How to Determine Sludge Concentration (MLSS)

The operation of activated sludge process needs many reasonable control parameters, including the control of activated sludge concentration (MLSS), which is one of the most commonly used indicators in the daily operation of sewage system.

1. Definition of Sludge Concentration (MLSS)

Activated sludge concentration refers to the content of suspended solids in the mixed liquid at the outlet of the aeration tank, which is expressed by the symbol MLSS. Its unit is mg/L, and it is used to measure the quantity of activated sludge in the aeration tank. The total amount of MLSS includes the following four aspects:

(1) Active microorganisms.

(2) Organic matter which can not be biodegraded by adsorbed on activated sludge.

(3) Residues of microbial self oxidation.

(4) Inorganic matter.

During the operation, it should be noted that MLSS only refers to the concentration of the mixture in the aeration tank without considering the concentration of the mixture in the secondary sedimentation tank. At the same time, when monitoring the concentration of the mixture in the aeration tank, it should be noted that the concentration of the activated sludge in the whole aeration tank is measured by the concentration of the mixture at the outlet end of the aeration tank.

2. Determination of Sludge Concentration (MLSS)

No matter which design calculation method is adopted, it is necessary to determine MLSS reasonably. When the other conditions are not changed, if MLSS is doubled, the capacity of aeration tank will be halved. If MLSS is halved, the capacity of aeration tank will be doubled. It will directly affect the infrastructure investment, so it needs to be determined carefully.

In the specification and manual, a selection range of MLSS value is recommended, such as 1.5-2.5kg/m³ for general aeration and 2.5-5.0kg/m³ for delayed aeration. The range of variation is relatively large, so it is difficult to determine. In order to select the appropriate MLSS value, it is necessary to find out the influencing factors.

MLSS can not be selected too low for three main reasons:

(1) If MLSS is too low, the volume of aeration will increase correspondingly, which is unfavorable in economy.

(2) If MLSS is too low, foam will be generated in the aeration tank. In order to prevent foam, the sludge concentration should be above 2kg/m³.

(3) When the sludge concentration is very low, the oxygen demand is less. If MLSS is too low, the volume of the aeration tank will be increased, thus the air supply will not be enough to meet the mixing requirement of the aeration tank.

MLSS can not be selected too high either, mainly because:

(1) In order to improve MLSS, it is necessary to increase the sludge reflux ratio, reduce the surface load of the secondary sedimentation tank and prolong the retention time of the secondary sedimentation tank, which requires increasing the volume of the secondary sedimentation tank and the energy consumption of the returned sludge. Considering the aeration tank, secondary sedimentation tank and return sludge pump room as a whole, in order to minimize the total cost and operating cost, the sludge return ratio is usually limited within 150%. For general municipal sewage, the return sludge concentration of the secondary sedimentation tank is usually 4-8kg/m³. If the maximum value is 8kg/m³, the MLSS in the aeration tank is 4.8kg/m³. In the actual design, the maximum MLL is not more than 4.5kg/m³.

(2) The nature of sewage and the operation of the aeration tank have a great influence on MLSS. If the composition of the sewage or the operation condition of the aeration tank is conductive to sludge bulking, and the sludge indicator SV1 stays high (e.g. SV1>180ml/g), the concentration of the returned sludge will be greatly reduced, so the MLSS must be a low value.

German ATV standard specifies the selection range of MLSS. When there is nitrification and no nitrification, the MLSS value is the same, which is not completely consistent with the specific situation of China. The concentration of pollution in municipal sewage in China is usually low. If the MLSS is too high without nitrification (sludge age is short), the residence time may be too short, which is not conducive to biochemical treatment. Therefore, the MLSS value without nitrification is reduced by 0.5kg/m³.