In-depth analysis of CASS process (circulating activated sludge process)

CASS process divides the reaction tank of sequencing batch reactor (SBR) into two parts along the length direction. The front part is the biological selection area, also called the pre-reaction area, and the back part is the main reaction area. A lifting decanter is installed at the back of the main reaction zone to realize the periodic circulation operation of continuous water inflow and intermittent drainage, integrating aeration, sedimentation and drainage. CASS process is an anaerobic/anoxic/aerobic alternating operation process, which has a certain effect of nitrogen and phosphorus removal. The wastewater is operated by plug flow, and all reaction zones are completely mixed to realize simultaneous nitrification and denitrification and biological phosphorus removal.

Introduction of CASS process flow

For ordinary urban sewage, CASS process does not need a high degree of pretreatment, but only needs to set up coarse grids, fine grids and grit chambers, and does not need primary sedimentation tanks and secondary sedimentation tanks, nor does it need a huge sludge return system (only about 20% of sludge returns in CASS reactor). The common CASS process flow in China is shown in figure 1.

(1) water filling-aeration stage

Aeration is carried out while water is fed, and the sludge in the main reaction zone flows back to the biological selection zone, with a general reflux ratio of 20%. At this stage, the aeration system provides oxygen to the reaction tank, which not only meets the oxygen demand of aerobic microorganisms, but also facilitates the full mixing and contact between activated sludge and organic matter, thus facilitating the oxidative decomposition of organic pollutants by microorganisms. At the same time, ammonia nitrogen in sewage is transformed into nitrate nitrogen by nitrification of microorganisms.

(2) precipitation stage

Stop aeration, and microorganisms continue to use the remaining dissolved oxygen in water for oxidative decomposition. With the further reduction of dissolved oxygen in the reaction tank, microorganisms change from aerobic state to anoxic state, and a certain denitrification occurs. At the same time, the activated sludge is precipitated and separated under almost static conditions, and the activated sludge sinks to the bottom of the tank and continues to play its role in the next cycle. The treated water is located in the upper part of the sludge layer, and sludge and water are separated by standing and settling.

(3) decapitation stage

After the sedimentation stage is completed, the decanter placed at the end of the reaction tank starts to work, and the supernatant is discharged layer by layer from top to bottom, and the decanter automatically resets after the drainage is completed. During the decanting period, the sludge return system works as usual, with the purpose of increasing the sludge concentration in the anoxic area, further denitrifying the nitrate nitrogen in the sludge returned to the area with the sludge, and releasing phosphorus.

(4) Idle stage

The idle period is generally short, mainly to ensure that the decanter rises to its original position during this period to prevent sludge loss. The actual decanting time is often shorter than the design time, and the remaining time is used to idle the sludge in the reactor and restore the adsorption capacity of the sludge.

Advantages of CASS process

(1) The process flow is simple, the floor space is small and the investment is low.

The core structure of CASS is the reaction tank, and there is no secondary sedimentation tank and sludge reflux equipment. Generally, there is no regulating tank and primary sedimentation tank.

(2) Biochemical reaction has great impetus.

The substrate concentration in the fully mixed continuous flow aeration tank is equal to the substrate concentration in the effluent of the secondary sedimentation tank, and the rate at which the substrate flows into the aeration tank is the substrate degradation rate. According to the principle of biochemical kinetic reaction, because the concentration of substrate in aeration tank is very low, the driving force of biochemical reaction is also very small, and the reaction rate and organic matter removal efficiency are relatively low. In an ideal plug-flow aeration tank, the mixed flow of sewage and reflux sludge enters from the head end of the tank, flows along the aeration tank in a plug-flow state and flows out at the tail end of the tank.

As the driving force of biochemical reaction, the substrate concentration gradually degrades from the highest concentration in influent to the lowest concentration in effluent, and the substrate concentration is not diluted during the whole reaction process, so as to maintain the driving force as much as possible. Here, there is only horizontal mixing on each section of the aeration tank, and there is no longitudinal backmixing.

From the degradation process of pollutants, when the sewage continuously enters the CASS tank with relatively low water volume, the CASS process is diluted by the mixed solution. Therefore, CASS process belongs to the category of fully mixed activated sludge process with variable volume in space. From the beginning of aeration to the end of drainage in CASS process, the substrate concentration is from high to low, the concentration gradient is from high to low, and the substrate utilization rate is from large to small. Therefore, CASS process is an ideal plug-flow reactor in time sequence, which has great impetus to biochemical reaction.

(3) the precipitation effect is good

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

(4) Flexible operation and strong impact resistance.

The CASS process has considered the factors of flow change in the design, which can ensure that the 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 influent and water quality by adjusting the operation cycle. When the influent concentration is high, it can also be discharged up to the standard by extending the aeration time to achieve the purpose of anti-impact load. During the rainstorm. Without an independent regulating tank, it can withstand the impact of peak flow of 6 times the normal average flow.

Years of operation data show that. When the impact of water flow and organic load exceeds 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 adjustment facilities, it is likely that the change of hydraulic load will lead to the loss of activated sludge, which will seriously affect the drainage quality. CASS process can not only strengthen the function of nitrogen and phosphorus removal, but also improve the effect of nitrogen and phosphorus removal by adjusting the working cycle and controlling the dissolved oxygen level in the reaction tank. Therefore, different treatment water quality can be achieved by adjusting the operation mode.

(5) Sludge bulking is not easy to occur.

Sludge bulking is a common problem in the operation of activated sludge process. Due to the poor settling performance of sludge, sludge and water cannot be effectively separated in the secondary sedimentation tank, resulting in sludge loss and poor effluent quality, which makes the sewage treatment plant unable to operate in serious cases. It takes some time to control and eliminate sludge bulking, which is lagging behind. Therefore, the choice of sewage treatment process that is not easy to cause sludge bulking is a problem that must be considered in the design of sewage treatment plant.

Because the specific surface area of filamentous bacteria is larger than that of bacterial micelles, it is beneficial to ingest low-concentration substrates, but the specific proliferation rate of filamentous bacteria is generally lower than that of non-filamentous bacteria. At high substrate concentration, both bacterial micelles and filamentous bacteria degrade and proliferate at a greater rate, but because micelles have a greater specific proliferation rate, their proliferation is also greater, which is better than filamentous bacteria.

However, there is a large concentration gradient in CASS reaction tank, which is in the alternating change of hypoxia and aerobic. Such environmental conditions can selectively cultivate bacteria in the micelle, make them become the dominant bacteria in the aeration tank, effectively inhibit the growth and reproduction of filamentous bacteria, overcome sludge bulking, and thus improve the operational stability of the system.

(6) Wide application range, suitable for phased construction.

CASS process can be applied to large, medium and small sewage treatment projects, and its application scope is wider than SBR process. On the one hand, the design and operation mode of continuous water inflow are convenient to match with the pretreatment structures, on the other hand, the control system is simpler than SBR process. For large sewage treatment plants, CASS reaction tank is designed as a combination of multi-tank modules, and a single tank can run independently. When the treated water quantity is less than the design value, it can be operated at the low water level of the reaction tank or in some reaction tanks. Since the main core structure of CASS system is CASS reaction tank, if the treated water volume increases beyond the designed water volume and cannot meet the treatment requirements, the CASS reaction tank can also be copied. Therefore, the construction of CASS sewage treatment plant can develop with the development of enterprises, and its phased construction and expansion is much simpler than the traditional activated sludge method.

(7) The amount of excess sludge is small and its properties are stable.

The sludge age of traditional activated sludge process is only 2~7 days, while that of CASS process is 25~30 days, so the sludge has good stability, good dewatering performance and less residual sludge. Removing 1.0kgBOD produces 0.2~0.3kg excess sludge, which is only about 60% of the traditional method. Because the sludge has been digested to a certain extent in CASS reaction tank, the oxygen consumption rate of the remaining sludge is only10mgo2/gmess? H, generally do not need to be stable, can be directly dehydrated. However, the traditional method has unstable residual sludge and poor sedimentation, and the oxygen consumption rate is greater than 20mgO2/gMLSS? H, must be stable before disposal.

Shortcomings of CASS process

CASS process is a single suspended sludge growth system, which uses mixed microbial populations to complete oxidation, nitrification, denitrification and phosphorus removal of organic matter in the same reactor. The interaction of various treatment functions limits its treatment efficiency in practical application, and also puts forward very strict requirements for control, so it is difficult to achieve stable and efficient operation of the process in engineering.

The complex relationship between (1) microbial populations needs to be studied.

The microbial community structure of CASS system is different from that of conventional activated sludge process, and the flora is mainly composed of nitrifying bacteria, denitrifying bacteria, phosphorus accumulating bacteria and heterotrophic aerobic bacteria. At present, there is not a good understanding of the complex relationship between survival competition and ecological balance among microbial populations in unsteady CASS system, and CASS technology theory is only analyzed and discussed from the technological process.

(2) It is difficult to improve the biological nitrogen removal efficiency.

On the one hand, nitration is difficult to complete. Nitrifying bacteria are chemoautotrophic bacteria, and the degradation of organic matter is completed by heterotrophic bacteria. When two kinds of bacteria are co-cultured, the growth of nitrifying bacteria will be limited due to the competition for substrate and DO, so it is difficult to become a dominant population, and the nitrification reaction will be inhibited. In addition, fixed aeration time may also make nitrification incomplete. On the other hand, the denitrification reaction is incomplete. In CASS process, about 20% of nitrate nitrogen is denitrified by returning sludge, and the rest is denitrified by simultaneous nitrification and denitrification, precipitation and idle sludge. During the sedimentation and idle period, because sludge and wastewater can't be mixed well, some nitrate nitrogen in wastewater can't contact with denitrifying bacteria, so it can't be reduced. In addition, during this period, because the organic matter has been fully degraded, the carbon source needed for denitrification is insufficient, which also limits the further improvement of denitrification efficiency.

(3) It is difficult to improve phosphorus removal efficiency.

The phosphorus release process of sludge in biological selector is greatly influenced by the concentration of nitrate nitrogen in the reflux mixture, so it is difficult to continuously improve the phosphorus removal efficiency.

(4) The control mode is relatively simple.

At present, CASS process in practical application is mainly based on time sequence control, but the quality of sewage is not constant, so it is not the best choice to adopt fixed reaction time.

Main technical characteristics of CASS process

(1) Continuous water inflow and intermittent drainage.

The traditional SBR process is intermittent water inflow and intermittent drainage, while the actual sewage discharge is mostly continuous or semi-continuous. CASS process can continuously feed water, which overcomes the shortcomings of SBR process, is more suitable for the actual drainage characteristics, and broadens the application field of SBR process. Although continuous water inflow is considered in CABS process design, even if there is intermittent water inflow in actual operation, it will not affect the operation of the treatment system.

(2) Operating time

CASS reaction tank is usually divided into four stages: aeration, sedimentation, drainage and idle.

(3) the unstable state of the operation process

In each working cycle, the liquid level of CANS pool is the highest at the beginning of drainage and the lowest at the end of drainage. The range of liquid level change depends on the drainage ratio, which is related to wastewater treatment concentration, discharge standard and biodegradation difficulty. The volume of mixed solution and the concentration of matrix in the reaction tank are changing, and the degradation of matrix is unsteady.

(4) Dissolved oxygen changes periodically with high concentration gradient.

CASS is aerated in the reaction stage, and the microorganism is in aerobic state, but it is not aerated in the precipitation and drainage stage, and the microorganism is in anoxic or even anaerobic state. Therefore. Dissolved oxygen in the reaction tank changes periodically, with large oxygen concentration gradient and high efficiency, which is conducive to improving nitrogen and phosphorus removal efficiency, preventing sludge bulking and saving energy. Practice has proved that for the same aeration equipment. Compared with the traditional activated sludge process, CASS process has higher oxygen utilization rate.

Comparison between CASS process and other processes

Comparison of 1, CASS and SBR

CASS reaction tank consists of pre-reaction zone and main reaction zone. The pre-reaction zone is controlled in anoxic state, which improves the removal effect of refractory organic matter. CASS water inlet process is continuous, so there is no solenoid valve control element on the water inlet pipeline, and a single tank can run independently, while SBR or CAST water inlet process is intermittent, and two or more tanks are used alternately in application, which increases the complexity of the control system. The displacement of CASS in each cycle generally does not exceed 1/3 of the total water in the pool, while the displacement of SBR is 1/2-3/4, so CASS has good impact resistance. CASS system is simpler than CAST system, but the effect of nitrogen and phosphorus removal is not as good as the latter.

CASS tank is divided into pre-reaction zone and main reaction zone. In the pre-reaction zone, microorganisms quickly absorb most of the soluble organic substances in sewage through the rapid transfer mechanism of enzymes, and experience a rapid accumulation process of high-load substrates, which plays a good buffer role on the water quality, water quantity, PH and toxic and harmful substances, and inhibits the growth of filamentous bacteria, which can effectively prevent sludge bulking; Subsequently, it undergoes a low-load substrate degradation process in the main reaction zone.

CASS process integrates reaction, precipitation, drainage and function. The degradation of pollutants is a push-flow process in time, and microorganisms are in the periodic changes of aerobic, anoxic and anaerobic, thus realizing the removal of pollutants and also having good nitrogen and phosphorus removal functions.

Through the simulation test, CASS biological treatment method has been successfully applied to the treatment of domestic sewage, food wastewater and pharmaceutical wastewater, and achieved good treatment results. The front part of the reactor is equipped with a biological selection area, and the back part is equipped with a lifting automatic decanting device, so as to minimize the disturbance of water flow to the sludge deposited at the bottom during drainage. Its working process can be divided into three stages: aeration, sedimentation and drainage, which are carried out periodically. Sewage continuously enters the pre-reaction zone and enters the main reaction zone through the bottom of the partition wall. Under the condition of ensuring oxygen supply, organic matter is degraded by microorganisms in the pool. The operation parameters can be adjusted according to the inlet water quality.

2. Compared with the traditional activated sludge process

(1) Low construction cost: the primary sedimentation tank, secondary sedimentation tank and sludge reflux equipment are omitted, and the construction cost can be saved by 10% ~ 25%. Taking a 6.5438+10,000-ton municipal sewage treatment plant as an example, the total investment of the traditional activated sludge process is about 65.438+0.5 billion, and that of the CASS process is about 65.438+0./kloc-0.0 billion.

(2) The process flow is short and the floor space is small: the main structures of the sewage plant are water collection tank, grit chamber, CASS aeration tank and sludge tank, but there is no primary sedimentation tank and secondary sedimentation tank, so the layout is compact and the floor space can be reduced by 20% ~ 35%.

(3) Operation cost saving: Because aeration is periodic, the dissolved oxygen concentration in the pond changes, and the dissolved oxygen in the sedimentation stage and drainage stage decreases. When aeration is resumed, the oxygen concentration gradient is large, the transmission efficiency is high, the energy saving effect is remarkable, and the operating cost can be saved 10% ~ 25%.

(4) High removal rate of organic matter and good effluent quality: According to the research results and engineering application, through reasonable design and good management, when the influent COD is 400 mg/L, the effluent of urban sewage is less than 30 mg/L. For biodegradable industrial wastewater, even if the influent COD is as high as 3000mg/L, the effluent can still reach about 50 mg/L. It is difficult for ordinary biological treatment processes to achieve such good water quality. Therefore, the investment of CASS process and secondary treatment can reach the water quality of tertiary treatment.

(5) Simple management and reliable operation: there are few types and quantities of equipment in the sewage treatment plant, the control system is relatively simple, and the process itself determines that sludge bulking will not occur.

(6) Low sludge yield and stable sludge properties.

(7) It has the function of nitrogen and phosphorus removal.

Design of CASS process

1 and main design parameters of CASS reactor

The maximum design water depth can reach 5m~6m, MLSS is 3500mg/L ~ 4000mg/L, the water filling rate is about 30%, the maximum supernatant decantation rate is 30mm/min, the solid-liquid separation time is 60min, and the design SVI 140ml/g and the single cycle time is 1. When treating municipal sewage, the volume ratio of biological selector, anoxic zone and main reaction zone in CASS is generally 1∶5∶30, which can be determined according to water quality and "module" test. Table 1 lists the reference design parameters of CASS process for treating municipal sewage of different scales.

2. Problems that should be paid attention to in 2.CASS design.

(1) water balance

The discharge of industrial wastewater and domestic sewage is usually uneven. How to give full play to the role of CASS reaction tank has a great relationship with the selected design process. If the design flow is not suitable, the water level will exceed the upper limit at the peak of water inflow, and the reaction tank will not be fully utilized when the water inflow is small. When the water quantity fluctuates greatly, it should be considered to set up a regulating pool.

(2) Selection of control mode

CASS process is characterized by program working system, which can adjust the working program according to the change of inlet and outlet water quality to ensure the effluent effect. The whole control system can be combined with field programmable controller (PLC) and microcomputer for centralized control. At the same time, in order to ensure the normal operation of CASS process, all equipment adopts manual/automatic operation mode, the latter is convenient for manual debugging and automatic control system failure, and the former is used for daily work.

(3) Selection of aeration mode

When choosing aeration head, we should try to adopt non-blocking aeration forms, such as perforated pipe, underwater aerator, umbrella aerator, spiral aerator, etc. When micropore aeration is used, high-strength rubber aeration disc or aeration pipe should be used. When the aeration is stopped, the micropores are closed and opened, which is not easy to cause micropore blockage. In addition, due to the characteristics of CASS process itself, the number of underwater aerators can be properly opened according to its operation cycle and DO, and the purpose of energy saving can be achieved on the premise of meeting the requirements of wastewater.

(4) Selection of drainage methods

The drainage requirements of CASS process are the same as SBR. At present, the commonly used equipment is the rotary skimmer, which has the advantages of uniform drainage, adjustable displacement, little interference to the bottom sludge and preventing floating objects on the water surface from being discharged with water. After CASS process, the supernatant should be discharged in time, and the discharge should be as uniform as possible, so as not to disturb the sludge layer deposited at the bottom of the tank. At the same time, floating objects on the water surface should be prevented from being discharged with the water flow, which will affect the quality of the effluent. At present, the common drainage methods are fixed drainage devices, such as a water outlet pipe without depth along the pool, which is opened from top to bottom in turn. Advantages are simple drainage equipment and low investment. Disadvantages are that there are many valves to be opened, and some sludge will accumulate in the drainage pipe, resulting in poor initial effluent quality. Although the floating drainage device and the rotating drainage device are expensive, they have the advantages of uniform drainage, adjustable displacement, little interference to the bottom silt and preventing floating objects on the water surface from being discharged with water. Therefore, these two drainage devices are widely used in front of the ear, especially the rotary drainage device, also known as decanter, which has flexible operation and high running stability.

(5) Other issues needing attention

1) the influence of winter or low temperature on CASS process and its control;

2) Determination of drainage ratio;

3) The influence and control of the rainy season on the water level in the pool;

4) Control the time and age of sludge discharge;

5) Size of pre-reaction zone and length-width ratio of reaction tank:

6) The matching of elevation and water quantity between intermittent drainage and subsequent treatment structures.

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