Difference between CASS and CAST processes in wastewater treatment technology

Currently, in the field of wastewater treatment engineering design in China, the abbreviations for the cyclic activated sludge process are often used interchangeably without distinction, CASS and CAST are often used interchangeably. Their specific process designs sometimes overlap and sometimes differ, leading to misunderstandings. Although both processes belong to the category of cyclic activated sludge processes, there are differences in specifics, mainly concerning whether the influent is continuous, and whether there is influent during decantation.

I. Difference in Continuous Influent

CAST equals Cyclic Activated System Technology Intermittent influent cyclic activated sludge technology, CASS=Cyclic Activated Sludge System Continuous influent cyclic aeration activated sludge system, CASS has continuous influent and intermittent effluent (actual wastewater discharge is mostly continuous or semi-continuous); CAST has intermittent influent and intermittent effluent. However, both indeed belong to cyclic activated sludge. But, generally, CASS the process should be a combination of multiple CASS basins operating , in alternation at different times to achieve continuous influent and effluent. For a single CASS basin, the influent is intermittent.

CASS process retains the ICEAS process advantages, both having continuous influent and intermittent effluent. Because CASS the process still has influent during the sedimentation stage, the sedimentation process can only be a non-ideal semi-static sedimentation, and the solid-liquid separation effect is not very stable. CAST the process does not have influent during the sedimentation stage, the sludge is not hydraulically disturbed during sedimentation, resulting in ideal sedimentation, more stable solid-liquid separation, and more flexible operation. This is CAST the CASS biggest difference.

CAST The reaction basin is ideally a plug flow reactor in terms of time, with a high organic matter removal rate. However, due to continuous influent, CASS partially loses the classic SBR process advantages of ideal plug flow, and also loses the characteristics of high removal rate and removal of refractory substances. From the perspective of currently operating projects, intermittent influent, i.e., the selection of CAST process, is more common. In short, when discussing cyclic activated sludge processes, in addition to distinguishing the specific influent — reaction — sedimentation — effluent operating cycles, the differences in English abbreviations should also be noted.

II. Differences in Composition

CASS is a single basin with two zones. CASS is based on SBR the reaction basin is designed in two parts along the length of the basin. The front part is the biological selection zone, also known as the pre-reaction zone. In the pre-reaction zone, microorganisms can rapidly adsorb most of the soluble organic matter in the wastewater through the rapid transfer mechanism of enzymes, undergoing a high-load rapid substrate accumulation process. This has a good buffering effect on the influent water quality, water quantity, PH and toxic and harmful substances, and inhibits the growth of filamentous bacteria, effectively preventing sludge bulking; then, it undergoes a lower-load substrate degradation process in the main reaction zone. The rear part is the main reaction zone, with a liftable automatic skimming device installed at the rear of the main reaction zone. The aeration, sedimentation, and drainage processes of the entire process are cyclically operated in the same basin.

CAST The entire process is completed in one reactor. The process follows “ influent — effluent ”、 “ aeration — non-aeration ” sequence to carry out the biodegradation and solid-liquid separation of organic pollutants. The reactor is divided into three zones: biological selection zone, anoxic zone, and main reaction zone. The biological selection zone operates under anaerobic and anoxic conditions, allowing the wastewater to contact the return sludge, making full use of the rapid adsorption of activated sludge to accelerate the removal of soluble substrates, and playing an acid hydrolysis role on refractory organic matter. At the same time, excess phosphorus absorbed by the sludge can be effectively released under anaerobic conditions. The anoxic zone mainly removes organic matter through the adsorption of regenerated sludge, while promoting further phosphorus release and enhancing nitrogen nitrification / denitrification, and aeration and idling can also restore sludge activity.

III. Differences in Process

1、 CASS The operating cycle is generally divided into four steps:

Aeration stage

        Oxygen is supplied to the reaction basin by the aeration device. At this time, organic pollutants are oxidized and decomposed by microorganisms, and the NH3-N in the wastewater is converted into NO3--N。

Sedimentation stage

        At this time, aeration is stopped, and microorganisms use the remaining DO for oxidation and decomposition. The reaction basin gradually transforms from aerobic to anoxic state, and denitrification reaction begins. Activated sludge gradually settles to the bottom of the basin, and the upper layer of water becomes clear.

Decantation stage

        After sedimentation, the decanter at the end of the reaction tank begins to operate, gradually discharging the supernatant liquid from top to bottom. At this time, the reaction tank gradually transitions to an anaerobic state to continue denitrification.

Idle phase

        The idle phase is the stage where the decanter rises to its original position.

2、 CAST The operating cycle is generally divided into five steps:

Inflow phase

CAST The influent first mixes with the sludge from the previous cycle's sedimentation phase in the biological selection zone. A large amount of influent forms a large substrate concentration gradient in this phase, allowing the permeases to quickly utilize the BOD in the influent under high sludge concentration conditions, forming a good anoxic anaerobic environment. Effective biological nitrogen removal and phosphorus removal are achieved by adjusting the reaction mode (influent time, influent volume, anoxic anaerobic reaction time) of the inflow phase. BOD is quickly utilized under high sludge concentration conditions, forming a good anoxic / anaerobic environment. Effective biological nitrogen removal and phosphorus removal are achieved by adjusting the reaction mode (influent time, influent volume, anoxic / anaerobic reaction time) of the inflow phase.

Aeration phase

        The wastewater from the inflow phase undergoes sufficient aerobic carbon removal and biological nitrification under sufficient aeration conditions.

Sedimentation phase

        No inflow, no aeration, no reflux allows the wastewater mixed liquor to obtain a quiescent flocculation sedimentation environment.

Decantation phase

        No inflow, no aeration, no reflux. The supernatant liquid is discharged through a floating decanter, and drainage stops when the liquid level drops to the lowest control water level.

Idle phase

        Inflow, no aeration, no reflux, depending on the specific operation. CAST System adjustment, which can serve as the entire CAST system adjustment