What Is Sludge Bulking?

Due to changes in certain factors, the quality of activated sludge has become lighter, expanded, and the sedimentation performance has deteriorated. The SV value has continued to rise, and normal sludge water separation cannot be performed in the secondary sedimentation tank. The loss of sludge causes the MLSS concentration in the aeration tank to be excessively reduced, thereby destroying the sludge of normal process operation. This phenomenon is called sludge swelling. Sludge swelling is a common anomaly in activated sludge systems.

Sludge swelling refers to the phenomenon that the structure of the sludge is extremely loose, its volume increases, and it floats, making it difficult to settle and separate and affect the quality of the effluent. Basically various types

The sludge structure is loose, the quality becomes lighter, and the precipitation compression performance is poor; the SV value increases, sometimes reaching 90%, and the SVI exceeds 300; a large amount of sludge is lost, and the effluent is turbid; secondary sedimentation is difficult to separate solid-liquid, and return to the sewage The mud concentration is low, and sometimes accompanied by the generation of a large amount of foam, the normal work of the biochemical treatment cannot be maintained.

Sludge swelling is one of the more serious anomalies in biochemical treatment systems, which directly affects the quality of effluent water and endangers the operation of the entire biochemical system.

The rate of sludge swelling is quite high. In almost 50% of urban sewage plants in Europe, sludge swelling occurs to varying degrees every year, and the rate is very high in China. Basically, sludge swelling occurs in various types of activated sludge processes. Sludge swelling not only has a high incidence rate, it is common, and once it is difficult to control, it usually takes a long time to adjust. There are many theories on sludge swelling, but they are not completely consistent, and there are even many contradictions, which causes a lot of trouble for water treatment workers.

Swelling of activated sludge can be divided into swelling of filamentous fungus sludge caused by excessive reproduction of filamentous fungi in sludge and expansion of non-filamentous fungus sludge without the presence of a large number of filamentous fungi. Filamentous sludge swells in most cases.

Filamentous sludge swelling

The normal activated sludge has a dense structure and good growth of bacterial micelles. The outer edges of the bacterial micelles are neat and clear, and ciliate protozoa can be found under a microscope. The sludge is alum-like, with good flocculation, sedimentation and concentration properties. The sludge volume index (SVI) is about 100, the sludge sedimentation volume (SV) is about 30%, and the water content is about 90%. From the perspective of the structure of the sludge, the activated sludge floc is a combination of bacterial micelles and filamentous bacteria. The filamentous fungus is like the skeleton of floc, and the bacteria micelles are adhered to the skeleton, intertwined with each other like the relationship between bone and meat. For normal activated sludge, there is a proper proportional relationship between them. If the filamentous fungus grows and reproduces too much, the growth and reproduction of the bacterial micelles will be inhibited. Many filamentous fungi stick out of the surface of the sludge, making the floc loose, the sedimentation performance deteriorates, the sludge volume expands, and the sludge settlement volume (%) And sludge volume index (SVI) are high, which is the expansion of filamentous fungal sludge. When the swelling is severe, almost the entire field of view under the microscope is filamentous bacteria. The volume index (SVI) of this filamentous fungal swollen sludge can generally reach 200-2000, depending on the degree of swelling. The appearance of filamentous fungal swelled sludge is different from normal sludge, and the supernatant is small but very clear.

The swelling of activated sludge caused by filamentous bacteria overproduction accounted for most of the sludge swelling. Therefore, people generally swell this filamentous fungus sludge, which is usually sludge swelling.

Non-filamentous sludge swelling

In addition to the above-mentioned types of activated sludge swelling, there is also a type that is not caused by the excessive proliferation of filamentous bacteria, which is called non-filiform bacteria sludge swelling. This swelling is caused by the accumulation of highly viscous polysaccharides outside the activated sludge bacteria. It can be seen that the difference between it and the above-mentioned type of sludge swelling is that the former is directly caused by microbial proliferation, while the latter is caused by the accumulation of metabolites (high-viscosity polysaccharides). Because this kind of high-viscosity metabolites (polysaccharides) has many hydroxyl groups in the molecule, it has a strong binding force with water, is hydrophilic, and is a very stable hydrophilic colloid. In addition, this highly viscous substance is covered with microorganisms in activated sludge and is generally in the form of a gel. Gels are characterized by the need to absorb large amounts of water to swell. Therefore, when high-viscosity expanded sludge occurs, its appearance volume increases significantly. It contains several times more bound water than normal activated sludge. Therefore, it is sometimes called swelling of water sludge or swelling of bacteria sludge. That is to say, in this swelling sludge floc, the ratio between the bacterial micelles and filamentous bacteria that contain high binding water is exactly the opposite of the aforementioned filamentous bacterial swelling sludge, that is, the bacterial micelles account for Most, filamentous bacteria are rarely or even invisible, and even if they are seen, there are very few filamentous bacteria. Therefore, the floc is loosened.

Unlike filamentous fungal swelling sludge, non-filamentous fungal swelling sludge has poor sedimentation and concentration performance due to the large amount of water contained in the bacterial micelles and the volume expansion, which reduces the sludge bulk density and compressibility. The sludge volume index (SVI) of this expanded sludge can also be as high as 400. In actual operation, the swelling of this type of sludge is relatively rare compared to the expansion of filamentous fungal sludge. Therefore, people generally refer to sludge swelling, which usually refers to the former (filamentous fungal sludge expansion), while the latter (non-filamentous fungal sludge expansion) is ignored.

Effect of sludge load on sludge expansion

It is generally believed that the growth of microorganisms in activated sludge conforms to the Monod equation:

In the operation of activated sludge process, swelling of activated sludge is a serious problem. Its cause is quite complicated. Many scholars from various countries have done a lot of research on this, but so far they have not obtained a satisfactory explanation. From the existing research results, the causes of activated sludge swelling can be summarized as follows:

After the sludge expansion occurs, the SS of the effluent of the secondary sedimentation tank will increase significantly until it exceeds the national discharge standard, and at the same time, the CODcr and effluent of the effluent will be increased.

Emergency measures

The main method of temporary emergency is to add drugs to enhance sludge sedimentation performance or directly kill filamentous bacteria. Add iron salt, aluminum salt, etc.

Flocculation

Swelling activated sludge is generally less dense than water. As an emergency treatment measure,

The domestic design of activated sludge process usually adopts medium load (0.3KgBOD5 / (kgMLSS · d)), but in practice people always use higher load from the economic point of view, so sludge expansion under high load is in China It has a wider meaning. Under high load conditions, the most common is insufficient DO, so first increase the gas-water ratio, strengthen the aeration, and use jet aeration at the head of the push 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 have been taken, you can consider adding soft filler to the head of the aeration tank. This part has a high removal rate of organic acids, thereby removing the growth-promoting factors of filamentous bacteria and helping the growth of flocculent bacteria. This method is more effective, but the cost is higher, and it causes inconvenience to future maintenance management. Or, a selector with a hydraulic retention time of about 15 minutes is set in front of the aeration tank, which can generally effectively inhibit the growth of filamentous bacteria.

For the SBR process with intermittent feed water, the reactor itself is completely mixed, and there is a concentration gradient of its pollutant matrix in time, so there is no need to set up another selector. The reason for sludge swelling in the intermittent SBR process is usually that the sludge concentration is too high and the influent organic matter concentration is low or the water volume is small, which results in a low sludge load. In this case, reducing the discharge ratio, increasing the initial concentration of the substrate, and forcing the SBR to discharge sludge, generally, the sludge swelling phenomenon can be effectively controlled. For SBR processes such as ICAS and CASS with continuous water inflow, if a sludge expansion occurs, it is necessary to set a pre-reaction zone or a bioreactor at the water inflow end.

Low-load activated sludge process

The substrate concentration in the aeration tank of the low-load activated sludge process is low, and filamentous bacteria can easily obtain higher growth efficiency, so it is the most likely to cause sludge swelling. In addition to thinking about water quality and aeration, the most fundamental and effective way is to divide the aeration tank into multiple compartments and run it in a push flow mode, or to add a small pre-aeration tank set as a compartment as a biological selector. The high sludge load is used in the selector to adsorb some organic matter and eliminate organic acids. This method not only helps to suppress sludge swelling, but also effectively improves the biochemical treatment effect. The method of adding filler in the aeration tank is also applicable to the low-load complete mixing process.

For the A / O and A2 / O processes, the hypoxic and anaerobic sections and the sludge return system can be set before the aerobic section, so that the mixed flora is alternately anoxic and aerobic, and the organic matter concentration cycle Change, which not only controls sludge swelling but also improves sludge settling performance. And the continuous water-feeding system such as alternate working oxidation ditch and UNITANK process, because it has the actual "selector" in time and space, it has a strong ability to control the sludge expansion. If sludge expansion occurs in these two processes, the sludge load and DO in the tank can be adjusted by adjusting the aeration to control the amount of dissolved oxygen and the amount of reflux sludge. After a period of improvement, the sludge expansion can generally be controlled. .

Due to the wide variety of filamentous bacteria in sludge swelling, the suitable environment for growth is also different. In the case of different processes and different water quality, the growth environment of microorganisms is very delicate, which requires that when sludge expansion occurs, water treatment workers need to perform a large number of practical experiments and analysis according to the actual situation, and bold practice can solve the problem of sludge expansion. .

Filamentous bacteria are an indispensable part of the growth treatment microorganism. The sludge swelling phenomenon is due to the excessive growth of filamentous bacteria. The fundamental reason for eliminating sludge swelling is to make the filamentous bacteria and activated sludge bacteria micelles grow in a balanced manner. The fully mixed type is more prone to sludge swelling and low sludge load than the push flow type Higher sludge load is prone to sludge swelling; influent water quality in terms of water temperature, pH, nutritional content, and whether there is a digestion reaction before treatment should be the first consideration of sludge swelling treatment; sludge swelling under high load is generally The reason is that the dissolved oxygen is inadequate; the sludge expansion under low load using a biological selector is an effective method. Due to the diversity of filamentous bacteria, there are still many inconsistencies in theoretical explanations and actual reports on sludge swelling. Bold practices are constantly summarized and widely communicated with peers to find effective solutions faster.