Introduction to the Cyclic Activated Sludge System (CASS/CAST) Process
The CASS process is a comprehensive development based on the dynamic mode and principles of biological reactions, combined with reasonable hydraulic conditions as the main approach. It features simple operation, flexible application, reliable performance, 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 primary method of treatment. Moreover, it occupies fewer land resources, has lower operating costs, and a high degree of automation, making it a wastewater treatment technology suited to China's national conditions and worthy of promotion.
The Cyclic Activated Sludge System was developed by American experts on the basis of the ICEAS process. It is a new evolution of the SBR process and is one of the most valued and focused aspects of social development today. The cyclic activated sludge method is the most critical link in wastewater treatment work, and is an operating system and workflow improved and optimized on the basis of environmental protection and energy saving. In its composition, the basic structure is formed on the basis of the sequencing batch activated sludge process (SBR), with the reaction tank designed in segments along the tank length and divided into two parts. The front half is the biological selection zone, also called the pre-reaction zone, while the main reaction zone is in the rear half, equipped with a liftable automatic decanting device, enabling the whole process of aeration, sedimentation, and drainage to operate in a comprehensive cycle, eliminating the secondary sedimentation tank of the conventional activated sludge process and thereby achieving a sludge return system, a sustainable inflow and intermittent drainage operating system.
The CASS (Cyclic Activated Sludge System) tank is the reaction tank of the cyclic activated sludge system; it is also called CAST (Cyclic Activated Sludge Technology), i.e., the cyclic activated sludge process. It integrates aeration and sedimentation functions; its working process is that aeration, sedimentation, and drainage are carried out sequentially in the same tank in periodic cycles, eliminating the secondary sedimentation tank of the conventional activated sludge process, and enabling programmatic control with a high degree of automation and easy operation. This technology was first applied in the United States: the wastewater treatment plant in Prairie City, Minnesota, the Toledo wastewater treatment plant in Ohio, and the district wastewater treatment plant in Michigan all achieved good results, with CODCr removal rates up to 85 percent and BOD5 removal rates up to 95 percent, and good nitrogen and phosphorus removal effects. There are dozens of engineering examples of industrial wastewater and domestic sewage treatment in Shanghai, Kunming, Beijing, and other places in China. [1]
The CASS process is now extremely widely used in current urban development and has become a major wastewater treatment system and technical measure. This wastewater treatment technology has the following advantages and merits:
Flexible operation with strong shock-load resistance.
Analyzing from the reaction status of the process, it works by analyzing and treating from the degradation of pollutants. When wastewater passes through the reaction tank during treatment, it uses microorganisms as the basis for a mixed-liquid dilution mode, thereby spatially forming a comprehensive, systematic reaction phenomenon, a passive management workflow. However, due to its strong comprehensive process mode, complete management control factors, and high mechanism concentration, it still has certain advantages in management work.
Because the CASS process is carried out in the sedimentation tank, its reaction effect is mainly sedimentation-based. This method not only facilitates the smooth and continuous progress of the sedimentation stage, but is also a working mode that can operate normally under influencing factors without special treatment.
When the CASS process operates, it runs in a cyclic manner in a single tank, and its entire working link is within the reaction tank. Therefore, it avoids other infrastructure and land occupation in the work, avoids the application of the secondary sedimentation tank and sludge recirculation equipment, and has the advantages of compact layout, small footprint, and high investment efficiency.
In the design process, the CASS process mainly achieves comprehensive control by considering constraint methods, ensuring that flow variations can meet the current design requirements of wastewater treatment, and ensuring that the residence time of the wastewater system during treatment meets the preset residence range; the process can also adapt to changes in water volume and quality by adjusting the operating cycle.
China is a country with a huge population, yet it is also a country extremely short of freshwater resources. Coupled with the serious environmental pollution caused by insufficient environmental protection awareness in past economic development, this has caused serious water resource pollution problems and further aggravated our country's water shortage. Therefore, in the work, attention must be paid to the following aspects for induction and summary, so as to achieve a comprehensive and systematic treatment process and measures.
In wastewater treatment work, since all tasks are carried out in the same reaction tank, it is necessary to fully select a reasonable design process and flow. Restrictions on the inflow peak often exceed the upper limit, causing the inflow to fail to be fully and effectively utilized on the reaction tank; therefore, attention must be paid to water balance requirements in the work.
The widespread use of the CASS process is attributed to the rapid development and popularization of information technology and automation technology, enabling it to form a systematic routine process in application. The characteristics of the CASS process are attributed to a process- and task-based model, and work optimization is carried out according to the inflow quality and status to ensure the effluent quality effect.
The CASS process can select a variety of aeration methods, but the aeration head should preferably use a non-clogging aeration form, such as underwater aerators, spiral aerators, perforated pipes, and umbrella aerators. When using microporous aeration, high-strength rubber aeration discs should be selected; when aerating, the micropores open, and when aeration stops, the micropores close, so as not to easily cause micropore clogging. [2]
