What is sludge return ratio
Sludge backflow ratio is the ratio of sludge backflow to the amount of water into the aeration tank. When the reflux water quality changes, it is hoped that the reflux ratio can be adjusted at any time. Sewage generally stays in activated sludge for more than 8h. After some adjustment of reflux ratio, the effect is often not immediately apparent and needs to be reflected after several hours. Therefore, by adjusting the reflux ratio, can not adapt to the change of sewage water quality and quantity at any time, generally keep the reflux ratio constant. But in the operation management of sewage treatment plant, adjusting reflux ratio is an effective emergency means to deal with emergencies.
Second, why the discharge of surplus sludge is generally kept constant
The discharge of excess sludge has a great effect on the function and treatment effect of activated sludge system, but the effect is very slow. For example, by regulating the discharge of surplus sludge to control the excessive reproduction of filamentous bacteria in activated sludge, the effect can usually be seen after 2-3 times of mud age. In other words, when the sludge age is 5 days, the control effect brought by adjusting the sludge discharge can be observed after 10-15 days.
Therefore, it is impossible to control or adapt to the daily change of inlet water quality and quantity through mud discharge operation. Even if mud discharge works, the changed sewage has already flowed out of the system, so the amount of mud discharge is generally kept constant. However, it is necessary to record the residual sludge emissions every day, and use F/M or SRT values and other methods to calculate daily, and summarize the regularity.
Three, what are the methods of adjusting the amount of reflux sludge
(1) Adjust according to the mud level of the secondary sedimentation tank.
This method can avoid the phenomenon of sludge loss caused by high sludge level in the secondary sedimentation tank, and the effluent water quality is relatively stable, but its disadvantage is that the concentration of reflux sludge is not stable.
(2) Determine reflux ratio according to sludge settling ratio
The calculation formula is:
R = SV/(100 - SV)
Sludge settling ratio measurement is relatively simple, rapid, with strong operability, its disadvantage is that when the sludge settling performance is poor, that is, when the sludge settling ratio SV is high, it is necessary to improve the sludge flow back, the result will make the concentration of reflux sludge decreased.
(3) Adjust the reflux ratio according to the concentration of reflux sludge and the concentration of mixed liquid sludge
The calculation formula is:
R = MLSS/(RSSS - MLSS)
It takes a long time to analyze the sludge concentration in the mixed liquid of reflux sludge and aeration using drying method, so it is not practical to direct the operation, so it is generally used as the method of checking the reflux ratio.
(4) According to the sludge sedimentation curve, determine the best sedimentation ratio of activated sludge in a specific sewage treatment plant.
By adjusting the sludge return flow, the residence time of the sludge in the secondary sedimentation tank is exactly equal to the time when the sludge reaches the maximum concentration through sedimentation. At this time, the concentration of the reflux sludge is the largest, and the return flow is the smallest. This method is simple and easy to use. It can obtain high reflux sludge concentration and the residence time of sludge in the secondary sedimentation tank is the shortest. This method is especially suitable for denitrification and phosphorus removal processes.
Four, there are several control methods of sludge reflux system
In order to realize the relative stability of sludge reflux concentration and sludge concentration of mixed liquid in aeration tank and convenient operation and management, there are three ways to control sludge reflux :(1) keep the reflux flow constant. (2) Keep the residual sludge discharge constant. (3) Backflow ratio and backflow are adjusted at any time.