Understanding Sludge Return Ratio: Key to Wastewater Management

1.Definition of SR

 The sludge return ratio is the ratio of the sludge return volume to the aeration tank inlet volume. When the return water quality and quantity change, it is hoped that the return ratio can be adjusted at any time. Sewage generally stays in the activated sludge for more than 8 hours. After a certain adjustment of the return ratio, the effect is often not immediately apparent and needs to be reflected after a few hours. Therefore, by adjusting the return ratio, it is impossible to adapt to the changes in sewage quality and quantity at any time, and the return ratio is generally kept constant. However, in the operation and management of sewage treatment plants, adjusting the return ratio as a means of dealing with emergencies is an effective emergency measure.

2.The discharge of excess sludge remains constant

The discharge of residual sludge has a great impact on the function and treatment effect of the activated sludge system, but this effect is very slow. For example, by adjusting the discharge of residual sludge to control the excessive reproduction of filamentous bacteria in the activated sludge, the effect is usually seen after 2 to 3 times the sludge age. In other words, when the sludge age is 5 days, it will take 10 to 15 days to observe the control effect brought by adjusting the discharge amount of sludge.

Therefore, it is impossible to control or adapt to the daily changes in the quality and quantity of the influent water through sludge discharge operations. Even if the sludge discharge is effective, the sewage that has changed has already flowed out of the system, so the sludge discharge volume generally remains constant. However, it is necessary to record the discharge of residual sludge every day, and use methods such as F/M or SRT values ​​to calculate it every day to summarize the regularity.

3.Method for adjusting the amount of return sludge

(1) Adjust according to the mud level of the secondary sedimentation tank.

This method can avoid the phenomenon of sludge loss caused by the excessively high mud level in the secondary sedimentation tank, and the effluent water quality is relatively stable. Its disadvantage is that the concentration of the return sludge is unstable.

(2) Determine the return ratio according to the sludge settling ratio

The calculation formula is:

                                  R=SV／(100—SV)

The determination of sludge settling ratio is relatively simple, rapid and easy to operate. However, its disadvantage is that when the sludge settling performance is poor, that is, when the sludge settling ratio SV is high, the sludge return volume needs to be increased, which will result in a decrease in the concentration of the return sludge.

(3) Adjust the return ratio according to the return sludge concentration and the mixed liquor sludge concentration

The calculation formula is:

                                 R=MLSS／(RSSS—MLSS)

The drying method is used to analyze the sludge concentration in the return sludge and aerated mixed liquor. It takes a long time and is not very realistic to directly guide the operation. It is generally used as a calibration method for the return ratio.

(4) According to the sludge settling curve, determine the optimal settling ratio of the activated sludge in a specific sewage treatment plant.

Then, by adjusting the sludge return volume, the sludge residence time in the secondary sedimentation tank is exactly equal to the time for the sludge to reach the maximum concentration through sedimentation. At this time, the return sludge concentration is the largest and the return volume is the smallest. This method is simple and easy to implement. While obtaining a high return sludge concentration, the sludge residence time in the secondary sedimentation tank is the shortest. This method is particularly suitable for denitrification and phosphorus removal processes.

4.Derivation of the calculation formula for SR

(1) Derivation of calculation formula based on sludge concentration

Ⅰ. Meaning of letters in the formula

Regarding the derivation of the calculation formula for sludge concentration, let us first determine the meaning of several letters in the formula, R (sludge return ratio; external return ratio), MLSS (mixed liquor sludge concentration), RSSS (return sludge concentration), Q (influent flow), QR (sludge return amount, note that Qr refers to the return amount of the mixed liquor, don't get it mixed up here!).

Ⅱ.Premise (assumption) conditions

Based on the role of sludge return, we assume that the amount of sludge loss is equal to the amount of sludge return, so we get the following formula:

                                         QR*RSSS=（Q+QR）*MLSS

Note: (Q+QR) is the actual water inflow, because most of the return sludge is water, so in the calculation of the water inflow, we will add up the return volume as the actual water inflow for sewage treatment!

Ⅲ.Derivation process

QR*RSSS=（Q+QR）*MLSS

↓

R*Q*RSSS=（1+R）Q*MLSS

↓

R*RSSS=（1+R）MLSS

↓

R*RSSS=MLSS+R*MLSS

↓

R*RSSS-R*MLSS=MLSS

↓

R（RSSS-MLSS）=MLSS

↓

R=MLSS/(RSSS-MLSS)

(2) Derivation of calculation formula based on sludge settling ratio

Ⅰ. Meaning of the letters in the formula

Regarding the derivation of the calculation formula of the sludge settling ratio, let's first determine the meaning of several letters in the formula, R (sludge return ratio; external return ratio), SV% (30-minute settling ratio of sewage mixture), SVR% (30-minute settling ratio of return sludge)

Ⅱ. Preconditions (assumptions)

There are actually two prerequisites for calculating using the sludge settling ratio. One is based on the concept of sludge settling ratio. The 30-minute sludge settles to the maximum compression limit, which is equivalent to SV being equal to the volume of sludge; the other is based on the role of sludge return. We assume that the volume of sludge loss is equal to the volume of sludge return, so we get the following formula:

QR*SVR%=（Q+QR）*SV%

Ⅲ. Derivation process

According to the concept of sedimentation ratio, the sludge return sedimentation ratio is the compression limit after 30 minutes. Theoretically, SVR=100%, so the formula is:

QR*100%=（Q+QR）SV%

↓

R*Q*100=（Q+R*Q）SV

↓

R*Q*100=（1+R）Q*SV

↓

R*100=（1+R）SV

↓

R*100=SV+R*SV

↓

R*100-R*SV=SV

↓

R(100-SV)=SV

↓

R=SV/(100-SV)

5.Control of reflux ratio under actual working conditions：

In actual process control operations, the positive operation and regulation of the return ratio is not very obvious, but it plays an important role in emergency regulation when the activated sludge system fails.

The normal control value of the activated sludge return ratio is 30%-70%, that is, the return flow rate of the activated sludge mixed liquor is between 30%-70% of the flow rate of sewage and wastewater in the aeration tank.

Therefore, it is necessary to control both high and low return ratios.

(1)Three situations in which the return ratio is controlled at a smaller value

Ⅰ. When the activated sludge has good settling compressibility in the secondary sedimentation tank, the return ratio can be lowered, because when the return ratio is lowered, the concentration of the returned activated sludge will increase, and the total amount that finally reaches the head of the aeration tank will remain basically unchanged.

Ⅱ. When the influent wastewater is in a high water volume state, the return ratio needs to be lowered to deal with it. The reason is that high-load influent sewage and wastewater usually show characteristics such as high concentration of organic pollutants and large water volume. The large water volume has a great impact on the activated sludge, which usually leads to poor sedimentation of the activated sludge in the secondary sedimentation tank. At the same time, increasing the return ratio of activated sludge under the condition of large influent sewage and wastewater will inevitably shorten the residence time of sewage and wastewater in the aeration tank. As a result, the time required for activated sludge to degrade excessive organic matter is shortened, the degradation effect is insufficient, and the activated sludge is not easy to enter the exhaustion period and has poor sedimentation.

Ⅲ. Controlling a smaller activated sludge return ratio is conducive to prolonging the static time of the activated sludge settled at the bottom of the secondary sedimentation tank. The final result is that the activated sludge will be in a very hungry state, and then it will return to the head of the aeration tank and will have an amazing ability to adsorb and degrade organic matter. This is particularly obvious when the activated sludge load is properly controlled. Whenever an excellent treatment effect is achieved, the activated sludge needs to be in the best adsorption and degradation state.

(2) Three situations where the return ratio is controlled at a larger value

Ⅰ. Controlling the return ratio at a larger value is conducive to inhibiting the occurrence of activated sludge aging under low load conditions. By accelerating the return of activated sludge in the secondary sedimentation tank to the head end of the aeration tank, the activated sludge can be prevented from staying in the secondary sedimentation tank for too long. If the residence time is too long, the activated sludge in the anoxic state is more likely to age.

Ⅱ. Controlling a high return ratio is conducive to increasing the activated sludge's ability to resist shock loads within a certain period of time. In particular, when a sudden surge in sludge load is found, it is necessary to adjust the return ratio to improve the activated sludge's ability to resist shock loads. The principle is that a significant increase in the return ratio will increase the activated sludge concentration at the head end of the aeration tank in a relatively short period of time, and respond to the impact of high loads in turn.

Ⅲ. Another reason for increasing the return ratio is when the aeration tank is subjected to conditions other than load shocks and the aeration tank mixed liquor needs to be diluted. If the pH value is too high or too low and affects the mixed liquid in the aeration tank, the reflow ratio can be increased to quickly dilute the mixed liquid in the aeration tank to reduce the impact of the pH change on the system.