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 rate in China is also very high. Basically, sludge bulking will occur in various types of activated sludge processes at present. Sludge bulking is not only high and common, but also difficult to control once it happens, and usually takes a long time to adjust. There are many theories about sludge bulking, but they are not completely consistent or even contradictory, which brings great trouble to water treatment workers. This paper will start with the internal factors of sludge bulking, sort out several mature and universal viewpoints, and summarize the general methods of sludge bulking control.
1, the cause of sludge bulking
Sludge bulking can be divided into filamentous bulking and non-filamentous bulking. The expansion of non-filamentous bacteria mainly occurs when the wastewater temperature is low and the sludge load is too high. At this time, the bacteria adsorbed a large amount of organic matter, and it was too late to metabolize it, and accumulated a large amount of highly viscous polysaccharide substances outside the cell, which greatly increased the surface attachments and made it difficult to precipitate and compress. However, when nitrogen is seriously deficient, it will also cause expansion. Because if nitrogen is lacking, microorganisms can't make full use of carbon sources to synthesize intracellular substances, and excess carbon sources will be converted into extracellular storage of polysaccharide, which is a kind of compound with strong hydrophilicity and easy to form bound water, thus affecting the settling performance of sludge and producing sludge bulking with high viscosity. When non-filamentous sludge bulking occurs, its biochemical treatment efficiency is still high, the effluent is still clear, and filamentous bacteria can not be seen in sludge microscopic examination. The expansion of non-filamentous bacteria is rare and the harm is not very serious, so I will not focus on it here.
Filamentous bacteria swelling is very common in daily practical work, and its reasons are also very complicated. There are many factors affecting filamentous sludge bulking, but first of all, we should realize that activated sludge is a mixed culture system, in which at least 30 kinds of filamentous bacteria may cause sludge bulking. Filamentous bacteria are an indispensable and important microorganism in the relationship with active micelle system. Its existence has played a very 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 play a role in filtering suspended solids in precipitation. Facts have also proved that sludge bulking will not occur when filamentous bacteria and bacteria in micelles are in equilibrium, and sludge bulking will only occur when the growth of filamentous bacteria is greater than that of bacteria in micelles.
1, the effect of sludge load on sludge bulking
It is generally believed that the growth of microorganisms in activated sludge conforms to Monod equation:
Where x refers to biological concentration, mg/l;
S—— growth limiting substrate concentration, mg/l;
μ —— growth limit substrate concentration, mg/l;
Ks-saturation constant, the value of which is μ=μmax/2 matrix concentration, mg/L;
μ max-the maximum specific growth rate of microorganisms at saturated concentration, d- 1.
It has been proved that the KS and μmax values of most filamentous fungi are lower than those of bacterial micelles. Therefore, according to the Monond equation above, filamentous fungi with low KS and μmax values have higher growth rate under the condition of low substrate concentration, while bacterial micelles with high KS and μmax values are dominant under the condition of high substrate concentration. There is also the hypothesis of surface area/volume ratio (A/V), that is, low load is beneficial to the growth of filamentous bacteria. 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 bacteria in micelles. When microorganisms are restricted and controlled by the substrate, filamentous bacteria with large specific surface area are more conducive to obtaining the substrate than bacterial micelles, and as a result, filamentous bacteria become the dominant bacteria in the aeration tank.
The viewpoint that low load is easy to cause sludge bulking has a mature explanation in practice and theory. However, in China, the load design of biochemical reaction is usually high, but a lot of sludge bulking occurs under high load conditions, which makes people doubt this theory. In fact, sludge bulking under high load is often caused by insufficient oxygen supply and the decrease of DO concentration in aeration tank. We will focus on the effect of dissolved oxygen DO on sludge bulking.
2. Effect of dissolved oxygen concentration on sludge bulking
The process of microbial degradation of organic matter is essentially a process of using oxygen. Dissolved oxygen is an important control parameter in the operation of activated sludge process, and 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 always been considered as one of the main factors causing filamentous sludge bulking. Because of the large specific surface area and low oxygen saturation constant of filamentous bacteria, at low DO concentration, filamentous bacteria proliferate faster than flocculants, which leads to filamentous sludge bulking. According to various research reactions, the critical value of the influence 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 process selection, tank type and influent type, and must be obtained according to the actual situation and experiments.
3. The influence of other factors on sludge bulking.
1) sewage type
Sewage types have obvious influence on sludge bulking. Generally speaking, sewage containing biodegradable and soluble organic components, especially low molecular weight hydrocarbons, sugars and organic acids, is easy to cause sludge bulking, such as brewery, dairy, petrochemical and paper-making wastewater.
2) Nutritional imbalance 3)
When N and P in sewage are insufficient, sludge bulking is easy to occur. All night, I thought that the appropriate ratio of N and P was 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 cause of sludge bulking due to lack of nutrients.
4) pH value and temperature
It is generally believed that low pH value is easy to cause filamentous bacteria to multiply. The influence of temperature on filamentous bacteria is also very common. For example, microsporidia is dominant in filamentous flora in winter, while Nocardia, 004 1 or Nostoc are easy to breed in warm season.
In addition, organic acids and hydrogen sulfide produced by early anaerobic digestion of sewage before entering the treatment system may also lead to sludge bulking. Sulfur bacteria, such as Thiobacillus bainite and Thiobacillus, can obtain energy from hydrogen sulfide oxidation. But this kind of bacteria proliferate in a very long filamentous form, sometimes as long as 1 cm, which leads to sludge bulking.
2. General solution of sludge bulking
Category I: Emergency measures
Suitable for temporary emergency, the main method is to add drugs to enhance sludge settling performance or directly kill filamentous bacteria. Adding coagulant such as iron salt and aluminum salt can directly improve the density of sludge and ensure the amount of precipitation. In addition, adding some chemicals, such as chlorine, to the returned sludge can also eliminate sludge bulking. Adding hydrogen peroxide and ozone can also destroy filamentous bacteria.
This method can generally reduce the SVI value quickly, but these methods can not fundamentally control the reproduction of filamentous bacteria. Once dosing is stopped, sludge bulking will come back. Moreover, adding chemicals may destroy the microbial growth environment of biochemical system, leading to the reduction of treatment effect, so this method can only be used as a temporary emergency.
The second category: improving the biochemical environment
When sludge bulking occurs in sewage plants, it can not be solved by changing the process flow, tank type and aeration mode. Only by changing the microbial growth environment in the biochemical pool on the basis of the operation process can the overgrowth of filamentous bacteria be inhibited or eliminated. In the case of different processes and water quality, it is difficult to have a universally applicable solution. However, some problems in biochemical technology must be paid attention to.
1) control of sewage properties
First of all, check and adjust the pH value. When the pH value is lower than 5, it is not only beneficial to sludge bulking, but also unfavorable to normal biochemical reaction, so it should be adjusted in time when the pH value is low. In addition, we must pay attention to the water temperature in winter in the cold areas in the north, and heat it when the water temperature is low, because low temperature will also lead to sludge bulking. Using blast aeration in winter can effectively raise the water temperature.
When the nutrients in sewage are insufficient or unbalanced, they should be added. The content of nitrogen and phosphorus should be controlled around BOD: N: P = 100: 5: 1.
If digestion occurs before the biochemical system of sewage treatment, low molecular weight organic acids will be beneficial to the growth of filamentous bacteria. At this time, the wastewater can be pre-aerated in the regulating tank to improve it. Generally, an 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/m3.hour of wastewater. Can keep the wastewater in the regulating tank fresh, and effectively prevent the odor generated by anaerobic.
2) For high-load biochemical systems, it is particularly important to keep enough dissolved oxygen in the pool; 3) In general, at least DO >； should be controlled; 2 mg/liter.
4) Sludge in the sedimentation tank should be discharged or refluxed in time; 5) Anaerobic phenomenon should be prevented. If there is anaerobic phenomenon, 6) all kinds of gases will be adsorbed on the sludge, 7) the sludge will also float, and 8) the sedimentation performance will become worse. 9) Anaerobic sludge reflux can also lead to the reproduction of filamentous bacteria. In this case, in addition to discharging the sludge, clearing the dead angle in the sedimentation tank (10) and shortening the residence time of the sludge in the tank, the DO value of the aeration tank (12) should also be increased to keep the dissolved oxygen in the water entering and leaving the sedimentation tank (13), or to carry out aeration regeneration before the sludge flows back to the biochemical tank. As shown on the left.
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 are some guiding methods for several common processes for the reference of sewage treatment workers.
A. high-load activated sludge process
At present, the design of activated sludge process in China usually adopts medium load (0.3 kg BOD5/(kg mlss d)), but in practice, people always adopt higher load from the economic point of view, so sludge bulking under high load has a wider significance in China. Under the condition of high load, the most common problem is insufficient DO, so firstly, we should improve the gas-water ratio, strengthen aeration, and adopt jet aeration at the head end of the plug-flow aeration tank, and observe for a period of time to find out the problem.
If the situation does not improve within a period of time after taking the above measures, soft packing can be considered at the head of aeration tank. This part has a high removal rate of organic acids, thus removing the growth promoting factors of filamentous bacteria and contributing to the growth of flocculants. This method is effective, but it is costly and inconvenient for future maintenance and 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 SBR process with intermittent water inflow, 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 up another selector. Usually, the reason of sludge bulking in batch SBR process is that the sludge concentration is too high, while the influent organic matter concentration is low or the water quantity is too small, resulting in low sludge load. In this case, sludge bulking can be effectively controlled by reducing the discharge ratio, increasing the initial concentration of substrate and forcibly discharging sludge from SBR. If sludge bulking occurs in SBR processes with continuous water inflow such as ICEAS and CASS, it is necessary to set a pre-reaction zone or bioreactor at the water inflow end.
B. low-load activated sludge process
The substrate concentration in the low-load activated sludge aeration tank is low, and filamentous bacteria are easy to obtain high growth efficiency, so sludge bulking is most likely to occur. In addition to finding ways to improve water quality and aeration, the most fundamental and effective way is to divide the aeration tank into multiple units and run it in a push-flow mode, or to add a small pre-aeration tank in the unit as a biological selector, so as to absorb some organic matters and eliminate organic acids by using high sludge load. This method not only helps to inhibit sludge bulking, but also effectively improves the effect of biochemical treatment. The method of adding filler in aeration tank is also suitable for low load complete mixing process.
For A/O and A2/O processes, an anoxic stage, an anaerobic stage and a sludge reflux system can be set before the aerobic stage, so that the mixed flora is alternately in anoxic and aerobic state, and the organic matter concentration changes periodically, which not only controls the sludge bulking, but also improves the sludge settling performance. However, systems with continuous water inflow, such as alternating oxidation ditch and UNITANK process, have a practical "selector" in time and space, so they have strong control ability on sludge bulking. If sludge bulking occurs in these two processes, the sludge load and DO in the tank can be adjusted by adjusting the aeration rate, controlling dissolved oxygen and controlling sludge reflux. After a period of improvement, sludge bulking can generally be controlled.
3. Summary
Generally speaking, sludge bulking is due to a wide variety of filamentous bacteria and suitable growth environment. Under the conditions 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 do a lot of practical experiments and analysis according to the actual situation, and boldly practice to solve the problem of sludge bulking. The following is a summary of the views of this article.
Filamentous bacteria are an indispensable part of growing and processing microorganisms. The phenomenon of sludge bulking lies in the excessive growth of filamentous bacteria, and the root of eliminating sludge bulking lies in the balanced growth of filamentous bacteria and activated sludge bacteria micelles; Full mixing is more likely to cause sludge bulking than plug flow, and both low sludge load and high sludge load are prone to sludge bulking. From the aspects of water temperature, pH value, nutrients and whether there is digestion reaction before treatment, the influent quality is the first problem to be investigated in treating sludge bulking. Sludge bulking under high load generally lies in insufficient dissolved oxygen; Using biological selector is an effective method for sludge bulking under low load. Due to the diversity of filamentous bacteria, there are still many inconsistencies between the theoretical explanation and the actual report on sludge bulking. Only by boldly summing up practice and communicating extensively with peers can we find an effective solution faster.