Difference Between Hydrolysis Acidification Tank And Anaerobic Tank In Aao Process: Part II Anaerobic Tank In Aao Process

Anaerobic tank in AAO process

The above process shows the traditional A/A/O process flow. The first section is an anaerobic tank. The main function of this tank is to release phosphorus by phosphorus accumulating bacteria. Secondly, hydrolysis acidification reaction can also occur in this tank.

Phosphorus accumulating bacteria, also known as phosphorus uptake bacteria and Phosphorus Removal Bacteria, are a special kind of bacteria in traditional activated sludge process. They can absorb excessive phosphorus in sewage under aerobic conditions, making the phosphorus content in the body several times higher than that of ordinary bacteria. Such bacteria are widely used for biological phosphorus removal.

1. Phosphorus release under anaerobic conditions

In the absence of dissolved oxygen or nitrate nitrogen, facultative bacteria can transform soluble BOD5 into low molecular weight volatile organic acid VFA through fermentation. Phosphorus accumulating bacteria absorb these fermentation products or VFA from raw sewage, and transport them to cells to assimilate into intracellular carbon energy storage material PHB. The required capacity comes from the hydrolysis of phosphorus accumulating bacteria and the fermentation of intracellular sugar, which leads to the release of phosphate.

2. Phosphorus uptake under aerobic conditions

Under aerobic conditions, the activity of phosphorus accumulating bacteria is restored, and more phosphorus than required for growth is stored in the form of phosphorus accumulation. Energy is generated through the oxidative metabolism of PHB for phosphorus absorption and phosphorus accumulation synthesis. Energy is trapped and stored in the form of high energy polyphosphate bonds, and phosphate is removed from water.

3. Discharge of phosphorus rich sludge

The generated phosphorus rich sludge is discharged in the form of surplus sludge, so as to remove phosphorus. From the perspective of energy, phosphorus accumulating bacteria release phosphorus under anaerobic conditions to obtain energy to absorb dissolved organics in wastewater, and degrade and absorb dissolved organics under aerobic conditions to obtain energy to absorb phosphorus.

After the raw water is mixed with the phosphorus containing sludge returned from the secondary sedimentation tank at the same time, under the action of facultative anaerobic fermentation bacteria, part of the macromolecular organics that are easy to biodegrade are converted into small molecules of volatile fatty acids (VFA). The phosphorus accumulating bacteria hydrolyze the phosphorus in the cells into orthophosphate, which is released into water. The released energy can be used for the transformation of aerobic phosphorus accumulating bacteria to survive in the anaerobic repression environment, At the same time, the small molecule organics after hydrolysis are absorbed to synthesize PHB and stored in the body.

The key to phosphorus removal is the setting of anaerobic zone. Under short-term anaerobic conditions, phosphorus accumulating bacteria can absorb low molecular substrate and rapidly assimilate and store these fermentation products, which means that anaerobic zone provides a competitive advantage for phosphorus accumulating bacteria.

In this way, phosphorus accumulating bacteria that can absorb large amounts of phosphorus can be selectively multiplied in the treatment system, and phosphorus can be removed by removing excess sludge with high phosphorus content. Another advantage of this selective proliferation is that it inhibits the proliferation of filamentous bacteria and avoids the possibility of producing sludge with poor sedimentation performance. Therefore, anaerobic/aerobic biological phosphorus removal processes generally do not produce sludge bulking.

Main factors affecting anaerobic tank:

1. Temperature

2. PH value

3. Dissolved oxygen

4. Nitrate nitrogen in anaerobic tank

5. Mud age

6. COD/TP

7. HRT