CASS process introduction
For general municipal wastewater, CASS process does not require a high degree of pretreatment, only the setting of coarse grating, fine grating and sand sedimentation tanks, without the primary sedimentation tank and the second sedimentation tank, there is no need for a huge sludge reflux system (only in the CASS reactor inside the sludge reflux of about 20%) common CASS process flow in China, as Figure 1 shown.
(1) Water filling - aeration stage
Water intake and aeration at the same time the sludge from the main reaction area back to the biological selection area, the general reflux ratio of 20%. In this stage, the aeration system supplies oxygen to the reaction tank, on the one hand, to meet the aerobic microorganisms on the need for oxygen, on the other hand, it is conducive to the activated sludge and the full mixing of organic matter and contact, which is conducive to the oxidation and decomposition of organic pollutants by microorganisms. At the same time, the ammonia nitrogen in the sewage is transformed into nitrate nitrogen through the nitrification of microorganisms.
(2) Precipitation stage
Stop aeration, microorganisms continue to use the remaining dissolved oxygen in the water for oxidative decomposition. With the further reduction of dissolved oxygen in the reaction tank, microorganisms from aerobic state to anoxic state, and some denitrification occurs. At the same time, the activated sludge in almost static conditions for precipitation separation, activated sludge sink to the bottom of the pool, the next cycle continues to play a role in the treated water is located in the upper sludge layer, static precipitation so that the mud and water separation.
(3) Decanting stage
After the completion of the precipitation stage, the decanter placed at the end of the reaction tank starts to work, discharging the supernatant layer by layer from top to bottom, and the decanter is automatically reset after the end of drainage. During the decanting period, the sludge reflux system works as usual, its purpose is to improve the concentration of sludge in the anoxic zone, with the nitrate nitrogen in the sludge refluxed to the zone for further denitrification and phosphorus release.
(4) Idle stage
The idle stage is generally shorter, mainly to ensure that the decanter rises to the original position during this stage to prevent sludge loss. The actual decanting time is often shorter than the design time, and its remaining time is used for sludge idling in the reactor as well as restoring the adsorption capacity of the sludge.
Advantages of CASS process
(1) Simple process, small footprint, low investment
The core structure of CASS is the reaction tank, there is no secondary sedimentation tank and sludge reflux equipment, and generally there are no regulating tanks and primary sedimentation tanks.
(2) biochemical reaction driving force
In the completely mixed continuous flow aeration tank in the substrate concentration is equal to the concentration of the substrate in the secondary sedimentation tank, the substrate flow rate into the aeration tank that is the substrate degradation rate. According to the principle of biochemical power reaction, due to the low concentration of substrate in the aeration tank, the biochemical reaction driving force is also very small, the reaction rate and the removal of organic matter efficiency is relatively low; in the ideal push-flow aeration tank, sewage and reflux sludge to form a mixture of streams from the first end of the pool into the state of the aeration tank into a push-flow flow along the aeration tank to the end of the pool outflow.
As the driving force of the biochemical reaction of the substrate concentration, from the highest concentration of influent water gradually degraded to the lowest concentration of effluent, the entire reaction process substrate concentration has not been diluted, as far as possible to maintain a greater driving force. During this time in the aeration tank of each cross-section only horizontal mixing, there is no longitudinal mixing back.
CASS process from the point of view of the degradation process of pollutants, when the sewage with a relatively low volume of water continuously into the CASS pool that is diluted by the mixture, so from the spatial point of view of the CASS process is a variable volume of completely mixed activated sludge method category; from the beginning of the aeration process to the end of the drainage of the entire cycle of CASS process, the substrate concentration from high to low, the concentration gradient from high to low, substrate utilization rate from large to small, therefore, the reaction process is the most effective way to achieve the desired results. Utilization rate from large to small, therefore, CASS process is an ideal time-ordered push-flow reactor, biochemical reaction driving force.
(3) good precipitation effect
CASS process in the precipitation stage of almost the entire reaction pool are playing a role in the precipitation, precipitation stage of the surface load is much smaller than the ordinary secondary sedimentation tank, although there is interference with the incoming water, but its impact is very small, the effect of precipitation is better. Practice has proved that when the winter temperature is low, sludge settling performance is poor, or in the treatment of some special industrial wastewater sludge cohesion performance is poor, will not affect the normal operation of CASS process. Experiments and projects have encountered SV up to 96% of the case, as long as the sedimentation phase of the time slightly extended, the system operation is not affected.
(4) flexible operation, shock resistance
CASS process has been designed to take into account the factors of flow changes, to ensure that the sewage in the system to stay in a predetermined treatment time by the precipitation discharge, especially CASS process can be adjusted by adjusting the operating cycle to adapt to changes in water intake and water quality. When the concentration of influent water is high, it can also be achieved by extending the aeration time to meet the discharge standards and achieve the purpose of shock load resistance. During heavy rainfall. Can withstand the usual average flow 6 times the peak flow impact, without the need for independent regulating pool.
Many years of operation data show that. In the flow shock and organic load shock exceeds the design value of 2 ~ 3 times, the treatment effect is still satisfactory. While the traditional treatment process has been equipped with auxiliary flow balance adjustment facilities, but also likely due to changes in hydraulic load resulting in the loss of activated sludge, seriously affecting the quality of drainage. When strengthening the function of nitrogen and phosphorus removal, 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 pool. Therefore, through the adjustment of the operation mode, different treatment water quality can be achieved.
(5) Sludge expansion is not easy to occur
Sludge expansion is a common problem encountered in the operation of activated sludge method, due to poor sludge settling performance, sludge and water can not be effectively separated in the second sedimentation tank, resulting in sludge loss, so that the effluent quality of water is worse, and in severe cases, so that the sewage treatment plant can not be operated, and to control and eliminate the expansion of sludge requires a certain period of time, with a lag. Therefore, the selection of sewage treatment process is not easy to sludge expansion is a sewage treatment plant design must be considered.
Since the specific surface area of the filamentous inoculum is larger than that of the inoculum, therefore, it is favorable to ingest low concentration of substrate, but generally the specific proliferation rate of the filamentous inoculum is smaller than that of the non-filamentous inoculum. In the case of high concentration of substrate in the inoculum and the filamentous inoculum are both degraded at a larger rate and proliferation, but due to the inoculum bacterial than the proliferation rate is greater, and the amount of proliferation is also greater, thus more than the filamentous inoculum is advantageous.
The CASS reaction pool in the existence of a large concentration of gradient, and in the anoxic, aerobic alternating changes, such environmental conditions can be selectively cultivated in the gelatinous group of bacteria, so that it becomes the dominant genus of aeration pool, effectively inhibit the growth and reproduction of filamentous Yin, to overcome the expansion of the sludge, so as to improve the stability of the system operation.
(6) Wide range of application, suitable for phased construction
CASS process can be applied to large, medium and small-scale sewage treatment works, than the SBR process is more widely applicable; continuous water intake design and operation, on the one hand, easy to match with the pre-treatment structures, on the other hand, the control system is simpler than the SBR process. For large-scale wastewater treatment plant, CASS reaction tank is designed as a combination of multi-tank module, and a single tank can be operated independently. When the amount of treated water is less than the design value, you can run in the low water level of the reaction pool or put into part of the reaction pool operation and other flexible operation methods; because the main core structure of the CASS system is the CASS reaction pool, if the amount of treated water is increased, more than the designed amount of water can not meet the treatment requirements, you can likewise replicate the CASS reaction pool, so the CASS method of wastewater treatment plant construction can be developed with the development of the enterprise and the Development, its stage construction and expansion is much simpler than the traditional activated sludge method.
(7) Small amount of residual sludge, stable nature
The mud age of traditional activated sludge method is only 2~7 days, while the mud age of CASS method is 25~30 days, so the sludge stability is good, the dewatering performance is good, and the residual sludge produced is less. Removing 1.0kgBOD produces 0.2~0.3kg residual sludge, which is only about 60% of the traditional method. Since the sludge has been digested to a certain extent in the CASS reactor, the oxygen consumption rate of the remaining sludge is only 10mgO2/gMISS?h or less, and generally does not need to be stabilized, and can be directly dewatered. While the traditional method of residual sludge is unstable, poor settlement, oxygen consumption rate is greater than 20mgO2/gMLSS?h, must be stabilized before disposal.
Disadvantages of the CASS process
CASS process for a single sludge suspension growth system, the use of mixed microbial populations in the same reactor to complete the oxidation of organic matter, nitrification, denitrification and phosphorus removal. The interaction of multiple treatment functions in practical application limits its treatment efficiency, but also puts forward very strict requirements for control, the project is difficult to achieve stable and efficient operation of the process.
(1) The complex relationship between microbial populations to be studied
The microbial population structure of CASS system is different from that of conventional activated sludge method, and the bacterial population is mainly composed of nitrifying bacteria, denitrifying bacteria, phosphorus-polymerizing bacteria and anaerobic aerobic bacteria. At present, the complex survival competition and ecological balance between microbial populations in the non-stationary CASS system is not well understood, CASS process theory is only from the process of some analytical discussions.
(2) It is difficult to improve the efficiency of biological denitrification
On the one hand, it is difficult to carry out the nitrification reaction completely. Nitrifying bacteria is a kind of chemoenergetic autotrophic bacteria, organic matter degradation is completed by heterotrophic bacteria. When two kinds of bacteria mixed culture, due to the existence of substrate and DO competition, the growth of nitrifying bacteria will be limited, difficult to become the dominant population, nitrification reaction is inhibited. In addition, the fixed aeration time may also make the nitrification incomplete. On the other hand, the denitrification reaction is incomplete.CASS process has about 20% of nitrate nitrogen denitrified by reflux sludge, and the rest of nitrate nitrogen is realized by synchronous nitrification denitrification and denitrification of sludge in sedimentation and idling period. In the settling and idling period, because the sludge and wastewater can not be mixed well, part of the nitrate nitrogen in the wastewater can not be contacted with denitrifying bacteria, so it can not be reduced. In addition, in this period, due to the organic matter has been fully degraded, denitrification of carbon source is insufficient, but also limit the denitrification efficiency of the further increase.
(3) Phosphorus removal efficiency is difficult to improve
The phosphorus release process of sludge in the bioselector is affected by the concentration of nitrate nitrogen in the reflux mixture, and it is difficult to continue to improve the phosphorus removal efficiency.
(4) control mode is relatively single
Currently in the practical application of the CASS process is basically based on timing control, but the quality of sewage water is not static, so the use of a fixed reaction time is bound to be not the best choice.
The main technical characteristics of the CASS process
(1) continuous water intake, intermittent drainage
The traditional SBR process for intermittent water intake, intermittent drainage, and the actual sewage discharge is mostly continuous or semi-continuous, CASS process can be continuous water intake, overcoming the shortcomings of the SBR process, more suitable for the actual characteristics of the drainage, and broaden the SBR process. Application areas. Although the design of CABS process are considered to be continuous water intake, but in actual operation even if there is intermittent water intake, does not affect the operation of the treatment system.
(2) the operation of the chronological
CASS reactor is usually according to the aeration, sedimentation, drainage and idle four stages according to the time sequence.
(3) The non-stationarity of the operation process
The liquid level in the CANS pool is highest at the beginning of the drainage in each working cycle, and the liquid level is lowest at the end of the drainage, and the magnitude of the change of the liquid level depends on the drainage ratio, which is related to the concentration of the treated wastewater, the discharge standard and the difficulty of biodegradation. The volume of the mixed liquid and the concentration of the substrate in the reaction tank are both variable, and the substrate degradation is non-stationary.
(4) Periodic changes in dissolved oxygen, high concentration gradient
CASS is aeration in the reaction stage, microorganisms are in an aerobic state, in the sedimentation and drainage stage is not aeration, microorganisms are in anoxic or even anaerobic state. Therefore. Dissolved oxygen in the reaction pool is a cyclical change, oxygen concentration gradient is large, more efficient, which is beneficial to improve the efficiency of nitrogen and phosphorus removal, prevent sludge expansion and save energy consumption. Practice has confirmed that for the same aeration equipment, CASS process has a higher oxygen utilization rate compared with the traditional activated sludge method.
CASS process and other process comparisons
1, CASS and SBR comparison
CASS reactor by the pre-reaction area and the main reaction area composed of pre-reaction area control in the anoxic state, therefore, the removal of difficult to degrade organic matter to improve the effect; CASS intake process is continuous, so there is no solenoid valve control components on the intake pipeline, a single cell can be run independently, while SBR or CAST water intake process is intermittent, the application is generally to be 2 or more than 2 pools alternately, the control system complexity increases.CASS each cycle of drainage is generally no more than 1/3 of the total amount of water in the pool, while the SBR is 1/2-3/4, CASS shock resistance is better.CASS is simpler than the CAST system, but the effect of denitrogenation and phosphorus removal is not as good as the The latter.
CASS pool is divided into pre-reaction area and main reaction area. In the pre-reaction area, microorganisms can quickly adsorb most of the soluble organic matter in the sewage through the rapid transfer mechanism of enzymes, and experience a high load of substrate rapid accumulation process, which plays a good buffer for the influent water quality, water quality, PH and toxic and harmful substances, and at the same time, inhibit the growth of filamentous bacteria, which can effectively prevent sludge expansion; and then in the main reaction area to experience a lower load of substrate degradation. The main reaction area then undergoes a low load substrate degradation process.
CASS process is a set of reaction, sedimentation, drainage, function in one, the degradation of pollutants in time is a push-flow process, while the microorganisms are in the aerobic, anoxic, anaerobic cyclic changes, so as to achieve the role of pollutant removal, but also has a better function of nitrogen removal, phosphorus removal.
CASS biological treatment method has been successfully applied to the treatment of domestic sewage, food wastewater and pharmaceutical wastewater after simulated experimental research, and has achieved good treatment results. A biological selection area is set up in the front part of the reactor, and a lifting automatic decanting device is set up in the back part, which minimizes the disturbance of the water flow on the bottom sedimentation sludge during drainage. Its working process can be divided into three stages of aeration, sedimentation and drainage, and the cycle is cyclic. 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. The operating parameters can be adjusted according to the quality of incoming water.
2, compared with the traditional activated sludge method
(1) low construction cost: the elimination of the primary sedimentation tank, secondary sedimentation tank and sludge reflux equipment, the construction cost can be saved 10% to 25%. Take 100,000 tons of urban sewage treatment plant as an example, the total investment of traditional activated sludge method is about 150 million, and the total investment of CASS method is about 110 million.
(2) short process, covers an area of less: sewage plant main structures for the collection tank, sand sedimentation tank, CASS aeration tank, sludge tank, and no primary sedimentation tank, secondary sedimentation tank, compact layout, covers an area of 20% to 35%.
(3) operation cost saving: because the aeration is cyclic, the concentration of dissolved oxygen in the pool is also changing, the precipitation stage and drainage stage of dissolved oxygen is reduced, restarting the aeration, the oxygen concentration gradient is large, the transfer efficiency is high, energy-saving effect is significant, the operation cost can be saved 10% to 25%.
(4) high organic matter removal rate, good effluent water quality: according to the results of research and engineering applications, through rational design and good management, for municipal wastewater, influent COD is 400mg/L, effluent is less than 30mg/L or less. For biodegradable industrial wastewater, even if the influent COD is as high as 3000mg/L, the effluent can still reach about 50m g/L. For general biological treatment process, it is difficult to achieve such good water quality. Therefore, for CASS process, the investment of the second level of treatment, can achieve the water quality of the third level of treatment.
(5) simple management, reliable operation: the type and number of sewage treatment plant equipment is less, the control system is relatively simple, the process itself determines that no sludge expansion.
(6) Low sludge production and stable sludge nature.
(7) It has the function of nitrogen and phosphorus removal.
Design of CASS process
1, the main design parameters of CASS reactor
Maximum design water depth up to 5m~6m, MLSS is 3500mg/L~4000mg/L, filling ratio is about 30%, maximum supernatant decantation rate is 30mm/min, solid-liquid separation time is 60min, design SVI is 140mL/g, single cycle is about 30%. 140mL/g, single cycle time (i.e. 1 operation cycle) is usually 4h (standard treatment module). Treatment of municipal wastewater, CASS in the biological selector, anoxic zone and the main reaction area of the volume ratio is generally 1:5:30, according to the specific water quality and "module" test to determine. Table 1 lists the CASS process for different sizes of urban wastewater treatment reference design parameters.
2, CASS design should pay attention to the problem
(1) water balance
Industrial wastewater and domestic sewage discharges are usually uneven, how to give full play to the role of the CASS reaction tank, and the choice of design flow rate has a great deal to do with the design, if the design flow rate is inappropriate, the water intake peak water level will be more than the upper limit of the intake of water when the tank can not take full advantage of the small reaction. If the design flow is not suitable, the water level will exceed the upper limit at the peak of water intake, and the reactor can not be fully utilized at the small water intake. When the water fluctuation is large, should consider setting up a regulating pool.
(2) the choice of control mode
CASS process is characterized by the program work system, according to changes in water intake and water quality to adjust the work program to ensure that the water effect. The whole set of control system can be used on-site programmable control (PLC) and microcomputer centralized control combined, at the same time in order to ensure the normal operation of the CASS process, all the equipment using manual/automatic operation, the latter is convenient for manual debugging and self-control system failure, the former for daily use.
(3) Selection of aeration
Selection of the aeration head should try to use non-clogging forms of aeration, such as perforated tubes, underwater aerators, umbrella aerators, spiral aerators and so on. The use of microporous aeration should use high strength rubber aeration disk or tube, when the aeration is stopped, the microporous closure, aeration is open, not easy to cause microporous clogging. In addition, due to the characteristics of the CASS process itself, the selection of underwater aerators can also be appropriate according to its operating cycle and DO and so on to open a different number of units, to achieve the premise of meeting the requirements of wastewater to save energy consumption.
(4) the choice of drainage
CASS process drainage requirements and SBR the same, at present, the commonly used equipment for the rotary skimmer, the advantages of uniform drainage, drainage can be adjusted, on the bottom of the sludge interference is small, but also to prevent water surface floatation with the discharge of the water. CASS process sedimentation needs to be the end of the supernatant in a timely manner, the discharge of water drainage should be as far as possible evenly, not disturbing the sedimentation at the bottom of the pool, and should not be disturbed. Can not disturb the sludge layer precipitated in the bottom of the pool, at the same time, should also prevent the surface of the floating matter with the water discharge, affecting the quality of the effluent. At present, the common drainage method has a fixed drainage device such as no depth along the pool device outlet pipe, from top to bottom sequentially open, the advantage of drainage equipment is simple, less investment, the disadvantage is that the opening of the valve is more, the drain will be part of the sludge will be accumulated, resulting in poor quality of the initial effluent water. Floating drainage device and rotary drainage device although the price is high, but the drainage is uniform, adjustable drainage, the bottom of the sludge interference is small, but also to prevent the surface of the water floating with the water discharge, therefore, the two drainage device ear before the application of more, especially the rotary drainage device, also known as the decanter, the operation of the flexible, high operational stability.
(5) other issues needing attention
1) the impact of winter or low temperature on the CASS process and control;
2) the determination of the drainage ratio;
3) the rainy season on the impact of the water level in the pool and control;
4) the timing of sludge discharge and the age of the mud control;
5) the size of the pre-reaction area and the reaction pool length and width ratio:
6)Matching the elevation and water volume of intermittent drainage with subsequent treatment structures.
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