Activated sludge is a general term of microbial community and its attached organic and inorganic substances, which can be divided into aerobic activated sludge and anaerobic granular activated sludge. Activated sludge contains a lot of bacteria, fungi, protozoa, metazoa and algae. All kinds of pollutants in sewage are decomposed and absorbed by activated sludge, and various indicators of activated sludge determine the quality of sewage treatment effect.
A large amount of activated sludge will be produced in the process of domestic sewage treatment. The amount of sludge is too low in the process of sewage treatment, and the pollutants in the sewage can not be fully decomposed, resulting in the effluent quality not up to standard. If the sludge content is too high, some activated sludge will die and decompose due to insufficient nutrition, and a large number of pollutants will be produced after the decomposition of activated sludge, which will affect the effect of sewage treatment. Therefore, it is necessary to control the content of activated sludge in sewage by discharging excess sludge, so as to achieve ideal sewage treatment effect.
The discharged excess sludge needs solid-liquid separation to reduce the sludge volume, so as to carry out the next step of harmless sludge treatment. Generally, centrifugal mud press, belt filter press and plate frame filter press are used in sewage treatment plant. At present, the belt filter press is also a common type. The following to explore the impact of the belt filter press mud.
1、 Working principle of belt filter press
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Briefly describe the working process of the belt filter press in our factory: the sludge is pumped from the sludge thickening tank to the filter press through the pipe mixer through the frequency conversion screw pump. In this process, the Cationic PAM (polyacrylamide) solution of a certain concentration is pumped into the pipeline mixer through the variable frequency screw pump, and the sludge in the pipeline mixer is fully mixed before entering the coagulation tank of the filter press. After mixing again in the coagulation tank, the sludge enters the sludge thickening section through gravity flow. At this stage, the water in the sludge flows out of the filter cloth by gravity to improve the sludge concentration in the thickening section. The concentrated sludge enters the sludge dewatering section through the rolling filter cloth. In the dewatering section, the sludge is extruded by the rolling filter cloth to remove the free water, thus further reducing the water content of the sludge. In the whole process, the filter cloth is flushed through the fixed spray head, so as to achieve the purpose of filter cloth permeating free water and filtering out more quickly.
2、 Factors affecting sludge discharge of belt filter press
The mud pressing of belt filter press is a dynamic operation process. There are several factors that affect the mud output and speed.
1. Water content of sludge in thickener
The moisture content of sludge in the thickening tank is lower than 98.5%, and the sludge discharge speed of the mud press is much higher than 98.5. If the moisture content of sludge is lower than 95%, the sludge will lose its fluidity, which is not conducive to mud pressing. Therefore, it is necessary to reduce the water content of sludge in thickening tank, but the water content should not be lower than 95%.
The water content of sludge in thickening tank is mainly affected by the volume of thickening tank, water content of excess sludge in secondary sedimentation tank and sludge settling performance. Increasing the volume of thickening tank can increase the settling time of sludge and reduce the water content of sludge in the thickening tank. Therefore, it should be considered to increase the volume of thickening tank appropriately in engineering design. The water content of the excess sludge in the secondary sedimentation tank is generally between 97.3% and 98.5%. If the sludge concentration in the secondary sedimentation tank is too high, the sedimentation pressure of the sludge in the secondary sedimentation tank will be increased, resulting in the sludge turning over in the secondary sedimentation tank. Therefore, it is generally not adopted to reduce the water content of the sludge in the thickening tank by reducing the water content of the sludge in the secondary sedimentation tank. The sedimentation performance of sludge seriously affects the water content of sludge in thickening tank. Under normal conditions, SV30 (sludge sedimentation ratio within 30 minutes) is 15% - 30%, but in special cases, SV30 can reach more than 70%. In this case, it is necessary to increase external conditions to speed up the sludge settling speed (generally adding a certain proportion of PAC or FeCl3). However, the effect of intermittent mud supplement is far less than that of the former.
2. Proportion of activated sludge in sludge
The particle size of activated sludge was larger than that of anaerobic nitrification, and the free water mixed with PAM was better separated from sludge. Through the mud pressing operation, it is found that when the proportion of anaerobic nitrification sludge in the thickening tank is high, the solid-liquid separation effect is not good after the sludge is mixed. The small sludge particles will cause the low water permeability of the filter cloth in the thickening section, increase the burden of solid-liquid separation in the pressure filtration section, and reduce the output of the mud press. When the proportion of activated sludge in the thickening tank is high, the solid-liquid separation effect is good in the thickening section of the mud press, which reduces the burden of solid-liquid separation of filter cloth in the pressure filter section. If the free water flows out more in the thickening section, the flow rate of sludge and drug mixture can be appropriately increased, so as to increase the sludge production of the sludge press in unit time.
3. Mud to drug ratio
After adding PAM, the sludge is initially mixed through the pipeline mixer, further mixed in the subsequent pipeline, and finally mixed through the coagulation tank. In the mixing process, most of the free water is separated from the sludge by the turbulent action in the flow, and then the solid-liquid preliminary separation effect is achieved in the concentration section. There should be no free PAM in the mud drug mixed solution.
If the PAM dosage is too large, PAM will be carried in the mixed solution, on the one hand, it will cause waste of PAM, on the other hand, PAM will stick on the filter cloth, which is not conducive to the washing of filter cloth by spray water, and the filter cloth will be blocked. If the dosage of PAM is too small, the free water in the sludge drug mixed solution can not be separated from the sludge, and the sludge particles block the filter cloth, so the solid-liquid separation can not be carried out.