Wastewater Treatment Processes Explained: the CASS Cyclic Activated Sludge Process
The CASS process is a dynamically driven, biologically based system developed through integrated design. Built around sound hydraulic conditions, it offers a simple, flexible and adaptable mode of treatment with reliable, wide-ranging performance — these are the defining features and advantages of CASS. It is applied extensively across every phase of construction, serves as a primary treatment method, occupies little land, carries low operating costs, and achieves a high degree of automatic control. It is a wastewater treatment technology that suits China's conditions well and is well worth promoting.
The Cyclic Activated Sludge System was developed by American experts building on the ICEAS process; it is an evolution of the SBR (sequencing batch reactor) process and one of the most closely watched developments in the field today. As the most critical step in wastewater treatment, it is an operating system and workflow improved and optimised on the basis of environmental protection and energy efficiency. Its basic structure derives from the sequencing batch reactor: the reaction tank is divided lengthwise into two zones. The front section is a biological selector, also called the pre-reaction zone, while the main reaction zone occupies the rear section and is fitted with a liftable automatic decanting device. This lets aeration, settling and drainage run in an integrated cycle, eliminating the secondary clarifier required by conventional activated-sludge processes and doing away with the sludge-return system — a continuously fed, intermittently drained operating system.
The CASS (Cyclic Activated Sludge System) tank is the reactor of the cyclic activated-sludge system; it is also known as CAST (Cyclic Activated Sludge Technology), i.e. the cyclic activated sludge process. It combines aeration and settling in one unit: aeration, settling and drainage proceed in sequence within the same tank, cycling period by period and removing the need for the secondary clarifier of conventional activated-sludge processes. It supports programmed control, a high level of automation, and easy operation. The technology was first applied in the United States; plants in Prairie, Minnesota, Toledo, Ohio and a district plant in Michigan all achieved good results, with CODCr removal around 85%, BOD5 removal around 95%, and effective nitrogen and phosphorus removal. In China, dozens of industrial-wastewater and domestic-sewage treatment projects in Shanghai, Kunming, Beijing and elsewhere have adopted it [1].
In design, the CASS process is controlled comprehensively by taking constraints into account, ensuring that flow variations still meet the design requirements of the wastewater treatment. During operation it keeps the hydraulic retention time within the preset range, and the cycle can be adjusted to accommodate changes in both flow and water quality.
In operation the CASS process cycles within a single tank; the entire sequence takes place in the reaction tank, so it avoids additional infrastructure and land footprint, dispenses with the secondary clarifier and sludge-return equipment, and offers the advantages of a compact layout, small footprint and high return on investment.
Looking at the reaction as it actually occurs, the process works through pollutant degradation. As wastewater passes through the reaction tank it is diluted by the mixed liquor on a microbial basis, forming a spatially integrated, systematic reaction — a passively managed workflow. Yet because the integrated process model is robust, the management and control factors are well developed, and the mixed-liquor concentration is high, the process still holds certain advantages in operation.
Because CASS settles within the tank and its reaction is dominated by settling, this approach not only keeps the settling phase running smoothly but also tolerates influencing factors that, without special treatment, would otherwise disrupt normal operation.
Flexible operation and strong shock-load resistance.
The CASS process is now used especially widely in urbanisation and has become a major wastewater treatment system and technical measure. The technology offers the following advantages:
China has a huge population yet is a country extremely short of freshwater resources. Combined with heavy environmental pollution caused by weak environmental awareness in past economic growth, this has made water-resource pollution severe and further aggravated the country's water shortage. We must therefore pay attention to the following points, drawing them together and summarising them, so as to achieve a comprehensive, systematic treatment flow and measures.
In wastewater treatment all operations take place within the same reaction tank, so a rational design flow and flow rate must be selected. Limits on peak inflow often exceed the upper bound, leaving the inflow unable to be used fully and effectively in the reaction tank; attention must therefore be paid to water-balance requirements.
The widespread use of the CASS process is thanks to the rapid development and popularization of information and automation technologies, which let it form a systematic routine workflow. The characteristics of CASS stem from a process-step-based model, with operation optimised according to influent quality and condition to guarantee effluent quality.
The CASS process can use various aeration methods, but non-clogging forms should be preferred for the diffusers — submerged aerators, spiral aerators, perforated pipe and umbrella-type aerators, for example. When fine-bubble aeration is used, high-strength rubber diffuser discs should be chosen; the micropores open during aeration and close when it stops, which prevents them from clogging [2].
