The operation of the activated sludge process requires the reasonable regulation of numerous 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 systems.
1. Definition of MLSS for sludge concentration
Activated sludge concentration refers to the content of suspended solids in the mixed liquid at the outlet of the aeration tank, represented by the symbol MLSS, with a unit of mg/L. It is used to measure the amount of activated sludge in the aeration tank. The total amount of MLSS includes the following four aspects:
Active microorganisms;
Organic matter adsorbed on activated sludge that cannot be biodegraded;
Residues from microbial self oxidation;
Inorganic substances.
During the operation, it is important to note that MLSS only refers to the concentration of the mixed liquid in the aeration tank, without considering the concentration of the mixed liquid in the secondary sedimentation tank. Meanwhile, when monitoring the concentration of mixed liquid in the aeration tank, it is important to note that the concentration of activated sludge in the entire aeration tank is measured based on the concentration of mixed liquid at the outlet of the aeration tank.
2. Determination of sludge concentration (MLSS)
Regardless of the design calculation method used, it is necessary to determine the MLSS reasonably. Under other unchanged conditions, Doubling the MLSS results in a doubling of the aeration tank capacity, Reducing MLSS by twice will double the capacity of the aeration tank, directly affecting infrastructure investment, so it needs to be carefully determined.
In the specifications and manuals, a recommended selection range for MLSS values is recommended, such as 1.5-2.5kg/m ³ for regular aeration and 2.5-5.0kg/m ³ for delayed aeration, both of which have significant variations and are difficult to operate during design. In order to select the appropriate MLSS value, it is necessary to understand the factors that affect it.
MLSS cannot be selected too low for three main reasons:
(1) If the MLSS is too low, the volume V of the aeration tank will correspondingly increase, which is economically unfavorable.
(2) The MLSS is too low, and foam is easily generated in the aeration tank. In order to prevent foam, it is generally necessary to maintain a sludge concentration of more than 2kg/m ³.
(3) When the sludge concentration is very low, less oxygen is required. For example, if the MLSS is too low and the tank capacity increases, the gas supply per unit tank capacity will be very small, which may not meet the mixing requirements in the tank and will inevitably require additional stirring power.
MLSS cannot be selected too high, mainly because:
(1) To improve MLSS, it is necessary to correspondingly increase the sludge reflux ratio, reduce the surface load of the secondary sedimentation tank, and extend the residence time of the secondary sedimentation tank. This requires increasing the volume of the secondary sedimentation tank and the energy consumption of the reflux sludge. Considering the aeration tank, secondary sedimentation tank, and reflux sludge pump room as a whole, in order to minimize the total cost and operating costs, the sludge reflux ratio is usually limited to within 150%. For general urban sewage, the reflux sludge concentration in the secondary sedimentation tank is usually 4-8kg/m3. If calculated based on the highest value of 8kg/m3, the MLSS in the aeration tank is 4.8 kg/m3 when the reflux ratio is 150%. In actual design, the maximum MLSS generally does not exceed 4.5kg/m3.
(2) The nature of sewage and the operating conditions of the aeration tank have a significant impact on MLSS. If the composition of sewage or the operating conditions of the aeration tank are conducive to sludge expansion, and the sludge index SV1 value remains high (such as SV1>180mL/g), the concentration of reflux sludge will be greatly reduced, MLSS must select a low value.
The German ATV standard specifies the selection range for MLSS, and the MLSS values are the same for both nitrification and non nitrification, which is not entirely in line with the specific situation in China. The concentration of pollutants in urban sewage in China is usually low. If the MLSS is too high in the absence of nitrification (short sludge age), it may have a short residence time, which is not conducive to biochemical treatment. Therefore, the MLSS value in the absence of nitrification is reduced by 0.5 kg/m3. The recommended MLSS values are listed in the table below.
Based on the above analysis, various aspects should be taken into consideration when selecting MLSS:
(1) Choose a higher value for long sludge age and low sludge load, a lower value for short sludge age and high sludge load, and a higher value for synchronous aerobic stabilization of sludge.
(2) Choose a lower value for the initial sedimentation tank and a higher value for the non initial sedimentation tank.
(3) When SV1 is low, select a higher value, and when it is high, select a lower value.
(4) When the sewage concentration is high, choose a higher value, and when it is low, choose a lower value.
(5) Joint construction of reaction tanks (such as SBR) does not have sludge reflux problems, choose a higher value or a higher value.
(6) Calculate whether the mixing power meets the requirements, and if it does not, make adjustments accordingly.
