Detailed Explanation of Wastewater-Treatment Processes: the Cyclic Activated Sludge Process (CASS)
The CASS process is a comprehensive development based on the dynamic mode and principle of biological reactions, combining reasonable hydraulic conditions as the main approach. It features simple work, flexible application, flexible operation, good reliability, and a wide scope of application-these are the main characteristics and advantages of the CASS process. It is widely applied in various construction stages and is a main treatment method; moreover, its application requires less land and lower operating costs and a high degree of automation, making it a sewage-treatment technology suited to China's national conditions and worthy of promotion.
The cyclic activated sludge process was developed by American experts on the basis of the ICEAS process. It is a new technology evolved from the SBR process and is also the most valued and focused link in current social development. The cyclic activated sludge process is the most critical link in sewage-treatment work, and is an improved and optimized working system and workflow based on environmental protection and energy savings. In its composition, its basic structure is formed on the basis of the sequencing batch activated sludge process, designed with the reaction tank segmented along its length into two parts. The front part is the biological selection zone, also called the pre-reaction zone, and the main reaction zone is in the rear part, equipped with a certain liftable automatic decanting device, enabling the whole process of aeration, sedimentation, and drainage to run in a comprehensive cycle, eliminating the secondary sedimentation tank of the conventional activated-sludge process and thus achieving a sludge-return system. It is a continuously-fed, intermittently-discharged working system.
The CASS (Cyclic Activated Sludge System) tank is the cyclic activated sludge system reaction tank, also called CAST (Cyclic Activated Sludge Technology), i.e., the cyclic activated sludge process. It integrates aeration and sedimentation functions; its working process is aeration, sedimentation, and drainage performed sequentially in the same tank in a periodic cycle, eliminating the secondary sedimentation tank of the conventional activated-sludge process and enabling programmatic control with a high degree of automation and convenient operation. This technology was first applied in the United States; the applications at the Prairie wastewater-treatment plant in Minnesota, the Toledo wastewater-treatment plant in Ohio, and the district wastewater-treatment plant in Michigan all achieved good results, with CODCr removal up to 85% and BOD5 removal up to 95%, and good nitrogen and phosphorus removal. There are dozens of engineering examples in China for industrial wastewater and domestic sewage treatment in Shanghai, Kunming, Beijing, etc. [1]
The CASS process is currently used especially widely in urban development and has become a main sewage-treatment system and technical measure. This sewage-treatment technology has the following advantages:
China is a populous country but also one with an extreme shortage of freshwater resources; coupled with insufficient environmental-protection awareness in past economic development causing serious environmental pollution, this has caused serious water-resource pollution problems and further aggravated China's water shortage. Therefore, in the work, attention must be paid to the following aspects for induction and summary, so as to realize a comprehensive and systematic treatment flow and measures.
Analyzing from the reaction status of the process, it analyzes and treats from the degradation of pollutants. When sewage passes through the reaction tank in the treatment work, it forms a comprehensive and systematic reaction phenomenon spatially based on microorganisms in a mixed-liquid dilution mode-a passive-management workflow. But because of its strong comprehensive process mode, complete management-control factors, and high mechanism concentration, it still has certain advantages in management work.
The CASS process is carried out in the sedimentation tank, and its reaction role is mainly sedimentation. This method not only facilitates the smooth and continuous progress of the sedimentation stage, but also allows the process to run normally under influencing factors that need no special treatment during application.
Flexible operation, strong shock-load resistance.
The CASS process in design mainly achieves comprehensive control by considering constraint modes, ensuring that flow variations can meet current sewage-treatment design requirements, and ensuring during treatment that the sewage-system residence time and the preset residence range are maintained, and the process can adapt to water-quantity and water-quality changes by adjusting the operating cycle.
The CASS process operates in a cyclic manner in one tank, and its entire working link is in the reaction tank, thus avoiding other infrastructure and footprint in the work, avoiding the application of the secondary sedimentation tank and sludge-return equipment, and featuring a compact layout, small footprint, and high investment efficiency.
In sewage treatment work, because all work is carried out in the same reaction tank, it is necessary to fully select a reasonable design flow and flow rate; the limitation of influent peak often exceeds the upper limit, causing the influent volume to fail to be fully and effectively utilized on the reaction tank, so in the work we must pay attention to water-balance requirements.
The widespread use of the CASS process is attributed to the rapid development and popularization of information and automation technologies, enabling the formation of a systematic routine flow in application. The characteristics of the CASS process are attributed to the basic mode of process-based fabrication and optimization according to influent quality and status, to ensure effluent quality.
The CASS process can choose a variety of aeration modes, but the aerator head should preferably use a non-clogging aeration form, such as submersible aerators, spiral aerators, perforated pipes, umbrella aerators, etc. When microporous aeration is used, high-strength rubber aeration discs should be selected, and when aerating, the micropores open, and when aeration stops, the micropores close, so as not to easily cause micropore clogging [2].
