The sustainable use of water has played a vital role in the long history of mankind. With the growth of population and the increase of economic and social activities, the global water demand is also continuing to rise, and the consequent water pollution is also more serious, pollution control and water environment remediation is one of the major problems that must be faced for a long time in the sustainable use of water resources, which has greatly promoted the development of sewage treatment technology, the use of advanced biotechnology to treat pollutants has attracted more and more general attention, biological sewage treatment system, the use of microbial characteristics to degrade organic matter, remove nutrients, the toxic substances into non-toxic substances, It plays an important role in wastewater treatment.
Fundamentals of aerobic biological treatment
Many microorganisms in nature have the ability to break down and transform contaminants. The method of treating sewage by using the oxidative decomposition of microorganisms is called biological treatment. At present, biological treatment method is mainly used to remove dissolved and colloidal organic pollutants and nutrients such as nitrogen and phosphorus in sewage, and can also be used for the treatment of some heavy metal ions and inorganic salt ions.
According to the different oxygen requirements of microorganisms acting in the treatment process, the biological treatment of sewage can be divided into two categories: aerobic biological treatment and anaerobic biological treatment. Aerobic biological treatment is carried out under the aerobic situation by the action of aerobic microorganisms. During the treatment process, the dissolved organic matter in the sewage is absorbed by the bacteria through the cell wall and cell membrane of the bacteria; Solid and colloidal organic matter is first attached to the bacterial cell in vitro, and the extracellular enzymes secreted by the bacteria are broken down into lytic substances before penetrating into the cells.
Bacteria through their own life activities oxidation, reduction, synthesis and other processes, a part of the absorbed organic matter into a simple inorganic substance, and release the energy required for bacterial growth and activity, and the other part of the organic matter into the necessary nutrients for the organism, the composition of new cell matter, so that bacteria gradually grow and multiply, producing more bacteria. After other microorganisms take up nutrients, the same biochemical reactions occur in their bodies.
When there is more organic matter in the sewage (when it exceeds the needs of microbial life), the synthetic part is increased, and the total amount of microorganisms increases rapidly; When there is not enough organic matter in the sewage, some microorganisms will die of hunger, and their corpses will become the "food" of another part of the microorganisms, and the total number of microorganisms will decrease. Although the cellular matter of microorganisms is also organic matter, but microorganisms are in a suspended state in water, relatively speaking, individuals are relatively large, but also easier to condense, can be with some other substances in the sewage (including some adsorbed organic matter and some inorganic oxidation products and excrement of bacteria, etc.) through physical cohesion in the sedimentation tank together with precipitation.
It can be seen that aerobic biological treatment is particularly suitable for the treatment of dissolved and colloidal organic matter, because this part of the organic matter can not be directly removed by precipitation, and the use of biological law can be converted into inorganic substances, and the other part can be converted into microbial cell matter and thus separated from sewage. However, it must be noted that the precipitated sludge (which contains a large number of microorganisms) is easy to decay in the absence of oxygen and should be properly disposed of.
The use of aerobic treatment of sewage, basically no odor, the time required for treatment is relatively short, if the conditions are suitable, generally can remove about 80% to 90% of bod5, sometimes even up to 95%. In addition to the activated sludge method mentioned above. Biological filters, biological turntables, sewage irrigation and stabilization ponds are also methods of sewage aerobic treatment. Customarily, the aerobic biological treatment of sewage is called biological treatment.
Activated sludge method
1. Microbial communities in aerobic activated sludge
1. Activated sludge ecology and common microorganisms
(1) The composition and properties of aerobic activated sludge
(1) Composition of aerobic activated sludge. Aerobic activated sludge is composed of a variety of aerobic microorganisms and facultative anaerobic microorganisms (with a small number of anaerobic microorganisms) mixed with organic and inorganic solid substances in the sewage to form a flocculent or velvety grain.
(2) The nature of aerobic activated sludge. All kinds of activated sludge have their own colors, and the moisture content is about 99%; its relative density is 1. 002 ~ 1.006, the mixture and reflux sludge are slightly different, the former is 1.002 ~ 1.003, the latter is 1.004 ~ 1.006; it has sedimentation properties; biological activity, has the ability to adsorb and oxidize organic matter; extracellular enzymes in aqueous solution, the macromolecular substances in the sewage are hydrolyzed into small molecules, and then absorbed into the body and oxidized and decomposed; It has the ability to self-propagate; the size of the velvet grain is 0.02~0.2mm, the specific surface area is 20~100cm2/mL, it is weakly acidic (pH value is about 6.7), and when the inlet water changes, it has a certain ability to withstand the change of the pH value of the inlet water.
(2) The presence state of aerobic activated sludge. Aerobic activated sludge in the fully mixed aeration tank, due to aeration agitation is always completely mixed with sewage, always existing in a suspended state, evenly distributed in the aeration tank and in intense motion. The microbial community of activated sludge removed from any point in the aeration tank is essentially the same. The microbial population and quantity between the segments in the push-flow aeration tank are different, and the microbial species increase with the direction of push-flow. At any point in each segment, the activated sludge microbial community is essentially the same.
(3) Microbial communities in aerobic activated sludge. The structural and functional center of aerobic activated sludge (velvet grains) is a bacterial mass formed by bacteria that can play a flocculation role, called a bacterial gel mass. Other microorganisms grow on it, such as yeast, mold, actinomycetes, algae, protozoa and some miniature offspring (rotifers and nematodes, etc.). Therefore, the activated sludge in the aeration tank is an ecosystem composed of the most suitable proliferation of flocculant bacteria centered on the most suitable flocculant bacteria under different nutrients, oxygen supply, temperature and pH values. Due to the constant artificial oxygenation and sludge reflux, the aeration tank is not suitable for the survival of some aquatic organisms. Especially those with larger populations than rotifers and nematodes and those with long life cycles. The main biological populations in activated sludge are bacteria, protozoa and nematodes. Other populations such as the genus Hydra , even some diptera larvae are occasionally seen. Algae are also visible in the mixture but are difficult to grow.
The main bacteria (dominant bacteria) of activated sludge (velvet grains) are derived from microorganisms in soil, river water, sewer sewage and air. Most of them are gram-negative bacteria, such as Gram and Cymbidium, which account for 70%, and other gram-negative and gram-positive bacteria. The bacteria of aerobic activated sludge can quickly stabilize organic pollutants in sewage, and have good self-cohesion ability and sedimentation performance. Butterfield isolated from activated sludge bacteria of the genus Geluginus that form velvet grains. In addition to the genus Ofoglium, McKinney also isolated several bacteria that can form velvet particles, such as E. coli and Pseudomonas, and found that many bacteria have the properties of agglomeration and velvet granulation.
The microbial population of activated sludge is relatively stable, but when the nutritional conditions (sewage type, chemical composition, concentration), temperature, oxygen supply, pH value and other environmental conditions change, the main bacterial population (dominant bacteria) changes. The activated sludge treating domestic sewage and hospital sewage will also contain pathogenic bacteria, pathogenic fungi, pathogenic amoeba (amoeba), viruses, rickettsia, mycoplasma, chlamydia, spirochetes and other pathogenic microorganisms.
(4) The concentration and quantity of microorganisms in aerobic activated sludge. The concentration of microorganisms in aerobic activated sludge is usually expressed in the 1L activated sludge mixture of how many milligrams of constant weight dry solids, that is, MIS (mixed liquid suspension solids, including inorganic and organic solids), or in lL activated sludge mixture contains how many milligrams of constant weight, dry volatile solids that are MLVSS (mixed liquid volatile suspended solids, that is, represent organic solids - microorganisms). In general municipal wastewater treatment, the ratio of MLVSS to MLSS is 0. 7 ~ 0.8 is appropriate. MLSS is maintained at 2000~3000mg/L. In the biological treatment of industrial wastewater, MLSS remains at about 3000mg/L. The MLSS for biological treatment of high concentration industrial wastewater is maintained at 3000~5000mg/L. There are 107 to 108 bacteria in the lmL aerobic activated sludge.
2. Mechanism of action of aerobic activated sludge purification of sewage
The purification effect of aerobic activated sludge is similar to the role of coagulants in water treatment engineering, which can floccinate organic and inorganic solid pollutants, and is known as "biological flocculant". It also absorbs and breaks down dissolved contaminants in water at the same time. Because it is composed of living microorganisms, it can reproduce itself, has biological "activity", and can be used continuously and repeatedly, while the chemical coagulant can only be used once, so the activated sludge is superior to the chemical coagulant. The purification mechanism of aerobic activated sludge is shown in Figure 1.
As can be seen from Figure 1, the relationship between microorganisms in the activated sludge velvet grain is the relationship between the food chain. The process of adsorption and biodegradation of organic matter by aerobic activated sludge velvet particles is divided into 3 steps. Step 1 is to adsorb organic matter from the sewage by flocculent microorganisms in the velvet particles of the activated sludge under aerobic conditions. Step 2 is the hydrolysis of the hydrolyzed bacterial macromolecular organic matter in the activated sludge velvet grains for small molecule organics, and at the same time, the microorganisms are combined with cells. The dissolved organic matter in the sewage is directly absorbed by the bacteria and oxidized and decomposed in the bacteria, where the intermediate metabolites are absorbed by another group of bacteria and then inorganized. Step 3 is when protozoa and miniature offspring absorb or swallow incompletely decomposed organic matter and free bacteria.
3. The basic process of activated sludge method
The activated sludge treatment process includes basic components such as aeration tank, sedimentation tank, sludge return and residual sludge removal system, as shown in Figure 2.
The sewage and the reflux activated sludge enter the human aeration tank together to form a mixture. The aeration tank is a bioreactor that is filled with air through an aeration device, and the oxygen in the air dissolves into the sewage to produce an aerobic metabolic reaction of the activated sludge mixture. The aeration equipment not only transmits oxygen into the mixture, but also acts as a stirring to make the mixture suspended (some aeration occasions additional stirring equipment is added). In this way, the organic matter, oxygen and microorganisms in the sewage can be fully transferred and reacted. Then the mixed liquid flows into the sedimentation tank, and the suspended solids in the mixed liquid are separated from the solid and liquid in the sedimentation tank, and the purified water that flows out of the sedimentation tank is purified. Most of the sludge in the sedimentation tank is returned to the aeration tank, called the reflux sludge, and the purpose of the reflux sludge is to maintain a certain concentration of suspended solids in the aeration tank, that is, to maintain a certain microbial concentration. The biochemical reaction in the aeration tank leads to the proliferation of microorganisms, and the proliferating microorganisms are usually removed from the sedimentation pond bottom sludge to maintain the stable operation of the activated sludge system, and the sludge discharged from the system is called residual sludge. The remaining sludge contains a large number of microorganisms, and should be effectively treated and disposed of before discharging the environment to prevent pollution of the environment.
4. The role of fungal gel groups
In the field of microbiology, it is customary to call the bacterial clumps formed by the genus Phytobacterium called bacterial gelocellums. In the field of water treatment engineering, the bacterial clumps in which all flocculating bacteria with pods or mucus or gelatin are coagulated with each other are also called bacterial gel groups, which are generalized bacterial micelles. As mentioned above, the fungal gel group is the center of the structure and function of the activated sludge (velvet particles), which is manifested in the absolute advantage of the quantity (except for the filamentous expansion of the activated sludge), which is the basic component of the activated sludge. Its role is manifested in the following aspects.
(1) There is a strong ability of biological flocculation, adsorption capacity and oxidative decomposition of substances. Once the fungal gel group is affected and destroyed by various factors, the removal rate of organic matter is significantly reduced, or even no ability to remove.
(2) Adsorption and decomposition of organic matter by fungal gel groups. It provides a good living environment for protozoa and miniature offspring, such as removing poisons, reducing oxygen consumption, increasing dissolved oxygen content in water, and also providing food.
(3) Provide attachment habitats for protozoa and miniature epizoans.
(4) It has an indicative effect. The performance of aerobic activated sludge can be measured by the color, transparency, number, particle size and tightness of the structure of the bacterial gel group. If the neobial micelle is light in color, colorless and transparent, and the structure is compact, it indicates that the bacteria gum mass has strong vitality, strong adsorption and oxidation capacity, that is, strong regeneration ability; Aging bacteria glue mass, dark color, loose structure, weak activity, poor adsorption and oxidation capacity.
5. The role of protozoa and miniature epizoans
Protozoa are an important part of activated sludge, and the number of them can reach 5000 /mL, which can account for 5% to 12% of the dry weight of the mixed liquid.
Protozoa and micro-epizoans play an active role in three aspects: biological treatment of sewage and water pollution and self-purification.
(1) Indication effect. Organisms evolved from the lower to the higher. Lower organisms are highly adaptable to the environment and are less sensitive to changes in environmental factors. Higher organisms, on the other hand, such as bellworms and rotifers, are particularly sensitive to dissolved oxygen and poisons. Therefore, in the sewage outlet in the water body, in the early stage of biological treatment of sewage or at the inlet of the push flow system, a large number of bacteria grow, and other microorganisms rarely or do not appear. With the increase of sewage purification and the self-purification degree of water bodies, many higher-level microorganisms have appeared accordingly. The sequence of protozoa and miniature epizoans is: bacterial one plant flagellar - carniospods (amoeba) one animal flagellar - swimming ciliates, and straw worms - solid ciliates and roundworms.
The indicative effect of protozoa and micro-epizoans is manifested in the following aspects:
(1) According to the succession of the above-mentioned protozoa and micro-epizoans, the degree of water quality and sewage treatment can be judged according to their activity laws, and the maturity of activated sludge culture can also be judged. The relationship between protozoa, micro-epizoans and activated sludge culture maturity is listed in Table 1.
The species of protozoa in the fully mixed activated sludge aeration tank are spatially observable (longitudinally in biological filters). With the gradual maturity of activated sludge, the dominant species of protozoa in the mixture will also change sequentially, starting with carnithopods and flagellar dominant animals. Swimming ciliates, reptile ciliates, and fixed ciliates appear sequentially.
(2) Judge the quality of activated sludge and treated water according to the protozoan species. Reptile ciliates and solid ciliates are closely connected to the activated sludge flocs, and once a certain density is reached, they will return to the aeration tank with the reflux activated sludge precipitated in the second sedimentation tank, and most of the rinsed off are flagellar dominant animals and swimming ciliates.
When the activated sludge reaches the mature stage and its protozoa develop to a certain number, the effluent water quality is significantly improved. Newly operated aeration tanks or poorly run aeration tanks, which mainly contain flagellar protozoa and rootpods, with only a small number of ciliates; Conversely, the mixture of aeration tanks with good effluent quality mainly contains ciliates, with only a small number of flagellar protozoa and amoeba. Ciliates become dominant species. Common examples include the genus Dipterocarpus, The Beanoid, Ciliate, some Solium, and Bellworm. This sludge state and the concomitant relationship between effluent quality and microbial species are the theoretical basis for using protozoa to indicate the effluent quality of activated sludge treatment plants.
Solid ciliates appear in the small bellworm genus, tired branch insect genus, cover fibrella, polycontolar genus, monocotopod genus, linguide genus, straw worm genus, roaming worm genus, endoculopod genus and rotifer, etc., indicating that the activated sludge is normal and the effluent quality is good. When the genus Beanworm, Grasshopper, Tetrachydiasis, House trichomoniasis, Eyeworm, etc. appear, indicating that the structure of activated sludge is loose and the quality of the effluent is poor. The presence of nematodes indicates a lack of oxygen.
(3) It can also be judged according to the changes in the individual morphology of protozoa in the harsh environment and its change process. Take the bell worm as an example: when dissolved oxygen is insufficient or other environmental conditions are harsh, a series of metamorphosis changes occur in the clock worm from normal worm body to cell sac. The tail stalk of the bell worm falls off first, and then the posterior end of the worm grows a secondary ciliary ring in a swimming state (usually called a swimming bell worm), or the worm body is deformed, or even oblong cylindrical, the front end is locked, the ciliary ring is shrunk into the body, relying on the secondary ciliary ring to swim in the opposite direction. If the sewage water quality is not improved, the insect body will become longer and longer, and finally shrink into a circular sac, if the sewage water quality improves, the insect body can return to its original state and restore its activity.
During the normal operation of sewage biological treatment, the normal treatment effect is often affected by sudden changes in inlet water flow, organic matter concentration, dissolved oxygen, temperature, pH, poison, etc., so that the quality of the effluent water does not meet the discharge standards. Changes in water quality can be known through water quality measurement, but the determination time of organic matter concentrations and toxic substances is long, so regular measurement is not easy to do. According to the regularity of protozoa consumption, the degree of sewage purification is preliminarily judged, or the inlet water quality and operating conditions are predicted according to the changes in the individual morphology and growth status of protozoa. Once the morphology and growth of protozoa are found to be abnormal, it is necessary to analyze which aspect of the problem is in time and solve it in time.
(2) Purification effect. There are 5000 to 20000 protozoa in the mixture of lmL normal aerobic activated sludge, of which 70% to 80% are ciliates, especially small-mouth bellworms, groove bellworms, ribbed fibral worms, roaming worms appear frequently and play an important role, and rotifers are 100 to 200. Some sewage rotifers grow and multiply, reaching 500 to 1000 in 1mL mixed liquid. Rotifers include rotifers, rotifers, vertebrates and so on. Protozoa have a variety of nutrient types, and saprophytic nutrients of the flagella absorb dissolved organic matter in sewage through osmosis. Most protozoa are animal nutrients, and they swallow organic particles and free bacteria and other tiny organisms, which play a positive role in purifying water quality. The number and metabolic pathway of protozoa are inferior to those of the fungal micelles, and the purification effect is not as large as that of the fungal gelloce. However, protozoa and micro-epizoans have no choice in eating food, they swallow bacterial gum closure in addition to eating organic particles, because their amount of ingestion does not affect the overall purification effect, so they do not endanger the purification effect. On the contrary, due to the presence of protozoa, especially ciliates, there is a significant improvement in the quality of effluent water. The purification effect of ciliates in the biological treatment of sewage is shown in Table 2.
6. Cultivation of aerobic activated sludge
The culture methods of activated sludge in the production plant are intermittent aeration culture and continuous aeration culture.
(1) Intermittent aeration culture
(1) Source of strains. Activated sludge taken from sewage treatment plants, activated sludge from different water quality sewage treatment plants, activated sludge taken from the same water quality sewage treatment plant, sludge from the lower foot sludge of the plant's collection pond or sedimentation tank, or river silt through which the plant's sewage flows for a long time has been expanded and cultivated.
(2) Domestication. All the activated sludge species used in different water quality sewage treatment plants from the plant must be domesticated before they can be used, and domesticated by intermittent aeration culture. Advanced low concentration sewage culture, aeration for 23h, precipitation for 1h, pour out the supernatant, and then into the same concentration of fresh sewage, continue aeration culture. Each concentration was run for 3 to 7 days, and an increase in the growth of activated sludge was observed by microscopy. One concentration can be adjusted up to operate with the previous concentration. After that, the sewage concentration is increased step by step until the original sewage concentration is raised. At the beginning of domestication, the structure of activated sludge was loose, there were more free bacteria, and flagellar and swimming ciliates appeared. At this time, the activated sludge has a certain sedimentation effect. During domestication, microscopic examination shows that protozoa are replaced from low to high. In the later stage of domestication, swimming ciliates were mainly present, and a small number of ciliates with certain pollution resistance, such as tired branches, appeared. The sedimentation performance of activated sludge is better, the boundary between the supernatant and the sedimentation sludge can be seen, and it is clearer, and the domestication is over. However, the inlet water flow still does not reach the design value.
(3) Cultivation. The domesticated activated sludge was changed to continuous aeration culture method to continue cultivation. During this period, the progress and maturity of activated sludge culture can be analyzed and measured by microscopic and chemically determined indicators: when the activated sludge is seen to form large particle flocs in an all-round way, its sedimentation performance is good, the volume settlement ratio (SV30) of the aeration tank mixture in the lL cylinder is more than 50%, and the sludge volume index (SVI, which is an indicator of the sedimentation performance of the activated sludge) is about 100mL/g; Microscopic examination shows that the structure of the bacterial gel group is tight and there are few free bacteria; Protozoa appear in large numbers, mainly fixed ciliates such as bell worms, and successively appear lintel worms, roaming worms, rotifers, etc.; The MLSS of the activated sludge in the aeration tank reaches about 2000mg/L, and when the inlet water reaches the design flow, the COD and BOD5 of the effluent are significantly reduced by the determination of chemical indicators, at which time the activated sludge culture enters the mature stage and can be transferred to the formal operation stage. If it is treated with industrial wastewater, when the inlet water BOD5 is 200~300mg/L, MLSS is maintained at about 3000mg/L, and dissolved oxygen is maintained at 2~3mg/L.
(2) Continuous aeration culture. In addition to intermittent culture, continuous culture can be used. When treating domestic sewage and industrial wastewater, when taking the activated sludge from the same water quality treatment plant as a strain, the activated sludge can be directly cultivated by continuous aeration culture. The amount of activated sludge inoculation is 5% to 10% of the effective volume of the aeration tank, and the first few days of start-up can be first smouldered, the dissolved oxygen is maintained at about lmg/L, and then the water is fed at a small flow rate, and each adjustment of the flow gradient should be maintained for about a week of running time. As the inlet water flow gradually increases, the concentration of dissolved oxygen gradually increases. When the inlet water flow reaches the design flow, if the inlet water BOD5 of industrial wastewater is 200~300mg/L, the MLSS is maintained at about 3000mg/L, and the dissolved oxygen is maintained at 2~3mg/L. If the inlet water BOD5 of domestic sewage is maintained at about 2000mg/L in the MLSS in the aeration tank of 150~250mg/L, the dissolved oxygen can be maintained at 1~2mg/L.
To determine whether the activated sludge is mature, it is also necessary to rely on microscopic examination and chemical determination and analysis of indicators. The microscopic judgment method is also to see the growth status of activated sludge in the early stage of culture, in the process of transition to maturity, whether the structure of the fungal gel group evolves from loose to compact, and whether the protozoa are replaced from low to high. When the inlet water flow reaches the design value, if the bacterial gel group structure is tight, forming large flocculent particles, and the protozoa such as bell worms appear in large quantities, and lintelworms, roaming insects, rotifers, etc. appear successively, that is, the mature stage of entering people.
7. Problems caused by microorganisms in the operation of activated sludge method
The most common failure of activated sludge in operation is the separation of sludge water in the second sedimentation tank. The causes of sludge settlement problems are sludge expansion, non-flocculation, tiny flocs, foaming and denitrification. This is just categorizing effects, which is actually not very precise and somewhat overlapping. All the problems of sedimentation of activated sludge are caused by the abnormal structure of the sludge floc. The size of activated sludge particles varies greatly, ranging from 0.5 to 5.0 μm of free individual bacteria to flocs with a diameter of more than 1000 μm (lmm). The maximum size of the floc depends on its adhesion strength and the size of the turbulence shear action in the aeration tank.
Floc structures fall into two categories: microstructures and macrostructures. The microstructure is a smaller floc (<75 μm in diameter), spherical, denser but relatively prone to rupture. Most of these flocs are composed of flocculent-forming bacteria. It is easy to be sheared into small particles under the turbulent conditions of the aeration tank. Although this floc can precipitate quickly, the small particles sheared from the large condensate require a long precipitation time and may be discharged with the effluent of the sedimentation tank, so that the BOD5 value of the final effluent rises and the turbidity increases significantly. When filamentous microorganisms appear, macrostructured flocs appear, and microorganisms condense around filamentous microorganisms to form larger irregular flocs, which have strong shear strength.
The following focuses on the formation and countermeasures of sludge expansion, and briefly explains other causes of sludge settlement problems.
(1) Non-condensing. Non-cohesion is a phenomenon caused by microstructured flocs. This is due to operational problems caused by the flocs becoming unstable and fragmented, or by turbulence formed by overaeration that cuts the flocs into pieces. It may also be that bacteria cannot coalesce into flocs, and microorganisms become free individuals or very small clumps. They are suspended in sedimentation tanks and flow out continuously with the effluent. Non-agglomeration is generally considered to be due to low dissolved oxygen concentrations, low pH, or shock loads. The sludge load should be greater than 0.4kg/(kg·d), otherwise the problem of non-coagulation will occur. Some toxic wastewater can also form tiny aggregates. Free-swimming protozoa, such as Nephropodium (unspecified species) and Grasshopper (unspecified species), in large numbers, do not affect the sedimentation performance of the sludge, but can also make the final water turbid.
(2) Tiny flocculents. The causes of the formation of microstructured flocs and the operational problems caused by them have been described above. Sludge containing tiny flocs does not form a high concentration in the effluent because its particles are much larger than non-condensing sludge. Discrete flocs can be observed with the naked eye in the effluent. Tiny flocs tend to form due to long mud age (>5 to 6 d) and low organic load [<0.2 kg/(kg·d)]. Therefore, this problem often occurs in delay aeration systems.
(3) Foaming. Since the use of non-degrading "hard" detergents, very thick white foams have often appeared in aeration tanks. The foam caused by microorganisms is another type of very dense, brown foam that sometimes appears in aeration tanks. This type of foam is formed due to the overgrowth of filamentous microorganisms of certain Nocardia species, and the bubbles of the aeration system enter the human population. This foam floats on the surface of the pool in the form of a dense and stable foam or a thick layer of scum. Bubbles that float the sludge may also be caused by denitrification.
The mechanism by which bubbles attach to nocardia is quite complex. In some cases, although the population density of this filamentous microorganism in the mixture is also very high, it does not cause sludge sedimentation quality problems. The reason for this is that Nocardia produces many branches, making the flocs a very strong macrostructure, producing a large, strong, and easily settled floc.
(4) Sludge swelling caused by filamentous bacteria. During the operation of the aeration tank, sometimes the sludge structure is loose, the settlement performance deteriorates, floats with the water, and overflows outside the pool, which is called sludge expansion. At the beginning, although the expansion sludge is slower than the normal activated sludge, the effluent quality is still very good. Even if the sludge has expanded severely, there is still a clear supernatant because the extended filamentous bacteria filter out the fine particles that form turbidity. Only when the sedimentation is very poor, the mud surface rises, so that the large floc also overflows the sedimentation tank, and finally the SS and B0D rise in the effluent. The main problem is that the sludge expansion makes the sludge compression performance worse, and the result is that a lot of thin sludge is returned to the aeration tank, which reduces the MLSS in the pool, which in turn causes the effluent quality to fail to meet the requirements and the operation of the aeration tank fails.
Good settling performance of ideal flocs; Extremely low ss and turbidity in the final effluent; Filamentous bacteria and floc formation are balanced; Filamentous bacteria all remain in the floc, which increases the strength of the floc and protects the fixed structure. Even if there are a small number of filamentous bacteria protruding from the sludge floc, they shrink the length enough to not affect the sludge settlement. In contrast, the expanded sludge has a large number of filamentous bacteria protruding from the floc.
There are two types of distinguishable expanded sludge flocs: the first is the filamentous bacteria with filamentous bacteria protruding from the flocs, which connect (or bridge) the individual flocs to form filamentous bacteria and flocnets; The second type is structures with more open (or diffusion), where bacteria coagulate along the filamentous bacteria to form fairly elongated flocs. The formation of flocs and the effect on precipitation depend on the type of filamentous microorganism.
About 25 filamentous bacteria are known to cause activated sludge to swell. Algae in activated sludge have not been found to cause sludge expansion.
The main causes of the expansion are low DO concentration, low sludge load rate, aeration tank inlet with more septic effluent, insufficient nutrition and low pH (<6.5).
Even if the species of filamentous microorganisms are known, there is currently no effective and practical way to control the dominant species. Therefore, the operator needs to operate the aeration tank according to the appearance of indicative filamentous microorganisms and control the operating conditions until the problem disappears. The main methods are as follows.
(1) Control the sludge load. The normal load of the general treatment system of the sewage treatment plant is 0.2~0.45kg/(kg·d). Sludge expansion may be exceeded when it occurs. To prevent expansion, the sludge load rate should always be controlled within the normal load range.
(2) Control the proportion of nutrients. The ratio of normal carbon (expressed in BOD5), nitrogen and phosphorus in a general aeration tank is BOD5:N :P=100:5:1. When BOD5:P is high, filamentous microorganisms can store excess parts in the body. When the nutrient concentration is insufficient, filamentous microorganisms are still stored, which enhances the competitiveness of filamentous microorganisms for floc-forming bacteria.
(3) Control DO concentration. In order to prevent the surge of filamentous microorganisms, the DO in the pool should generally be controlled above 2.0mg/L. Because the minimum DO concentration to prevent sludge expansion is a function of the sludge load F/M. So when F/M increases, the minimum DO concentration should be increased accordingly.
(4) Add chlorination, ozone or hydrogen peroxide. These chemical agents are used to selectively control the excessive growth of filamentous microorganisms.
(5) Add coagulant. Lime, ferric chloride or polymer flocculants can be added to improve flocculation of the sludge while also increasing the strength of the flocs.
(5) Sludge swelling caused by non-filamentous bacteria. Sludge swelling sometimes occurs when filamentous microorganisms are not present. This swelling is related to the decoagulation effect, when free bacteria produce a bacterial gum mass matrix, it will lead to sludge expansion, often called bacterial gel mass expansion or viscous expansion. This failure is due to the large number of extracellular polymers produced in the floc microstructure, which has a paste-like or jelly-like appearance, which can be clearly distinguished from normal flocs with Indian ink reverse staining. After normal floc staining, the ink will penetrate deeply into the human floc, while the floc with extracellular polymers can resist the penetration of impregnation.