Cyclic Activated Sludge System (CASS): Process Principles and Applications
The CASS process is a comprehensively developed technology based on biologically driven principles and rational hydraulic conditions, forming a simple-to-operate and flexible treatment mode; flexible operation, good reliability, and a wide application scope are its main characteristics and advantages. It is widely applied in various construction stages and is a major treatment method; based on its application, 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 conditions and worthy of promotion.
The Cyclic Activated Sludge System (CASS) was developed by U.S. experts on the basis of the ICEAS process; it is a new technology evolved from the SBR process and is currently the most valued and watched link in social development. Cyclic activated sludge is the most critical link in wastewater treatment and is a work system and flow improved and optimized on the basis of environmental protection and energy saving. In its composition, its basic structure is formed on the basis of the sequencing batch reactor activated sludge process, with the reaction tank designed in sections along its length and divided 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, on which a certain liftable automatic decanting device is installed, so that the whole process of aeration, sedimentation, and drainage operates in an integrated cycle, eliminating the secondary sedimentation tank of the conventional activated sludge process and thereby achieving a sludge-return system - a work system with continuous influent and intermittent effluent.
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 - aeration, sedimentation, and drainage - proceeds sequentially in the same tank in periodic cycles, eliminating the secondary sedimentation tank of the conventional activated sludge process, enabling programmed control, a high degree of automation, and convenient operation. This technology was first applied in the United States; applications at the Prairie City 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 of 85%, BOD5 removal of 95%, and good nitrogen and phosphorus removal. In China there are dozens of engineering cases for industrial and domestic wastewater treatment in Shanghai, Kunming, Beijing, and other places [1].
Because the CASS process is carried out in the sedimentation tank, its reaction is mainly sedimentation-based; this method not only facilitates the smooth continuation of the sedimentation stage but also operates normally without special treatment for the influencing factors encountered in application.
The CASS process operates cyclically in a single tank, and its entire working stages are within the reaction tank, thus avoiding other infrastructure and land space in operation, avoiding the use of the secondary sedimentation tank and sludge recirculation equipment, and offering the advantages of compact layout, small footprint, and high investment efficiency.
Analyzing the reaction status of the process, its work is analyzed and treated from the perspective of pollutant degradation. When the sewage passes through the reaction tank in treatment, a mixed-liquor dilution mode based on microorganisms is formed, creating a comprehensive and systematic reaction phenomenon in space - a passive-management workflow; however, due to its strong integrated process mode, complete management and control factors, and high mechanism concentration, it still has certain advantages in management.
The CASS process is especially 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 in application:
Flexible operation and strong shock-load resistance.
In the design of the CASS process, comprehensive control is achieved mainly by considering the constraints, ensuring that flow variation can meet the current wastewater treatment design requirements, and ensuring during treatment that the wastewater system's retention time falls within the preset retention range; the process can adapt to changes in water quantity and quality by adjusting the operating cycle.
China is a populous country but one extremely short of fresh water resources; coupled with serious environmental pollution caused by insufficient environmental awareness in past economic development, this has led to severe water-resource pollution and further aggravated China's water shortage. Therefore, in practice, attention should be paid to summarizing the following aspects to achieve a comprehensive and systematic treatment process and measures.
In wastewater treatment, because all work is carried out in the same reaction tank, it is necessary to select reasonable design flow and capacity; the limit of influent peak loading often exceeds the upper bound, causing the influent volume to be insufficiently and effectively utilized in the reaction tank, so in practice we must pay attention to the water-balance requirement.
The widespread use of the CASS process is attributed to the rapid development and popularization of information and automation technologies, enabling a systematic routine flow in application. The characteristics of the CASS process stem from a process-unit-based mode, and its operation is optimized according to influent quality and status to ensure effluent quality.
The CASS process can adopt various aeration modes, but the diffusers should preferably be non-clogging types such as submerged aerators, spiral aerators, perforated pipes, and umbrella aerators. When microporous aeration is used, high-strength rubber diffuser 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].
