What are the effective methods to solve the sludge expansion in the traditional activated sludge method?

The key technology of activated sludge method is the good or bad settling performance of activated sludge, which directly affects the quality of effluent water, and sludge expansion is an important reason for deteriorating the quality of treated water.

1 The concept of sludge expansion and measurement index

1.1 The concept of sludge expansion

Activated sludge is one of the abnormalities of activated sludge treatment system in the process of operation, and its apparent phenomenon is that the structure of activated sludge flocs and normal flocs compared to some loose, volume expansion, water content increases, and is not conducive to the absorption and degradation of the nutrients in the sewage sludge substrate, and affects the subsequent processes. It is not favorable to the absorption and degradation of nutrients in sewage by the sludge substrate, and it affects the precipitation effect of the subsequent processes.

Sludge expansion is generally defined in the following three aspects: poor settling performance, small regional settling velocity; loose sludge, not dense, large sludge index; sludge expansion caused by filamentous bacteria, the total length of filamentous bacteria is greater than 1 × 104 m/g.

1.2 Theory of sludge expansion

Chudoba proposed the selectivity theory in 1973, which is based on the growth kinetics of microorganisms, and the theory is based on the growth kinetics of microorganisms, and the theory is based on the growth kinetics of microorganisms. The theory is based on microbial growth kinetics and analyzes the competition between filamentous bacteria and bacterial colony bacteria based on the difference in the maximum growth rate μmax of different species of microorganisms and the value of their saturation constant Ks. The theory is that there are two types of microbial populations in activated sludge, A and B. Filamentous bacteria belong to type A; with low Ks and μmax values, they have high growth rate and dominate at low substrate concentration; while bacteriophage belong to type B, with high Ks and μmax values, they have high growth rate and dominate at high substrate concentration. 1980 Plam expanded the theory and considered that the theory also applies to dissolved oxygen, and that the theory also applies to dissolved oxygen. In 1980, Plam extended the theory and concluded that the theory also holds true for dissolved oxygen, i.e., the concentration of DO, like the carbon substrate, affects the growth rate and dominance of the two types of bacteria.

The selectivity theory gives a reasonable explanation of the sludge expansion phenomenon on the basis of microbial growth dynamics, which has been widely accepted and become the main theory in the field of sludge expansion research. Under the guidance of this theory, a selective reactor process has been successfully developed to control sludge expansion.

1.3 Measurement index

Sludge settling ratio: the volume of settled sludge formed by taking the mixture in the activated sludge reactor after standing for 30 min as a percentage of the volume of the original mixture. Normal activated sludge after 30 min of static precipitation, generally close to its maximum density, reflecting the concentration of activated sludge in the secondary sedimentation tank.

Sludge volume index: aeration tank at the outlet of the mixture, after 30 min of static precipitation, each gram of dry sludge formed by the sedimentation of sludge occupied by the volume. Can indicate the activated sludge in the bacterial colloid binding water rate of high and low.

Sludge layer settling speed: the mixture is left to stand for a period of time, the formation of a clear mud-water demarcation line, and thereafter enter into the layer precipitation stage, the demarcation line of the uniform decline in the speed of the sludge layer settling speed.

Filamentous bacteria length: the length of filamentous bacteria in the unit volume of activated sludge, this indicator is used to indicate the content of filamentous bacteria.

2 types of sludge expansion

Sludge expansion is divided into two categories of filamentous bacteria expansion and non-filamentous bacteria expansion. 90% of them are caused by filamentous bacteria expansion. 90% of them are caused by filamentous bacteria and only about 10% are caused by non-filamentous bacteria. Activated sludge system in the biological in dynamic equilibrium, the ideal flocculent precipitation performance is good, filamentous bacteria and bacterial colloid bacteria compete with each other, interdependent, the presence of filamentous bacteria in the floc is conducive to the protection of floc structure has been formed and can increase its strength. However, under the triggering of sludge expansion, filamentous bacteria prevailed in the competition with the bacterial colloid, and a large number of filamentous bacteria stretched out of the floc, destabilizing it.

There are two types of sludge swelling flocs that can be recognized: the first type has long filamentous bacteria protruding from the floc, and such filamentous bacteria connect individual flocs to form a network of filamentous bacteria and flocs; the second type has a more open structure, where the bacteria coalesce along the filamentous bacteria to form an elongated floc.

3 Causes of sludge expansion

3.1 Causes of filamentous bacteria sludge expansion

3.1.1 Inlet water quality

(1) Insufficient nutrient content in raw water. Activated sludge treatment of sewage (wastewater) water process, is the sludge microbial populations continue to absorb and utilize the pollutants in the water, in their own proliferation at the same time, the pollutants to be degraded process. With the reaction needs a variety of nutrients to ensure its normal metabolic activities, and to maintain the dynamic balance and activities of the organisms. If the microorganisms do not have enough food, it will make the low-nutrient microorganisms Silk Sulfur Bacteria and Bay Sulfur Bacteria overpopulate, and dominate in the competition with the bacteria of the bacterial colloid cluster.

(2) High content of carbohydrates and soluble substances in raw water. Filamentous bacteria have some of their own characteristics compared with other strains of bacteria, it is weak hydrolysis of macromolecular substances, it is more difficult to absorb insoluble substances. Therefore, when the wastewater contains a larger amount of soluble organic matter, it is favorable to the propagation of filamentous bacteria in the substrate. In addition, when wastewater contains excessive amounts of sugar carbohydrates, filamentous bacteria such as Coccidioides spp. can directly absorb and utilize glucose, lactose and other saccharides as an energy source, and at the same time secrete highly viscous substances to cover the surface of the bacterial gelatinous mass bacteria, thus greatly improving the water binding rate of the sludge.

(3) High sulfide content. Normal activated sludge sulfur metabolism filamentous bacteria content is not much, if the sewage sulfide content is high (this situation exists in industrial wastewater), it is easy to cause such as sulfur bacteria, 021N type bacteria, Bei's sulfur bacteria and other sulfur metabolism filamentous bacterial proliferation, resulting in triggering the sludge expansion.

(4) water fluctuations. Water fluctuation refers to the raw water into the activated sludge reactor in the flow rate as well as the concentration of organic matter, the type of change. If the concentration of organic matter in the aeration tank is suddenly increased, the dissolved oxygen content will be reduced due to the rapid respiration of microorganisms, at this time, the filamentous bacteria in the competition for oxygen prevail, a large number of reproduction, causing sludge expansion.

3.1.2 Reactor environment

(1) temperature. Each bacterium in the reactor substrate has its own optimal growth temperature, and at the optimal growth temperature, its reproduction is vigorous and competitive. If the temperature is low, the microbial metabolism in the sewage is slow, it will accumulate a large number of highly viscous polysaccharides, so that the surface attached water of activated sludge greatly increases, the SVI value increases, which may cause sludge expansion. The effect of temperature on filamentous bacteria is also very common, filamentous bacteria expansion of temperature sensitivity, in other conditions are equal, 10 ℃ to produce a serious sludge expansion phenomenon; will be raised to 22 ℃ reactor temperature, no longer produce sludge expansion. This is one of the reasons why most activated sludge swells or swells more severely in winter.

(2) Dissolved oxygen. Dissolved oxygen, as one of the three elements (gas, water and sludge) that make up the activated sludge mixture, is necessary for many biodegradation reactions. Bacterial gelatinous group of bacteria and planktonic coccobacilli and other filamentous bacteria on dissolved oxygen needs a relatively large difference, bacterial gelatinous group of bacteria are aerobic bacteria, while the vast majority of filamentous bacteria are adaptable micro-anaerobic bacteria. Therefore, if the dissolved oxygen content is insufficient, the growth of the bacterial colloid bacteria is inhibited, while the filamentous bacteria can still utilize the organic matter normally, and prevail in the competition.

(3) pH value. A low pH value will lead to the propagation of filamentous fungi and cause sludge expansion. The optimal pH range for activated sludge microorganisms is 6.5~8.5; when the pH value is lower than 6.5, it is favorable for the growth and reproduction of fungi; when the pH value is as low as 4.5, the fungi will be completely dominated, the activated sludge floc will be destroyed, and the treated water quality will be deteriorated[9].

(4) BOD-sludge load. BOD sludge load is an important indicator for designing the activated sludge reactor and controlling its operation.

3.2 Causes of non-filamentous sludge expansion

There are fewer studies on non-filamentous sludge expansion, and it is generally believed that non-filamentous sludge expansion occurs due to abnormalities in the physiological activities of the flocs.

3.2.1 The feed water contains toxic substances

Because the feed water contains more toxic substances, resulting in bacterial poisoning can not secrete enough viscous material, it is difficult to form flocs, or even if the formation of flocs, but the structure of the loose, the settling performance is not good.

3.2.2 Lack of nutrients or imbalance

Lack of nutrients or imbalance of nutrients in the feed water, in addition to triggering the expansion of filamentous bacteria, will also lead to the expansion of non-filamentous bacteria sludge. As the feed water contains a large amount of dissolved organic matter, so that the sludge load is too high, and the feed water and the lack of sufficient N, P or dissolved oxygen is insufficient, the bacteria will soon be a large amount of organic matter into the body, and can not be metabolized in a timely manner to decompose, to the outside of the secretion of too many sugar substances, hydroxyl contained in this type of material has a strong hydrophilicity, which can be made to the activated sludge combined with the rate of water as high as 400%, a viscous gel

4.

4 Control methods of sludge expansion caused by filamentous bacteria

4.1 Control of sludge expansion by dosing method

Early control methods of sludge expansion mainly rely on the addition of chemicals (such as disinfectants) to kill the filamentous bacteria directly or the addition of inorganic or organic coagulants to increase the density of sludge flocs to improve the settling performance of sludge flocs. At present, such methods are still used in some sewage treatment plants.

4.1.1 Addition of oxidizing agents

(1) Addition of Cl2 or bleach. The traditional oxidant used to control sludge expansion is Cl2. Jenkins et al. showed that Cl2, HOCl and hypochlorite with oxidizing power penetrate into the cells and destroy the enzyme system in the bacteria, leading to cell death. The vast majority of said filamentous bacteria can be controlled by the addition of chlorine. Generally cast in the reflux sludge, chlorination point of Cl2, concentration should be controlled at less than 35 mg / L, chlorination amount is most suitable to control in 10 ~ 20 mg / L-d, casting too much instead of killing the bacterial colloid bacteria, resulting in the disintegration of flocs. When the SVI value is gradually reduced and the swelling is constantly relieved, the dosage should be gradually reduced.

(2) H2O2 dosing, hydrogen peroxide in the control of sludge filamentous bacteria expansion is also widely used, Keller et al. found that the minimum dosage of filamentous bacteria control is 0.1 g/kg-d (H2O2/MLSS), it will destroy the effect of dephosphorylation, dosing after a period of time (about 10 days) dephosphorylation will be slowly restored. The toxicity of H2O2 has only a small effect on nitrogen removal, and no significant changes in ammonia, nitrogen and nitrate nitrogen were found in the test.

(3) O3 injection: Ozone injection can also control the sludge swelling caused by filamentous bacteria, ozone can also effectively improve nitrification and increase the removal rate of difficult to degrade organic matter, ozone dosage of about 4 g/kg-d (H2O2/MLSS), generally added in the aerobic zone.

4.1.2 Adding coagulants

Adding synthetic organic polymers, iron salts, aluminum salts and other coagulants can increase the specific gravity of sludge through its coagulation to improve the compactness of sludge; adding kaolin, calcium carbonate, calcium hydroxide, etc., can also be improved by increasing the compactness of the sludge to improve the settling of sludge performance. Practice has shown that there is no primary sedimentation tank sewage plant, its SVI value is relatively low, so there is a primary sedimentation tank sewage plant sludge expansion, part of the sewage is sent directly to the aeration tank is also a way to control sludge expansion.

When sludge expansion occurs, the above method can reduce the SVI value faster, but not fundamentally control the propagation of filamentous bacteria. Once the dosing is stopped, sludge swelling may occur again. Dosing changes the growth environment of microorganisms, will undoubtedly have a negative impact on the stable operation of the sewage treatment plant, so it can only be used as a temporary emergency only.

4.2 Improvement of environmental control of sludge expansion

Through the sludge expansion mechanism and further understanding of the role of filamentous bacteria, the control of sludge expansion from simple dosing and other methods to the application of the principles of ecology to adjust the operating conditions of the treatment process and the reactor environmental conditions, through the coordination of the bacterial gelatinous group of microorganisms with the filamentous bacterium **** biological relationship, to eliminate the root cause of sludge expansion. Relationship, fundamentally eliminate the problem of filamentous bacterial expansion of sludge.

4.2.1 Add a biological selector

As early as in the 1970s, it was found that when the mixture in the aeration tank is pushed to flow and form a clear substrate concentration gradient, it is not easy for sludge expansion to occur. The design principle of bio-selector is to make the ecological environment in the aeration tank is conducive to the selective development of the bacterial colloid bacteria, the application of the mechanism of biological competition to control the excessive proliferation of filamentous bacteria, so as to control the sludge expansion. We can set up a small pool before the aeration tank to locally increase the F/M value, or set up the front end of the aeration tank as a high load contact area to selectively cultivate the bacterial colloid bacteria to become the dominant species.

Selector can be divided into aerobic, anoxic and anaerobic three types. The working principle of aerobic selector is to utilize the bacterial colloid bacteria can rapidly multiply and store these substrates in high load substrate concentration, while the growth rate of filamentous bacteria at this time can not be significantly increased. In the aeration reaction after the high load contact, the bacterial colony bacteria utilize the stored substrate to multiply and grow in large quantities, and the growth of filamentous bacteria is inhibited due to the lack of food. The working principle of the anoxic selector is that most of the bacterial colloid bacteria can utilize the nitrate in the chemical oxygen as the oxygen source to grow and multiply. And filamentous bacteria this function is weak, so the growth is inhibited.J.Wanner et al. through the anaerobic selector experimental analysis confirmed that the bacterial colloid bacteria due to the release of phosphorus reaction and the acquisition of energy to be able to use the organic matter in anaerobic conditions for reproduction and storage, the subsequent aeration reaction of the substrate concentration of the bottom, so that the filamentous bacteria are inhibited, thus preventing the occurrence of sludge expansion.

4.2.2 Adoption of SBR process

From the change of substrate concentration in the reaction stage of SBR method, it can be seen that the SBR method is not easy to happen sludge expansion. If the ordinary activated sludge method in the mixture of the flow state expressed in terms of "dispersion", then it is in the complete mixing of infinity, in the ideal push flow is zero. SBR method of the reaction stage of the change in substrate concentration is equivalent to the ordinary sludge aeration tank compartment number of an infinite number of cases (because of the ordinary sludge treatment method of the number of compartments of the aeration tank closer to the more close to the Push-flow type). This favors the bacterial colony bacteria to be in a competitive advantage. In addition, the advantages of the SBR method are: the influent and the beginning of the reaction stage of the reactor is in an anaerobic state, which is conducive to the inhibition of the excessive growth of filamentous bacteria; SBR method of sludge age is short, than the value-added rate of the filamentous bacteria can not be very good propagation; can be eliminated in the primary sedimentation tank relative reduction of the proportion of dissolved substrate in the wastewater, and at the same time increased the amount of total suspended solids. It can be seen that the SBR itself is a good selector to prevent sludge expansion.

4.2.3 Return sludge regeneration method

This method is mainly used in the denitrification and phosphorus removal process, the return sludge discharged from the second sedimentation tank is discharged into a separately set up aeration tank for aeration to oxidize the microbial body of the stored material, so that the bacterial colloid bacteria have the maximum adsorption and storage capacity to make the sludge is fully regenerated and restored to activity, so it can be in the competition with the filamentous bacterial Gain advantage and inhibit the overpopulation of filamentous bacteria.

5 Control methods for sludge swelling caused by non-filamentous bacteria

Non-filamentous bacteria swelling, also known as high viscosity swelling, has been reported in few studies in China. Nutrient deficiency is an important factor leading to sludge swelling. The study of Gao Chundi et al. showed that adding sufficient nitrogen and phosphorus sources and increasing the sludge load appropriately could control the occurrence of sludge expansion. If it is caused by the lack of trace metals, sludge expansion can be eliminated by supplementing the amount of trace metals in the effluent. In addition, the addition of enzymes can also control the occurrence of sludge expansion.

6 Conclusion

With the increasing depth of practice, the research on the problem of sludge expansion has been deepened, and new research results are constantly published, but there is no uniform and absolute answer to the question of the causes of sludge expansion. Many researchers through experiments have reached inconsistent or even opposite conclusions. In engineering practice, triggered by the sludge expansion of the triggers can not be a single, only to analyze the main reasons for its generation, in order to find a key solution to the problem.