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During the operation of wastewater treatment by activated sludge process, there are many reasons that can inhibit the sludge activity in the biochemical system (aeration tank), lead to the sludge floating in the biochemical system and the sludge bulking in the sedimentation tank, so as to reduce the removal rate of organic matter.
The problem of sludge bulking and floating is a thorny problem that has been accompanied and often occurs since the production of activated sludge. Its main characteristics are: loose sludge structure, light weight, bulky volume and poor sedimentation compression performance; The SV value increases, sometimes up to 90%, and the SVI reaches more than 400; A large amount of sludge is lost and the effluent is turbid; The two sedimentation tank is difficult to separate from solid and liquid, and the concentration of reflux sludge is low, sometimes accompanied by a large number of bubbles, which can not maintain the normal operation of biochemical treatment. Sludge bulking and floating is one of the serious abnormal phenomena in the biochemical treatment system. It directly affects the effluent quality and endangers the operation of the whole biochemical system.
Once the sludge activity in biochemical tank (aeration tank) is inhibited, it will lead to the change of microbial properties and groups and the decrease of organic substrate removal rate. Excessive growth of some microorganisms, such as filamentous bacteria, will form froth or scum. When operating, mechanical stress and air bubbles will reduce the proportion of activated sludge and float upward. The influx of the two settling tank will cause sludge expansion in the two sedimentation tank, not only increasing the suspended solids in the effluent, but also greatly reducing the biological reaction system (aeration tank). Activity and quantity of activated sludge in.
The incidence of sludge bulking is quite high. Nearly 50% of urban sewage plants in Europe have different degrees of sludge bulking every year, and the incidence in China is also very high. Basically, sludge bulking occurs in various types of activated sludge processes. Sludge bulking not only has a high incidence and common occurrence, but also takes a long time to adjust once it is difficult to control. There are many theories on sludge bulking, sludge floating and inhibition of sludge activity in biochemical system, but they are not completely consistent. Based on reading a large number of literature, this paper discusses the causes, detection methods and control technologies of activated sludge activity inhibition, bulking and floating, and sorts out several mature and universal viewpoints, which are summarized as follows.
one Main factors causing activated sludge to float up
1.1 inlet water quality
1.1.1 Excess surfactant and grease compounds
These substances can affect the stability and permeability of cell plasma membrane, make some essential components of cells lose, and lead to microbial growth stagnation and death. When the aeration tank contains a large amount of these substances, a large number of bubbles (bubbles) will be generated. These bubbles can easily accumulate on the micelles and reduce the proportion of activated sludge. In addition, when the oil content in the influent is too high, after aeration and mixing, the oil will agglomerate on the surface of the bacterial micelle, causing the bacteria to die of hypoxia, resulting in the reduction of specific gravity and floating up.
1.1.2 pH impact
Too high or too low pH value will affect the catalysis of extracellular enzymes of activated sludge microorganisms and enzymes existing in cytoplasm and cell wall, as well as the absorption of nutrients by microorganisms. When the pH in the continuous flow aeration reaction tank is less than 4.0 or pH > 11.0, in most cases, the microbial activity in the activated sludge is inhibited, or loses activity, or even dies, resulting in sludge floating. The experimental results of treating beer wastewater and chemical wastewater by SBR method show that when the influent pH is 2.5-5.0 and 10.0-12.0, the lower (or higher) the pH value, the more serious the inhibition of sludge activity and the more floating sludge. During the reaction cycle of controlling low pH value (3.5-7.0), the pH value remains unchanged, and the activated sludge of the two kinds of wastewater begins to float when pH ≤ 5.5. On the other hand, with the increase of pH value, the flocculation of activated sludge increases due to the increase of ionized functional groups of extra cellular polymer (although the negative charge of the band increases), but when the pH value exceeds a certain range, the flocculation decreases. It can be seen that the increase of electric repulsion at this time will also cause deflocculation (suspension, non flocculation, deflocculation) and floating of activated sludge [6].
1.1.3 effect of salt content
Adjusting the pH value of the influent cannot eliminate the influence of alkalinity on activated sludge. Adjusting the pH value of the alkaline influent neutralizes the alkaline substances, but produces salt. The osmotic pressure is also different with different salt solution concentrations, and osmotic pressure is one of the important factors affecting the survival of microorganisms. If the osmotic pressure of the solution in which the microorganisms are located changes suddenly, it will lead to cell death.
1.1.4 Water temperature overheating
The suitable temperature range for microorganisms constituting activated sludge is generally 15-35 ℃. When it exceeds 45 ℃, most microorganisms in activated sludge will die and float upward (except for long-term domesticated or special microorganisms) In addition, Klaus kriebitzsch et al. Also found that the enzyme activity was better when the temperature was 20 ℃, 30 ℃ and 40 ℃, and decreased significantly when the temperature was greater than 50 ℃.
1.1.5 Toxic substrate
The substrates with toxic effect on aerobic activated sludge microorganisms mainly include COD and organic substances (phenols and their derivatives, alcohols, aldehydes and some organic acids, etc.) with high content , sulfide, heavy metal and halide. High substrate concentration can form stable compounds with the activity center of cell enzymes, resulting in inaccessible matrix, unable to be degraded, and even cell poisoning and death. After entering human cells, heavy metal ions mainly combine with - SH group on enzymes or proteins to inactivate or denature them. Trace heavy metal ions can also accumulate continuously in cells and eventually cause micro damage The most common halides are iodine and chlorine. Iodine irreversibly reacts with bacterial proteins (or enzymes) It combines with tyrosine to produce diiodine tyrosine to inactivate the bacteria. Chlorine and hydration form hypochlorous acid, which decomposes to produce strong oxidant. Moreover, the mutation of organic matter in wastewater reduces or disappears the domesticated microorganisms that can degrade organic poisons.
one point two Process operation
1.2.1 Excess aeration
The microorganism is in starvation and causes its own oxidation into the aging period, and the dissolved oxygen concentration (do) in the tank increases; or due to poor sludge activity, the linear speed of aeration impeller is too high, and there is too much oxygen supply. In short, if do increases, the sludge activity may be very good in the short term, because the metabolism is fast and the organic matter decomposes quickly, but over time, the sludge will be light and broken (but without bubbles) This kind of sludge has light color, poor activity, decreased oxygen consumption rate, increased sludge volume and sludge index, and significantly reduced the treatment effect.
1.2.2 Sludge floating caused by hypoxia
The sludge is gray, black if anoxic for too long, and often with small bubbles.
1.2.3 Sludge floating caused by denitrification
When the content of organic ammonia compounds in wastewater is high or ammonia nitrogen is high, it can be oxidized to NO3 - by nitrate bacteria and nitrite bacteria under appropriate conditions. If the sludge is accumulated in the secondary sedimentation tank or the residence time is too long, the N2 produced by NO3 - reduction will be adsorbed by the activated sludge flocs, making the activated sludge float upward.
1.2.4 Sludge floating caused by too large return flow
The sudden increase of return flow will lead to incomplete separation of gas and water, and the bubbles in the aeration tank will float up in the sedimentation area. This kind of sludge is granular and the color remains unchanged. The upward turning direction is turbulent turning from the wall of diversion area to the wall of sedimentation area.
1.2.5 Sludge floating caused by sludge accumulation at the bottom of secondary sedimentation tank
If the bottom mud of the secondary sedimentation tank is fermented, the CO2 and H2 generated will also agglomerate on the activated sludge, reducing the specific gravity of the sludge and floating up. After the sludge is decomposed to produce CH4 and H2S, small bubbles escape from the water surface first, followed by black sludge floating up.
one point three Excess growth of filamentous bacteria in activated sludge and its control
1.3.1 Temperature and load
The best growth condition of microfilament (mocrothrix patvicella) is that the temperature is 12-15 ℃, and the sludge load is less than 0.1kg / (kg · d). Its natural hydrophobicity will cause poor dehydration of activated sludge, up to 490ml / g. after the temperature is higher than 20 ℃, M. parvicella does not increase in value even if the sludge load is 0.2kg / (kg · d). It is broken into 30-80 μ M fragments, floating up in the form of scum.
1.3.2 Surfactant, lipid compounds and mechanical stress
The most frequent filamentous bacteria causing low load bulking and sludge floating are microfilaments, type 0092 and type 0041. The increase of surfactant and lipid compound concentration in influent, inoculation and mechanical stress will also cause the growth of actinomycetes. Kappeleretal observed mechanical stress (such as centrifugal pump) Damage the compact activated sludge flocs and lead to excessive growth of microfilaments [9].
1.3.3 Excessive addition of filamentous bacteria inhibitor
After hydrogen peroxide is injected into the outflow tank of the aeration tank, the filamentous bacteria disappear a few days later, and the SVI decreases from 580ml / g to 178ml / g. moreover, hydrogen peroxide also has the effect of ensuring the do of the aeration tank and removing the odor of H2S. However, if too much hydrogen peroxide is added, the activity of activated sludge will be inhibited and sludge will float up.
2. Causes of sludge bulking in sedimentation tank (secondary sedimentation tank)
Sludge bulking is divided into filamentous bacteria bulking and non filamentous bacteria bulking. Non filamentous bacteria bulking mainly occurs when the water temperature of wastewater is low and the sludge load is too high. At this time, bacteria adsorb a large amount of organic matter and have no time to metabolize. A large amount of highly viscous polysaccharide substances are accumulated outside the cells, resulting in a large increase of surface attachments and difficult precipitation and compression. And bulking can also occur when nitrogen is seriously lacking Swelling phenomenon. If nitrogen is deficient, microorganisms are easy to work and cannot make full use of carbon source to synthesize cellular substances. Excess carbon source will be turned into polysaccharide extracellular storage. This storage is a highly hydrophilic compound and is easy to form bound water, which will affect the sedimentation performance of sludge and produce high viscous sludge bulking. When non filamentous sludge bulking occurs, its biochemical treatment efficiency is still low High, the effluent is also relatively clear, and filamentous bacteria can not be seen in sludge microscopic examination. Non filamentous bacteria bulking occurs less and the harm is not very serious, so we don't focus on research here.
Filamentous bacteria bulking is common in daily practical work, and its causes are very complex. There are many factors affecting filamentous bacteria sludge bulking, but we should first recognize that activated sludge is a mixed culture system, in which there are at least 30 filamentous bacteria that may cause sludge bulking. Filamentous bacteria are an indispensable category in the symbiotic relationship with activated micelle system Important microorganism. Its existence plays a good role in purifying sewage. It is of great significance in maintaining the floc structure of sludge, maintaining the purification efficiency of biochemical treatment, and filtering suspended solids in sedimentation. It has also been proved that sludge bulking will not occur when filamentous bacteria are in balance with bacterial micelle bacteria. Only when filamentous bacteria grow more than bacterial micelle bacteria Sludge bulking will occur.
2.1 effect of sludge load on Sludge Bulking
Studies have shown that most filamentous bacteria have KS and μ The value of Max is lower than that of bacterial micelle. Therefore, according to the above monond equation, it has low KS and μ The filamentous bacteria with max value had high growth rate and high KS and μ The micelle with max value is dominant only under the condition of high substrate concentration. Similarly, the theory that low load is beneficial to the growth of filamentous bacteria is also the surface area / volume ratio (A / V) hypothesis. Here, the surface area and volume refer to the surface area and volume of microorganisms in activated sludge. This hypothesis holds that the specific surface area (A / V) of filamentous bacteria extending outside the flocs When the microorganism is limited and controlled by the substrate, the filamentous bacteria with large specific surface area are more favorable than the bacterial micelle in obtaining the substrate. As a result, the filamentous bacteria become the dominant bacteria in the aeration tank.

one point two Process operation
1.2.1 Excess aeration
The microorganism is in the starvation state and causes its own oxidation into the aging period, and the dissolved oxygen concentration (do) in the pool increases; Or due to poor sludge activity, the linear speed of aeration impeller is too high and oxygen supply is too much. In short, with the increase of do, the sludge activity may be very good in the short term, because the metabolism is fast and the decomposition of organic matter is fast, but over time, the sludge is light and broken (but there are no bubbles), floating all over the surface of the sedimentation tank like fog and flowers, and walking with the water flow. This kind of sludge has light color, poor activity, decreased oxygen consumption rate, increased sludge volume and sludge index, and significantly reduced treatment effect.
1.2.2 Sludge floating caused by hypoxia
The sludge is gray, black if anoxic for too long, and often with small bubbles.
1.2.3 Sludge floating caused by denitrification
When the content of organic ammonia compounds in wastewater is high or ammonia nitrogen is high, it can be oxidized to NO3 - by nitrate bacteria and nitrite bacteria under appropriate conditions. If the sludge is accumulated in the secondary sedimentation tank or the residence time is too long, the N2 produced by NO3 - reduction will be adsorbed by the activated sludge flocs, making the activated sludge float upward.
1.2.4 Sludge floating caused by too large return flow
The sudden increase of return flow will lead to incomplete separation of gas and water, and the bubbles in the aeration tank will float up in the sedimentation area. This kind of sludge is granular and the color remains unchanged. The upward turning direction is turbulent turning from the wall of diversion area to the wall of sedimentation area.
1.2.5 Sludge floating caused by sludge accumulation at the bottom of secondary sedimentation tank
If the sediment in the secondary sedimentation tank is fermented, the generated CO2 and H2 will also agglomerate on the activated sludge, reducing the specific gravity of the sludge and floating up. After the sludge is decomposed to produce CH4 and H2S, small bubbles first escape from the water surface, and then black sludge floats up.
one point three Excess growth of filamentous bacteria in activated sludge and its control
1.3.1 Temperature and load
The optimum growth condition of microfilament (mocrothrix patvicella) is that the temperature is 12-15 ℃, and the sludge load is less than 0.1kg / (kg · d). Its natural hydrophobicity will cause poor dehydration of activated sludge, up to 490ml / g. When the temperature is higher than 20 ℃, M. parvicella does not increase in value even if the sludge load is 0.2kg / (kg · d). It broke into 30-80 μ M fragments, floating up in the form of scum.
1.3.2 Surfactant, lipid compounds and mechanical stress
The most frequent filamentous bacteria causing low load bulking and sludge floating are microfilaments, type 0092 and type 0041. In the influent, the increase of the concentration of surfactant and lipid compounds, inoculation and mechanical stress will also cause the growth of actinomycetes. Kappeleretal observed that mechanical stress (such as centrifugal pump) damaged tight activated sludge flocs and led to excessive growth of microfilaments [9].
1.3.3 Excessive addition of filamentous bacteria inhibitor
After hydrogen peroxide was injected into the outflow tank of the aeration tank, the filamentous bacteria disappeared and the SVI decreased from 580ml / g to 178ml / g. Hydrogen peroxide can also ensure the aeration tank do and remove H2S odor. However, if too many people are added, the activity of activated sludge will be inhibited and the sludge will float up.
2. Causes of sludge bulking in sedimentation tank (secondary sedimentation tank)
Sludge bulking can be divided into filamentous bacteria bulking and non filamentous bacteria bulking. The expansion of non filamentous bacteria mainly occurs when the water temperature of wastewater is low and the sludge load is too high. At this time, the bacteria adsorb a large amount of organic matter and have no time to metabolize. A large amount of highly viscous polysaccharide substances are accumulated outside the cells, resulting in a large increase in surface attachments, which is difficult to precipitate and compress. When nitrogen is seriously deficient, expansion can also occur. Because in case of lack of nitrogen, microorganisms are easy to work and cannot make full use of carbon source to synthesize cellular substances. Excess carbon source will be turned into polysaccharide extracellular storage. This storage is a highly hydrophilic compound and is easy to form bound water, which will affect the sedimentation performance of sludge and produce high viscosity sludge bulking. When non filamentous sludge bulking occurs, its biochemical treatment efficiency is still high, the effluent is still relatively clear, and filamentous bacteria can not be seen in sludge microscopic examination. The occurrence of non filamentous bacteria swelling is less, and the harm is not very serious, so we don't focus on it here.
Filamentous swelling is common in daily practical work, and its causes are also very complex. There are many factors affecting filamentous bacteria sludge bulking, but we should first realize that activated sludge is a mixed culture system, in which there are at least 30 filamentous bacteria that may cause sludge bulking. Filamentous bacteria are indispensable and important microorganisms in the symbiotic relationship with active micelle system. Its existence plays a good role in purifying sewage. It is of great significance to maintain the floc structure of sludge, maintain the purification efficiency of biochemical treatment, and filter suspended solids in sedimentation. Facts have also proved that sludge bulking will not occur when filamentous bacteria are in balance with bacterial micelles. Sludge bulking will occur only when filamentous bacteria grow more than bacterial micelles.
2.1 effect of sludge load on Sludge Bulking
Studies have shown that most filamentous bacteria have KS and μ The value of Max is lower than that of bacterial micelle. Therefore, according to the above monond equation, it has low KS and μ The filamentous bacteria with max value had high growth rate and high KS and μ The micelle with max value is dominant under the condition of high substrate concentration. The same theory that low load is beneficial to the growth of filamentous bacteria is the surface area / volume ratio (A / V) hypothesis. The surface area and volume here refer to the surface area and volume of microorganisms in activated sludge. This hypothesis holds that the specific surface area (A / V) of filamentous bacteria extending outside the floc is much larger than that of bacterial micelles. When the microorganism is limited and controlled by the substrate, the filamentous bacteria with large specific surface area are more favorable than bacterial micelles in obtaining the substrate. As a result, the filamentous bacteria become the dominant bacteria in the aeration tank.
The view that low load is easy to lead to sludge bulking has been well explained both in practical operation and in theory. However, in China, the load design of biochemical reaction is usually high, and a large amount of sludge bulking occurs under high load conditions, which has aroused people's doubt about this theory. In fact, sludge bulking under high load conditions is often caused by insufficient oxygen supply and reduced DO concentration in the aeration tank. We will focus on the effect of dissolved oxygen do on sludge bulking. See for details http://www.dowater.com More relevant technical documentation.
2.2 effect of dissolved oxygen concentration on Sludge Bulking
The process of microbial degradation of organic matter is essentially the process of oxygen utilization. Dissolved oxygen is an important control parameter in the operation of activated sludge process. The concentration of do in aeration tank directly affects the removal efficiency of organic matter and the growth of activated sludge. Low DO concentration has been considered as one of the main factors causing filamentous sludge bulking. Filamentous bacteria proliferate faster than flocculent bacteria at low DO concentration due to their large specific surface area and low oxygen saturation constant, resulting in filamentous sludge bulking. According to various research reactions, the critical value of the effect of do on sludge bulking is uncertain. The requirement of DO concentration is closely related to sludge load. The higher the load, the greater the corresponding critical value. The determination of this value is closely related to the process selection, tank type and inlet type, which can be obtained only according to the actual situation and combined with the experiment.
2.3 impact of other aspects on Sludge Bulking

2.3.1 Sewage type
The type of sewage has an obvious effect on sludge bulking. Generally speaking, sewage containing easily biodegradable and dissolved organic components, especially low molecular weight hydrocarbons, sugars and organic acids, is easy to cause sludge bulking, such as brewery, dairy, petrochemical and papermaking wastewater.
2.3.2 Imbalance of nutrients
When n and P in sewage are insufficient, it is easy to cause sludge bulking. It is generally considered that the appropriate ratio of N and P is BOD5: n: P = 100:5:1. Many studies have shown that many filamentous bacteria have strong affinity for nutrients N and P, which may be the reason for sludge bulking caused by lack of nutrients.
2.3.3 pH and temperature
It is generally believed that low pH is easy to cause the mass reproduction of filamentous bacteria. The effect of temperature on filamentous bacteria is also very common. For example, microthix parvicella is dominant in filamentous flora in winter, while nocardiaform, 0041 or nostocoida limnicda are easy to reproduce in large numbers in warm seasons.
In addition, the organic acid and hydrogen sulfide produced by the early anaerobic digestion of sewage before the influent treatment system may also lead to sludge bulking. Sulfur bacteria, such as Thiobacillus Baini and thiomyces, can obtain energy from hydrogen sulfide oxidation. Such bacteria proliferate in very long filaments, sometimes up to 1 cm, resulting in sludge bulking.
3. Detection method of sludge activity inhibition and floatation in biochemical system (aeration tank)
three point one Determine the oxygen consumption rate (our) and ATP of sludge
Measuring the oxygen consumption rate (our) of activated sludge can judge the non-toxic flow in, load conditions and sludge discharge balance. If adenobitter triphosphate (ATP) is determined at the same time, the microbial biomass and activity can also be analyzed quantitatively from the aspect of treatment function. According to the research of P.E. Jorgensen et al., the determination of ATP content and our is a reliable method to detect biomass activity.
three point two Diagnosis of activated sludge status and performance by indicator biology
Microscopic examination of microorganisms in activated sludge is carried out by microscope. Protozoa and metazoa (collectively referred to as micro animals) are relatively larger than individual bacteria, which are easy to observe, identify and count under the microscope, and are more sensitive to the changes of external environmental conditions. As an indicator organism to diagnose the state and performance of activated sludge, they have been widely used in engineering practice.
4. Technical measures for controlling sludge floating in biochemical system
① The most feasible and economical way to stabilize the inlet water quality of the aeration tank is the final water return, which is used to dilute and adjust the organic matter concentration in the inlet water of the aeration tank to make it stable within a certain range. The prerequisite for the final water return is that the treatment capacity of the sewage treatment plant must be greater than the actual inlet water.
② The sewage treatment plant shall consider setting a large volume regulating tank (homogenizing tank) and controlling the liquid level of homogenizing tank (regulating tank). Due to high liquid level, the water buffer capacity of homogenization tank will be reduced or even lost; The low liquid level operation not only has poor homogenization effect, but also is easy to make the oil and impurities at the bottom of the homogenization tank enter the aeration tank, causing the activated sludge to float upward under impact. The liquid level should be controlled at 50% - 70%.
③ Add nutrients reasonably. Due to the imbalance of nutrient proportion in industrial wastewater, carbon source is often sufficient and nutrients such as nitrogen and phosphorus are insufficient. Therefore, additional nutrients must be added when treating industrial wastewater. Urea and phosphate are generally used as nitrogen and phosphorus sources, but the dosage should not be excessive.
④ The aeration tank is equipped with neutralization tank and pH automatic regulation system composed of alkali tank, acid tank, pH detector and pH automatic regulation valve to control the pH value of the inlet water of the aeration tank within the required range.
⑤ Pure oxygen aeration is adopted. The pure oxygen aeration device imported from West Germany has never seen sludge floating since it was put into operation for 5 years.
⑥ Sludge floating caused by sludge poisoning can increase aeration, reduce water inflow and remove dead sludge.
⑦ The microbial composition of activated sludge mainly depends on wastewater composition, flow form, operating conditions and appropriate design. Because it is almost difficult to control the composition of wastewater in the actual treatment process, it is very important to optimize the operation conditions and reactor design.
5. General solution to sludge bulking in secondary sedimentation tank
5.1 emergency measures
It is suitable for temporary emergency. The main method is to add drugs to enhance sludge sedimentation performance or directly kill filamentous bacteria. Adding coagulants such as iron salt and aluminum salt at the inlet of aeration tank can directly improve the flocculation and compactness of sludge and ensure the precipitated effluent. In addition, add some agents that can kill filamentous bacteria, such as chlorine, ozone, hydrogen peroxide, etc. Chlorine addition in the return sludge can also eliminate sludge bulking. When the effective chlorine is 10-20mg / L, it can effectively kill jerseys bacteria and sulfur bacteria; Higher than 20mg / l may cause harm to flocculate forming bacteria. Therefore, when using chlorine, it must be added reasonably according to the allowable range of dosage. Adding hydrogen peroxide and ozone can also destroy filamentous bacteria. Ozone, hydrogen peroxide and other oxidants have killing effect on Jersey bacteria only under high metering conditions.
Generally, this method can quickly reduce the SVI value, but this method does not fundamentally control the reproduction of filamentous bacteria. Once the dosing is stopped, the sludge bulking phenomenon can make a comeback. Moreover, the dosing may damage the microbial growth environment of the biochemical system and reduce the treatment effect. Therefore, this method can only be used for temporary emergency.
5.2 improving biochemical environment
When sludge bulking occurs in the sewage plant, it can not be solved by changing the process flow, tank type and aeration mode. It can only inhibit or eliminate the excessive reproduction of filamentous bacteria by changing the microbial growth environment in the biochemical tank on the basis of the running process. In the case of different processes and water quality, it is difficult to have a universal solution. However, several problems that should be paid attention to in biochemical process must be paid attention to.
5.2.1 Control of sewage properties
Firstly, the pH value should be checked and adjusted. When the pH value is below 5, it will not only be beneficial to sludge bulking, but also be harmful to normal biochemical reaction. Therefore, when the pH value is low, it should be adjusted in time.
In addition, the water temperature has a certain impact on sludge bulking. The suitable temperature range for microorganisms constituting activated sludge is generally 15-35 ℃, and the microbial metabolic rate increases by twice every 1 ℃ increase in temperature. When the temperature exceeds 40 ℃, most microorganisms in activated sludge will die and lead to sludge bulking. In cold areas in the north, attention should be paid to the water temperature in winter. If the water temperature is low, it should be heated, because low temperature will also lead to sludge bulking. Blast aeration can effectively increase the water temperature in the aeration tank.
When the nutrient content in sewage water is insufficient or unbalanced, it shall be added. N. P content shall be controlled at BOD: n: P = 100:5:1.
If the digestion phenomenon occurs before the sewage treatment biochemical system, the low molecular organic acids produced will be conducive to the growth of filamentous bacteria. At this time, the wastewater can be improved by pre aeration in the regulating tank. Generally, the air diffuser is used to aerate the regulating tank with an effective water depth of 3-5M, and the air supply can be controlled at 0.5-1.0m3 (air) / m3 wastewater · H. It can keep the wastewater in the regulating tank fresh and effectively prevent the odor caused by anaerobic.
5.2.2 maintain sufficient dissolved oxygen in the biochemical tank, generally controlled at 0.3 --- 2mg / L; For high load biochemical system, do should be controlled at least 2 mg / L.
5.2.3 adjust the sludge load in the aeration tank. The operation experience shows that filamentous sludge bulking is easy to occur if the sludge load exceeds 0.35kgbod / kgmlss. D.
5.2.4 shorten the hydraulic retention time of the sedimentation tank and timely discharge or return the sludge in the sedimentation tank to prevent anaerobic phenomenon. In case of anaerobic phenomenon, various gases generated will be adsorbed on the sludge, which will also make the sludge float upward and deteriorate the sedimentation performance. Moreover, anaerobic sludge reflux will also lead to the proliferation of filamentous bacteria. In this case, in addition to discharging sludge, removing dead corners in the sedimentation tank and shortening the residence time of sludge in the tank, it is also necessary to increase the do value of the aeration tank to keep the water in and out of the sedimentation tank relatively dissolved oxygen, or aerate and regenerate before the sludge flows back into the biochemical tank.
After solving the above problems, if the sludge bulking phenomenon is still not controlled, it must be analyzed according to the actual situation. The following puts forward some guiding methods for several common processes for reference.
A. High load activated sludge process
At present, the domestic design of activated sludge process usually adopts medium load (0.3kgbod5 / (kgmlss · d)), but in practice, people always adopt higher load from the economic point of view, so the sludge bulking under high load has a wide significance in China. In the case of high load, the most common problem is the lack of do, so first increase the gas-water ratio, strengthen aeration, and adopt jet aeration at the head end of the plug-flow aeration tank. Observe for a period of time to find out the problem.
If the situation does not improve for a period of time after the above measures are taken, it can be considered to add soft filler at the head of the aeration tank. This part has a high removal rate of organic acids, so as to remove the growth promoting factors of filamentous bacteria and help the growth of flocculent bacteria. This method is more effective, but the cost is high, and it is inconvenient for future maintenance management. Or a selector with hydraulic retention time of about 15min is set in front of the aeration tank, which can generally effectively inhibit the growth of filamentous bacteria.
For the SBR process with intermittent influent, the reactor itself is completely mixed, and there is a concentration gradient in the matrix of pollutants in time, so there is no need to set another selector. Generally, the reason for sludge bulking in intermittent SBR process is that the sludge concentration is too high, while the influent organic matter concentration is low or the water volume is too small, resulting in low sludge load. In this case, reducing the discharge ratio, increasing the initial matrix concentration and forced sludge discharge of SBR can generally effectively control the sludge bulking phenomenon. For continuous influent SBR processes such as ICEAS and CASS, if sludge bulking occurs, it is necessary to set up a pre reaction zone or bioreactor at the influent end.
B. Low load activated sludge process
The substrate concentration in the aeration tank of low load activated sludge process is low, and filamentous bacteria are easy to obtain high growth efficiency, so it is the most prone to sludge bulking. In addition to finding ways in terms of water quality and aeration, the most fundamental and effective way is to divide the aeration tank into multiple cells and operate in a push flow mode, or add a small pre aeration tank set in cells as a biological selector. In this selector, high sludge load is used to adsorb some organic substances and eliminate organic acids. This method not only helps to inhibit sludge bulking, but also can effectively improve the effect of biochemical treatment. The method of adding filler in aeration tank is also applicable to low load complete mixing process.
For a / O and A2 / O processes, the anoxic section, anaerobic section and sludge reflux system can be set before the aerobic section to make the mixed flora alternately in anoxic and aerobic state, and make the organic concentration change periodically, which not only controls the sludge bulking, but also improves the sludge sedimentation performance. The continuous influent systems such as alternating working oxidation ditch and UNITANK process have a practical "selector" in time and space, so they have a strong ability to control sludge bulking. If sludge bulking occurs in these two processes, the sludge load and do in the tank can be adjusted by adjusting aeration, controlling dissolved oxygen and controlling return sludge. Generally, sludge bulking can be controlled through improvement over a period of time.
6. Summary
In general, sludge bulking is due to a wide variety of filamentous bacteria, and the suitable growth environment is also different. In the case of different processes and different water quality, the growth environment of microorganisms is very subtle, which requires that when sludge bulking occurs, water treatment workers need to make a large number of practical experiments and analysis according to the actual situation and bold practice in order to solve the problem of sludge bulking.