Wastewater Treatment Basic Terminology: What Is Sludge Bulking
Sludge bulking refers to the phenomenon where the sludge structure becomes extremely loose, its volume increases and rises to the surface, making sedimentation and separation difficult and affecting effluent quality. Essentially all types of activated sludge processes experience sludge bulking, and once it occurs it is difficult to control, usually requiring a long time to adjust. The incidence of sludge bulking is quite high: in Europe nearly 50% of municipal wastewater plants experience sludge bulking of varying degrees every year, and the incidence in China is also very high. Regarding sludge bulking, there are many theories from various perspectives, but they are not entirely consistent, and some even contradict each other, causing great trouble to water treatment workers.
The sludge structure is loose, its mass becomes light, and its settling and compression performance is poor; the SV value increases, sometimes reaching 90%, and the SVI exceeds 300; large amounts of sludge are lost, and the effluent is turbid; secondary sedimentation makes solid-liquid separation difficult and the return sludge concentration is low, sometimes accompanied by the generation of large amounts of foam, making it impossible to maintain the normal operation of the biochemical treatment.
Sludge bulking is one of the more serious abnormal phenomena in biochemical treatment systems. It directly affects effluent quality and endangers the operation of the entire biochemical system.
The incidence of sludge bulking is quite high: in Europe nearly 50% of municipal wastewater plants experience sludge bulking of varying degrees every year, and the incidence in China is also very high. Essentially all types of activated sludge processes experience sludge bulking. Sludge bulking not only has a high incidence and is widespread, but once it occurs it is difficult to control, usually requiring a long time to adjust. Regarding sludge bulking, there are many theories from various perspectives, but they are not entirely consistent, and some even contradict each other, causing great trouble to water treatment workers.
In addition, this technology also has broad application and practice in related fields.
The KS and umax values of most filamentous bacteria are lower than those of zoogloeal flocs, so, according to the Monod equation above, filamentous bacteria with low KS and umax values have a high growth rate under low substrate concentration conditions, while zoogloeal flocs with higher KS and umax values only dominate under high substrate concentration conditions. The surface area/volume ratio (A/V) hypothesis is another theory that also holds that low load is favorable to filamentous bacterial growth. Here, surface area and volume refer to the surface area and volume of microorganisms in the activated sludge. This hypothesis holds that the specific surface area (A/V) of filamentous bacteria extending outside the flocs is much larger than that of zoogloeal bacteria. When microorganisms are in a substrate-limited and controlled state, filamentous bacteria with a larger specific surface area have an advantage over zoogloeal flocs in obtaining substrate, and as a result filamentous bacteria become the dominant bacteria in the aeration tank.
The view that low load easily leads to sludge bulking has a relatively mature explanation both in actual operation and in theory. But in China, the load design of biochemical reactions is usually high, and much sludge bulking occurs under high-load conditions. In fact, sludge bulking under high-load conditions is often caused by insufficient oxygen supply and reduced DO concentration in the aeration tank.
The degradation of organic matter by microorganisms is essentially the process of utilizing oxygen. Dissolved oxygen (DO) is an important control parameter in the operation of the activated sludge process; the DO concentration in the aeration tank directly affects the removal efficiency of organic matter and the growth of activated sludge. Low DO concentration has long been considered one of the main factors causing filamentous sludge bulking. Because of their larger specific surface area and lower oxygen saturation constant, filamentous bacteria multiply faster than floc-forming bacteria at low DO concentrations, leading to filamentous sludge bulking. According to various research responses, the critical value of DO's influence on sludge bulking is not certain. The requirement for DO concentration is closely related to the 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 by combining experiments with actual conditions.
On this basis, industry experts have also conducted extensive research and improvement.
When the content of soluble organic matter in wastewater is high, sludge bulking also easily occurs. Generally, the soluble organic matter referred to here is mainly low-molecular soluble organic matter, including the above-mentioned monosaccharides and disaccharides. In fact, in the treatment of dairy production wastewater, fermentation wastewater, and sugar wastewater (containing large amounts of soluble organic matter), sludge bulking easily occurs. Generally speaking, compared with other free bacteria, filamentous bacteria in activated sludge have weaker hydrolysis ability for high-molecular substances and are also difficult to absorb insoluble substances. Therefore, when wastewater contains a lot of soluble organic matter, filamentous bacteria can easily utilize and reproduce with it. This also makes filamentous bacterial sludge bulking prone to occur. For highly viscous non-filamentous bulking, because the wastewater contains more soluble saccharide substances, activated sludge microorganisms can also easily utilize them to produce more highly viscous polysaccharide substances, which also leads to the occurrence of this type of sludge bulking.
Activated sludge microorganisms, in order to carry out normal growth and reproduction, in addition to carbon sources, also need nitrogen, phosphorus and other nutrients. There should be an appropriate proportion between nitrogen, phosphorus and carbon; the generally experienced proposed proportion is usually: BOD5:N:P=100:5:1. When the nitrogen and phosphorus content in wastewater is insufficient, sludge bulking also easily occurs. For example, in activated sludge, the surface area of filamentous bacteria is relatively larger than that of other microorganisms, making it easy to take up substrates. Therefore, when the proportion of nitrogen and phosphorus relative to BOD5 is insufficient, filamentous bacteria, with the above characteristics, can more easily utilize substrates than other microorganisms and still live normally and grow and reproduce. Under such circumstances, other microorganisms in the activated sludge, because nitrogen and phosphorus are not satisfied, gradually decline. As a result, filamentous bacteria increase greatly, leading to the occurrence of filamentous bacterial sludge bulking.
In the operation of the aeration tank, the dissolved oxygen content of the mixed liquor is also an important issue, because different microorganisms have different requirements for dissolved oxygen. Judging from past practical experience, it is unfavorable if the DO concentration in the aeration tank is too low, and sludge bulking easily occurs. Although filamentous bacteria are aerobic bacteria, they are different from other aerobic bacteria in activated sludge: under low DO conditions in activated sludge, most aerobic bacteria can hardly continue to grow and reproduce, but filamentous bacteria can still adapt to this environment and continue to grow and reproduce. Thus filamentous bacterial sludge bulking easily occurs. Moreover, even if they are kept in an anaerobic state for a considerable time, they will not lose vitality; once the aerobic state is restored, they will grow and reproduce again.
According to relevant experience, when the DO of the aeration tank mixed liquor is below 0.5 mg/L, a large number of sulfur bacteria (Beggiatoa and Thiothrix) are found in activated sludge microscopy, but few sheath-bearing filamentous bacteria (Sphaerotilus) are found. For example, in Shanghai at the turn of spring and summer and in midsummer, the water temperature is high (up to over 30 degrees C), the oxygen partial pressure is low, and it is also the peak electricity consumption period with tight power supply, so the aeration tank often shows hypoxia with low DO concentration, and filamentous sludge bulking often occurs in the activated sludge; the microscopic examination result is that it is caused by excessive growth of Thiothrix and Beggiatoa. Therefore, when the DO is low (generally below 0.5 mg/L) and the water temperature is high (generally 30-36 degrees C), it is suitable for the growth and reproduction of Thiothrix and Beggiatoa, and the sludge bulking is a sulfur-bacterial filamentous bulking. In autumn, when the water temperature is between 20 and 28 degrees C and the DO concentration rises slightly, the filamentous bulking of activated sludge is found to be the result of excessive growth of Beggiatoa and Sphaerotilus. In fact, at a higher DO concentration, such as as high as 7 mg/L, filamentous bulking of sludge can still be found, in which Sphaerotilus dominates among the filamentous bacteria. This shows that both high and low DO concentrations can cause filamentous bulking mainly caused by Sphaerotilus.
It is worth noting that the technology and standards in this field are also continuously developing and improving.
According to practical experience, if the pH value of the aeration tank liquor is kept below 6.0 for a long time, filamentous microorganisms in the activated sludge will occupy a dominant position, and the sludge volume index SVI value will increase, leading to filamentous bacterial sludge bulking. Because when the pH value is in the range of 5.8-8.1, it is suitable for the growth and reproduction of Sphaerotilus natans. In addition, Geotrichum candidum may also proliferate in the pH range of 3-12. According to this situation, it can be said that acidic conditions are especially favorable for the growth and reproduction of filamentous bacteria and become an inducement for filamentous bacterial sludge bulking.
On this basis, industry experts have also conducted extensive research and improvement.
When the load rate is too low, filamentous bacterial sludge bulking may also occur, mainly because filamentous microorganisms may still gain a competitive advantage on such occasions [1].
After sludge bulking occurs, the SS of the secondary settling tank effluent will increase substantially until it exceeds the national discharge standard, and at the same time it causes the effluent CODcr and BOD5 to also exceed standards. If control measures are not taken immediately, continuous sludge loss will sharply reduce the number of microorganisms in the aeration tank, failing to meet the normal needs of decomposing organic pollutants, leading to a decline in the performance of the entire system and even collapse. If recovery is needed, it must start from cultivating and acclimatizing the activated sludge again.
The main temporary emergency method is to add chemicals to enhance sludge settling performance or directly kill filamentous bacteria. Adding coagulants such as iron salts and aluminum salts can directly improve the compactness of the sludge and ensure settled effluent. In addition, adding some chemical agents, such as chlorine, into the return sludge can also eliminate the sludge bulking phenomenon. Adding hydrogen peroxide and ozone can also have the effect of destroying filamentous bacteria.
This method can generally reduce the SVI value quickly, but these methods do not fundamentally control the reproduction of filamentous bacteria; once the dosing stops, the sludge bulking phenomenon may come back. Moreover, dosing may destroy the growth environment of microorganisms in the biochemical system, leading to reduced treatment effect, so this method can only be used as a temporary emergency.
When sludge bulking occurs in a sewage plant, it is generally impossible to solve it by changing the process flow, tank type and aeration method; it can only be solved by changing the growth environment of microorganisms in the biochemical tank on the basis of the running process to inhibit or eliminate the excessive reproduction of filamentous bacteria. Under different processes and water qualities, it is difficult to have a universally applicable solution. However, several problems that are often encountered in biochemical processes must be paid attention to.
In addition, from the perspective of industrial development, market demand is also driving technological progress.
Prevent the occurrence of anaerobic phenomena. If anaerobic phenomena occur, various gases produced will adsorb on the sludge, causing the sludge to float and worsening its settling performance. Moreover, the return of anaerobic sludge will also trigger a large amount of filamentous bacterial reproduction. In this case, in addition to sludge discharge and removal of dead corners in the sedimentation tank, and shortening the retention time of sludge in the tank, the DO value of the aeration tank should also be increased. Keep the water entering and leaving the sedimentation tank at a higher dissolved oxygen level. Or aerate and regenerate the sludge before it returns to the biochemical tank [2].
Filamentous bacteria are an indispensable part of the microorganisms in treatment. The sludge bulking phenomenon lies in the excessive growth of filamentous bacteria; the fundamental way to eliminate sludge bulking is to enable filamentous bacteria and activated sludge zoogloeal flocs to grow in a balanced manner; completely mixed type is more prone to sludge bulking than plug flow type; low sludge load is more prone to sludge bulking than high sludge load; the influent water quality in terms of water temperature, pH, nutrient composition and whether there is predigestion before treatment is the first issue to be investigated in treating sludge bulking; sludge bulking under high load is generally due to insufficient dissolved oxygen; 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 theoretical explanations and actual reports on sludge bulking. Only by daring to practice, continuously summarizing, and extensively communicating with peers can we find effective solutions more quickly.
In a pilot device with a treatment capacity of 50 L/h, cationic polyacrylamide was added to make its concentration reach 10, 20, 30, 40, 50 and 60 mg/L respectively, and the change in the SV value of the sludge was observed. The addition of polyacrylamide has a certain effect on improving the settling performance of the sludge, and there is an optimal dosage, but the effect is not very ideal. The reclaimed water reuse system uses a new type of submerged composite membrane bioreactor, with large aeration volume and strong hydraulic stirring, and the aggregated floc particles are easily damaged, resulting in unsatisfactory coagulation effect; when the dosage is higher than the optimal dosage, in addition to neutralizing the negative charge of the colloid, the excessive positive charge makes the colloidal ions carry a positive charge and re-stabilize. In a pilot device with a treatment capacity of 50 L/h, ferrous sulfate solution was added to make its mass concentration vary between 10 and 180 mg/L, and the change in the SV value of the sludge was observed; the state of the zoogloeal flocs before and after dosing.
After adding ferrous sulfate solution, the sludge settling performance was significantly improved, and the SV value decreased by about 15%. However, after exceeding 60 mg/L, the sludge settling performance did not improve further, so the ferrous sulfate dosage in actual operation was determined to be 60 mg/L. Before and after adding ferrous sulfate (60 mg/L), the measured mixed liquor pH value dropped from 7.63 to 7.07, and the negative impact on sludge activity was very small. The dosing effect of cationic polyacrylamide is limited by hydraulic conditions and other factors and is not very ideal, and its monomer is toxic and difficult to degrade, posing secondary pollution problems, and its economic benefit is worse than that of ferrous sulfate. Ferrous sulfate is cheap and easy to use, has no negative impact on the membrane and sludge, and its effect on sludge density is effective, but it cannot fundamentally solve the problem of nutritional imbalance, so it can only be used as an emergency control measure.
On this basis, industry experts have also conducted extensive research and improvement.
Due to the high content of detergent in the raw water, coupled with the strong aeration intensity, white, viscous foam often appears and accumulates more and more. When sludge bulking occurs, the harm is greater. In addition to adding defoamer, a hydraulic defoaming method is adopted. Install nozzles above the reaction tank and use the effluent of the MBR reactor to spray the upper part of the reaction tank to control the harm of bulking sludge and foam to the reactor, which will achieve good results.
In addition, from the perspective of industrial development, market demand is also driving technological progress.
For the SBR process with intermittent influent, the reactor itself is completely mixed, and there is a concentration gradient of pollutants in time, so there is no need to set up an additional selector. Usually the cause of sludge bulking in the intermittent SBR process is that the sludge concentration is too high, while the influent organic concentration is low or the water volume is small, leading to a low sludge load. For this situation, reducing the discharge ratio, increasing the initial substrate concentration, and forcing sludge discharge from the SBR can generally effectively control the sludge bulking phenomenon. For continuous influent SBR such as ICEAS and CASS, if sludge bulking occurs, it is necessary to set up a pre-reaction zone or biological reactor at the influent end.
The low-load activated sludge process has a low substrate concentration in the aeration tank, and filamentous bacteria can easily obtain a higher growth efficiency, so it is most prone to sludge bulking. In addition to finding ways in water quality and aeration, the most fundamental and effective method is to divide the aeration tank into multiple grids and operate them in plug-flow mode, or add a small pre-aeration tank set as a biological selector, which uses a high sludge load to adsorb part of the organic matter and eliminate organic acids. This method not only helps inhibit sludge bulking, but also effectively improves the biochemical treatment effect. The method of adding fillers in the aeration tank is also applicable in the low-load completely mixed process.
For the A/O and A2/O processes, a biological selector can be formed by setting an anoxic section and an anaerobic section before the aerobic section and a sludge return system, so that the mixed bacterial group alternately stays in anoxic and aerobic states, and the organic matter concentration changes periodically, which both controls sludge bulking and improves the settling performance of the sludge. The alternating oxidation ditch and UNITANK process and other continuous influent systems, because they already have a practical 'selector' in time and space, have a strong control ability over sludge bulking. If sludge bulking occurs in these two processes, it can be controlled by adjusting the aeration to control the dissolved oxygen and controlling the return sludge volume to adjust the sludge load and DO in the tank, and after a period of improvement, the sludge bulking phenomenon can generally be controlled.
The density of bulking activated sludge is generally smaller than that of water. As an emergency measure, consider adding coagulants to improve its settling performance. Commonly used polymer coagulants - cationic polyacrylamide and inorganic coagulant - ferrous sulfate were preliminarily selected for comparative tests.
