CASS process technology features Shijiazhuang Worcester Environmental Protection
Shijiazhuang Wooster Environmental Protection Technology Co., Ltd. is a professional manufacturer of microporous aerators, ABS pipes, U-PVC pipes, pipe fittings, water treatment and environmental protection fillers. The company is an aerator with design, development, manufacturing, sales and service, water treatment and environmental protection filler production enterprises. The main products are diaphragm aerator, microporous aerator, rotary mixing aerator, tube aerator, aeration hose, aeration head, diffuser aerator, ABS pipeline, UPVC pipeline and Accessories, composite packing, solid elastic packing, honeycomb inclined tube packing, hollow ball packing, Pall ring packing, suspension ball packing, step ring, drainage cap, etc.
2 Main technical features of CASS process
2.1 continuous water intake, intermittent drainage
The traditional SBR process is intermittent water, intermittent drainage, and the actual sewage discharge is mostly continuous or semi-continuous, CASS process can continuously enter the water, overcome the shortcomings of SBR process, more suitable for the characteristics of actual drainage, broaden the application of SBR process field. Although the CASS process is designed to be continuously influent, it does not affect the operation of the treatment system even if there is intermittent water in actual operation.
2.2 Timing in operation
The CASS reaction tank is usually carried out in sequence according to the four stages of aeration, sedimentation, drainage and idle.
2.3 Unsteady state of operation
The liquid level zoi in the CASS pool is high at the beginning of drainage in each working cycle. When the drainage is finished, the liquid level is low. The variation of the liquid level depends on the drainage ratio, and the drainage ratio and the concentration of the treated wastewater, emission standards and biodegradation are difficult. The degree of easiness and so on. Both the volume of the mixture and the concentration of the matrix in the reaction cell are varied, and the degradation of the matrix is ​​non-steady.
2.4 Dissolved oxygen periodically changes, high concentration gradient
CASS is aerated in the reaction stage, the microorganisms are in aerobic state, no aeration in the sedimentation and drainage stages, and the microorganisms are in anoxic or even anaerobic state. Therefore, the dissolved oxygen in the reaction tank is periodically changed, the oxygen concentration gradient is large, and the transfer efficiency is high, which is advantageous for improving the efficiency of nitrogen and phosphorus removal, preventing sludge expansion, and saving energy. Practice has confirmed that for the same aeration equipment, the CASS process has a higher oxygen utilization rate than the conventional activated sludge process.
3 main advantages of CASS process
3.1 Simple process, small footprint and low investment
The core structure of CASS is the reaction tank. There is no secondary sedimentation tank and sludge return equipment. Under normal circumstances, there is no adjustment tank and primary sedimentation tank. Therefore, the sewage treatment facilities are compact in layout, low in land occupation and low in investment.
3.2 Biochemical reaction driving force
The substrate concentration in the fully mixed continuous flow aeration tank is equal to the effluent substrate concentration in the secondary settling tank, and the rate at which the bottom stream enters the aeration tank is the substrate degradation rate. According to the principle of biochemical kinetics, because the concentration of the substrate in the aeration tank is very low, the biochemical reaction driving force is also small, and the reaction rate and organic matter removal efficiency are relatively low; in the ideal push-flow aeration tank, the sewage and the sewage The mixed stream formed by the return sludge enters from the head end of the pool, flows into the aeration tank in a push flow state, and flows out to the end of the tank. As the substrate concentration of the biochemical reaction, the concentration of zui from the influent water gradually degraded to the low concentration of zui at the time of water discharge, and the substrate concentration of the whole reaction process was not diluted, and the driving force was maintained as much as possible. There is only lateral mixing on each section of the aeration tank, and there is no longitudinal back mixing.
The CASS process is based on the degradation process of pollutants. When the sewage enters the CASS tank continuously with relatively low water volume, it is diluted by the mixed liquid. Therefore, the CASS process is a variable-mixed fully-activated activated sludge process. From the beginning of the CASS process to the end of the drainage cycle, the matrix concentration is high to low, the concentration gradient is from high to low, and the matrix utilization rate is from large to small. Therefore, the CASS process is ideal for chronological push. In the flow reactor, the biochemical reaction has a large driving force.
3.3 Good precipitation effect
In the precipitation stage, almost all the reaction tanks in the CASS process are precipitated. The surface load in the sedimentation stage is much smaller than that in the ordinary secondary sedimentation tank. Although there is interference from the influent, the effect is small and the sedimentation effect is good. Practice has proved that when the winter temperature is low, the sludge sedimentation performance is poor, or when some special industrial wastewater sludge treatment performance is poor, it will not affect the normal operation of the CASS process. In experiments and engineering, SV30 has been encountered as high as 96%. As long as the time of the sedimentation phase is slightly extended, the system operation is not affected.
3.4 Flexible operation, strong impact resistance, and can achieve different processing targets
The CASS process has taken into account the factors of flow change during design, which ensures that the sewage will be discharged after sedimentation in the system for a predetermined treatment time. In particular, the CASS process can adjust the operating cycle to adapt to the ratio of water inflow and water quality. When the influent concentration is high, the discharge can be achieved by extending the aeration time to achieve the purpose of impact resistance. In heavy rain, it can withstand the peak flow impact of the average average flow rate of 6 letters without the need for independent adjustment. Years of operation data show that the treatment effect is still satisfactory when the flow impact and organic load impact exceed the design value of 2-3 letters. Although the traditional treatment process has been equipped with auxiliary flow balance adjustment facilities, it is also likely that the activated sludge will be lost due to the change of hydraulic load, which seriously affects the drainage quality.
When strengthening the function of nitrogen and phosphorus removal, the CASS process can improve the effect of nitrogen and phosphorus removal by adjusting the working cycle and controlling the dissolved oxygen level of the reaction cell. Therefore, different treatment water quality can be achieved by adjusting the operation mode.
3.5 It is not easy to cause sludge expansion
Sludge expansion is a problem often encountered in the operation of activated sludge process. Due to the poor sedimentation performance of sludge, sludge and water cannot be effectively separated in the secondary settling tank, resulting in sludge loss and poor effluent quality. The sewage treatment plant cannot operate, and it takes a certain time to control and eliminate the sludge expansion, which has hysteresis. Therefore, selecting a sewage treatment process that is less prone to sludge expansion is a problem that must be considered in the design of a sewage treatment plant.
Since the specific surface area of ​​the filamentous bacteria is larger than that of the bacterial micelles, it is advantageous for ingesting low-concentration substrates, but in general, the specific growth rate of filamentous bacteria is smaller than that of non-filamentous bacteria, and the micro-substrate concentration is high. The bacteria all degrade the substrate and proliferate at a relatively high rate, but because of the larger proliferation rate of the micelle bacteria, the proliferation amount is larger, which is superior to the filamentous bacteria. However, there is a large concentration gradient in the CASS reaction tank, and it is in the alternation of hypoxia and aerobic. Such environmental conditions can selectively culture the bacteria micelles, making them the dominant bacteria in the aeration tank. Genus, effectively inhibiting the growth and reproduction of filamentous bacteria, overcoming sludge expansion, thereby improving the operational stability of the system.
3.6 Wide application range, suitable for phased construction
The CASS process can be applied to large, medium and small sewage treatment projects, which is more widely used than the SBR process. The design and operation mode of continuous water inlet is easy to match with the pre-treatment structure on the one hand, and the control system is more than the SBR process on the other hand. simple.
For large sewage treatment plants, the CASS reaction tank is designed as a multi-pool module combination, and the single tank can be operated independently. When the treated water volume is less than the design value, it can be operated at a low water level of the reaction site or put into a part of the flexible operation mode such as the operation of the reaction tank; since the main core structure of the CASS system is the CASS reaction tank, if the amount of treated water increases, the amount of water exceeding the design cannot be exceeded. When the processing requirements are met, the CASS reaction tank can be replicated. Therefore, the construction of the CASS wastewater treatment plant can be developed with the development of the enterprise, and its stage construction and expansion is much simpler than the traditional activated sludge process.
3.7 Small amount of excess sludge, stable in nature
The traditional activated sludge method has a sludge age of only 2-7 days, while the CASS method has a sludge age of 25-30 days, so the sludge has good stability, good dewatering performance, and less residual sludge. Removal of 1.0kg of BOD produces 0.2 to 0.3 kg of excess sludge, which is only about 60% of the conventional method. Since the sludge has been digested to a certain extent in the CASS reaction tank, the oxygen consumption rate of the excess sludge is only below 10 mgO2/g MLSS.h, and generally does not need to be stabilized, and can be directly dehydrated. However, the residual sludge in the traditional method is unstable, the sedimentation is poor, and the oxygen consumption rate is greater than 20 mgO2/g MLSS.h, which must be stabilized before disposal.
4 issues that should be noted in CASS design
4.1 Water balance
The discharge of industrial wastewater and domestic sewage is usually uneven. How to fully exert the role of the CASS reaction tank has a great relationship with the selected design flow. If the design flow rate is not suitable, the water level will exceed the upper limit at the peak of the influent water, and the water intake will react to the hour. The pool cannot be fully utilized. When the water volume fluctuates greatly, consider setting the conditioning tank.
4.2 Choice of control methods
The increasingly widespread use of CASS processes has benefited from the development of automation technology and its application in wastewater treatment engineering. The CASS process is characterized by a program working system, which can adjust the working procedure according to the changes of the influent and effluent water quality to ensure the water output effect. The whole control system can be combined with on-site programmable control (PLC) and centralized control of the microcomputer. At the same time, in order to ensure the normal operation of the CASS process, all devices adopt manual/automatic operation modes, and the latter is convenient for manual debugging and automatic control system failure. The former is used for daily work.
4.3 Selection of aeration mode
CASS process can choose a variety of aeration methods, but when choosing an aeration head, try to use a non-blocking aeration form, such as perforated pipe, underwater aerator, umbrella aerator, spiral aerator. When using microporous aeration, a rubber aeration disc or tube with high strength should be used. When the aeration is stopped, the micropores are closed and opened during aeration, which is not easy to cause micropore blockage. In addition, due to the characteristics of the CASS process itself, the underwater aerator can be used to open different numbers according to its operating cycle and DO, so as to save energy consumption while meeting the wastewater requirements.
4.4 Choice of drainage method
The drainage requirements of the CASS process are the same as those of the SBR. At present, the commonly used equipment is a rotary water slinger, which has the advantages of uniform drainage, adjustable displacement, low interference to the bottom sludge, and prevention of water surface floating matter being discharged with water.
The supernatant of the CASS process should be discharged in time. The drainage should be discharged as evenly as possible. The sludge layer deposited at the bottom of the tank should not be disturbed. At the same time, the floating matter on the 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 include fixed drainage devices, such as setting water outlet pipes at different depths along the pool, and opening them from top to bottom. The advantage is that the drainage equipment is simple and the investment is small. The disadvantage is that there are many open valves and some sludge will be accumulated in the drainage pipes. , resulting in poor initial effluent quality. Although the floating drainage device and the rotary drainage device have high price, the drainage is uniform, the displacement is adjustable, the interference to the bottom sludge is small, and the floating surface floating material is prevented from being discharged with the water. Therefore, the two drainage devices are currently used more. Especially the rotary drainage device, also known as the water purifier, is favored by designers and users for its flexible operation and high operational stability.
4.5 Other issues to be aware of
1. Influence and control of CASS process in winter or low temperature
2. Determination of drainage ratio
3. Impact and control of the rainy season on the water level in the pool
4, mud timing and mud age control
5. The size of the pre-reaction zone and the aspect ratio of the reaction cell
6. The problem of matching the elevation and water volume of intermittent drainage and subsequent treatment structures.
5 CASS economy
Practice has proved that the daily processing water volume of the CASS process is a few hundred cubic meters, and hundreds of thousands of cubic meters. As long as the design is reasonable, it has certain economic advantages compared with other methods. It saves 20%-30% of investment compared with traditional activated sludge process and saves more than 30% of land. When multiple processes are required to be used in series, such as when there are other treatment processes after the CASS process, it is usually necessary to increase the intermediate pool and lifting equipment, which will affect the overall economic advantage. At this time, a detailed technical and economic comparison is needed to determine The CASS process is used or other aerobic treatment processes.
Because the aeration of the CASS process is intermittent, which is conducive to the transfer of oxygen, the aeration time can be flexibly adjusted according to the changes in water quality and water volume, all of which create conditions for reducing operating costs. Overall, the CASS process runs at a slightly lower cost than conventional activated sludge processes.
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