is turbid).
If the solid content of the supernatant is large, it indicates that the dry solid load of the differential centrifuge is large, so the differential speed should be increased accordingly. After the differential speed is increased, reduce the residence time of the sludge in the centrifugal concentration, and quickly push the dehydrated dry sludge out of the centrifuge, so that it has no chance to return to the liquid phase, which will increase the output of dry sludge and reduce the solid content of the supernatant. Some types of equipment have the function of automatically accelerating slag discharge, that is, when the set torque reaches a certain limit value, the equipment will automatically reduce the amount of mud and medicine, increase the differential speed, and quickly push out the accumulated mud ring layer. After the torque is reduced to a certain value, the flow and differential will automatically return to normal. Therefore, the differential speed should be determined according to the material properties, processing capacity, processing requirements and centrifuge structural parameters. In other words, the best processing capacity and the differential value range required by the processing effect shall be found on the site according to the situation, so as to achieve the highest processing capacity as far as possible under the condition of meeting the degree of mud biscuit. The differential adjustment range is generally changed by increasing or decreasing by 1 each time, and the range should not be too large.
In short, there is a contradiction between processing capacity and processing effect. To improve processing capacity, it is necessary to increase the differential ratio, but it may reduce the degree of mud biscuit; In order to improve the degree of sludge biscuit, it is necessary to reduce the degree of difference, so as to reduce the treatment capacity. Therefore, the on-site commissioning work is to find the best equipment operating condition parameters that meet the actual sludge property conditions on site, so as to achieve the dual purpose of maximum equipment operating efficiency and best treatment effect. There is no simple data to calculate. We can only rely on long-term actual debugging to accumulate experience and adjust in time according to changes.
2.6 aspect ratio
When the drum speed is constant, the greater the length diameter ratio, the greater the processing capacity, the longer the relative settlement time of solid materials in the centrifuge drum, the lower the moisture content in the solid materials of the banking office, and the better the separation effect; On the contrary, the worse the separation effect.
3. Problems encountered in unit operation
4.2 sludge moisture content
The principle of the rigid dehydration machine under study is to further reduce the water content of the sludge after centrifugal dehydration by heating, and the water content can be reduced to 60%.
(4) Internal water, accounting for 3%, exists in the interior of sludge particles or microbial cells. Biological method is used to destroy the cell membrane to remove the water in the cells, or high-temperature heating method is used.