At present, the abbreviation of cyclic activated sludge process is often not distinguished in the field of domestic sewage treatment engineering design. CASS and CAST are often mixed, and their specific process designs are sometimes the same and sometimes different, which has caused misunderstandings in people's understanding. In fact, although both of these processes belong to the category of cyclic activated sludge process, there are differences in specific details, mainly focusing on whether continuous water inflow and whether water inflow occurs during decanting. 1、 The difference between continuous water intake and intermittent discharge (actual sewage discharge is continuous or semi continuous); CAST is intermittent water intake and intermittent drainage. But they do belong to the category of cyclic activated sludge. However, the general CASS process should involve multiple sets of CASS tanks operating in combination, with alternating inflow and outflow of water at different times to achieve continuous inflow and outflow. For a single CASS pool, it is intermittently filled with water. The CASS process retains the advantages of the ICEAS process, which involves continuous water inflow and intermittent drainage. Due to the fact that the CASS process still receives water during the sedimentation stage, its sedimentation process can only be a semi static sedimentation in a non ideal state, and the separation effect of sludge and water is not very stable. The CAST process does not introduce water during the sedimentation stage, and the sludge does not experience hydraulic interference during the sedimentation process, making it an ideal sedimentation process. The sludge water separation effect is more stable and the operation is more flexible. This is the biggest difference between CAST and CASS. The CAST reaction tank has ideal flow rate in terms of time and high organic matter removal rate. However, due to continuous water inflow, CASS partially loses the ideal push flow advantage of the classical SBR process, as well as the characteristics of high removal rate and removal of difficult to degrade substances. From the actual running projects now, it can be seen that most of them use intermittent water intake, that is, CAST technology is more commonly used. In summary, when discussing the cyclic activated sludge process, in addition to distinguishing its specific operating cycle of inflow reaction sedimentation drainage, attention should also be paid to the differences in English abbreviations.
2、 The difference in composition between the two is that CASS has one pool and two zones. CASS is based on SBR, and the reaction tank is designed in two parts along the length direction of the tank. The front part is the biological selection zone, also known as the pre reaction zone. In the pre reaction zone, microorganisms can quickly adsorb most of the soluble organic matter in the sewage through the rapid transfer mechanism of enzymes, and undergo a high load substrate rapid accumulation process. This has a good buffering effect on the quality, quantity, pH, and toxic and harmful substances of the influent water, and also inhibits the growth of filamentous bacteria, effectively preventing sludge expansion; Subsequently, it undergoes a lower load substrate degradation process in the main reaction zone. The rear of the main reaction zone is equipped with an adjustable automatic skimming device. The aeration, sedimentation, and drainage processes of the entire process are cyclically operated in the same tank. The entire CAST process is completed in one reactor, and the process involves the biodegradation of organic pollutants and the separation of sludge and water in the order of "inflow outflow" and "aeration non aeration". The reactor is divided into three zones, namely the biological selection zone, the facultative zone, and the main reaction zone. The biological selection zone operates under anaerobic and facultative conditions, allowing the wastewater to come into contact with the returning sludge. It fully utilizes the rapid adsorption of activated sludge to accelerate the removal of soluble substrates and acidify and hydrolyze recalcitrant organic matter. At the same time, it can effectively release excess phosphorus absorbed in the sludge under anaerobic conditions. The facultative zone mainly removes organic matter through the adsorption of regenerated sludge, while promoting further release of phosphorus and enhancing nitrification/denitrification of nitrogen. Through aeration and idle, sludge activity can also be restored.
3、 Differences in Process
1. The CASS operation cycle can generally be divided into four steps:
During the aeration stage, oxygen is introduced into the reaction tank by the aeration device. At this time, organic pollutants are oxidized and decomposed by microorganisms, while NH3-N in the wastewater is converted into NO3-- N through microbial nitrification.
During the sedimentation stage, aeration is stopped and microorganisms utilize the remaining DO in the water for oxidative decomposition. The reaction tank gradually transitions from an aerobic state to an anoxic state and begins denitrification reaction. The activated sludge gradually settles to the bottom of the tank, and the upper water becomes clear.
After the settling stage is completed, the decanter placed at the end of the reaction tank begins to work, gradually discharging the supernatant from top to bottom. At this point, the reaction tank gradually transitions to an anaerobic state to continue denitrification.
The idle stage is the stage when the decanter rises to its original position.
2. The CAST operation cycle can generally be divided into five steps:
The inlet section CAST first mixes with sludge from the previous sedimentation stage in the biological selection zone. A large amount of incoming water forms a large substrate concentration gradient in this section, and the BOD in the incoming water is quickly utilized under high concentration sludge conditions through osmotic enzymes, forming a good anaerobic/anoxic environment. Effective biological denitrification and phosphorus removal can be achieved by adjusting the reaction mode of the inlet section (inlet time, inlet volume, anoxic/anaerobic reaction time).
The sewage in the inlet section of the aeration section undergoes sufficient aerobic decarbonization and biological nitrification under sufficient aeration conditions.
The sedimentation section does not allow water, aeration, or backflow to create a static coagulation and sedimentation environment for the sewage mixture.
The decanting section does not allow water to enter, aerate, or reflux. The supernatant is discharged through a floating decanter, and the drainage stops when the liquid level drops to the lowest control water level.
The idle section is not aerated or refluxed, and the adjustment of the CAST operation system depends on the specific operating conditions. It can be used as the adjustment of the entire CAST operation system.
