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CASS process principle
The CASS pool is divided into a pre-reaction zone and a main reaction zone. In the pre-reaction zone, microorganisms can rapidly adsorb most of the soluble organic matter in the sewage through the rapid transfer mechanism of enzymes, and undergo a high-load matrix rapid accumulation process, which plays a good buffering role in the influent water quality, water quantity, PH and toxic and harmful substances, and at the same time inhibits the growth of filamentous bacteria, which can effectively prevent sludge swelling, and then undergoes a matrix degradation process with a lower load in the main reaction zone. The CASS process integrates reaction, precipitation, drainage and function, and the degradation of pollutants is a push-flow process in time, while microorganisms are in the periodic changes of aerobic, anoxic and anaerobic, so as to achieve the removal of pollutants, and also have good denitrification and phosphorus removal functions. CASS biological treatment method is the abbreviation of periodic cycle activated sludge method, which was first produced in the United States and introduced into China in the early 90s. After simulation and experimental research, it has been successfully applied to the treatment of domestic sewage, food wastewater and pharmaceutical wastewater, and has achieved good treatment results, laying a good foundation for the popularization and application of CASS method in mainland China.
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The CASS method is an evolution of the batch activated sludge method (SBR method):
A biological selection area is set up at the front of the reactor, and an automatic decanting device that can be lifted and lowered is set up at the rear. Its working process can be divided into three stages: aeration, sedimentation and drainage, and the cycle is carried out. The sewage continuously enters the pre-reaction area and enters the main reaction area through the bottom of the partition wall, so that the organic matter is degraded by the microorganisms in the pool under the condition of ensuring oxygen supply. Depending on the quality of the influent water, the operating parameters can be adjusted.
Features of the CASS method:
The advantages of the CASS method over SBR are:
The reaction cell is composed of a pre-reaction zone and a main reaction zone, so the removal effect of refractory organic matter is better.
The inlet process is continuous, so there is no need for control elements such as solenoid valves on the inlet pipe, and a single pool can operate independently, whereas the SBR inlet process is intermittent and generally requires two or more pools to be used alternately.
The drainage is completed by a liftable weir decanter, which gradually decreases with the water surface and discharges the treated clean water evenly, which minimizes the disturbance of the water flow to the sedimented sludge at the bottom during drainage.
The discharge volume of each cycle of the CASS method generally does not exceed 1/3 of the total water in the pool, while the SBR is 3/4, so the CASS method has better impact resistance than the SBR method.
Compared with the traditional activated sludge method, the advantages of the CASS method are:
Low construction cost: The primary sedimentation tank, secondary sedimentation tank and sludge return equipment are omitted, and the construction cost can be saved by 10-25%. Taking a 100,000-ton municipal sewage treatment plant as an example, the total investment of the traditional activated sludge method is about 150 million yuan, and the total investment of the CASS method is about 110 million yuan.
Short process flow and small footprint: The main structures of the sewage plant are water collection tank, grit tank, CASS aeration tank and sludge tank, but there is no primary sedimentation tank and secondary sedimentation tank, and the layout is compact, and the floor area can be reduced by 20-35%. Taking the 100,000-ton municipal sewage plant as an example, the traditional activated sludge method covers an area of about 180 acres, and the CASS method covers an area of about 120 acres.
Save operating costs: Because the aeration is cyclical, the concentration of dissolved oxygen in the pool also changes, the dissolved oxygen in the precipitation stage and the drainage stage is reduced, and when the aeration is restarted, the concentration gradient of oxygen is large, the transfer efficiency is high, the energy saving effect is significant, and the operating cost can be saved by 10-25%.
High removal rate of organic matter and good effluent quality: According to the research results and engineering application, through reasonable design and good management, for urban sewage, when the influent COD is 400mg/L, the effluent is less than 30mg/L. For biodegradable industrial wastewater, even if the influent COD is as high as 3000mg/L, the effluent can still reach about 50mg/L. For general biological treatment processes, it is difficult to achieve such good water quality. Therefore, the investment in the CASS process, secondary treatment, can achieve the water quality of tertiary treatment.
Simple management and reliable operation: The type and quantity of equipment in the sewage treatment plant are small, the control system is relatively simple, and the process itself determines that no sludge expansion occurs. Therefore, the system is simple to manage and reliable to operate.
The sludge yield is low and the sludge properties are stable.
It has the function of nitrogen and phosphorus removal.
No odor.
CASS process characteristics
The equipment is easy to install, the construction period is short, it has good water resistance and anti-corrosion ability, and the equipment has a long service life. Car washing and other related miscellaneous uses; the treatment process is at the advanced level at home and abroad, the equipment has a high degree of automation, and can be operated and controlled by microcomputer; the whole process is relatively simple to operate, the maintenance is convenient, and the odor and mosquitoes that pollute the environment are not produced in the treatment plant; the investment is relatively low, the treatment cost is low, and the process has the value of popularization and application.
The CASS operation cycle can be generally divided into four steps:
In the aeration stage, the reaction tank is oxygenated by the aeration device, and the organic pollutants are oxidized and decomposed by microorganisms, and the NH3-N in the sewage is converted into NO3--N through the nitrification of microorganisms.
Sedimentation phase At this time, aeration is stopped, and microorganisms use the remaining DO in the water for oxidative decomposition. The reaction cell gradually transitions from an aerobic state to an anoxic state, and the denitrification reaction begins. The activated sludge gradually sinks to the bottom of the pond and the upper layer of water becomes clear.
Decanting phase After the precipitation is over, the decanter placed at the end of the reaction tank starts working and gradually drains the supernatant from top to bottom. At this time, the reaction cell gradually transitions to an anaerobic state and continues denitrification.
Idle Phase The idle phase is the stage when the decumber rises to its original position.
CAST process
Introduction to the CAST process
The CAST process is the abbreviation of Cyclic Activated Sludge Technology, which is based on the SBR process, adding selectors and sludge return facilities, and making some adjustments to the timing, thus greatly improving the reliability and efficiency of the SBR process. The main structure of the CAST process is composed of an SBR reaction cell, which is mainly divided into a selection zone and a reaction zone. In a CAST system, at least two pools should be installed to enable the system to achieve continuous water intake. In general, water is ingested and aerated in the first pool, sedimentation and decanting in the other, and vice versa. In a multi-pool system, the effluent can be made continuous through a reasonable selection of the circulation process.
CAST process features
1. Flexible and reliable operation
● The biological selector can operate in three ways: aerobic, anoxic and anaerobic according to the water quality of the sewage. The selector can be operated with constant or variable volume
● The state can be adjusted arbitrarily to give full play to the physiological characteristics of different microorganisms
● The selector volume is variable to avoid sludge expansion and improve the reliability of the system
● Strong anti-shock load capacity, suitable for industrial wastewater and urban sewage treatment
2. There are few processing structures and the process is simple
● The total volume of the pool is reduced, and the cost of civil works is low
● There is no need to set up a secondary sedimentation tank and its sludge scraping equipment, nor do you need to set up a return sludge pumping station
3. Phosphorus and nitrogen removal can be realized
● Adjust the aeration and non-aeration sequences of the variable volume of the biological selector to improve the effect of biological phosphorus and nitrogen removal
4. Save investment
● Fewer structures and less floor space
● The equipment and control system are simple
● Low aeration intensity, no need for large amount of air supply equipment
● Low operating costs
Scope of application
1. Processing scale
The maximum scale can reach 200,000 m3/d
2. Treat water quality
It has a wide range of applications and can be used to treat all kinds of domestic sewage and industrial wastewater
SBR process advantages
1. The ideal pushing process increases the driving force of the biochemical reaction, improves the efficiency, and the anaerobic and aerobic in the pool are in an alternating state, and the purification effect is good.
2. The operation effect is stable, and the sewage is precipitated in an ideal static state, which requires short time, high efficiency, and good effluent quality.
3. Resistant to impact load, there is retained treated water in the pool, which has the effect of dilution and buffering on sewage, and effectively resists the impact of water volume and organic dirt.
4. Each process in the process can be adjusted according to the water quality and water quantity, and the operation is flexible.
5. There are few processing equipment, simple structure, easy to operate and maintain management.
6. There is a concentration gradient of DO and BOD5 in the reaction tank, which can effectively control the expansion of activated sludge.
7. The SBR system itself is also suitable for the combined construction method, which is conducive to the expansion and transformation of the wastewater treatment plant.
8. Denitrification and phosphorus removal, properly control the operation mode, realize the alternation of aerobic, anoxic and anaerobic states, and have a good effect of denitrification and phosphorus removal.
9. The process flow is simple and the cost is low. The main equipment has only one sequential batch reactor, no secondary sedimentation tank, sludge return system, regulating tank and primary sedimentation tank can also be omitted, compact layout, and saving floor space.
ICEAS process
The basic unit of the ICEAS process is a reactor in a group of two rectangular cells. Each pool is divided into two parts: the pre-reaction area and the main reaction area, the pre-reaction area is generally in a hypoxic state, and the main reaction area is the main body of the aeration reaction.
The advantage of ICEAS is that it uses a continuous inlet system, which reduces the complexity of the operation and is suitable for large-scale wastewater treatment, but it also loses the five advantages listed in Table 1 at the same time as the process improvement, and only retains the structural characteristics of the SBR reactor.
Compared with the classic SBR process, the ICEAS process has the following characteristics:
a. Different precipitation characteristics
The sedimentation of ICEAS is disturbed by the influent water, which destroys the conditions under which it becomes an ideal precipitation. In order to reduce the disturbance caused by the inlet water, the tank is generally designed as a rectangle, so that the effluent is similar to the advection sedimentation tank.
b. Ideal flow pushing performance and sludge expansion control
Due to the continuous water intake, ICEAS loses the advantages of the ideal flow and high removal rate of refractory substances of the classical SBR, and cannot control the occurrence of sludge expansion, so it is necessary to set up a selection area.
c. Suitable for larger sewage treatment plants due to continuous water intake
The continuous water inlet does not need to be switched between the inlet valves, and the control is simple, so it can be used for larger sewage treatment.