Characteristics of activated sludge in three ditch oxidation ditch

The sludge age of the three ditch oxidation ditch is long, which is designed to be 26 days. The operation load is low, for example, the average sludge load of oxidation ditch in 1996 is 0.055 kgbod5 / (kgmlss · d). The hydraulic retention time is 14.6h, so most of the bacteria in the ditch are in the internal respiration period. When the operation is normal, a large number of microorganisms are mainly swimming or fixed ciliates and rotifers, while flagellates and other lower protozoa are rare.

Because there is no primary sedimentation tank in the plant, and there are many kinds of microorganisms in the urban sewage directly enter into the oxidation ditch, which can make the microorganisms in the activated sludge adapt to the raw water environment to the maximum extent while having the inoculation function. Therefore, the purification capacity and operation stability of the system are further improved, and the types and quantity of microorganisms are greatly increased. According to statistics, 56 species of microorganisms have been observed since the establishment of the plant, 12 of which are dominant species in daily life (see Table 2).

The above dominant species accounted for about 97% of the total, and the occurrence rate was more than 95%. In protozoa, ciliates are the dominant species, and swimming ciliates are the most common species, followed by sessile ciliates, followed by epicellus and rotifers.

Effect of mud age on oxidation ditch system

In the design, the stable sludge age of the three ditch oxidation ditch is 26 days, and the activated sludge involved in the process reaction accounts for 55% of the total sludge in the three ditches. In the actual operation, the age of sludge varies with the amount of sludge discharged. See Fig. 2 for the denitrification and COD removal of oxidation ditch system by mud age in different periods.

It can be seen from Figure 2 that the removal rate of NH3-N increases with the increase of sludge age, indicating that the longer the activated sludge age is, the less food will enter the oxidation ditch, and the system will operate under low load with complete nitrification. Cod did not change significantly in the range of mud age (18-95 days).

However, the TN removal rate does not always increase with the increase of sludge age. When the sludge age reaches a certain value (about 87 days), the TN removal rate begins to decline, because at this time, the activity of sludge decreases, and the denitrification efficiency decreases. In addition, the longer the sludge retention time is, the larger the total amount of sludge in the oxidation ditch will inevitably increase, so the oxygen demand of the mixed liquid of activated sludge will increase, and the energy consumption of water treatment will increase.

If the sludge age of oxidation ditch system is too short, because the generation cycle of nitrifying bacteria is about 10 days, when the sludge age is less than 10 days, the ditch is not suitable for the reproduction and survival of nitrifying bacteria, resulting in incomplete nitrification and reduced nitrogen removal. Therefore, it is very important for the normal operation of the oxidation ditch system to control the appropriate mud age.