Abstract: By monitoring the operating indicators of high-density sedimentation tanks, the reasonable operating conditions of high-density sedimentation tanks were explored. The results indicate that turbidity can be used to quickly estimate the concentration of sludge water entering the mixing zone; Both dosing and reflux should be controlled and adjusted according to the concentration of sludge in the reaction zone; The inflow rate of high-density sedimentation tanks has a significant impact on the turbidity of the effluent. It is advisable to reduce the inflow rate as much as possible while maintaining a certain amount of treated dry solids.
Keywords: high-density sedimentation tank, operating conditions, sludge water discharge, reflux, sedimentation
The high-density sedimentation tank (hereinafter referred to as the high-density tank) in the concentration process of the sludge water system of Tianjin Xinkai River Water Plant consists of four parts: mixing, reaction, concentration/sedimentation, and external sludge circulation. The maximum designed sludge inflow for a single tank is 960m ³/h, and the designed sludge concentration is 2.0g/L. During operation, it was found that when the sludge concentration was higher than 1.5 g/L, sludge would float up in the high-density tank. In order to solve this problem, the author conducted regular sampling and monitoring of each area of the high-density pool to explore its optimal operating conditions.
1.3 External sludge recirculation zone can accelerate the growth of alum flowers and increase their density, in order to maintain the high sludge concentration required for uniform flocculation. However, due to the instability of sludge level changes and the suction effect of the sludge layer near the suction port of the reflux pump, the concentration of the reflux sludge is very uneven, ranging from 0-28.7 g/L, and in most cases, the sludge concentration is low. The actual reflux rate during operation is always 300m3/h, without considering the size of reflux concentration and inlet slurry concentration. The reflux of clean water has little effect on the reaction zone, only reducing the influent concentration by 5%; But when the concentration of returning sludge is high, the influent concentration will increase several times, resulting in insufficient dosing and small flocs. Therefore, the reflux rate should be adjusted appropriately based on the inlet mud concentration, inlet mud flow rate, and reflux concentration. The reflux concentration is related to the height of the sludge bed. When the reflux sludge concentration is high, it indicates that the sludge level is high, and the reflux rate should be reduced to slow down the rate of sludge layer increase in the sedimentation zone. The reflux rate can also be adjusted according to the settling ratio of the reaction zone. When the settling ratio is large (>15), the reflux rate should be reduced. This is because the sludge concentration in the reaction zone is high enough to ensure flocculation effect, and there is no need to reflux to increase concentration, which also avoids the waste of reagents and the acceleration of sludge layer growth. The continuous scraping of the bottom scraper of the high-density tank in the 1.4 sedimentation zone promotes the concentration of sludge in the sedimentation zone. The inclined plate is placed at the top of the sedimentation tank to remove residual alum and produce qualified effluent. The supernatant from the sedimentation zone flows back to the distribution well. If the turbidity of the supernatant is high, it will affect the water purification process and waste coagulants. After monitoring, it was found that the flow rate has a significant impact on the turbidity of the effluent, as shown in Table 1. In the experiment, it was found that regardless of whether FeC13 was added in the mixing zone or not, the turbidity of the effluent supernatant was higher at higher flow rates; At low inflow rates, the turbidity of the effluent supernatant is also lower.
During the monitoring process, there was a PAM dosing pump malfunction that lasted for a total of 5 hours. When the inlet flow rate is 300m ³/h and the sludge concentration is 0.6g/L, the flocs in the reaction zone of the high-density tank are very small, but the maximum turbidity of the effluent is 2.4 NTU. In addition, when the actual concentration of the incoming water was 0.5g/L and the dosing system was operating normally, there was a local uplift phenomenon in the sedimentation area of the high-density tank. In the first few minutes of the uplift, the incoming water flow rate suddenly increased from 300m ³/h to 600m ³/h, indicating that the uplift was caused by flow shock. Therefore, the flow rate has a significant impact on the turbidity of the effluent in the sedimentation zone. According to the Stokes formula, the settling velocity of a 0.5mm floc is calculated to be 12.67g/h, which is much higher than the rising velocity of 5.56g/h (when the flow rate is 300m ³/h). To ensure the turbidity of the effluent, the inflow rate should be controlled at an appropriate dosage; At the same time, in order to prevent the occurrence of sludge flotation, sudden changes in flow rate should be prevented. Conclusion 1: Turbidity can be used to quickly estimate the concentration of sludge water in the mixing zone of high-density sedimentation tanks, and there is a good linear relationship between the two indicators within the same water quality period The reflux ratio and reflux concentration have a significant impact on the concentration of influent sludge, and the reflux process should be strictly controlled to ensure the formation and precipitation of flocs The dosage should be adjusted according to the sludge concentration in the reaction zone to minimize drug consumption and obtain better settling flocs In practical operation, the sedimentation ratio can be used to estimate the concentration of sludge in the reaction zone, and adjust the dosage and reflux ratio accordingly The flow rate has a significant impact on the turbidity of the effluent, and it should be operated at an appropriate inlet flow rate as much as possible. The flow rate should be adjusted slowly to prevent possible upward fluctuations caused by sudden changes in flow rate.
