Operating principle of CASS process

CASS process is a sequencing batch activated sludge process (SBR) reaction tank is divided into two parts along the length direction, the front part is the biological selection zone, also known as the pre-reaction zone, the back part is the main reaction zone. A lifting decanting device was installed at the rear of the main reaction zone to realize the periodic operation of continuous intake and intermittent drainage, integrating aeration precipitation and drainage. CASS process is an alternate anaerobic/anoxic/aerobic process, which has a certain effect of nitrogen and phosphorus removal. Wastewater is run in the way of push flow, and each reaction zone is run in the form of complete mixing to realize simultaneous nitrifying denitrification and biological phosphorus removal.

The process flow

For general municipal wastewater, the CASS process does not require a very high degree of pretreatment, only the installation of coarse grid, fine grid and sand settling tank, no primary settling tank and secondary settling tank, also does not require a large sludge return system (only in the CASS reactor about 20% of the sludge return).

The operation process of CASS process consists of four stages: water filling - aeration, precipitation, decanting and idling. The specific operation process is as follows:

(1) Water filling - aeration stage

At the same time, the sludge in the main reaction zone is reflux to the biological selection zone, with a general reflux ratio of 20%. At this stage, the aeration system supplies oxygen to the reaction tank, which on the one hand meets the oxygen needs of aerobic microorganisms, and on the other hand is conducive to the full mixing and contact of activated sludge and organic matter, thus facilitating the organic pollutants to be oxidized and decomposed by microorganisms. At the same time, the ammonia nitrogen in the wastewater is transformed into nitrate nitrogen through the nitrification of microorganisms.

(2) Sedimentation stage

The aeration is stopped and the microorganisms continue to use the remaining dissolved oxygen in the water for oxidative decomposition. With the further decrease of dissolved oxygen in the reaction tank, the microorganisms change from aerobic state to anoxic state, and denitrification occurs to a certain extent. At the same time, the activated sludge is sedimentation and separation under almost static conditions. The activated sludge sinks to the bottom of the tank and continues to play its role in the next cycle. The treated water level is at the top of the sludge layer and the sludge and water are separated by static sedimentation.

(3) Decanting stage

After the sedimentation stage is completed, the water decanter at the end of the reaction pond begins to work, the supernatant is discharged layer by layer from top to bottom, and the water decanter is automatically reset after the drainage. During the decanting period, the sludge reflux system works as usual to increase the sludge concentration in the anoxic zone, with the sludge reflux to the zone for further denitrification of nitrate nitrogen and release of phosphorus.

(4) Idle stage

The time of idling stage is generally short so that the decanter can rise to the original position and prevent sludge loss. The actual decanting time is usually shorter than the design time, and the remaining time is used to idle the sludge in the reactor and restore the sludge adsorption capacity.

Advantages of Process

(1) Simple process, small footprint and low investment

The core structure of CASS is the reaction tank, and there is no secondary settling tank and sludge reflux equipment. In general, there is no regulating tank and primary settling tank. So. Sewage treatment facilities are compact, occupying an area of province and low investment.

(2) biochemical reaction driving force

The substrate concentration in the complete mixed continuous flow aeration tank is equal to the bottom discharge concentration of the secondary sedimentation tank and the rate of the substrate flowing into the aeration tank is the substrate degradation rate. According to the principle of biochemical dynamic reaction, due to the low concentration of substrate in the aeration tank, the driving force of biochemical reaction is very small, and the reaction rate and removal efficiency of organic matter are relatively low. In an ideal push-flow aeration tank, the mixed flow formed by sewage and returned sludge enters from the first end of the tank, flows along the aeration tank in a push-flow state, and flows out at the end of the tank. The substrate concentration, which is the driving force of biochemical reaction, gradually degrades from the highest concentration of inlet water to the lowest concentration of outlet water. The substrate concentration is not diluted during the whole reaction process, and the greater driving force is maintained as far as possible. There is only transverse mixing on each section of the aeration tank, and there is no longitudinal backmixing.

From the point of view of the degradation process of pollutants in CASS process, when the sewage continuously enters the CASS tank with a relatively low amount of water, it will be diluted by the mixed liquid. Therefore, from the perspective of space, CASS process belongs to the category of variable volume and complete mixed activated sludge process. From the perspective of the whole cycle of CASS process from the beginning of aeration to the end of drainage, the substrate concentration changes from high to low, the concentration gradient changes from high to low, and the substrate utilization rate changes from large to small. Therefore, CASS process is a push-flow reactor in an ideal time sequence, with a large driving force of biochemical reaction.

(3) good precipitation effect

In CASS process, almost the whole reaction tank plays a sedimentation role in the sedimentation stage, and the surface load of the sedimentation stage is much smaller than that of the ordinary secondary sedimentation tank. Although there is the interference of water, the influence is small, and the sedimentation effect is good. The practice has proved that the normal operation of CASS process will not be affected when the temperature is low in winter and the sludge settling performance is poor, or when the sludge coagulation performance is poor in the treatment of some special industrial wastewater. In the experiment and engineering, SV was as high as 96%. As long as the time of precipitation stage was slightly extended, the system operation was not affected.

(4) Flexible operation and strong impact resistance

CASS process has been designed to consider the factors of flow changes, which can ensure that sewage stays in the system for a predetermined treatment time and then is discharged through precipitation. In particular, CASS process can adapt to the changes of inflow and quality of water by adjusting the operation cycle. When the influent concentration is high, the standard discharge can be achieved by prolonging the aeration time to achieve the purpose of anti-impact load. In a storm. It can withstand peak flow shocks up to 6 times the usual average flow, without the need for a separate regulating pool. Years of operation data show. When the flow impact and organic load impact exceed the design value by 2~3 times, the treatment effect is still satisfactory. Although the traditional treatment process has been equipped with auxiliary flow balance regulation facilities, it is still likely to cause the loss of activated sludge due to the change of hydraulic load, which seriously affects the drainage quality. When the function of denitrification and dephosphorization is strengthened, CASS process can improve the effect of denitrification and dephosphorization by adjusting the working cycle and controlling the level of dissolved oxygen in the reactor. Therefore, through the adjustment of operation mode, can achieve different treatment of water quality.

(5) sludge bulking is not easy to occur

Sludge bulking is frequently during operation of activated sludge system problems, due to the sludge settling performance is poor, sludge and water cannot be in the second pond for effective separation, resulting in the loss of the sludge, use water quality is poor, serious when the sewage treatment plants can't run, but it takes time to control and eliminate the sludge expansion, with a lag. Therefore, the selection of sewage treatment technology which is not prone to sludge bulking is a problem that must be considered in the design of sewage treatment plant. Due to the specific surface area of the filamentous bacterium is bigger than wormwood micelle, therefore, is conducive to absorb low concentration of substrate, but usually filamentous bacterium than proliferation rate is smaller than the filamentous bacterium, under the high concentration of substrate wormwood micelles and filamentous bacterium with larger degradation rate and proliferation, but because of the micelle rate is larger than bacterial, its proliferation is bigger also, thus the filamentous bacterium is preferred. And CASS reaction pool there is a large concentration degree, and under anoxic and aerobic change alternately, such environmental conditions can be selectively develop of micelle bacteria, make it become the advantages of aeration tank of genus, effectively inhibit the growth of filamentous bacterium and reproduction, overcome the sludge expansion, thus improve the running stability of the system.

(6) wide range of application, suitable for phased construction

CASS process can be used in large, medium and small sewage treatment projects, which is more applicable than SBR process. The design and operation of the continuous inflow is easy to match with the pre-treated structure, and the control system is simpler than the SBR process on the other hand. For large sewage treatment plants, the CASS reaction tank is designed as a combination of multi-tank modules, and a single tank can operate independently. When the treated water quantity is less than the designed value, it can be operated in the low water level of the reaction tank or put into the partial reaction tank to run a variety of flexible operations. Because the main core structure of the CASS system is the CASS reaction tank, the CASS reaction tank can also be copied if the amount of treatment water increases and the designed amount of water cannot meet the treatment requirements. Therefore, the construction of the CASS sewage treatment plant can be developed with the development of the enterprise, and its stage construction and expansion are much simpler than the traditional activated sludge process.

(7) small amount of residual sludge, stable nature

The sludge age of the traditional activated sludge process is only 2~7 days, while the sludge age of the CASS process is 25~30 days, so the sludge has good stability, good dewatering performance, and less residual sludge. The removal of 1.0kgBOD produced 0.2~0.3kg of residual sludge, which was only about 60% of that of the traditional method. Since the sludge has been digested to a certain extent in the CASS reaction tank, the oxygen consumption rate of the remaining sludge is only below L0MgO2 / Gmiss H, which generally does not need to be stabilized and can be directly dehydrated. However, the residual sludge from the traditional process is unstable, with poor settling and oxygen consumption rate greater than 20mGO2 / GMLSS h, which can only be disposed of after stabilization.