Definition of sludge return
   Sludge reflux is a secondary sedimentation (or sedimentation area) separated activated sludge that flows back to the aeration tank. Sometimes the sludge flows back into the regeneration tank in front of the aeration tank for re-aeration to restore the adsorption capacity of the activated sludge. The sludge return ratio is the ratio of sludge return flow to the water inlet of the aeration tank. When the quality and quantity of the returned water changes, it is hoped that the return ratio can be adjusted at any time. Sewage generally stays in activated sludge for more than 8 hours, and after some adjustment of the reflux ratio, its effect often cannot be seen immediately, and it needs to be reflected after several hours. Therefore, by adjusting the reflux ratio, it is impossible to adapt to the change of sewage water quality and quantity at any time, and generally keep the reflux ratio constant. However, in the operation and management of sewage treatment plants, adjusting the return ratio is an effective emergency means to deal with emergencies.
  Since sludge reflux is a major process link in sewage treatment plants to ensure the quality of effluent, how to control the return flow and return ratio of activated sludge has become the key to operation in the case of large changes in water volume and water quality. Through sludge return, the sludge precipitated with the effluent from the biochemical tank into the secondary sedimentation tank is returned to the system to maintain a stable amount of activated sludge (MLSS) and normal treatment process. In the process and actual operation, because the flow rate of the backflow sludge has a large relationship with the inlet volume, the ratio of the amount of backflow sludge to the influent flow rate is often used to explain the size of the flow, which is called the sludge return ratio, expressed in R.
About the control of sludge return system
There are four specific methods:
1. Keep the sludge return flow constant
   Constant return flow is the most commonly used and simplest control method. The amount of sludge returned can be controlled by two different setpoints during the day and at night.
   This method does not consider the change of influent load, but controls the sludge backflow according to a certain flow rate, so it is not the most ideal control method, and it is more suitable for the situation that the influent flow Q is relatively constant or does not fluctuate much.
For example, in general large sewage treatment plants, the inlet flow rate is relatively small, and the water can be stored in the water collection well and pipeline in front of the pump to achieve balanced inlet water, and the constant flow rate within the range of the liquid level can be tolerated. However, if the influent flow rate changes greatly, it will cause the sludge to be redistributed in the secondary sedimentation tank and the biological reaction tank, resulting in a series of problems. When the Q increases, the sludge in some bioreactor tanks will be transferred to the secondary sedimentation tank, which reduces the MLSS, but the bioreactor tank needs a higher MLSS to treat the increased sewage. At the same time, the increase of sludge in the secondary sedimentation tank will lead to an increase in the mud level, and there is a possibility of sludge loss. When the Q decreases, part of the sludge will be transferred from the secondary sedimentation tank to the bioreaction tank, increasing the MLSS, but the bioreactor tank does not need a high MLSS at this time.
2. Keep the sludge return ratio R constant
   If the return sludge concentration RSSS remains unchanged, then the MLSS can also remain unchanged. However, according to the relationship between the RSSS of the reflux sludge concentration and the surface load Q/A of the secondary sedimentation tank and the sludge sedimentation performance parameters k and n, we can find that the concentration of reflux sludge changes with the change of the surface load of the secondary sedimentation tank and the sludge sedimentation performance, and it is difficult to maintain the MLSS unchanged. In order to analyze the effects of constant QR control and R control on the activated sludge system, the surface load Q/A of the secondary sedimentation tank was assumed to be 0.6, the reflux ratio R=40%, and the MLSS=4000mg/L, and the sludge sedimentation performance remained unchanged (k=20, n=0.3). See the table below for specific data:
   Influence of Sludge Return Control of Constant Return Flow and Constant Return Ratio with Water Volume Change (SVI constant)
   From the experimental data, whether the constant return flow control or the constant return ratio control is implemented, the changes in MLSS and F/M caused by the change of water volume are consistent, namely:
The influent flow rate increased, the MLSS decreased, and the F/M increased rapidly.
The inlet flow rate decreases, the MLSS rises, and the F/M decreases rapidly.
   From the perspective of the amplitude of change, when the constant sludge return flow is controlled by QR control, the change of water volume causes a large change in MLSS and F/M, which is not conducive to the stable operation of the biological reaction tank, but the solid load of the secondary sedimentation tank can remain unchanged, which is conducive to the stable operation of the secondary sedimentation tank.
   Compared with the constant sludge reflux ratio R control, although the buffer capacity of the biological reaction tank to water volume change is improved, the impact on the operation of the secondary sedimentation tank is relatively large.
   To sum up, whether it is constant QR or constant R, it is not the best choice for sludge return control, and operators should also adopt more active control strategies according to changes in process conditions to ensure the stable and efficient operation of sewage treatment plants.
3. Keep the concentration of sludge mixture MLSS constant
   In the activated sludge method, MLSS is usually controlled at about 3000mg/L. The so-called constant MLSS control refers to maintaining a certain target value of the MLSS as much as possible.
   The operator can calculate the return flow required to achieve the target value of the MLSS based on the target value of the influent flow Q, the return sludge concentration RSSS and the mixed sludge concentration MLSS, and then control this amount.
It is worth mentioning that the control range and effective control time of constant MLSS control are limited by the amount of sludge stored in the secondary sedimentation tank.
   If the water intake is very small, the sludge in the secondary sedimentation tank may cause the sludge to float or the quality and activity of the sludge to decrease.
   When the inlet flow is large, the secondary sedimentation tank cannot provide sufficient amount of backflow sludge.
   Therefore, strict constant MLSS control can only be achieved by setting up a return sludge storage tank.
4. Keep the sludge load F/M constant
   The so-called constant F/M control is a control method to keep the ratio of organic matter amount F and microbial weight M within an appropriate range.
This control requires more setup of a reflux sludge storage tank than a constant MLSS control. However, even so, it is difficult to achieve fixed F/M control when the amount of inlet water varies greatly (e.g. more than 20%).
    It is worth mentioning that whether it is constant MLSS control or constant F/M control, the return ratio is adjusted according to RSSS and MLSS.
    Relevant studies compare constant MLSS control and fixed F/M control, assuming that the surface load Q/A of the secondary sedimentation tank is 0.6, the reflux ratio R=40%, MLSS=4000mg/L, the sludge sedimentation performance remains unchanged (k=20, n=0.3), the MLSS or F/M is maintained when the water volume changes, the required reflux ratio R, and other effects are as follows.
Influence of MLSS and F/M sludge backflow control when water volume changes (SVI constant)
   From the above table data, it can be seen that when constant MLSS control is implemented, both the biological reaction tank and the secondary sedimentation tank are affected to a certain extent, but they can adapt to large flow changes, which requires the secondary sedimentation tank to have a strong sedimentation storage capacity. When the constant F/M control is implemented, it is suitable for the stability of the biological system when the water volume changes little, but it has a certain impact on the secondary sedimentation tank. Adjusting the return flow rate or return ratio regularly or at any time according to the situation can undoubtedly make the system always in the best state, but it correspondingly increases the difficulty of operation control.
Comparison of sludge backflow control methods
    The effects of four sludge return control methods on system performance were comprehensively analyzed by four sludge return control methods, namely constant return flow QR control, constant return ratio R control, constant MLSS control and constant F/M control, on the system performance when the water volume changed, and the results were as follows: The effects of different sludge return strategies on the system performance when the water volume changed
1. Keep the sludge return flow constant for control
    The stability of the secondary sedimentation tank is mainly considered, but it has a great impact on the biological reaction tank and the adaptability to water volume change is not strong.
2. Keep the reflux ratio R constant for control
    It is a good choice, compared with the constant MLSS control, it has a greater impact on the sludge load F/M, which may affect the treatment effect, but has little effect on the secondary sedimentation tank. This control strategy has a certain adaptability to changes in water volume.
3. Keep the MLSS constant for control
    It is undoubtedly a better control strategy, which has good adaptability to water volume changes and has little impact on the overall system, but it has certain requirements for the secondary sedimentation tank, which needs to be able to withstand a large solid surface load and have sufficient mud storage space.
4. Keep F/M constant for control
    Although it can ensure the relative stability of sludge load and is beneficial to biological treatment, it has a great impact on other performance of the system, and the requirements for facility capacity are also very high, so it is not suitable for large changes in water volume, so the use of this control strategy has great limitations.
    In conclusion, constant return ratio R control and constant MLSS control are both good sludge reflux control strategies.
1. If the process design allows certain changes in F/M, the constant return ratio R control strategy can be selected;
2. If there is a surplus capacity of the secondary sedimentation tank, the constant MLSS control strategy can be selected;
3. If the water volume does not change much and the capacity of the return system meets the requirements, you can choose to use constant return flow QR control and constant F/M control.
Source: Hongtai Huarui Technology Group Co., Ltd